[HN Gopher] Webb and Hubble confirm Universe's expansion rate
___________________________________________________________________
Webb and Hubble confirm Universe's expansion rate
Author : thunderbong
Score : 605 points
Date : 2024-03-11 20:49 UTC (1 days ago)
(HTM) web link (www.esa.int)
(TXT) w3m dump (www.esa.int)
| JohnMakin wrote:
| This sort of brushes on it but for a long time there was hope at
| resolving the Hubble Tension by saying that Hubble telescope's
| measurements were incorrect, because that would be the most
| simple explanation. This was not the case, so if anything, the
| mystery deepens. I don't know for certain but I believe Hubble's
| estimation has been widely accepted for a while though, because
| we've been using the 13.8 billion cosmological age estimate ever
| since I started brushing up my layman's understanding of the
| subject.
| orra wrote:
| > we've been using the 13.8 billion cosmological age estimate
| ever since I started brushing up my layman's understanding of
| the subject.
|
| I remembered the age of the universe as as 13.7 billion years,
| but I wasn't sure why that was.
|
| Well, the initial WMAP results in 2003 supported an age of 13.7
| billion years. Later results nudged this upwards to 13.8
| billion years. Of course, all the results have error bars.
| explaininjs wrote:
| > Of course, all the results have error bars.
|
| Not to mention the underlying philosophical assumptions, such
| that the "rate of time" has been constant across all of...
| time.
|
| Aka: How do we know a "year" 13 Billion "years" ago bears any
| resemblance to one now? What would it mean for it not to?
| fooker wrote:
| Philosophical indeed, as it's impossible to define the idea
| of a rate of time, when the idea of rate is defined in
| terms on time itself.
| explaininjs wrote:
| Yes, an external reference clock is needed.
|
| If a computer chugs along doing { counter++; } at 1 clock
| cycle per clock cycle for 13.7 billion clock cycles, it
| will think 13.7 billion clock cycles have passed when
| counter is 13.7 billion.
|
| On the other hand, if a computer chugs along at one clock
| cycle per clock cycle for 1 clock cycle, and reads
| &counter and sees 13.7 billion, it will think 13.7
| billion clock cycles have passed.
|
| Either way it's perfectly capable of introspecting it's
| source code and logically stepping back until the memory
| location was 0 to see how many clock cycles would have
| been required to reach it's current state, but that sort
| of reasoning is completely devoid of meaning without both
| perfect knowledge of what the true start state was, and a
| guarantee that no external influences have occurred.
|
| Here in reality, we know neither the our start state nor
| our isolation level, but the hubris of many is too string
| to not at least try finding some logical step-back
| functions and iterating them until they don't know how to
| go any further, then proudly proclaiming that "The
| Start". (after all - how could it _not_ be The Start,
| look, I can iterate the inverse of my step-backward
| function from then to now and it matches! QED!)
| jMyles wrote:
| I agree with you 100% on your assessment of what can be
| known, but I think I disagree nearly as strongly on your
| assessment of the humanity that springs from attempting
| to know it.
|
| To exist in the state you describe - with neither start
| state nor isolation level measurable in any tamper-proof
| way - and to yet still dedicate one's life to observing
| and pondering the complexity of the resulting cosmos is,
| to my eye, laudable and beautiful.
|
| Where you see hubris, I see humility. Everyone who
| attempts to expand the corpus of human understanding of
| cosmology knows that the endgame is somewhere short of
| perfection. And yet they are inspired to carry on. It
| seems to me that matters of state, economics, medicine,
| technology, and many other fields will benefit from a
| similar disposition.
| explaininjs wrote:
| I think the hubris comes in when folks assume that
| because that answer is presented by Science/Observation
| rather than Religion/Philosophy, it's somehow guaranteed
| to be "more accurate", when in reality both are guesses
| with equally unknowable error bars.
| fooker wrote:
| This reminds me of old computer games that started to run
| faster on newer computers, because of strong assumptions
| about clock speeds!
|
| For a hypothetical entity inside the game, nothing would
| have changed.
|
| https://www.vogonswiki.com/index.php/List_of_CPU_speed_se
| nsi...
| dartos wrote:
| I'd assume that we have some notion of how the laws of
| physics have changed, if at all, since the Big Bang.
|
| We measure time in vibrations of a cesium isotope IIRC
| db48x wrote:
| No, we measure the frequency of vibrations of _light_,
| not of a type of atom. Specifically, it is light emitted
| by cesium atoms that are transitioning from one specific
| energy state to another specific energy state. Although
| this is arbitrary, it is highly reproducible and would
| give precisely the same measured lengths of time at any
| point since the big bang.
| elashri wrote:
| That assumes that fundamental laws of physics did not
| change (will not change). This is what we believe and
| have no evidence otherwise. This is important since we
| rely on measuring atomic transitions of cesium atoms
| which itself were formed/forming billions of years after
| the big bang itself.
|
| The laws of physics invariance under time is a core to
| our understanding. It would be very disrupting if we
| found otherwise.
| not2b wrote:
| Right, but most deviations one can think of (like,
| changes over time to physical "constants") would have an
| observable effect, so ancient galaxies would look much
| more different from modern galaxies and spectra wouldn't
| look the same other than a red shift, which moves all the
| lines in a uniform way.
| yongjik wrote:
| Fun fact: the Oklo reactor, a _naturally occurring_
| nuclear reactor that was active more than a billion years
| ago, was used to test if physical constants were the same
| in ancient times.
|
| https://en.wikipedia.org/wiki/Natural_nuclear_fission_rea
| cto...
| drjasonharrison wrote:
| And we can split hairs and conclude that for "the last
| two billion years, on this planet, in this galaxy, the
| physics affecting nuclear decay have not changed"
|
| It's great to know that say dating using carbon-14 decay
| is still useful over those time ranges on planet earth (I
| don't know if that is something we care about given that
| fossils don't tend to contain much carbon, but coal and
| oil deposits are around 400 million years old).
|
| I don't want to imply that this is too small of a sample
| size, but I will imply that nuclear decay, and the
| movement of galaxies across the universe might be
| unrelated. Don't know. Not sure how we'd measure that.
| Supernova observations would tell us about nuclear fusion
| and it's limits. Does it tell us about nuclear fission? I
| don't know.
| mr_toad wrote:
| "Before" the Big Bang it's not even clear that there were
| laws of physics.
| speak_plainly wrote:
| Time in this context is just an arbitrary measurement. Like
| extrapolating the calendar back to the Big Bang which is
| when space/time began, another way to think about time.
| dwattttt wrote:
| How do we know <anything>? Observations we can make plus
| models that relate those to the things we can't observe
| directly.
| vlovich123 wrote:
| Unfortunately we can't observe fundamental things like
| "what are the rules of physics and time at the beginning
| of the universe?". We look for clues and make large
| assumptions, but given that the universe experienced a
| 10^78 factor expansion during the Big Bang, assuming that
| actually happened, then why would it make sense to assume
| that the rules of the universe today are the rules for
| the very early beginning of the universe? A strand of DNA
| would become 10 light years. Given that relativity
| redefined our understanding of basic physics but only
| applies as we approach the speed of light, it would stand
| to reason that the rules of physics would be different
| from our current models based on today's observations
| when the matter of the universe is packed much more
| tightly together.
| itishappy wrote:
| Entirely reasonable assumptions! Our models match
| surprisingly well though... The CMB has a blackbody
| spectrum that aligns with predictions, we see galaxies
| more or less when and where we'd expect them, stellar
| populations look like what we'd expect for a universe
| made of hot hydrogen, and more! It's not quite perfect,
| but modern physics explains stuff really quite well even
| billions of years ago!
| explaininjs wrote:
| Modern physics is guided by those observations, they
| can't be then used as an argument for its veracity.
|
| Let's say I walk into a room and observe someone writing
| a tally mark on a chalk board once every second. I count
| 4x10^17 tally marks. I might assume that 4x10^17 seconds
| ago that same person entered the room and started
| tallying. I might even observe for the next 4x10^17
| seconds they continue to tally. Heck I might even see a
| recorder going that when I play back at what I assume to
| be 1x speed, has chalk scratches at regular intervals for
| 4x10^17 seconds. I still don't have any actual evidence
| that they started those 8x10^17 seconds ago.
| vlovich123 wrote:
| The inflationary epoch where it expanded by 10^78 in
| volume happened in the first 10^-32 seconds. The furthest
| galaxy we can see (fairly poorly) is 300M years after the
| Big Bang. It's likely if time or the rules were
| different, 300M years was enough for things to mostly die
| down to steady state. And as you say, they match more or
| less but those errors could easily hide remnants of when
| things were different. Of course, these are all numbers
| that assume the Big Bang theory is correct which is
| difficult to impossible to falsify since we can't
| possibly observe or test anything from that long ago.
| We'll have to wait to see if refinements to our model
| that clear up contradictions change what we think about
| the beginning of the universe and other boundary
| conditions.
| Veserv wrote:
| Well for one, if the laws of physics are not time invariant
| (i.e. the laws of physics are not the same at all points in
| time) then energy can be created or destroyed [1]. So that
| would be quite a shocker.
|
| [1] https://en.m.wikipedia.org/wiki/Noether%27s_theorem
| itishappy wrote:
| Doesn't dark energy do exactly that? 70% of the energy of
| the universe doesn't seem to want to play by our rules!
| digging wrote:
| Yes, but my understanding is that dark energy doesn't
| play by the same rules, it's an exception. I certainly
| can't explain why but also it may not be known exactly
| why, given dark energy is an unexplained phenomenon.
| cygx wrote:
| The party line is that energy is not conserved at
| cosmological scales. However, it's more of a semantic
| question: We can tell you exactly by how much it gets
| violated (that's basically the first Friedmann equation),
| and if you prefer, you can attribute the missing energy
| to the gravitational field. A lot of physicists don't
| like that approach as it isn't possible to write down a
| corresponding stress-energy tensor, ie gravitational
| energy cannot be properly localized.
| explaininjs wrote:
| Do you think the current amount of energy in the universe
| is 0? If not, how was it created?
| baq wrote:
| That's a completely different question, to which the
| answer is 'we don't know'.
| explaininjs wrote:
| It's really not. If there is nonzero energy, and our
| current models don't allow for energy to be created,
| either we need new models that do allow for energy to be
| created (time-variant physics, for instance), or we need
| the existing models to propose a way it was initialized.
| Without that there's a gaping hole in the model the size
| of all of the energy in the universe.
|
| Certainly you can't 1) have a model where energy is
| constant 2) believe there is nonzero energy in the
| universe and 3) dismiss any model where energy _can_
| change as bogus out of hand because you believe it should
| be constant, _without_ counter-proposing how it even got
| here.
| mrguyorama wrote:
| >or we need the existing models to propose a way it was
| initialized.
|
| We have that, it's the big bang.
| explaininjs wrote:
| How remarkably unsatisfactory.
|
| The argument being:
|
| You can't propose physics ever changed, that'd mean
| energy could have been created at some point!
|
| Energy was created at some point, wouldn't it be nice to
| know when?
|
| Everything created at the Big Bang exactly as it is now,
| stop asking questions.
| baq wrote:
| Nobody argues it wouldn't be nice. Physics argues we
| can't ever know. It's a rather significant difference and
| if this gets proven wrong, you'll find that physicists
| are the ones partying the hardest.
| orra wrote:
| You're asking to go too far back in time.
|
| The difficulty is we don't know what happened in the
| first planck second after the big bang, let alone before
| the big bang (if that's meaningful).
|
| We haven't unified quantum mechanics and relativity.
| Hence we can't be certain that singularities exist, or
| that the universe started off in a singularity.
| explaininjs wrote:
| I'm aware. What I'm saying is if that's how much you
| don't know, you'd better not make any claims like "X
| can't be accurate because it doesn't match what I think I
| do know".
| hnfong wrote:
| Saying "X is true because otherwise, it would be quite a
| shocker" isn't really a proof.
|
| It's almost an appeal to common sense. (which,
| admittedly, is often the best argument we have). IMHO
| there's often a bit of over-confidence among scientists
| about the universe being 13 billion years old and what
| happened during the Big Bang, if it just relies on such a
| common sense argument.
|
| I know it's _unscientific_ to suggest maybe laws of
| physics are not time invariant across such scales,
| because until we have a time machine we can 't test this
| theory, but then flipping the argument (that laws of
| physics are _definitely_ time invariant) is also
| technically unscientific -- we 're basically assuming
| this without strong evidence.
|
| This goes back to the old debate on the problem of
| induction in science, (see David Hume, Karl Popper, etc.)
| and I think it isn't emphasized enough in modern
| discussions that, perhaps, there's a small chance that
| these foundational concepts in physics could be invalid.
| Veserv wrote:
| I did not present proof. They asked what it would mean
| and a consequence of physical laws not being time
| invariant would be that energy does not need to be
| conserved.
|
| To the extent that relates to epistemology that would be
| more like anti-proof I guess? It does not prove that the
| laws are time invariant, rather it raises the bar to
| demonstrate that the laws are not time invariant because
| that means energy need not be conserved. You can not get
| one without the other without attacking even deeper
| fundamentals of modern scientific models. So you must
| either demonstrate the linked claims, which are pretty
| foundational themselves, or you must overturn basically
| everything; both of which demand very robust evidence.
| wavefunction wrote:
| I have had the same thought that primordial reality had the
| same timestream as us but it was much longer, like the
| first "year" of reality was far longer than one year today,
| just the thinking "could time have been shorter or longer,
| why not"
| denton-scratch wrote:
| > the first "year" of reality was far longer than one
| year today
|
| This talk about the "rate of time" doesn't make any sense
| to me. A second takes one second, and always has.
|
| Isn't this like asking whether the length of a metre
| might have changed over time? Or the mass of a
| kilogramme? It looks to me like a category error.
