[HN Gopher] Astronomers posit that we live in an area with below...
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Astronomers posit that we live in an area with below average
density
Author : samizdis
Score : 86 points
Date : 2023-12-01 16:01 UTC (7 hours ago)
(HTM) web link (phys.org)
(TXT) w3m dump (phys.org)
| ryandvm wrote:
| We should call it "space"
| ranting-moth wrote:
| The management called. It's too long. They'd like to shorten it
| to " " to keep it cool with the kids.
| bozhark wrote:
| Can _ be copyrighted or trademarked?
| askiiart wrote:
| IANAL, but I _think_ you can trademark letters, and that
| similarly to colors, given the letter is known to
| correspond to your business in a particular market segment,
| and with the caveat that the trademark would only apply in
| that market segment.
|
| Theoretically a space/" " is just another character, so
| maybe. But practically speaking, it would probably be
| legally filed as "space", and the company's trademark would
| be on either them naming their company "space", or on them
| stylizing it as a " ". That's my best guess, at least.
| layer8 wrote:
| Space does not imply low(er)-density, void does.
| dylan604 wrote:
| It depends on if you are looking at the return type of the
| object, or what chars it is full of
| gryfft wrote:
| Don't say that too loud or somebody will show up and say we
| should call it "tab."
| LightHugger wrote:
| tab is less keypresses after all
| adammichaelc wrote:
| Why would distance from earth influence the speed of expansion of
| a distant galaxy?
|
| Does that suggest that our awareness of the galactic bodies
| itself is somehow influencing them, as in the double-slit
| experiment?
| 317070 wrote:
| > our awareness of the galactic bodies itself is somehow
| influencing them, as in the double-slit experiment
|
| In the double slit experiment, the awareness is not influencing
| the outcome. The act of measuring is. Pretty sure the act of
| measuring the galactic bodies has no impact on them in any
| meaningful way.
| I_Am_Nous wrote:
| No, but there is a cool blending of the two concepts when
| light is bent by large gravity wells so that it actually
| shows the same star multiple times. When we observe which
| copied star "produced" a certain photon, it technically
| collapses the quantum possibilities _backwards in time_ as
| that photon was emitted potentially billions of years ago.
| rightbyte wrote:
| > it technically collapses the quantum possibilities
| backwards in time as that photon was emitted potentially
| billions of years ago
|
| There is no way to verify that. Big parts of quantum
| physics is more or less pseudoscience.
| I_Am_Nous wrote:
| Perhaps it could be verified using time delayed lensing
| experiments[1]?
|
| 1. https://www.nature.com/articles/s41550-021-01450-9
| rightbyte wrote:
| Seems like measuring "bending" of light, rather? But
| honestly I am not really qualified to say anything about
| astronomy.
| I_Am_Nous wrote:
| In this case it's light bending around a gravity well
| which produces multiple "copies" of the star, but the
| light from each copy arrives at different points in time.
|
| So they could capture the light and attempt to capture
| the "same" light again later to try to verify or change
| the result they received.
|
| I did some more reading about it after posting, and the
| original experiment was intended to test if light
| "chooses" to be a wave OR a particle in a way they could
| affect, and that it could only be one or the other at a
| time. The truth seems to be more that it acts as both at
| the same time and whatever sensor equipment you use to
| pick it up is what it acts like.
| matkoniecz wrote:
| > Big parts of quantum physics is more or less
| pseudoscience.
|
| [citation needed]
| cypherpunks01 wrote:
| I'm not sure about that. I suppose it really depends on
| your interpretation of the results, but I didn't think the
| cosmic interferometer experiment convinced physicists of
| retrocausality, by-and-large.
| I_Am_Nous wrote:
| Functionally it means we can trace a single photon back
| to the source which emanated it, lensed or unlensed. That
| said, if quantum effects are not bound by time or space
| technically the photon ALWAYS came from an individual
| source and our clarifying which one it came from just
| collapses the wave from superposition of where the photon
| could go to where it did go.
|
| As I understand the double slit experiment, this is a
| fundamental property of light as a photon exhibits wave-
| particle duality. If so, retrocausality in this case
| would just mean the fundamental wave function can be
| collapsed into actuality without time or space being
| involved.
|
| Am I far off base?
| M2Ys4U wrote:
| > Why would distance from earth influence the speed of
| expansion of a distant galaxy?
|
| It wouldn't - but it may influence how we measure distance. If
| we're using the wrong distance measurements then we're
| calculating the speed of expansion incorrectly.
| anon25783 wrote:
| At any point in space, a celestial object at distance d will
| tend to appear to move away from you faster than an object at a
| distance less than d. The only thing special about the Earth
| here is that it happens to be where we live.
| cwmma wrote:
| no it's because space itself is expanding which causes things
| more distant from each other to be moving away from each other
| faster, the wikipedia article about hubbles law has a graphic
| illustrating the priciple with bread
| https://en.wikipedia.org/wiki/Hubble%27s_law
| edgyquant wrote:
| Wikipedia is not a proper source and I've no idea why people
| stopped considering this and now use it as such. The graphic
| may be fine, but provide a real source when pushing back
| against another commenter.
| johndunne wrote:
| To be fair to Wikipedia, it's come on a long way since its
| early days when anyone could make whatever edits they
| wanted. The most popular entries on the site are very well
| vetted, though not perfect. Just my 2 cents.
| zlg_codes wrote:
| Pages contain other sources, however, which makes WP a good
| place to begin research on a topic.
|
| This place isn't debate club and nobody's getting a prize
| for winning an Internet argument.
| itronitron wrote:
| It's distance between _any_ two points, however the distance
| measuring techniques to which we have access can only be
| performed by astronomers on Earth. Hence, one of those points
| will be Earth.
| Turing_Machine wrote:
| Right. A common analogy is the dough for a loaf of raisin
| bread. Consider the raisins on the surface of the dough. Pick
| one raisin to be your point of view (analogous to Earth, in
| this case). If the dough rises and expands from (say) 50 cm
| to 100 cm in diameter, another raisin adjacent the "Earth"
| raisin won't move very much, but the distance to a raisin at
| the diametrically opposed point will increase from about 78
| cm to about 157 cm. The distant raisin will thus appear to be
| moving at a higher velocity than the adjacent raisin.
| gizmo686 wrote:
| This observation is the reason we think the universe is
| expanding.
|
| As an analogy, consider the 2 dimensional surface on the
| surface of a balloon. As you inflate the balloon, the distance
| between any 2 points increases, and it increases more the
| farther away the points are from each other.
| Udo wrote:
| > Does that suggest that our awareness of the galactic bodies
| itself is somehow influencing them, as in the double-slit
| experiment?
|
| "Awareness" is not a thing, not even in the double slit
| experiment. The term 'measurement' refers to a specific kind of
| interaction that bridges quantum systems with classical
| systems, although I believe a good case could be made that
| these waveforms never actually fully collapse.
|
| Likewise, if there is anything special about Earth's position
| in the greater cosmos, it would be a trick of perspective or
| perception - unless there are any completely disruptive new
| discoveries about the nature of reality. However, my money
| would be on the fact that the universe is simply not as uniform
| as we thought.
| chongli wrote:
| Galaxies themselves aren't expanding, they're gravitationally
| bound. Galaxies are moving apart from one another, however.
|
| The issue is that not all galaxies are moving away from us. The
| ones that are closer to us have a lot of peculiar velocity [1].
| This means they can be moving toward us or moving tangentially
| to us or any other direction. If we want to characterize the
| expansion of the universe as a whole, we need to account for
| this in our models. It turns out to be a lot more complicated
| than we previously thought.