| explaininjs wrote:
| `This talk about the "rate of execution" doesn't make any
| sense to me. A cycle takes one cycle, and always has.`
|
| The root of the question is whether there exists an
| external "system clock", and what that would even mean.
| denton-scratch wrote:
| > whether there exists an external "system clock"
|
| I'll ignore your analogy with a CPU, which I don't think
| is apposite.
|
| Whether there's a 'system clock', a sort of reference
| clock that can tell you how fast time is passing, so you
| can calibrate other clocks against it, seems to be the
| same category error. As far as I can see, either time is
| all there, all at once; or one second lasts exactly one
| second. (I've perpetrated the same category error there,
| because a second doesn't 'last' for some period of time;
| it just is).
|
| [Edit] That's not very clear. I mean: if you are
| measuring a distance, you use a calibrated ruler. If you
| doubt the calibration of your ruler, you might
| recalibrate for precision. But you don't have a clear
| idea of what one foot is, it doesn't make any sense to
| ask whether your one-foot ruler has grown or shrunk; how
| would you tell if the purported 'fact' is true or false?
|
| And if some fact about the Universe means that it has
| grown or shrunk, how would you tell? If you can't in
| principle tell whether a 'fact' is true or false, it
| follows that the 'fact' isn't a fact, because it has no
| effect on anything.
|
| See Russells Teapot.
| explaininjs wrote:
| You're certainly welcome to completely ignore the
| question at hand, it's just surprising that you'd use so
| many characters to do so.
|
| The question is if there's a different sort of _thing_
| beyond our idea of time. We have our concept of N caesium
| oscillations is the base reference for everything, and
| the duration of all action is derived from it. Could
| there be more depth to the rate at which things occur
| than that, especially on cosmic scales? Could other
| processes, which operate at different base clock levels,
| be interacting with the universe we observe in ways we
| don 't yet understand? Could those processes have clock
| levels that vary over history with respect to our caesium
| definition?
|
| You can claim not, that's its precisely that shallow. But
| neither of us can provide evidence either way, and your
| belief is simply much less interesting to me.
| mr_toad wrote:
| Rates are measured with time as the divisor. What does rate
| of time even mean?
| perihelions wrote:
| Here's a graph of the contradictory measurements (JWST data not
| yet included),
|
| https://en.wikipedia.org/wiki/Hubble%27s_law#Determining_the...
| (caption: _" Value of the Hubble constant in (km/s)/Mpc,
| including measurement uncertainty, for recent surveys[54]"_)
| gregorymichael wrote:
| What are the implications?
| kromem wrote:
| Basically it means that we can't regard the Hubble result as a
| mismeasurement and the age of the universe seems to be
| different depending on how you measure it.
|
| From the article:
|
| "The bottom line is that the so-called Hubble Tension between
| what happens in the nearby Universe compared to the early
| Universe's expansion remains a nagging puzzle for cosmologists.
| There may be something woven into the fabric of space that we
| don't yet understand."
| washadjeffmad wrote:
| Sounds like they don't want to spoil everyone's research
| grants!
| jdiff wrote:
| Outside of the tinfoil, it just sounds like the universe is
| complex and not always predictable.
| washadjeffmad wrote:
| I meant it as a comment on the phrasing. Potentially
| jeopardizing a field's cash flow is a legitimate worry,
| and I see a few have felt that, as well.
| bigbillheck wrote:
| There's a whole lot of open problems in cosmology,
| nobody's going to be out of work if they solve this one.
| harywilke wrote:
| It's amazing that barely a 100 years ago The Great Debate
| in astronomy was weather the Milky Way was the extent of
| the universe or things like Andromeda were their own
| 'island universes'. In the 1920s, Edwin Hubble showed
| that Andromeda was far outside the Milky Way by measuring
| Cepheid variable stars. These are the same stars that we
| are measuring today in this debate.
| https://en.wikipedia.org/wiki/Great_Debate_(astronomy)
| chadcmulligan wrote:
| And not much longer before that the discussion was how
| long would the sun last - 5,000 years or so was the
| estimate if it was a big ball of burning gas (source: A
| scientific American article I read, wish I could find it
| again, hoping someone here knows)
| yokoprime wrote:
| I think it's this one you're referring to
| https://www.scientificamerican.com/article/experts-doubt-
| the...
| chadcmulligan wrote:
| That's the one, thanks, I've been looking for it for ages
| techwiz137 wrote:
| I just think it means the expansion rate is not a constant,
| but a variable.
| rihards__ wrote:
| I kind of agree with this conclusion.
|
| Before we know better it can be just that spacetime was
| expanding at a different rate (we still would need at least
| one another Planck that operates in roughly same range to
| confirm this).
|
| Hubble wavelenght range - 0.1 to 0.8 mm Webb wavelenght
| range - 0.6 to 28.3 mm Planck wavelenght range - 330 to
| 10000 mm
|
| My understanding is that Planck was observing photons that
| have happened much more earlier.
| mr_mitm wrote:
| Either we live in an unusually under dense region of the
| universe or our models are wrong ("new physics").
| amethyst wrote:
| Or it's a simulation and someone keeps pushing changes to
| production.
| cwillu wrote:
| Which would also count as new physics.
| yreg wrote:
| With even more literal meaning of new.
| lajr wrote:
| I wonder if their introspection is good enough to have our
| population on a Grafana dashboard somewhere
| queuebert wrote:
| Somewhere aliens are making fun of how shoddy our
| simulation is coded.
| brcmthrowaway wrote:
| It's definitely a simulation at this point
| ducttapecrown wrote:
| All the expert software engineers agree this is the most
| likely explanation. Have physicists looked into this?
| NateEag wrote:
| > All the expert software engineers agree this is the
| most likely explanation.
|
| That's quite a strong claim. I'm skeptical. Sources?
|
| > Have physicists looked into this?
|
| They shelved it right next to "God Made The Universe" in
| the "Unfalsifiable Propositions" section, under the title
| "Grad Students Made The Universe."
| kaashif wrote:
| I'm reading their comment as a joke about how software
| engineers tend to overestimate their own expertise on
| things like physics and are not actually anywhere close
| to experts.
|
| Software engineers presenting weird pseudo science as
| serious physics is one way this manifests.
|
| I could be wrong.
| imzadi wrote:
| Someone keeps running gparted on our partition
| nsilvestri wrote:
| This URL is a stub, and the full article can be read at
| https://www.esa.int/Science_Exploration/Space_Science/Webb/W...
| cwillu wrote:
| Added bonus: it has 50% less animated "responsive" design.
| jjbinx007 wrote:
| I wonder if the mods can change the main article link to the
| one you provided instead?
| dang wrote:
| Indeed we can!
| dang wrote:
| Thanks! We've changed to that from
| https://www.esa.int/ESA_Multimedia/Images/2024/03/Webb_Hubbl...
| above.
| FredPret wrote:
| Unbelievable how much we can work out from just the odd photon
| hitting us from somewhere in the great unknown.
| golemotron wrote:
| Occam's Razor does a lot of heavy lifting.
| daxfohl wrote:
| It's astounding to me that space is so empty that in the
| billions of light years from here to the edge of the universe,
| there's orders of magnitude less total interference than what
| you get from a small cloud.
|
| It's also astounding that celestial objects emit enough photons
| that thousands per second travel within the arc that goes the
| distance from the star to somewhere inside the radius of your
| pupil.
|
| And if this wasn't the case, then we'd never even be aware of
| any of this.
| suzzer99 wrote:
| Also that the light and gravitational waves from colliding
| neutron stars can travel 100 million light years and arrive
| at earth within a second of each other. That's a mind-
| bogglingly small amount of drag.
| CobrastanJorji wrote:
| "The odd photon hitting us" is also a pretty good description
| of eyesight, radio, and fiber optics.
| fooker wrote:
| Clearly the even photon is for parity checking and error
| correction ;)
| westurner wrote:
| Hubble's law: https://en.wikipedia.org/wiki/Hubble%27s_law
|
| Expansion of the universe:
| https://en.wikipedia.org/wiki/Expansion_of_the_universe :
|
| > _While objects cannot move faster than light, this limitation
| only applies with respect to local reference frames and does not
| limit the recession rates of cosmologically distant objects_
|
| Given that v is velocity in the opposite direction, and c is the
| constant reference frame speed of light; do we account for
| velocity in determining whether light traveling at c towards
| earth will ever reach us? v - c < 0 if v>c
| v + c > c if v>0
|
| Are tachyons FTL, is there entanglement FTL?
|
| How far away in light years does a mirror in space need to be in
| order to see dinosaurs that existed say 100 million years ago?
| blackbear_ wrote:
| > do we account for velocity in determining whether light
| traveling at c towards earth will ever reach us?
|
| As far as I know this is not necessary because the speed of
| light is constant regardless of the velocity of both the source
| and the observer (this is Einstein's special relativity:
| https://en.m.wikipedia.org/wiki/Special_relativity)
| arbitrage wrote:
| Tachyons aren't a thing. Tachyons are sci-fi nonsense.
|
| Nothing in the universe can travel faster than the speed of
| light. This does not hold for the universe itself. It can and
| does expand faster than the speed of light, using specific
| reference frames (i.e., big enough).
|
| So, space can increase FTL. Particles do not travel faster than
| light tho, that is nonsense.
| rthnbgrredf wrote:
| Well, we have (virtual) particles that can travel backwards
| in time, without breaking causality. There's no proof that
| Tachyons exist, they are purely hypothetical, but they are
| not outright nonsense.
| cbolton wrote:
| Take a star in a region of the universe that recedes from us
| at 3c. In what sense is the star not traveling faster than
| the speed of light relative to us?
| a_random_canuck wrote:
| > do we account for velocity in determining whether light
| traveling at c towards earth will ever reach us?
|
| No, because the speed of light is constant for all observers.
| From our frame of reference on earth, light from distant
| receding galaxies is always moving towards us at exactly the
| speed of light, c. Those galaxies also observe the light moving
| away from them at exactly c.
|
| That seems contradictory and unintuitive, that two observers
| moving away from each other both measure light moving c
| relative to themselves, but that's reality.
|
| It's another measurement that changes: if c is always constant,
| then it must be the passage of time and the distance travelled
| that we observe differently.
| pigpang wrote:
| > That seems contradictory and unintuitive, that two
| observers moving away from each other both measure light
| moving c relative to themselves, but that's reality.
|
| Light is a wave. Photon is a complex thing (Hopfion?), but
| it's a wave, so it waves something, a medium. Speed of wave
| propagation in a medium is constant. IMHO, it's intuitive.
| mrguyorama wrote:
| >but it's a wave, so it waves something, a medium.
|
| Which was unobserved in the michleson-morely experiment and
| other followup experiments because....?
| pigpang wrote:
| LIGO/Virgo are better version of Michelson-Morely
| experiment. Gravitational waves are found, so existence
| of _a_ medium is confirmed.
| mrguyorama wrote:
| Gravitational waves existing doesn't confirm the
| existence of a medium any more than discovering light
| behaves like a wave confirms the existence of the aether.
|
| If you want to posit that light has a medium, you need to
| redo 100 years of physics, so start showing your work.
| You mention elsewhere "doing a calculation", and that's
| just not nearly good enough. You want to overturn
| perfectly working physics, you NEED to show up with
| receipts. That's table stakes.
| tzs wrote:
| A little background based on a few articles about this plus my
| recollection of PBS Space Time videos on this: There are at least
| two ways to try to figure out the rate of expansion of the
| universe (which is called the Hubble Constant).
|
| * From variations in the cosmic microwave background (CMB) which
| are the result of certain conditions in the early universe it is
| possible to figure out what the expansion rate should be now.
|
| * From looking at very distant galaxies and noting how far away
| they are and how fast they are receding from us the expansion
| rate can be calculated.
|
| Theory says that these should give the same expansion rate. When
| the rates were first found using those two methods they gave
| different results, but the error bars on both were large enough
| to overlap. People expected then that further refinement of both
| methods to decrease the error bars would converge to some common
| value.
|
| That did not happen. Refinement of the CMB measurements got to 67
| +/- 0.5, and refinement of the galaxy distance/speed method got
| to 73 +/- 1. Those do not overlap.
|
| This non-overlap between the possible ranges given by the two
| methods is called the Hubble tension, and it is one of the most
| irksome problems in cosmology.
|
| Possible explanations include:
|
| * Some sort of error in how we measure the variations in the CMB.
|
| * Some sort of error in the distant galaxy distance or speed
| measurements, which until the James Webb telescope were almost
| entirely Hubble telescope measurements.
|
| * We're missing something in our understanding of the physics.
|
| These new results add a bunch of data from the James Webb
| telescope, which observes in different wavelengths than Hubble.
| These results fit with the Hubble measurements.
|
| They do _not_ resolve the Hubble tension. What they do is remove
| most doubt that the distant galaxy results involve some sort of
| Hubble measurement error. I believe cosmologists are pretty
| confident of the CMB measurements, and so this will be
| interpreted as telling us that the Hubble tension is not just a
| problem with our measurements. There is either physics that we
| got wrong or physics we need to discover.
| rf15 wrote:
| This, together with the endless philosophising around dark
| energy, dark matter and whatnot paints a pretty strong arrow
| towards our models having some flaws when it comes to their
| large-scale application. I hope to live long enough to see
| where we made our mistake and get a better model.
| pishpash wrote:
| Almost every model we build of more trivial things based on
| observation always turns out to be not really right. I cannot
| imagine why one of the universe that has had multiple version
| updates in the last 100 years to not also be grossly
| mistaken. I also don't expect the full model to be simple or
| beautiful. We may be thinking wishfully based on massive
| extrapolation and cutting corners to suit our narrow view
| into the world.
| CuriouslyC wrote:
| I think the truth is likely that we live in a very complex
| universe that can be approximated by these sweeping laws in
| general, but is very "messy" close up. Consider that while
| the universe roughly obeys probabilities, it also has will
| and intent (to a degree we can debate in another
| conversation).