|
| The crisis in cosmology (aka the Hubble tension [2]) is that
| our two means of characterizing the expansion of the universe,
| models of the cosmic microwave background (CMB) and
| measurements on the cosmic distance ladder using standard
| candles (Cepheid variables [3] for up-close measurements, Type
| Ia supernovae [4] for more distant measurements) disagree with
| one another, and that disagreement is getting worse, not
| converging.
|
| [1] https://en.wikipedia.org/wiki/Peculiar_velocity#Cosmology
|
| [2] https://en.wikipedia.org/wiki/Hubble%27s_law#Hubble_tension
|
| [3] https://en.wikipedia.org/wiki/Cepheid_variable
|
| [4] https://en.wikipedia.org/wiki/Type_Ia_supernova
| sesm wrote:
| One alternative explanation is that the expansion of the
| universe is slowing down.
| verisimi wrote:
| Without twerking the laws of gravity, I posit I do not live in a
| giant void.
|
| Am I right?
| NoMoreNicksLeft wrote:
| Please tweak the laws of gravity, but never twerk them.
| verisimi wrote:
| I'm _not_ the one twerking them.
| ranulo wrote:
| Interesting solution.
|
| I have a theory in my head for years now. It is probably wrong,
| but here it comes:
|
| In an infinite universe the total amount of gravity that affect
| us in one point in space is defined by the event horizon if we
| assume that gravity travels with light speed. Every atom in the
| universe has a very small influence on us. But this event horizon
| expands with light speed all the time. I wonder if this could
| lead to very small but permanent increasing gravitational pull
| from all directions at once. In other words, and increasing
| inflation.
| I_Am_Nous wrote:
| Sort of like a pair of ice climbers, where one climbing up and
| securing themselves allows the other climber to safely climb
| higher. Eventually we can't see the ice climbers anymore, but
| that doesn't mean they aren't still helping each other climb
| higher.
| _boffin_ wrote:
| it grow. we grow?
| jprete wrote:
| Increasing gravitational force would result in a contracting
| universe, not an expanding one, I think.
| pdonis wrote:
| "Gravitational force" doesn't work for describing the
| dynamics of the universe as a whole.
|
| What would result in a contracting universe is a large enough
| density of matter (about a factor of 20 larger than the
| actual density in our universe if we just look at ordinary
| visible matter). But this does not mean "increasing
| gravitational force". As I have pointed out in other posts
| upthread, the "gravitational force" on a given piece of
| matter due to the rest of the matter in the universe (if we
| leave out local influences, like our solar system or galaxy
| for us here on Earth) is zero. This is true regardless of the
| current state of expansion or contraction.
| yodon wrote:
| You're on the path to coming up with Mach's Principle[0].
|
| [0] https://en.m.wikipedia.org/wiki/Mach%27s_principle
| kabouseng wrote:
| The force of gravity is reduced by the inverse square of the
| distance (newton's law). Thus as space expands, and matter red
| shift away from us, the force of gravity reduce over time. The
| maximum force of gravity was just after the big bang.
| pdonis wrote:
| All of this is wrong. Newtonian gravity does not work for
| describing the universe as a whole. As I pointed out in
| response to the GP upthread, the "force of gravity" on us due
| to the overall matter distribution of the universe is _zero_.
| ReptileMan wrote:
| Does this account for all the extra space being created - from
| what I have understood from "always right" youtube videos -
| parts of the universe are moving away from us with speed
| greater than the speed of light (or more precise the space
| between the points is increasing at rate higher than C, no
| actual movement is being done)
| pdonis wrote:
| _> Does this account for all the extra space being created_
|
| As I have pointed out upthread, the GP's "theory" is not
| correct, so it doesn't account for anything.
|
| "Expansion of space" is just a consequence of the overall
| spacetime geometry of the universe, which is due to its
| overall average matter distribution (and to dark energy,
| which is what is causing the expansion to accelerate).
| pdonis wrote:
| I don't know what theory of gravity you are trying to use, but
| it isn't the correct one.
|
| In our actual model of the universe, using the correct theory
| of gravity, the "total amount of gravity" affecting us (or any
| point) from the rest of the universe (i.e., once we factor out
| local influences like our solar system and our galaxy) is
| _zero_. That is because the average matter distribution in the
| universe is the same in all directions from us, so the
| "gravity" from it cancels out. The average matter distribution
| in the universe affects its overall rate of expansion over
| time, but this is not the same as the kind of "gravity" you are
| thinking of.
|
| Also, while our universe does have a cosmological horizon (due
| to accelerating expansion), this horizon does not work the way
| your hypothetical "event horizon" does.
|
| In short, your "theory" is not even wrong, because it doesn't
| even start from a correct underlying theory of gravity.
| mxkopy wrote:
| > That is because the average matter distribution in the
| universe is the same in all directions from us, so the
| "gravity" from it cancels out.
|
| Can't gravity have an infinitesimally small effect, which
| means this matter distribution has to be perfectly balanced
| for its gravity to cancel out to zero at some point?
| unsupp0rted wrote:
| I don't think we know how small of an effect gravity can
| have over a large distance. Is there a lower Planck-like
| limit?
| 015a wrote:
| > while our universe does have a cosmological horizon (due to
| accelerating expansion), this horizon does not work the way
| your hypothetical "event horizon" does.
|
| I think this statement is really the crux of the counter-
| argument. Your statement that matter has average uniform
| density in all directions around us is obviously only correct
| in sufficiently large frames of reference; there are galactic
| voids, and galactic-super-strands, uniformity really only
| exists within the "mathematically and hypothetically
| infinite" frame.
|
| You should expand on why the cosmological horizon does not
| function in the same way the GP's "event horizon" analogue
| does; and/or possibly, expand on how large the frame would
| have to be to achieve reasonable uniformity.
| unsupp0rted wrote:
| Sure, and whenever Warren Buffet gives a speech to 1000
| college students, the average wealth of every person in the
| auditorium briefly jumps to millionaire-level, until Buffet
| leaves the room.
| gumballindie wrote:
| I dont know why but my mind enters an endless loop when thinking
| about the vastness of space and where that space exists. Almost
| as if it reaches the end of human imagination. I can imagine the
| multiverse, but i cant imagine the "nothingness" in which they
| exist. I cant imagine where everything came from and where it
| goes. It makes no sense. I understand the explanation of physics
| and time, the big bang, expansion, but where the heck does it all
| exist in? The void of what? What are they expanding into? I
| understand matter existing in the universe, but where is the
| universe expanding in? What are the laws of that "in"? That void
| exists in something but what does the something exist in? It's
| almost as weird as if nothing ever existed yet it exists.
| tux3 wrote:
| It is not expanding inside of something.
|
| The picture I've been given is living on the surface of a
| balloon. Draw a few dots on the surface, blow in the balloon.
| Every points expands away from every other point.
|
| Of course with a balloon there is always an outside, but the
| point is that it isn't expanding like if it were pushing
| against an external wall.
|
| It's expanding everywhere. It's expanding in the space between
| your fingers. If you look there, it's not pushing against any
| outside. There is just more space per space, the longer you let
| it age.
| ninkendo wrote:
| > It's expanding in the space between your fingers
|
| That's not the case (at least not yet [0].) Anything that's
| bound together by local forces such as gravity or
| electromagnetism isn't affected by the universe's expansion.
| You have to get to the scale of the space between galaxies
| (and even then, galaxies which aren't orbiting/colliding with
| one another) before the expansion has any effect.