| kaashif wrote:
| What do you mean by will and intent?
|
| I guess if humans have will and are part of the universe,
| then the universe has it too, but I don't think that's
| what you meant.
| CuriouslyC wrote:
| Hopefully we can agree that at least part of the universe
| "chooses" outcomes, rather than them occurring randomly.
| You could assume that "choosing" is what random outcomes
| feel like but I don't think that's productive.
|
| Given that we have evidence of physical systems evolving
| their states by conscious choice, that begs the question
| "Why would large, complex physical systems have a
| governing force that doesn't exist in smaller physical
| systems?" Occam's Razor suggests that consciousness and
| will drive the evolution of all physical systems, and our
| elegant equations are merely statistical approximations
| following the law of large numbers given a population
| with a natural variance. Of course we can debate what it
| means for a low information system to be conscious and
| have will, but I think that's a more meaningful debate
| than trying to pinpoint the exact moment that a system
| becomes complex enough for the magical emergence fairy to
| sprinkle consciousness dust on inert matter.
| qup wrote:
| It's probably spaghetti code
| cowgoesmoo wrote:
| We know for sure that there are issues with our current
| theories. Our two best theories, general relativity and
| quantum mechanics, are not compatible with each other.
|
| https://en.wikipedia.org/wiki/General_relativity#Relationshi.
| ..
| dclowd9901 wrote:
| Interesting to think we only got the "middle" size physics
| right, but we can't reconcile it with micro or universal
| physics.
| ethbr1 wrote:
| I think about it as getting practical / low-tech
| observable physics right, then expanding out from there.
|
| < atomic and > planetary have _very_ important
| applications!
|
| But I'd argue not nearly as many as "Here are all the
| formula that govern a falling apple."
| hinkley wrote:
| I still have $20 down on "We are living in a universe inside
| a black hole."
| hughesjj wrote:
| I mean effectively we are regardless. The cosmological
| event horizon is a thing.
|
| Also, I forget off the top of my head, but there's some
| oddities depending on your chosen frame of reference when
| looking @ hawking radiation (skip to part about relativity
| of the vacuum) that I think could apply to the cosmic event
| horizon as well
|
| https://youtu.be/isezfMo8kWQ?si=5m_L6JtZ7p7Ls6xH
| superjan wrote:
| And don't forget cosmic inflation.
| michaelsbradley wrote:
| "Humason assembled spectra of the nebulae and I attempted to
| estimate distances." So wrote Hubble of his colleague Milton
| Humason in 1935 by which time spectra had been obtained for
| over 150 nebulae. Hubble was a stern warner of using the
| Doppler effect for galaxies and argued against the recessional
| velocity interpretation of redshift, convincing Robert
| Millikan, 1923 recipient of the Nobel Prize for Physics and
| director of physics at the California Insitute of Technology,
| that the redshift interpretation as an expanison of the
| universe was probably wrong, the year before both of their
| deaths in 1953.
|
| Hubble ended his book Observational Approach to Cosmology[+]
| with the statement:..."if the recession factor is dropped, if
| redshifts are not primarily velocity-shifts, the picure is
| simple and plausible. There is no evidence of expansion and no
| restriction of time-scale, no trace of spatial curvature, and
| no limitation of spatial dimensions. Moreover, there is no
| problem of internebular material. The observable region is
| thoroughly homogeneous; it is too small a sample to indicate
| the nature of the universe at large. The univers might even be
| an expanding model, provide the rate of expansion, which pure
| theory does not specify, in inappreciable. For that matter, the
| universe might even be contracting."
|
| [+] https://ned.ipac.caltech.edu/level5/Sept04/Hubble/paper.pdf
|
| source: https://plasmauniverse.info/people/contributors.html
| xeonmc wrote:
| Could it be that nearby galaxies are akin to small angle
| approximations in spacetime trajectory, but as you get really
| far away (e.g. CMB) the perspective distortion increases
| hyperbolically? I notice that if you normalize the Hubble data
| by their Lorentz factor you get back a constant expansion rate:
| https://www.desmos.com/calculator/llhnja1ocb
| rhelz wrote:
| For real? That is a very interesting observation.
| scotty79 wrote:
| Probably accidental?
|
| Redshifts can be way higher than 1 that in your scaling is a
| magic value.
|
| https://en.wikipedia.org/wiki/List_of_the_most_distant_astro.
| ..
| throwawaymaths wrote:
| There's a 'fringe' theory that the CMB is not the echo of the
| big bang but rather the redshifted black body radiation of
| intergalactic dust (apparently the numbers are about right).
| Although the primary proponent of this theory is trying to
| justify some sort of cyclic universe model with it, if my
| understanding is correct it would still be compatible with a
| 'standard' big bang that has the "longer" (aka galactic
| expansion) timeframe.
|
| And in any case physics still needs to explain what happened to
| that blackbody radiation.
| sebzim4500 wrote:
| How could that be true?
|
| Wouldn't that smear the spectrum so that it no longer so
| matches the black body radiation of a single object of a set
| temperature?
| throwawaymaths wrote:
| No. According to the standard big bang theory the CMB
| should look like black body radiation.
|
| As for smearing, IIRC black body curve has the central
| limit property
| sebzim4500 wrote:
| > According to the standard big bang theory the CMB
| should look like black body radiation.
|
| Yes, that's my point.
|
| >As for smearing, IIRC black body curve has the central
| limit property
|
| Surely that can only be true for a very specific
| distribution of temperatures? To go to an extreme, if you
| combine the spectrum of two objects of different
| temperatures you do not get anything like a black body
| curve.
| raattgift wrote:
| I agree with you; you can't build up a perfect blackbody
| spectrum by superposing different near-blackbody spectra,
| and that led to the cosmological thermalization problem.
| Furthermore, baryonic intergalactic dust isn't like dark
| matter: we see redshifted spectral features of components
| of such dusts imprinted on all sorts of backgrounds from
| quasars to the CMB. The dusts tend to be greybodies. (I'm
| ignoring the warm-hot intergalactic medium here).
|
| The CMB has narrow spectral lines imposed on it by
| galaxies, dust clouds, and similar objects. Those lines
| can be used to measure the CMB effective temperature at
| an object's redshift. A specific example of how this
| works is linked at the bottom of this comment. If a
| similar highly dusty object is found in the foreground of
| a bright quasar (merely nearby quasars were used in the
| case below), we could get an even tighter measurement of
| the CMB effective temperature at the dust, and perhaps
| look in new ways at how the CMB deviated (at the dusty
| object) very slightly from a blackbody spectrum. This
| amounts to a new line of evidence in the study of the
| expansion history.
|
| More generally, constraining the evolution of the CMB
| temperature in the matter-dominated era (T_cmb > 4 K, z >
| ~ 0.4) will prove fatal to ideas like some in this thread
| (which is already in trouble because of other
| spectroscopy e.g. the Lyman-alpha forest and Gunn-
| Peterson trough).
|
| I don't really understand the summary of the "'fringe'
| theory" at the top of the thread (and I can't identify
| which specific theory throwawaymaths means). I guess the
| idea could be that some unknown mechanism might
| homogenize the emissive components of baryonic
| intergalactic dust but not circumgalactic medium spectral
| features. I suppose one could try to investigate a non-
| adiabatic expansion to explain those features found in
| the CMB in a nonstandard way. Non-adiabatic components in
| the expansion have been examined by various theoretical
| teams in the context of quantum cosmology and/or "fifth
| force" dark energy, although mostly at much larger
| lookback times than the the standard surface of last
| scattering.
|
| I don't understand what was meant by '"longer" (aka
| galactic expansion) timeframe'.
|
| Finally, "echo of the big bang" isn't really helpful in
| understanding the origin of the CMB photons, why they're
| now filling space with a thermal Planck spectrum, and why
| we detect small variations in that spectrum in narrow
| views along different directions, but like "fabric of
| spacetime" I guess we're kinda stuck with the expression.
|
| - --
|
| (Open Access) Riechers, D.A., Weiss, A., Walter, F. et
| al. Microwave background temperature at a redshift of
| 6.34 from H2O absorption. Nature 602, 58-62 (2022).
| https://doi.org/10.1038/s41586-021-04294-5
| throwawaymaths wrote:
| I am not a physicist, but iirc the idea of the
| alternative explanation of the CMB is that if you time
| evolve backwards intergalactic dust over expansion, you
| reach a point where the dust gets really dense and
| effectively opaque.
|
| So this isn't summing all of the dust you go through, but
| rather the redshift all the way back to an era when the
| universe was super dusty (with the expected thermal
| signature of that era)
| raattgift wrote:
| Maybe you can dig up a reference, because you're on the
| verge of telling the conventional cosmology in a time-
| reversed way.
|
| Under time reversal eventually the very first stars
| disintegrate into mostly atomic hydrogen, which
| compresses and heats adiabatically. Eventually it becomes
| hot enough that the hydrogen ionizes. Ignoring processes
| which alter photon number, the hot nuclei and electrons
| form a dense fog.
|
| As with a fog here on earth, where incoming light can get
| bounced in a random direction off a fog droplet ("Mie
| scattering"), photons scattering off these hot charged
| particles are scattered in a random direction ("Compton
| scattering" at high energies, "Thomson scattering" at low
| energies). When we densify the fog, the free-streaming
| length of light decreases, increasing the fog's opacity.
|
| Under the normal direction of time, therefore, the CMB is
| when de-ionization enormously shrinks the analogue of the
| fog droplet size ("Thomson cross-section"), and expansion
| increases the distance between the fog droplet analogues.
| The free-streaming length can become effectively
| infinite, like clear night air after a fog dissipates.
| The spectrum of the light at that point encodes the
| effective temperature of the scattering medium. In an
| expanding universe, that spectrum will lose energy (i.e.,
| redshift).
|
| The electrically neutral mostly-hydrogen then takes three
| principal forms: collapsing clouds of gas, which
| eventually form the first low-metallicity stars; cold
| dusts of neutral atoms at various sparser densities; and
| a warm-to-hot sparse intergalactic medium. The latter two
| is where we should find the so-called "missing baryons",
| the large fraction of atoms not found in stars and
| galaxies.
|
| Backlighting by active galactic nuclei and UV-hot stars
| ruins the idea that a microwave-bright diffuse dust of
| electrically neutral atoms or molecules could generate
| the CMB with its spectral features. You'd have to keep
| the stars from heating the dust elements, while keeping
| the dust dense enough to generate the CMB photon-density.
| How do you preserve that density during expansion?
|
| At the top of the thread you said "the primary proponent
| of this theory is trying to justify some sort of cyclic
| model", which doesn't escape this point. (It also didn't
| lead me to a reference).
|
| However, if instead whoever you are struggling to
| remember is the proponent of an _eternal and static_
| cosmology -- with no expansion, ever -- some of these
| problems with the idea that the CMB can be the product of
| cold dust could be overcome. The idea might be that we
| still have a speed of light lookback, with more distant
| galaxies being older. The older galaxies being redder
| could be some sort of dust that is very thick in the
| distant past, thick enough to completely shroud whole
| galaxies like a lampshade, turning hot thermal Planck
| spectra into cold thermal Planck spectra. Then you have
| to (a) get rid of the dust over time without sending more
| or hotter photons in our direction, and (b) add spectral
| features to the cold thermal Planck spectra before it
| gets to us. I don 't see how either could be done without
| very different atomic and/or gravitational physics than
| we have in our solar system. Adding in supernovae --
| whose light-curves we see redshifted as per the article
| at the top -- makes this idea even harder, because then
| you need a lampshade-dust that down-converts photon
| energies in even more ways.
|
| I'm not aware of any published or even serious attempt to
| do this, although I didn't look too deeply into the
| literature beyond confirming that this wasn't something
| proposed by Jayant Narlikar.
| throwawaymaths wrote:
| > if you combine the spectrum of two objects of different
| temperatures you do not get anything like a black body
| curve.
|
| Also true for two normally distributed variables and yet
| the normal distribution has the central limit property.
|
| Presuming the hypothesis, do you suspect that you'd be
| looking at exactly two grains of dust, one of which has a
| crazy high temperature and the other has a crazy low
| temperature in any given pixel of the CMB?
| theoreticalmal wrote:
| I can't believe it. First they take Pluto from us, next
| they're going to tell me the static on my tv set isn't the
| remnants of the Big Bang, but just hot dust? My childhood is
| crumbling!
| taneq wrote:
| The hot dust is also remnants of the Big Bang, if that
| helps. Of course, so are we. :)
| orlp wrote:
| Your comment made me realize how some younger people have
| no idea of, or at least no first-hand experience of static
| on TV and radio since it is all digital nowadays.
| LilBytes wrote:
| My great grandparents had a black and white TV when I was
| a toddler, and I'm 36!
|
| The amount of change millennials and the generations
| before and after us have seen is boggling.
|
| I was out in country Victoria, Australia a few months
| back and the internet was TERRIBLE. I'm talking, JPG's
| loading line by line terrible. And this was on 'alleged'
| 4G.
|
| I felt pretty nostalgic for all of 2-3 minutes before I
| started pulling my hair out. I feel nostalgic about it
| again now.
| denton-scratch wrote:
| See also https://en.wikipedia.org/wiki/White_Dot
| brabel wrote:
| I have a super modern flatscreen. Sometimes, when it
| can't connect to the laptop it shows what I believe is
| "simulated" static noise! I love that :D
| sebzim4500 wrote:
| The static on your TV set is <1% CMB, mostly it is
| interference from other man-made sources and IIRC there is
| some coming from the sun.
| xwolfi wrote:
| https://www.sciencedaily.com/releases/2023/12/231201123626.h...
| lagrange77 wrote:
| Interesting, thanks!
|
| Out of curiosity: Can we actually differentiate between the
| expansion of space itself and the drifting of objects within it
| in the same direction?
| dmd wrote:
| You would have to come up with an explanation for such
| drifting to be occurring across the entire visible universe
| in precisely the same way, across billions and billions of
| light years.
| xeonmc wrote:
| Just a fringe speculation without any physical basis: maybe
| the Big Bang emits a 4-dimensional spherical shell of
| matter at _c_?
|
| With the direction in which we are traveling at _c_ called
| "time", and the other three which we 're _not_ moving
| through called "space";
|
| diverging rays having spatial velocity relative to each
| other equal to tan(alpha) i.e. flat projection;
|
| and at any given point on the 4-sphere's 3D surface
| everything appear to be diverging in 3D because the
| emission is spherically symmetric;
|
| and trajectories sufficiently paraxial to us fits the small
| angle approximation but those diverging widely has much
| more apparent perspective distortion,
|
| with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) =
| 1/Lorentz_Factor?
| dmd wrote:
| > without any physical basis
|
| Exactly - the "universe is expanding" explanation has the
| benefit of having tons of physical evidence for it,
| whereas your explanation has the drawback of being
| meaningless not-even-wrong word salad.
| dvsfish wrote:
| While its probably totally wrong, I think your tone and
| general attitude to people throwing around imaginative
| and creative ideas is extremely unproductive and frankly
| rude. If you want to stifle creative discussions, this is
| exactly how you'd do it. I found the post to be very
| stimulating regardless of its validity. And it's not like
| they're trying to spread it as gospel. There was a lot of
| disclaimer.
| mrguyorama wrote:
| If you want creativity, take an art class. This is
| cosmology, ostensibly a science. Pulling wild ideas out
| of thin air and basically ad-libbing a "theory" is not.
| dvsfish wrote:
| So we have to put everyone who dares to imagine down
| about it? I see value in sharing a wild idea purely for
| the inspiration it may give others to think about things
| from a unique perspective. A lot of great science started
| as a strange idea.
| mrguyorama wrote:
| What's the value in """daring to imagine""" about
| cosmology? This is nothing more than reality based fan
| fiction.
|
| >A lot of great science started as a strange idea.
|
| Disagree. A lot of great science started with "This weird
| math I'm doing is the only way to fit this data", which
| is an extremely different thing.
|
| Do you not think physicists and cosmologists and their
| nerdy friends have thought of wild ideas? Do you not
| think undergrad students spend drunken friday nights
| theorycrafting universes?
|
| It's pretty disrespectful to "why don't you just" a
| profession without showing that you at least did your
| homework first, or that you have looked at whoever
| definitely asked that question before you.
|
| There are people in this post reinventing Tired light and
| Aether based theories for crying out loud, theories with
| fairly conclusive reasons we abandoned them.
|
| When you just ass-pull theorycrafting without any of the
| fundamentals, all you do is talk in circles. It's no
| better than all the dumb "the brain is like a computer"
| arguments by analogy in neuroscience posts.
|
| It's not even wrong.
|
| We live in a world where the people who had to cheat off
| me in high school biology angrily insist that the vaccine
| doesn't work. Do we really benefit from playing such
| anti-scientific games?
| mr_mitm wrote:
| Thank you. It's good to know I'm not the only one who
| feels disrespected from people on the sidelines who
| clearly never opened a cosmology textbook.
|
| It's one thing to ask curious questions, but "why don't
| you just do the thing that was ruled out 100 years ago"
| and expecting a detailed rebuttal is a bit rich. It's
| like asking "why don't they cure cancer by cutting it out
| of the body". Or "why don't you just apply basic set
| theory to P=NP".
|
| It's always the same on these HN threads about cosmology.
| Even worse with dark matter, probably because you can
| understand rotating curves with high school physics. No
| one ever invents or roots for fringe theories on baryonic
| acoustic oscillations in the angular power spectrum of
| the cosmic microwave background.
| raattgift wrote:
| > No one ever invents ... fringe theories on baryonic
| acoustic oscillations
|
| BAOs are clearly the sucker-marks made when the great and
| mighty Kroll <https://tardis.fandom.com/wiki/Kroll>
| seized the walls of the universe!
|
| https://insidetheperimeter.ca/wp-
| content/uploads/2021/05/Bar...
|
| I'm not nearly as bothered by people typing out what they
| themselves admit are essentially physics fantasies
| (especially when prefaced with "I'm not a physicist" or
| the like), as by people who try to convince readers with
| even less exposure to physics that some alternative to
| good theories is clearly better but that the powers that
| be of theoretical physics are somehow suppressing it, and
| then clearly have little understanding about how the good
| theories work. There's a lot of that, especially in DM
| threads.
| root_axis wrote:
| I think if you're going to dismiss the comment in such a
| flippant and condescending manner you should at least
| explain what's wrong with it.
| dmd wrote:
| How does what they said differ from simply saying the
| universe is expanding - but in a word-salady way that
| obfuscates it?
| jabits wrote:
| This is a really interesting thought. I will be noodling
| on this tonight. Thanks
| arandomusername wrote:
| I think we observed some of the galaxies moving away from us
| at rates faster than speed of light, multiple times faster,
| which wouldn't work with objects drifting away if we hold by
| that speed of light is max speed matter can move at.
| brabel wrote:
| That's physically impossible. You can't observe something
| that's beyond your "light cone", as any galaxy "moving"
| (it's not really moving, it's the space that's getting
| expanding) faster than light would be. What you're
| referring to is the fact that we can confidently predict
| that galaxies at the edge of the observable universe, which
| we currently see moving away from us really fast, but as
| they were in the distant past due to their light taking
| billions of years to arrive at us, are currently, if we
| could actually see them where they are right now (again: we
| can't), "moving" faster than light away from us.
|
| Once a galaxy has moved beyond or "light cone", it's lost
| forever: you won't see it again even if you try moving
| towards it at light speed for all eternity.
| cygx wrote:
| The Hubble sphere (the place where recession velocities
| hit the speed of light) is not the same as the particle
| horizon (our past lightcone at current cosmological time,
| the boundary of the observable universe) or the cosmic
| event horizon (our past lightcone at infinite
| cosmological time, the boundary of the asymptotically
| observable universe).
|
| Cf the last paragraph of
| https://en.wikipedia.org/wiki/Hubble_volume
|
| _Observations indicate that the expansion of the
| universe is accelerating, and the Hubble constant is
| thought to be decreasing. Thus, sources of light outside
| the Hubble horizon but inside the cosmological event
| horizon can eventually reach us. A fairly counter-
| intuitive result is that photons we observe from the
| first ~5 billion years of the universe come from regions
| that are, and always have been, receding from us at
| superluminal speeds._
| cygx wrote:
| This is correct: For example, at time of emission of the
| light we receive today, GN-z11 had a recession velocity
| above 4c. A redshift of 1090 (which is the approximate
| redshift of the cosmic microwave background) corresponds to
| a recession velocity on the oder of 60c.
| BunsanSpace wrote:
| The speed of light, or C, is the max speed information can
| move through our 3d space.
|
| Having objects moving away from us at a speed greater than
| C, isn't weird. The observable universe is a 3d subspace of
| a higher dimensional object. A good analogy is a balloon,
| where there's a 2d subspace on a 3d object that's being
| inflated. Even if you can only move at a certain velocity,
| the balloon can inflate such that the 2d surface expands
| faster than the max velocity we prescribe for it, and would
| grow faster at the beginning even if we pumped a constant
| amount of air into said balloon.
|
| Great analogy for gravity too! because you could create
| dimples in the balloon (gravity wells) which would curve a
| straight trajectory, while being unnoticed to an observer
| on the 2d subspace.
| mr_mitm wrote:
| Yes, the "drifting" component you speak of is called the
| peculiar velocity. It's responsible for the "finger of god"
| effect if we don't account for it (galaxies appear to be
| aligned along the line of sight of the observer) and becomes
| negligible at large enough distances. I guess technically
| it's hard to differentiate because it's usually random, but
| it enters the error analysis and is accounted for.
| Kerbonut wrote:
| Could it be the rate of expansion is not constant?
| mr_toad wrote:
| https://en.wikipedia.org/wiki/Quintessence_(physics)
| rmbyrro wrote:
| The scale of the universe sometimes feels terrifying to me
| swader999 wrote:
| It's not if you consider we can only move at less than the
| speed of light or that we go on forever.
| ChrisMarshallNY wrote:
| The thing that strikes me, is the detailed image description.
| That looks like good fodder for an ML visitor.
| harywilke wrote:
| Dr.Becky goes over this in a video [0] from a year ago about the
| divergent results obtained by the two main ways we measure the
| rate of expansion. Cosmic Microwave Background Vs. Supernovae. As
| the accuracy of each method has improved, the end results have
| diverged.
|
| [0] 'theJWST just made the "Crisis in Cosmology" WORSE'
| https://www.youtube.com/watch?v=hps-HfpL1vc&t=858s
| pigpang wrote:
| Unfortunatelly, title of article on HN contradicts content of
| article, so many HN readers skip article because of "confirmed"
| (in other words: nothing new).
|
| IMHO, a Tired Light theory will better explain facts, but it
| will require paradigm shift, so there will be a lot of
| resistance before revolution.
| luc4sdreyer wrote:
| > but it will require paradigm shift, so there will be a lot
| of resistance before revolution.
|
| I think the main point of resistance is the incompatibility
| with observations. All Tired light models have been
| falsified: https://en.wikipedia.org/wiki/Tired_light#Specific
| _falsified...
|
| If you're aware of a model that can fit some or all of our
| observations, please share it!
| pigpang wrote:
| I have my own theory, which is not disproved yet:
| gravitational background noise slow down light a bit.
| Gravitation affects whole stream of photons in uniform way,
| not individual photons. Moreover, it doesn't change
| direction of photons, so no blur or scattering. My napking
| maths, which I did few months ago, tells that gravitational
| delaying should case effect of same magnitude as in red
| shift, (I did calculation for one frequency only, for
| proper calculation I need to know the temperature of the
| noise).
| luxuryballs wrote:
| those are some pretty old energy waves
| arcastroe wrote:
| I've asked this question before, but I don't think I received a
| good answer, so figured I'd try asking again.
|
| How do we know that galaxies are accelerating away from us and
| not moving at constant speed? People often point to the
| observation that the further away a galaxy is, the faster it
| appears to be moving away from us, implying acceleration.
|
| However, wouldn't we expect to see the same observation even
| without any acceleration? Imagine there are some objects in space
| all moving in random directions and speeds, relative to Earth.
| After long enough time, all objects will appear to be moving away
| from Earth, even if they were moving towards it initially. And
| after long enough time, the objects that move fastest should be
| farthest away, by the simple definition of speed!
|
| In short, even if galaxies weren't accelerating, we would still
| see that the further away a galaxy is, the faster it recedes.
| Steuard wrote:
| You're right: the "more distant galaxies are moving away
| faster" point is just Hubble's original observation of an
| expanding universe. It's not an argument for cosmic
| acceleration. (If you see people making that claim, they're
| probably either speaking carelessly or not experts themselves.)
|
| The conclusion that the expansion is accelerating was a quite
| recent result: 1990s, I believe. It's based on careful
| measurements of supernova explosions of a type with computable
| intrinsic brightness in increasingly distant galaxies, and the
| exact pattern seen in their apparent redshifts vs. apparent
| brightness. It was a shocking discovery when it came out, with
| two separate teams announcing the result pretty much neck and
| neck. There's also independent and compatible evidence for
| acceleration from the exact pattern of variations in the
| temperature of the cosmic microwave background seen at
| different points in the sky.
| runeb wrote:
| Their light is more red shifted the farther away they are. I'm
| no expert on this, but I believe in a constant-speed scenario
| they would have equal red shift no matter the distance
| willis936 wrote:
| The assumption made here is that relative velocity is the
| only method that would redshift light. Gravitational redshift
| is a thing and our model of gravity is incomplete.
| pdonis wrote:
| _> The assumption made here is that relative velocity is
| the only method that would redshift light._
|
| Not in our actual model of the universe, no. The redshift
| of light is determined by the spacetime geometry and the
| worldlines of the emitter and receiver. That is a general
| formula that works in any spacetime.
|
| _> Gravitational redshift is a thing_
|
| Not for the universe as a whole, no. Gravitational redshift
| is only meaningful in certain kinds of spacetimes, namely
| stationary spacetimes (which, roughly speaking, describe
| objects that either don't change with time at all, or which
| are periodic, like a rotating planet or star). The
| spacetime that describes our universe as a whole is not
| stationary and there is no meaningful concept of
| gravitational redshift.
|
| _> our model of gravity is incomplete_
|
| In the sense that we do not have a quantum theory of
| gravity, yes. But that does not affect anything under
| discussion here. Our current theory of gravity, GR, works
| fine for treating the expansion of the universe and whether
| or not it is accelerating.
| willis936 wrote:
| >Our current theory of gravity, GR, works fine for
| treating the expansion of the universe and whether or not
| it is accelerating.
|
| Then why are there phantoms in the data that need dark
| matter and dark energy to make the supposed working model
| fit them?
| pdonis wrote:
| I'm not sure what you mean by "phantoms in the data". The
| distribution of stress-energy is a free parameter in GR;
| it _has_ to be inferred from observations.
|
| The terms "dark matter" and "dark energy" are just names
| for, respectively, "stress-energy that acts like the
| matter we can see, but we can't see it", and "stress-
| energy that acts like a cosmological constant". Neither
| of those things poses any problem for GR, since both
| types of stress-energy are allowed for in the theory.
|
| "Dark matter" poses a problem for _particle physicists_ ,
| who have so far been unable to find any fundamental
| particles that would produce the observed properties.
| "Dark energy" only poses a problem if for some reason you
| don't like having a nonzero cosmological constant.
| willis936 wrote:
| >"Dark matter" poses a problem for particle physicists,
| who have so far been unable to find any fundamental
| particles that would produce the observed properties.
|
| It's clear to me why we haven't made any new discoveries
| in cosmology in the past two decades. It's this exact
| attitude of "the model is the truth". All models are
| wrong. The data can help you improve it, but you have to
| at least want to improve it.
| sophacles wrote:
| What do you think they do now?
|
| How do you propose they do it differently?
|
| What evidence do you have that what they are doing now
| doesn't work, and does the all the evidence of how they
| work support your hypothesis?
|
| Be detailed, because your comment just has some
| motivational speaker nonsense but no depth. For example,
| in the last 20 years cosmology has:
|
| + Refined its model of stellar formation based on
| observational data of the number of planets found
| observationally, and used this to validate and invalidate
| several model adjustments.
|
| + Observed galaxies that appear not to have dark matter,
| and by their existence and behavior validate some
| theories of dark matter, and validated others, which
| predict such galactic behavior. (e.g. some theories
| attempting to update gravity).
|
| + Run simulations of stellar and glactic formation that
| predicted structures in the universe that were later
| observed.
|
| Everywhere they look they are finding things the models
| don't explain well, and refining the models - that is
| literally using the data to improve the models.
|
| If you think you can come up with something better, then
| do it - all you gotta do is make up some mumbo jumbo and
| write down any old equation. It probably should:
|
| - provide the same results as were observed when the
| plugging in the experimental parameters of existing
| experiments.
|
| - explain "wierd stuff" in the data that existing models
| couldn't.
|
| - predict future observations of the known phenomena with
| the same or better accuracy as the old model
|
| - predict currently unobserved and unpredicted phenomena
|
| Go ahead and take a stab real quick - I'm sure you can do
| it. I mean Gallieo did it, so did Newton and Einstein.
| Next up is willis936.
| hnfong wrote:
| People can be right about problems with a process or way
| of thinking without being the next Einstein. There's no
| need to get personal.
| sophacles wrote:
| But in this case you and your friend are not right about
| the process. There was a claim that "the models aren't
| being updated based on the new data" which is
| categorically false. It's not that they pointed out
| problems in the process, it's that they flat out lied
| about things - such dishonesty doesn't help solve any
| problems you imagine you see, its just trolling.
| willis936 wrote:
| Why should I? I am dedicating my life to my biggest
| passion, which is not cosmology.
|
| If one does commit themselves to their passion they
| should strive to push the field further forward, not
| stagnate it.
|
| It can be done.
|
| https://arxiv.org/abs/2402.19459
| jMyles wrote:
| > we haven't made any new discoveries in cosmology in the
| past two decades
|
| Is that true?
|
| At least for my hobbyist understanding of the progress of
| cosmology, quite a lot seems to have happened in the past
| two decades. Confirmation of the Higgs Boson at CERN [0]
| kept me up all night to watch the press conference; I
| found it extremely exciting. (Maybe you count this
| strictly as observational particle physics and not
| cosmology, but I might appeal for it to be allowed in the
| context of your critique).
|
| And what of TFA? Isn't what we're reading now a new
| discovery in cosmology?
|
| What about the rush of exoplanet discoveries?
|
| What about the dramatically different galactic properties
| now observed in increasingly strange corners of the
| observable universe (including some which perhaps give
| insight into some of the properties of "dark matter" or
| whatever it ends up being)?
|
| 0: https://home.web.cern.ch/news/news/physics/new-
| results-indic...
| oneshtein wrote:
| Yes, it's true. Mainstream science refuses to accept
| anything radically new because of huge baggage. Nobody
| wants to look stupid, then relearn, recalculate,
| republish, reteach everything, or lose their tenures,
| grants, etc. It's why science advances in small
| incremental steps. AFAIK, there is a team of scientists
| secretly working on radically new set of theories (I got
| contact but cannot join because of war).
| mr_mitm wrote:
| That notion is ridiculous. Finding something radically
| new is every scientist's dream. Look at how Einstein is
| perceived, who arguably found one of the most radically
| new theories. Nobody thought he looked stupid or lost
| tenure. No one goes into science hoping to simply confirm
| what everybody already thought was true.
| mr_mitm wrote:
| The true reason we have made little progress in the past
| 20 or so years (and a 20 year slouch is historically
| nothing unusual) is that pretty much all data we
| collected in that time frame confirmed the standard
| model. It's the one big dilemma cosmology has. The
| standard model (LambdaCDM) works unreasonably well. Our
| problems with it are largely theoretical. New data is
| also hard to come by. Look at how long it took to plan,
| build and launch Euclid, cosmology's big hope of finding
| new physics. The hubble tension from the OP's article is
| already the most interesting discovery since 1998 when
| evidence for dark energy was first seen.
|
| And trust me, all scientists know that all models are
| wrong. This isn't some unique insight that is beholden to
| amateur scientists on the sidelines.
| px43 wrote:
| As I understand it, if the expansion was constant, farther
| away stuff would still be more red shifted. Stuff twice as
| far away appears to be moving twice as fast. It helps me to
| imagine the expansion of a metal cookie sheet, where the two
| edges are moving apart faster relative to each other compared
| to the speed that they're moving away from the center.
|
| The surprising bit is that the far away stuff seems to be
| even more red shifted than that, so we're not just expanding,
| but the rate of expansion seems to be accelerating.
| colechristensen wrote:
| If your idea was the case there would always be new things from
| very far away heading towards us, this is not the case. If the
| universe is flat and infinite there would be no end in supply
| of new galaxies with all velocities and you would always have
| the same mix as the initial mix of velocities. That's not what
| we see.
| NemoNobody wrote:
| I'm so glad "the infinite universe" as an idea is finally
| falling off. It works great in the Hitchhikers Guide, I love
| the floopy mattresses and planets that grow screwdrivers but
| nothing real is infinite.
|
| I didn't even realize how many people held that belief til
| that article about how the universe isn't as big as we
| thought
| colechristensen wrote:
| Huh? Every evidence points to a flat infinite universe.
| Nothing but speculation points to anything otherwise.
| snowwrestler wrote:
| > After long enough time, all objects will appear to be moving
| away from Earth, even if they were moving towards it initially.
| And after long enough time, the objects that move fastest
| should be farthest away, by the simple definition of speed!
|
| Sure, but to argue that this explains what we observe today,
| you would need to show that as of today it has been "long
| enough," which is its own can of worms to open.
|
| You might say "obviously it has been long enough for full
| sorting, because we observe a fully sorted data set of speed
| correlated with distance." But that would be begging the
| question.
| digging wrote:
| It's because the movement is ascribed to the expansion of space
| itself, not the individual galaxies. We don't have any reason
| to believe galaxies are moving in random directions at random
| speeds (not at scales that explain the redshifts we call the
| expansion of the universe).
|
| In your explanation, I think we'd expect to see some very
| distant, very slow-moving galaxies moving _toward_ us. And
| there may be some very fast-moving galaxies close to us that
| just started really far away. Objects would be entering our
| local universe from outside it, and that simply doesn 't
| happen.
| arcastroe wrote:
| > In your explanation, I think we'd expect to see some very
| distant, very slow-moving galaxies moving toward us
|
| Thank you, but I suppose I'm not really questioning the big
| bang piece. My question was mostly in regards to the
| continued acceleration piece. Feel free to disregard the "in
| random directions" part of my original post.
|
| I'm picturing more of an explosion in empty space. A firework
| or granade of sorts. Any individual dust/shard of the
| explosion still sees all other objects moving away from it
| and the rest of my question stands. But I suppose this would
| imply a "center" to the explosion, which I've also heard is
| not the case.
|
| Theres a few other comments offering more clarity to the
| acceleration piece. Thank you everyone!
| pdonis wrote:
| _> I 'm picturing more of an explosion in empty space._
|
| No, that's not what the big bang is.
|
| _> this would imply a "center" to the explosion, which
| I've also heard is not the case_
|
| That's correct. The big bang does not work like anything
| ordinary that you are used to imagining. The math is
| straightforward and unambiguous, but there is no good
| ordinary language description that corresponds to the math.
| alexb23 wrote:
| I find helpful this analogy of the space-time
| (4-dimension) expansion from the big bang: the surface
| (2D) of an expanding bubble. YMMV.
| lp4vn wrote:
| As I understand it, before the big bang the whole
| observable universe was contained in a small sphere and
| then it started to expand metrically. Is this
| interpretation correct?
|
| Another thing: suppose I point a laser beam to the space
| and by chance this laser beam never finds any kind of
| matter in its way, where is this laser going to? To an
| infinite void? Is it correct to say that stars radiate
| energy to the infinite then?
| pigpang wrote:
| It's just an interpretation. Your interpretation is
| similar to the Big Bang model of visible Universe
| expansion. If you can convince us that your model is
| better than other models, then we will use your model,
| but nobody can _prove_ than a model is correct, unless we
| will find a hidden recorder somewhere which was turned on
| for few dozens of billion years.
|
| Photon will hit something, or will travel until it will
| be redshifted to obvilion, or will travel until end of
| the medium (photon is a wave, so it waves something).
| denton-scratch wrote:
| > (photon is a wave, so it waves something)
|
| That sounds suspiciously like postulating the 'ether'.
| Surely what a photon 'waves' is the electromagnetic
| field, which is not a medium, and which fills the whole
| of spacetime. There is no 'end of the medium'.
| pigpang wrote:
| "Field" means 3d array of numbers. Spacetime means 4d
| array of numbers. You are talking about mathematical
| model of Universe, while I'm talking about physics.
| Mountain is not just an excitement in a height field. If
| photon is not waving something, then it's not a wave.
| Physicists prove that photon is a wave.
| michaelsbradley wrote:
| > If photon is not waving something, then it's not a
| wave.
|
| No, you've simply hit the limits of needing/wanting to
| understand something in terms of something else similar
| or more familiar.
|
| Makes the same point on a related matter:
| https://youtu.be/Q1lL-hXO27Q
| pdonis wrote:
| _> It 's just an interpretation_
|
| No, it's not, it's our best current model's description
| of the actual physical reality of our universe.
|
| _> nobody can prove than a model is correct_
|
| That's true, but it's also true that we _can_ show models
| to be _incorrect_ , as in, falsified by the data. For
| example:
|
| _> Photon will hit something, or will travel until it
| will be redshifted to obvilion, or will travel until end
| of the medium_
|
| For the scenario that was posed, a laser beam that never
| hits anything, none of your statements here are true. The
| first is ruled out by the scenario; the second is known
| to be false because there is no "gravitational redshift"
| of light in the universe as a whole (because models in
| which there would be such a redshift are known _not_ to
| correctly model our data on the universe as a whole), and
| there is no "end of the medium" (again, models in which
| there would be an "end of the medium", i.e., where the
| universe stopped containing matter and started being just
| vacuum, are known _not_ to correctly model our data).
|
| I have described what actually happens in my own response
| to the GP upthread.
|
| _> (photon is a wave, so it waves something)._
|
| Light is an electromagnetic wave; what "waves" is the
| electromagnetic field. (If you use a "photon" model, you
| are using the _quantum_ electromagnetic field as opposed
| to its classical approximation.) There doesn 't have to
| be any other "medium"; the electromagnetic field is
| present everywhere.
| pigpang wrote:
| > No, it's not, it's our best current model's description
| of the actual physical reality of our universe.
|
| OK, it's our best model, but it doesn't invalidate other
| models, less complete or less popular, it compete with
| them.
|
| > That's true, but it's also true that we can show models
| to be incorrect, as in, falsified by the data.
|
| Yep. The article is about the Huble Tension, which
| invalidates Big Bang model. We still use it.
|
| > the second is known to be false because there is no
| "gravitational redshift" of light in the universe as a
| whole (because models in which there would be such a
| redshift are known not to correctly model our data on the
| universe as a whole)
|
| The Big Bang model is incomplete too: galaxies with FTL
| speeds, different speeds of expansion, no center of bang,
| no flows, no source of energy, it stretches time and
| space, etc.
|
| I assume that the only infinite thing in infinite
| Universe is Universe itselft. All other things are
| finite. Thus, a photon has finite life, like any other
| wave.
|
| > there is no "end of the medium"
|
| The right-hand rule in EM suggests that we are in north
| hemisphere of something, so south hemisphere will have
| symmetrical rule, unless you believe that God chose
| right-hand rule for the whole infinite universe. If we
| are in a sphere, then that sphere rotates and have a
| boundary.
|
| > what "waves" is the electromagnetic field.
|
| "Field" is an array of numbers. You are mixing model and
| reality.
| pdonis wrote:
| _> it doesn 't invalidate other models_
|
| One model can't invalidate other models. Only _data_ can
| invalidate a model.
|
| What other models do you have in mind?
|
| _> the Huble Tension, which invalidates Big Bang model_
|
| No, it doesn't. It means we have more work to do, to
| figure out why two _calculations_ of the Hubble constant,
| by different routes, give different answers.
|
| Invalidating the Big Bang model would be finding evidence
| that there was no Big Bang at all. The Hubble tension is
| nothing of the sort.
|
| _> The Big Bang model is incomplete too: galaxies with
| FTL speeds, different speeds of expansion, no center of
| bang, no flows, no source of energy, it stretches time
| and space, etc._
|
| None of these are issues at all. The model accounts for
| them all in a perfectly self-consistent fashion.
|
| Also, your nomenclature is biased: for example, the "FTL
| speeds" you refer to are coordinate speeds, which have no
| physical meaning. "FTL" in General Relativity means
| "moving outside the light cones", and that does not
| happen.
|
| _> The right-hand rule in EM suggests that we are in
| north hemisphere of something_
|
| The right-hand rule is a human convention. It tells us
| nothing about physics.
|
| _> You are mixing model and reality._
|
| No, you are incorrectly assuming that the word "field"
| can only refer to the model. That's not the case.
| Physicists commonly use the word "field" to refer to both
| the mathematical object in the model _and_ the actual
| physical thing that is being modeled. Light is "waves of
| the electromagnetic field" in the latter sense.
| pdonis wrote:
| _> before the big bang the whole observable universe was
| contained in a small sphere and then it started to
| expand_
|
| We have no evidence of any time when the universe was
| _not_ expanding. At the earliest times we have evidence
| of, the universe was already expanding (extremely rapidly
| --much, much, much more rapidly than it is now). At those
| times, our observable universe was indeed contained in a
| very small volume.
|
| _> suppose I point a laser beam to the space and by
| chance this laser beam never finds any kind of matter in
| its way, where is this laser going to?_
|
| Since the universe is spatially infinite in our best
| current model, the laser beam will just keep on going
| forever.
|
| _> To an infinite void?_
|
| According to our best current model, no, the laser beam
| will never stop passing by matter, of approximately the
| same average density as the matter we can see.
|
| _> Is it correct to say that stars radiate energy to the
| infinite then?_
|
| Yes, as long as you recognize that "the infinite" never
| becomes a "void".
| lp4vn wrote:
| >According to our best current model, no, the laser beam
| will never stop passing by matter, of approximately the
| same average density as the matter we can see.
|
| I didn't understand this part, why would the laser beam
| would never stop passing by matter? Because of the metric
| expansion of the universe? Isn't it reasonable to assume
| that there is a skirt of the universe where matter keeps
| expanding into nothingness?
| AnimalMuppet wrote:
| I'm not an expert, but I think it's like this:
|
| If the universe were expanding uniformly, we would see galaxies
| moving away from us. The further away they are, the faster they
| would move. Distance and velocity would have a linear
| relationship, with the Hubble Constant as the scaling factor.
|
| But what we actually see is that, if we measure precisely
| enough, galaxies further away are moving faster than that. The
| conclusion is that the expansion is accelerating.
| pdonis wrote:
| _> galaxies further away are moving faster than that_
|
| No, you have it backwards. Accelerating expansion means,
| roughly speaking, that we see galaxies further away moving
| away _slower_ than a "uniform" expansion would predict.
| Remember that we are seeing galaxies further away as they
| were a longer period of time ago--so "accelerating expansion"
| means the universe was expanding _slower_ then, when the
| light was emitted, than it is now.
|
| Actually, though, we don't observe the distance to a galaxy
| directly. We infer it from other observations. The actual
| observed quantities are redshift, brightness, and angular
| size, and the relationship between those three observed
| quantities is what tells us the expansion history of the
| universe.
| pdonis wrote:
| _> How do we know that galaxies are accelerating away from us
| and not moving at constant speed?_
|
| More precisely, we see that galaxies started accelerating away
| from us a few billion years ago; before that they were
| decelerating (moving away from us but with the "speed"
| decreasing instead of increasing).
|
| _> People often point to the observation that the further away
| a galaxy is, the faster it appears to be moving away from us,
| implying acceleration._
|
| That observation tells us that the universe is expanding, but
| by itself it does _not_ tell us whether the expansion is
| accelerating or decelerating or neither. So you are correct
| that that observation alone is not sufficient to show that the
| expansion is accelerating.
|
| What we look at to see how the expansion rate changes with time
| is a _comparison_ of three pieces of observed data, galaxy by
| galaxy: redshift, brightness, and angular size. The
| relationship between these three quantities is what
| cosmologists use to construct a model of the expansion history
| of the universe, which in turn tells us things like what I said
| above, that the expansion has been accelerating for the last
| few billion years but before that it was decelerating.
| AlecSchueler wrote:
| By few billion are you talking like 3 billion? Why the
| change?
| pdonis wrote:
| Meaning, why did the expansion change from decelerating to
| accelerating a few billion years ago? Because that was when
| the density of matter, which had dominated the dynamics
| until then, became smaller than the density of dark energy,
| which has dominated the dynamics since then. The dark
| energy density doe not change with time, but the density of
| matter decreases as the universe expands.
| 1d22a wrote:
| How is is that dark energy density does not change with
| time? Surely the total amount of dark energy has to be
| constant (energy can't be created or destroyed, and all
| that), and then as the universe expands, that's then the
| same amount of energy over a larger volume, right?
| denton-scratch wrote:
| /me not a cosmologist.
|
| I think the story is that dark energy is indeed created,
| in the new emptiness resulting from the expansion of
| space.
|
| <mumble> I believe it's supposed to be _spacetime_ that
| expands, not 'space'. But it's beyond me to explain what
| that even means; as I understand it, _spacetime_ refers
| to the whole Universe, across all of time. To 'expand'
| means 'to become larger over time'. But if the thing
| that's expanding includes time itself, then I'm
| bewildered.
| raattgift wrote:
| It is in fact the metric expansion _of space_. The
| spatial part of the Robertson-Walker spacetime metric
| expands equally along the time axes of a family of
| freely-falling future-directed worldlines (we can call
| them "Eulerian observers", and individual clusters of
| galaxies are good approximations).
|
| That is, in the past, galaxy clusters are relatively
| close together, and in the future they are relatively
| very far apart.
|
| If you need an image, think of a vase of cut flowers,
| with the stems tightly bound together at the bottom of
| the vase, and the flowers loosely separated at the top.
| Time is in the direction away from the vase's bottom. A
| super thin slice through a stem represents a snapshot of
| a galaxy cluster at a particular time in its existence.
|
| https://s3-eu-
| west-1.amazonaws.com/images.linnlive.com/de6e1...
|
| <https://media.istockphoto.com/id/578833902/vector/expans
| ion-...> : time increases from the left to the right.
| mr_mitm wrote:
| Dark energy may be the energy of vacuum itself, that's
| why it's constant. And no, energy conservation does not
| apply in this case. There is a good blog article on
| precisely this question by Sean Carroll: https://www.prep
| osterousuniverse.com/blog/2010/02/22/energy-...
| pdonis wrote:
| _> How is is that dark energy density does not change
| with time?_
|
| Because that's how a cosmological constant works.
|
| There are alternate models where there is "dark energy"
| (as in, stress-energy that causes accelerated expansion)
| whose density does change with time (for example, a "Big
| Rip" model in which the dark energy density _increases_
| with time), but such models do not match our best current
| data.
| ijustlovemath wrote:
| One thing that is not often mentioned is that this effect only
| applies outside the local supergroup; within the supergroup
| gravity overrides the expansion of spacetime and holds us
| together (for now!)
| narag wrote:
| _How do we know that galaxies are accelerating away from us and
| not moving at constant speed?_
|
| There's a more basic question: How do we know the galaxies are
| moving? It seems (I haven't seen any other response, like...
| ever) that we have _one and only one_ way to measure the speed
| of galaxies: the red shift.
|
| It's impossible to triangulate those huge distances and the
| time scale would also be a barrier, so no way of confirming the
| red shift calculations with a different method. That means that
| if the red shift was caused by any other effect, say the light
| "degrading" after millions of years of travelling the void, all
| the calculations would be invalid.
|
| I've asked about this many times and the answers are in the
| line of "we don't know any other reason for the light to red
| shift" and "the current theoretical frame is consistent", even
| if there isn't any other measure to be consistent with.
|
| There was a prediction (expansion is related to Big Bang) that
| the far away galaxies, being younger, would have a different
| composition. This prediction seems to be failing, but advances
| in instrumental could give us a more precise answer in the
| future.
| k7sune wrote:
| "Degrading" sounds very intuitive to me. Can the frequency of
| the light waves simply slow down over a very long
| distance/time? Or maybe the speed of light simply slows down
| over an unimaginably long distance? We don't have any model
| to describe such behavior, but everyday objects around us all
| slow down one way or another, what makes light so different?
| Sprocklem wrote:
| IIRC, this was one of the explanations proposed when the
| existence of a red shift was first noted: that the light is
| somehow slowly losing its energy over very long distances,
| becoming "redder" as it did so. It ultimately lost out to
| the dark energy / space-time expansion theory, although I
| do not recall why. Presumably there was some observation
| that precluded "degrading" light from being the _sole_
| explanation.
| hnfong wrote:
| https://en.wikipedia.org/wiki/Tired_light
| semi-extrinsic wrote:
| There are several challenges for "tired light", or indeed
| any theory that's an alternative to expansion of the
| universe.
|
| The theory has to explain why the light gets redshifted,
| but does not get blurred, and the spectral lines do not
| get broadened. This severely restricts the type of
| interactions possible. Also the theory has to explain the
| consistency between redshifts within our own galaxy, to
| that of far-away galaxies.
| pigpang wrote:
| My theory is that redshifting caused by gravitational
| background noise. I did calculation somewhere on HN or
| Youtube few months ago and numbers are of same magnitude.
| javajosh wrote:
| _> It's impossible to triangulate those huge distances_
|
| Galaxies are BIG. Andromeda is faint, but the same angular
| size as the moon. It's 2.5Mly away, but it's also 150kly
| across. Over a long enough time line you could do
| triangulation on it. In fact it's moving toward our galaxy,
| but very, very slowly compared with its diameter at 110kps.
| But yeah, in theory you could do triangulation on it over a
| very long period of time.
| mr_toad wrote:
| For triangulation to work you need to move, not your
| target. Triangulation is only used for objects within about
| 1000 parsecs, where we can triangulate using the movement
| of the Earth along its orbit.
| garrettgarcia wrote:
| The American astronomer, Halton Arp, had a theory he called
| "intrinsic redshift". My limited understanding is that he saw
| evidence of "very close" and "very far away" structures that
| are connected to each other in space, which makes no sense.
| He theorized that redshift may also be indicative of the age
| of a galaxy, rather than only indicating velocity.
|
| The interpretation of redshift as velocity is also the
| primary reason cosmologists think the universe is expanding.
| the-mitr wrote:
| His book Seeing Red which raises several interesting and
| troublesome questions for the standard big bang model is
| worth a read.
| __turbobrew__ wrote:
| Degrading light sounds like an interesting idea to me, you
| could call it the "cosmological damper" if you will. Thought
| experiment: imagine you have light particles in a perfectly
| circular orbit around a black hole. Does the light ever fall
| into the black hole or does it orbit for eternity?
| oneshtein wrote:
| There a whole set of theories, called "Tired light"
| theories, which tries to explain Cosmological Red Shift by
| degradation of photons with time. Buy they require whole
| set of different cosmological principles: a medium for
| light propagation is required to dump lost energy into, no
| Big Bang. But even with a tired light theory, galaxies are
| accelerating toward attractors, see
| https://arxiv.org/pdf/1702.02483v1.pdf
| https://www.youtube.com/watch?v=NpV0GQo3P0c
| at_a_remove wrote:
| You're not going to get a simple answer because the answer is
| quite complex.
|
| Astronomy is the paleontology of photons. You should take an
| astronomy course if you really want to know, but essentially
| a "ladder" was built of distances, starting with the very
| near and slowly building outward using various techniques and
| discoveries of physics as they became available. This is
| called _the cosmic distance ladder_. You start with stellar
| parallax, then after that you go farther with "standard
| candles" (particular types of variable stars). But then you
| have to get even further out, where you can no longer see an
| individual star, and then you rely on specific breeds of
| supernovae. Only then do you get to redshift, and compounding
| tons of data from step three seems to verify the redshift
| estimates. By the time you get to the Hubble constant, it was
| a _huge_ rift between two communities over what was still a
| factor of two difference.
|
| It's quite fascinating, but I can't really dump out an entire
| book into a comment.
| ganzuul wrote:
| > "we don't know any other reason for the light to red shift"
|
| https://en.wikipedia.org/wiki/G%C3%B6del_metric
|
| There are versions of this which do provide another reason
| for red shift.
|
| Personally I keep this in mind as a means to free my thinking
| from a single narrative.
| mr_toad wrote:
| > It's impossible to triangulate those huge distances
|
| https://en.wikipedia.org/wiki/Cosmic_distance_ladder
| EnterpriseTell wrote:
| >In short, even if galaxies weren't accelerating
|
| -Galaxies are not accelerating, space is expanding.
|
| > Imagine there are some objects in space all moving in random
| directions and speeds, relative to Earth...
|
| -No, In your scenario then end result would be a most static
| average distance between all objects in the universe. As an
| infinite number of objects come from infinite distances, there
| would ALWAYS be objects in the neighborhood.
|
| I think what you're imagining is a bunch of objects in a box,
| give them random vector and then remove the box. If they
| maintain course, all will eventually move outside the original
| box boundaries, and away from each other. (not the way the
| universe is).
|
| They know space is expanding. The primary mechanism we know
| this is the speed with with we measure an object (moving away)
| is redshifted. Objects at the same distance from Earth, but
| opposite regions of space are moving at the ~SAME measured
| velocity.
|
| There simply is no existing theory which can account for what
| we are seeing besides space expanding. I'm not big on thinking
| we understand it all, but in this particular measurement, there
| is basically zero doubt. Space is expanding, which has the
| affect of accelerating all objects in the universe away from
| you, with an acceleration relative to the distance. The more
| space between you, the more opportunity to expand.
| munksbeer wrote:
| What does "space is expanding" mean? That the distance
| between objects is increasing? How can you tell the
| difference between "space expanding" and "objects moving in a
| non-expanding space"? Is there any way to tell the difference
| or is it just that all objects are moving away at the exact
| speeds that satisfies the "space expanding" explanation and
| nothing else?
|
| But then, I'm back to what does "space is expanding" mean?
| What is doing the expansion?
| baq wrote:
| space expanding means what it literally says: there is more
| space everywhere at once. there was less a moment ago and
| now there's more. the longer the distance, the more space
| gets added in between, thus the effect is extreme on
| universe scale and undetectable on planetary scales.
| munksbeer wrote:
| Well yes, but did you read my post in detail? I asked
| what the difference was between "space expanding" vs
| "objects moving away from each other".
|
| And secondly, what is the mechanism for "space
| expanding"? What is "space" in this context? What
| actually is expanding?
| pigpang wrote:
| Spacetime is 4D array of points. It's like a movie file,
| but with 3D frames instead of 2D frames (pictures).
| Expansion of space means that coordinates of points in a
| frame changed to move away from us, to match movie
| (model) with reality.
| munksbeer wrote:
| An array of 4D points implies some sort of construct upon
| which the points exist. As far as I know, the that was
| the concept of "the ether" and that fell out of fashion
| long ago.
|
| So again, how can you tell the difference between space
| expanding or two things actually just moving away from
| each other (to keep it to a very simple example of two
| bodies).
| pigpang wrote:
| Ether now known as "physical vacuum". As I told you
| already, expansion of spacetime is the mathematical tool,
| like a shader in OpenGL.
| effingwewt wrote:
| Let me try to explain- galaxies moving would be like
| balls floating in a pool. The galaxies are moving across
| the water.
|
| Space moving would be like balls placed on a bed sheet
| and the sheet expanding. The galaxies aren't moving- the
| sheet is.
|
| What's actually happening is more like galaxies moving
| around on a sheet that's being pulled further at all
| sides.
| munksbeer wrote:
| How can you tell the difference?
| baq wrote:
| things are moving away from other things the faster the
| farther away they are, uniformly across the universe.
| everything is moving away from everything, not just two
| particular points.
|
| is there a difference if there isn't a difference?
| tvshtr wrote:
| Imagine a deflated baloon with two dots drawn close to each
| other. Now inflate it; the dots didn't move but the plane
| that they were drawn/positioned on did.
| munksbeer wrote:
| I understand that concept. I'm asking, how can you
| materially tell the difference?
| m3kw9 wrote:
| If we can travel faster than light does it blow up the theory
| that the universe expands? Because if we can travel faster than
| light the universe is theoretically be infinite if I can go to a
| point pas the furthest reaches of stars/matter.
| not2b wrote:
| If we can travel faster than light, that would blow up all the
| theories that say that we cannot. And FTL travel would imply
| time travel (if in your reference frame, something is traveling
| faster than light, there's another reference frame where it is
| going backwards in time). A lot of science fiction just sweeps
| this under the table and pretends that we have Newtonian
| absolute time to go along with the FTL travel.
| m3kw9 wrote:
| If we can do FTL it may just prove that light speed can be
| achieved by brute force, or an alternative method. No laws
| needs to be broken and new laws can be found. Like quantum
| physics and classical physics.
| mjfl wrote:
| It is very fortunate that the universe is expanding. This
| provides a virtually unlimited source of energy.
| digging wrote:
| Without being able to harvest dark energy, it's actually the
| exact opposite.
| mjfl wrote:
| The extra energy comes from the increase in gravitational
| potential energy between objects. The accelerating increase
| in this potential energy implies the total energy of the
| universe is increasing. I'm not talking about harvesting dark
| energy directly.
| antod wrote:
| I'm no physicist, but isn't the gravitational potential
| energy between two objects _inversely_ proportional to the
| distance between them?
| mjfl wrote:
| it is, in the negative direction though. So increasing
| the distance increases the potential energy by making
| -1/r closer to zero.
| antod wrote:
| Heh, I had to go off and read some stuff to get my head
| around what "negative" potential energy would even mean
| (high school physics was decades ago).
|
| So... the negative is just a convention to represent work
| done against the "field", and positive is work done by
| the field? ie more of a vector than actually being
| negative energy? I think I get that bit now.
|
| So now I'm wondering if that still applies to an
| expanding universe vs eg a rocket leaving earth. If
| things aren't moving further apart by work (ie force x
| distance) being done against their gravitational fields
| so much as space time expanding, is there an increase in
| potential energy? And then if objects are moving apart
| faster than escape velocity, could that still be seen as
| increasing potential energy?
|
| I think I'm confusing myself further...
| puzzledobserver wrote:
| What would a hypothetical dark energy engine look like?
| undersuit wrote:
| It would harness dark energy. /s
|
| We haven't detected dark matter or dark energy outside of
| their visible effects on the larger universe. Maybe we
| can't interact with it with baryonic matter. A dark energy
| engine made out of dark matter would be invisible.
| mjfl wrote:
| two black holes in highly elliptical orbit, so they they
| nearly collide when they come together. at the center of
| mass is a cloud of iron atoms. When the black holes nearly
| collide tidal forces across iron nuclei rip them apart.
| These ripped apart atoms can then be re-harvested and used
| as fuel in a fusion engine. The energy loss from ripping
| the atoms apart is then recovered by the black holes by
| taking advantage of dark energy when they go apart, which
| increases the distance between them, increasing the
| gravitational potential energy.
| digging wrote:
| I could definitely be wrong, but I don't see how dark
| energy would enter this scenario. DE doesn't seem to play
| a role in gravitationally bound systems like two black
| holes orbiting each other.
| mjfl wrote:
| dark energy increases the distance between objects,
| adding to the system's energy by increasing the
| gravitational potential energy. Without the contribution
| from dark energy, after several cycles the pull from the
| iron atoms would cause the black hole orbit to decay.
| fsckboy wrote:
| given that a telescope conceived while Bill Clinton was president
| in the 1990's got named so as to complete the name "Webb Hubble",
| let's please please please call the next telescope "Chelsea".
|
| https://pagesix.com/wp-content/uploads/sites/3/2020/02/chels...
|
| https://www.wnd.com/wp-content/uploads/2015/10/Webb-Hubbell_...
|
| (Chelsea Clinton's resemblance to Clinton family friend Webb
| Hubble has prompted speculation...)
| yen223 wrote:
| I now really want to know what prompt was used to generate this
| text
| acyou wrote:
| Even though the title is copied from the article, we should
| change the title to "New data indicates the Webb and Hubble
| telescopes agree on the universe's expansion rate, but not with
| the cosmic microwave background measurement based expansion rate"
| arbitrage wrote:
| That's a mouthful.
|
| The title should be "Hubble Tension almost certainly not caused
| by measurement error."
| russdill wrote:
| Even the article seems confusing at first. The new measurements
| don't seem to be shedding light on anything or removing any
| confusion. They just confirm that hubble measurements already
| made. The Hubble tension remains as confounding a problem as
| ever.
| dbtc wrote:
| Can someone help me understand - point me toward some reading or
| ELI5 - what is the universe expanding into? (Or probably, why is
| that question not formulated well?)
| digging wrote:
| The universe is not expanding into anything. It is infinite.
| However, new space is being created in the voids between
| objects. It isn't super intuitive or particularly easy to
| grasp, things are just getting farther apart.
| andreareina wrote:
| One of the quantities described by Einstein's equations of
| General Relativity is the metric tensor--a matrix of matrices--
| that describes how "far" things are in space and time given the
| stuff in the local environment. One of the things that the
| equation tells us is that objects that are not gravitationally
| bound will tend to get farther away from each other as time
| passes. We call this the expansion of space. As far as we can
| tell, there isn't anything "outside" the universe is expanding
| "into"; distances just become larger somehow.
| modelofdemocray wrote:
| it's not going to expand faster than what you're able to observe
| sudom82 wrote:
| This article is titled "Webb and Hubble confirm Universe's
| expansion rate", however I don't see the expansion rate actually
| listed there, or in the comments. I see a mention of age, but not
| the rate. Does anyone know the rate? I clicked through to the
| paper itself[1], but wasn't able to interpret it from the details
| I could see
|
| [1]: https://iopscience.iop.org/article/10.3847/2041-8213/ad1ddd
| russdill wrote:
| It seems like a better title would be "Webb confirms Hubble's
| measurement of Universe's expansion rate"
| ta8645 wrote:
| Don't know why, but I recently got this gentleman's channel in my
| feed, who believes that there is no expansion at all:
|
| https://youtu.be/TGpjGVNVYEg?t=397
|
| It's beyond my depth to explain why he's wrong.
| pmayrgundter wrote:
| Alexander Unziker. Also Eric Lerner, Pierre Robitaille
| beaned wrote:
| Stupid question: how do we know that the universe is strictly
| exponentially expanding, and not both expanding and contracting
| in perpetuity like a sin wave? And could such an idea have
| anything to do with the Hubble tension?
| Vecr wrote:
| That's the "big crunch" theory. It's been pretty dubious for
| quite a while now though.
| physicles wrote:
| The article mentions the cosmic distance ladder, which is one of
| my favorite things in all of science. How do we know how far away
| the really far stuff is? It's non-trivial and I find the history
| fascinating.
|
| It all started with knowing the distance from the earth to the
| sun. Nobody had a clue until Richer and Cassini got within 10% in
| 1672. Then we nailed it down in 1769 with James Cook's voyage to
| Tahiti, the primary purpose of which was to observe the transit
| of Venus from the other side of the world.
|
| From there if you know basic geometry, you can observe the nearby
| stars shift a bit when the earth goes around the sun (parallax),
| but that only works to about 10k light years.
|
| Then, we discovered a couple unbelievably convenient astrophysics
| hacks: Cepheid variables (Henrietta Swan Leavitt, 1908) and Type
| 1A supernovae (Subrahmanyan Chandrasekhar, 1935, the namesake of
| the Chandra X-Ray Observatory). These allowed us to move out a
| couple more rungs on the ladder.
|
| From there, the relationship between redshift and distance
| becomes significant and that takes us to the edge.
|
| https://www.uwa.edu.au/science/-/media/Faculties/Science/Doc...
| rexer wrote:
| Great comment! Maybe you can help me with a book
| recommendation?
|
| I was recently looking for a book which was basically your
| comment, but more in depth and covered the last couple thousand
| years. I wanted a to read about the history of astronomy -
| yknow, what was the state of the art in, say, 1350 or whatever.
| If you know of anything, I'd be super interested!
| physicles wrote:
| Unfortunately I don't have any books to recommend. I don't
| remember where I learned about Cepheid variables and type 1a
| supernovae (maybe science shows, maybe youtube, ...) but I
| learned about the transit of Venus stuff on a big Wikipedia
| rabbit hole one evening.
|
| I think the pre-quantum mechanics era for physics and
| astronomy is super interesting. People figured out so much
| with such primitive tools, and it's all very accessible and
| easy to understand.
| tails4e wrote:
| Terence Tao did a great lecture on this,
| https://youtu.be/kY1gfrhNUIg?si=9u9k8of6-jRybwCG
| HarVard93 wrote:
| ya
| severila wrote:
| You might be interested in "Unrewarded" by "Ben Moore" which
| has an interesting take by telling the history of astronomy
| through the lives of those that made these discoveries but
| were not awarded a Nobel Prize.
| zuzun wrote:
| _Big Bang: The Origin of the Universe_ by Simon Singh.
| nolta wrote:
| You might like "Coming of Age in the Milky Way".
| rexer wrote:
| This looks perfect, thank you!
| hummingn3rd wrote:
| If you speak French or don't mind translating, there is this
| great video that goes through these techniques in layman's
| terms https://www.youtube.com/watch?v=FGwmAEMabm4&t=1
| tails4e wrote:
| I've often thought about this myself. I'm sure scientists
| involved are aware of the compounding errors with each step and
| build that in, but I'd love to see an analysis that breaks that
| down. When I first saw it I thought the errors due to cephids
| must be a large component of uncertainty, but really I've no
| idea how well contained that is.
| sanxiyn wrote:
| Error analysis of cosmic distance ladder is fiercely
| technical subject. https://arxiv.org/abs/1103.2976 Table 5 is
| titled "H0 Error Budget for Cepheid and SN Ia Distance
| Ladders". (It is old and the field is moving fast, but this
| is what I happened to remember.)
|
| Among total error of 3.1%, Cepheid reddening is 1.4% and the
| second largest source of uncertainty. SN Ia statistics is the
| largest with 1.9%. Rarely discussed in popular treatment is
| anchor distance, the third largest source with 1.3%. It is
| uncertainty of bottom lungs of the ladder, eg the distance to
| Large Magellanic Cloud before Cepheid and SN Ia are involved.
| raattgift wrote:
| > Rarely discussed in popular treatment is anchor distance
|
| Yes, and I'm sad that it's so rare.
|
| The Megamaser Cosmology Project is incredibly awesome;
| combining line of sight acceleration, velocity, velocity
| gradient, and observer angle on the sky is very "anchor".
|
| https://safe.nrao.edu/wiki/bin/view/Main/MegamaserCosmology
| P...
|
| Gaia seems to be making good progress on direct parallax up
| to kiloparsecs.
|
| And always happy to plug ASAS-SN
| <https://www.astronomy.ohio-state.edu/asassn/index.shtml>
| (The deleted @SuperASASSN was one of the best astro follows
| on twitter some years ago), who have found an awful lot of
| detached eclipsing binaries: <https://academic.oup.com/mnra
| s/article/517/2/2190/6695108?lo...>.
| luc4sdreyer wrote:
| > Nobody had a clue
|
| In the 3rd century BC, Aristarchus calculated that the Sun was
| between 18 and 20 times farther away from the Earth than the
| Moon, and proposed the Heliocentric model as a result. The true
| value is instead approximately 400 times. But it's incredible
| given that he didn't have lenses, the value of Pi, and that the
| Geocentric model was considered correct until 1800 years after
| his death.
|
| https://en.wikipedia.org/wiki/Aristarchus_of_Samos#Distance_...
|
| Nice video about the cosmic distance ladder by Terence Tao:
| https://www.youtube.com/watch?v=7ne0GArfeMs
| whimsicalism wrote:
| yeah i would call estimating it as 20x when it is actually
| 400x firmly within "not having a clue".
|
| He didn't say nobody had a clue about heliocentrism.
| dexterdog wrote:
| But I believe he was the winner for many years under price
| is right rules.
| nsxwolf wrote:
| Smugly: "1 mile."
| justsid wrote:
| Can you claim to be a winner under the rules of a game
| that won't be invented yet for hundreds of years?
| nashashmi wrote:
| Yes. The fact that even this was a thought and he was
| charting the space bodies and trying to establish
| distances between them. The rules were invented. The
| players just were not famous.
| namaria wrote:
| * thousands
| cwmma wrote:
| Plus they didn't know the distance to the moon
| Cthulhu_ wrote:
| I appreciate that he worked on it at least. Around the same
| era, someone else calculated the circumference of the earth
| (and that it was round) in a pretty accurate fashion
| (between -2.4% and +0.8% off) based on measuring shadows on
| equally sized posts at different locations on the same
| date. Googled, it was Eratosthenes, the cities were
| Alexandria and Syene/Assuan.
| Etherlord87 wrote:
| Can we call it an estimation if it's a range (18 to 20 times
| further than the Moon) and yet it is incorrect?
|
| If he said "between 2 and 1'000'000 times farther than Moon",
| it would be very imprecise, but not incorrect. If he said "20
| times farther" - it would be an extremely inaccurate
| estimate.
| risfriend wrote:
| The distance was also known in 16th century as per Hindu hymn
| of hanuman chalisa -
| https://hinduism.stackexchange.com/questions/10370/did-our-a...
| cygx wrote:
| Not sure that's convincing: Why would I multiply a unit of
| time (Yug) with unit of distance (Yojan) to arrive at the
| distance to the sun? Also note that per Wikipedia, the
| historical value of the Yogan can range from 3.5km (~2.2
| miles) to 15km (~9.3 miles). How was the value of 8 miles
| chosen?
| whimsicalism wrote:
| Strongly doubt this
| QuadmasterXLII wrote:
| I'm sure your goal is to boost the intellectual reputation of
| ancient hindu philosophy, but you're mostly scuffing the
| intellectual reputation of hinduism.stackexchange.com
| Scubabear68 wrote:
| It is amazing how much of basic science is rooted in simple
| geometry.
| joshjje wrote:
| I imagine there must be lots of gravity lensing going on as
| well, not sure how they deal with that.
| quijoteuniv wrote:
| Is it 42?
| merek wrote:
| For those interested in the cosmic distance ladder, David Butler
| has an excellent youtube series "How far away is it?", detailing
| the methods used to estimate distance along the cosmic distance
| ladder, the history, and examples. I highly recommend.
|
| https://www.youtube.com/playlist?list=PLpH1IDQEoE8QWWTnWG5cK...
| jon_adler wrote:
| Thank you. This series of videos is extremely helpful for
| understanding the concepts referred to in the article.
| 3cats-in-a-coat wrote:
| The universe is not expanding. The atoms are getting smaller.
| pantulis wrote:
| Play player Euclid.
| pfannkuchen wrote:
| I don't really understand how we can make conclusions about the
| entire universe when literally all of the data is collecting from
| one point in space. Couldn't there be local effects that
| obfuscate global behavior?
|
| Is there an implicit caveat asterisk on all such statements that
| is like "as far as we can possibly tell from the data we have"
| and the reality is we really, really don't know for sure?
| luc4sdreyer wrote:
| >from one point in space
|
| - We have multiple observatories on and around the planet - The
| Earth is moving around the Sun - The Sun is moving around the
| centre of the galaxy - The galaxy is moving towards the great
| attractor, etc
|
| The Copernican principle states that humans, on the Earth or in
| the Solar System, are not privileged observers of the universe,
| that observations from the Earth are representative of
| observations from the average position in the universe. This
| has been tested in various ways:
| https://en.wikipedia.org/wiki/Copernican_principle#Tests_of_...
|
| A nice PBS spacetime video about the topic:
| https://www.youtube.com/watch?v=q-6oU3jXAho
|
| You can also ask how do we know that the laws of physics
| haven't changed over time. We don't. But at some point you have
| to make a few basic assumptions in order to have any hope of
| making scientific progress.
| mr_mitm wrote:
| > "as far as we can possibly tell from the data we have"
|
| Isn't that self evident in all cases?
|
| In cosmology, the implicit assumptions are the Corpernican
| principle and that GR is correct. That is covered in
| introductory texts, but unless stated otherwise, it is usually
| not mentioned, especially not in press releases.
|
| I mean, maybe you're just a Boltzmann brain floating in space
| and nothing is real. Fun to think about, sure, but expecting to
| state the basics in any and all cases is unrealistic.
| pfannkuchen wrote:
| I'm not talking about physics being different in different
| places. I'm talking about the signal we are getting from far
| away quite likely being altered between its origin and when
| it gets to us.
|
| Different branches of science have vastly different degrees
| of certainty. I don't feel there is sufficient effort put
| towards communicating this well to the public, especially
| with respect to the Big Bang, and that annoys me because lay
| people seem to use it as a factual version of an origin myth
| when really it probably isn't any more factual than any other
| random myth you could pick.
|
| It's very irresponsible to replace the cultural origin myth,
| which is psychologically important to humans. The whole
| "explosion from nothing" myth may actually be contributing to
| modern people's feelings of meaninglessness.
| SkyBelow wrote:
| Yes, there is such an implicit caveat on all of science.
|
| Science at its core runs on a repeated process of collect some
| data, explain it, collect some more data, explain it as well,
| and keep repeating forever. The scientific model you are likely
| more familiar with refines this a bit where you use the current
| model and data to direct where you spend your time/funding
| collecting more data, but that is mostly a heuristic. Sometimes
| a person going off on their own will find something amazing to
| bring back, but we have to remember that many others spend
| their entire lives and end up finding nothing new or
| noteworthy.
|
| One problem with science literacy is that too many people treat
| the existing models as true, even when they aren't. In some
| cases we know they aren't correct because there are
| discrepancies in experiments that shouldn't exist, but no model
| better fits the data and resolves the discrepancies.
|
| At no point can science say "this model is how reality must
| work". But that is somewhat scary. It is scary to think the
| truth is unknowable and at best we will have an ever better
| approximation, so people find it simpler to treat that
| approximation as truth.
| isolli wrote:
| [Naive question warning]
|
| What if the cosmological constant (from which, if I understand
| correctly, the Hubble constant is derived) is not constant? Could
| a changing comological "constant" explain the discrepancy between
| the various methods of calculating the Hubble constant?
|
| From Wikipedia [0]: The cosmological constant "was revived and
| reinterpreted as the energy density of space, or vacuum energy,
| that arises in quantum mechanics. It is closely associated with
| the concept of dark energy."
|
| Do vacuum energy and dark energy have to be constant?
|
| [0] https://en.wikipedia.org/wiki/Cosmological_constant
| kmm wrote:
| The Hubble constant is not necessarily derived from the
| cosmological constant. To be clear, it's not even a constant
| either, it's a proportionality factor between the recessional
| velocity of distant objects and their distance from us. Though
| in some usages it refers to the value of this factor at the
| current time, which would in fact make it a constant.
| Regardless, even without a cosmological constant, after the Big
| Bang you'd still have an expanding universe, possibly
| collapsing or endlessly expanding, and I like to think of the
| Hubble constant/parameter in this case as representing the
| "momentum" the matter in the universe has left from the Big
| Bang.
|
| The cosmological constant does in fact have to be constant
| within the constraints of general relativity. The mathematical
| machinery of GR only allows two parameters: Newton's constant
| G, representing the coupling of matter to gravity, and the
| cosmological constant. Both have to be true constants, numbers
| with a unit.
|
| However, this is only true of the most basic theory of dark
| energy, where you directly add a constant to the general
| relativistic lagrangian. More complicated theories, like, for
| example, quintessence, involve adding new dynamical fields to
| the theory. The "effective cosmological constant" associated to
| such theories, quantifying the effect these fields are having
| on the expansion of the universe, can dynamically change over
| time, and some of these theories are proposed to solve the
| Hubble tension. To be clear, although none of these theories
| are fringe or pseudoscience, they haven't been accepted as a
| final explanation either, resolving these issues is still a
| work in progress, at this point they're all simply interesting
| hypotheses.
| mr_mitm wrote:
| Dark energy is a constant in the standard model (equivalent to
| the cosmological constant) and all observations I'm aware of
| besides the Hubble tension are consistent with it being
| constant. But nothing stops you from going beyond the standard
| model, and people have done that and proposed models with a
| dynamic dark energy density, for example quintessence models.
| They'd solve some theoretical issues with the standard model,
| but so far no observations have been precise enough to
| differentiate between those models and the standard model.
|
| Having a dynamic dark energy is a proposed solution to the
| Hubble tension: https://arxiv.org/abs/2103.01183
|
| That is, besides a whole family of potential other solutions:
|
| Dark energy in extended parameter spaces [289] Early Dark
| Energy [235] Early Dark Energy [229] Dynamical Dark Energy
| [309] Phantom Dark Energy [11] Decaying Warm DM [474]
| Metastable Dark Energy [314] Dynamical Dark Energy [11, 281,
| 309] Neutrino-DM Interaction [506] PEDE [392, 394] GEDE [397]
| Interacting dark radiation [517] Elaborated Vacuum
| Metamorphosis [400-402] Vacuum Metamorphosis [402] Self-
| Interacting Neutrinos [700, 701] IDE [314, 636, 637, 639, 652,
| 657, 661-663] IDE [314, 653, 656, 661, 663, 670] IDE [656]
| Self-interacting sterile neutrinos [711] Critically Emergent
| Dark Energy [997] Unified Cosmologies [747] Generalized
| Chaplygin gas model [744] f (T ) gravity [814] Scalar-tensor
| gravity [856] Galileon gravity [876, 882] "Uber-gravity [59]
| Modified recombination [986] Power Law Inflation [966]
| Reconstructed PPS [978] Super LCDM [1007] f (T ) [818] Coupled
| Dark Energy [650]
| isolli wrote:
| Thank you to both commenters, who pointed me to this theory:
| https://en.wikipedia.org/wiki/Quintessence_(physics)
| jug wrote:
| I can recommend Dr. Becky's introduction to the issues
| surrounding the Hubble Tension and recent JWST measurements in
| this 16 minute clip, as she introduces laymen to the problems
| well: https://www.youtube.com/watch?v=hps-HfpL1vc
|
| It's an older clip, but it checks out... ;) (still JWST
| discoveries and she most importantly introduces the viewer to the
| still remaining conflicts)
| ptmargah wrote:
| https://hotidol.vip/#/home/?code=KXWZ5V
| kovacs_x wrote:
| I'm 1000% sure, in 20, 50, 100, 500 years humans will look back
| on "science of 20/21th century" as we look now on say 16/17th
| century- there were some things that were "mostly" right, but
| most of it was incorrect / inprecise / incomplete and was a
| fantasy of earlier thought up models.
|
| It's been like that always. Don't think it'll be different this
| time. Though people of every time thought they had (almost)
| complete understanding of the universe. I remember reading that
| at the beginning of 20th century science was largerly considered
| complete... we know how it turned out! (think relativity, quantum
| physics, etc.)
|
| Think about that!
|
| (ps. me personally I just cannot stomach that universe is only
| <15B "years" old. Not saying there was no BANG! (ie. large cosmic
| event) back then and we're products of it :), but don't think it
| was "the beginning of everything". Seems very much like "god
| created universe!" type of thinking)
| Vecr wrote:
| Why? Sure, maybe a better number to give is "somewhere around
| 20", but it's difficult to tell how the "true number" could be
| over 100 or even 50.
| miramba wrote:
| Explanation needed. How can you, a primitive ape on a tiny
| remote planet with an average lifespan of 80 years (not meant
| as an insult, I am too), be sure about what happened 15, 20
| or 100 BILLION years ago? I think this is what parent meant
| with: "people of every time thought they had (almost)
| complete understanding of the universe." - "Don't think it'll
| be different this time."
| Vecr wrote:
| There's good evidence for it, inferred temperature of the
| early universe included. Obviously not perfect evidence,
| but even if you assume "tired light", it's another thing
| all together to come up with a new version of
| thermodynamics.
| mr_mitm wrote:
| We can literally see into the past 13 billion years ago by
| building space crafts that collect microwave radiation,
| because we aren't as primitive as you may want us to
| believe.
| physicles wrote:
| Sure, the frontier is always messy. Scientists talk about
| "quarks" and "dark energy" but those are just placeholders for
| "we kinda get what they do but we have no idea what they
| actually are."
|
| 100 or 500 years from now, QM and GR will turn out to be
| approximations, or shadows, of some deeper theory. But that
| won't change how insanely accurate their predictions are.
|
| Our understanding of reality isn't just moving forward, it's
| getting asymptotically closer to ground truth. New theories may
| upend the conceptual framework, but they still just add decimal
| points -- they have to, otherwise they're worse theories than
| what we already have.
| swamp40 wrote:
| I cannot for the life of me understand how you look at the cosmic
| background radiation - which appears equally in every direction
| you look, for as far away as you can see - and say this is
| evidence of a "big bang", originating from a single spot at a
| single point in time. And the universe just "expanded faster than
| light" to cover _everywhere_ with the same consistent drab layer
| of cosmic background radiation.
|
| It seems like a child's fairy tale to me.
| SkyBelow wrote:
| >originating from a single spot at a single point in time
|
| That's based on a simplified model of the big bang which isn't
| what scientist use. It is everywhere, and time itself isn't
| defined. One major result of this is that many people treat the
| visible universe as the universe, but those are two different
| concepts and while the visible universe is finite, the universe
| might not be (it might be infinite, finite unbounded, or finite
| unbounded).
|
| Also, science isn't saying "This is what happened". It is
| saying "Our best model to date says this is happening". In the
| end it is only a model and the model is still open to being
| refined and there is always the possibility of data resulting
| in the entire model being overturned, though often this leads
| to an ever more complex model that is harder to work with but
| better fits the experimental data (such as the wave/particle
| nature of light which is hard for a person to conceptualize).
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