|
| [0] One of the hypotheses (of many: https://en.wikipedia.org/
| wiki/Future_of_an_expanding_univers...) of the future of the
| expanding universe predicts that the expansion rate will
| increase indefinitely, do the point where the expansion
| energy (dark energy) actually _does_ become enough to expand
| the space between stars, planets, and even matter (and
| eventually the atoms in your body), but we don 't really have
| any proof of this. This theory is known as the "big rip":
| https://en.wikipedia.org/wiki/Big_Rip
| layer8 wrote:
| My understanding is that it does expand on small scales
| (such as between your fingers), but at that scale the
| expansion is so weak/slow that it is immediately
| compensated by the local attractive forces.
| ninkendo wrote:
| Sure. You could model it as being there being an
| extremely weak expansion force happening between your
| fingers, but the force not being enough to overcome any
| other local forces (gravity, molecular bonds, etc)...
| said force is not strong enough to overcome planetary
| orbits either, or orbits of stars around the galactic
| center, or the orbits of nearby galaxies around each
| other... but once you start getting into intergalactic
| scales where the force attracting galaxies together is
| _very_ weak, the expansion force starts to actually make
| a difference.
|
| But the point is the space between your fingers isn't
| literally expanding. It's not like the universe is just
| expanding everywhere uniformly, which is what tux3 seemed
| to imply. The distance between your fingers is staying
| the same, and so is everything else nearby.
| layer8 wrote:
| My point though is that yes space is literally expanding
| everywhere uniformly, it's just expanding "through your
| fingers" (i.e. your fingers don't expand along with it)
| because the local forces keep your fingers (and the
| planet and the galaxy) together.
|
| This is roughly similar to how planets don't collapse due
| to gravity, because the stronger electromagnetic
| repulsion makes them stable bodies, but this doesn't mean
| that the gravitational field still isn't there throughout
| the planet.
| slothtrop wrote:
| I've seen this tied up with one idea where space-time
| itself is ripped apart, leaving nothing that _can_ expand
| and instead "everything" once against occupying the same
| space less than an atom (if it can even be quantified),
| reduxing a Big Bang.
|
| Otherwise we increasingly approximate a transition to
| "nothing" into infinity.
| globular-toast wrote:
| I understand what you mean and I can easily get this feeling
| when looking at the night sky on a clear night. I have to
| quickly abort the loop because it's not very pleasant.
| PcChip wrote:
| FYI their definition of "void" is "slightly below average
| density"
| nuz wrote:
| "Slightly" can mean such gigantic numbers still on this scale.
| Still interesting if this is the case
| 1270018080 wrote:
| But then back to human scale it's only a few less hydrogen
| atoms per cubic meter.
| dang wrote:
| Thanks--I've replaced the void with that in the title above.
| hprotagonist wrote:
| But apparently we don't live in a void, we live in a big glob of
| higher density galaxies, the Laniakea supercluster.
|
| https://arxiv.org/pdf/2311.00215.pdf
|
| MOND hasn't been having the rosiest of time lately.
| https://academic.oup.com/mnras/article/527/3/4573/7342478?lo...
| due-rr wrote:
| Dr. Becky has a nice explainer video of the first paper.
|
| https://youtu.be/-kTe0xRAU1w
| OnACoffeeBreak wrote:
| Doesn't sound like the paper's (the one you linked) authors
| don't necessarily agree with your summary. From the discussion
| on page 16:
|
| These results seemingly worsen the recently established tension
| between the inferred value of H0 from early and late times
| Universe probes, which has been argued to potentially be the
| sign of new cosmological physics (see for example Refs.
| [78-83]). This might appear to be in contradiction with the
| possibility, explored for example in Refs. [74, 84- 93], that
| local gravitational physics could alleviate the Hubble tension.
| Amongst these, a class of models achieve a lowering of H0 under
| the assumption that we live in an underdense region, whose
| inner expansion rate is on average larger than the background
| one. Some results in the literature, see for example refs.
| [94-96], seem to corroborate the latter assumption finding
| evidence of local voids which averaged on spheres of r [?]
| 100Mpc have density contrasts of d <= -0.1, unexpected within
| the LCDM model. Computing the average density contrast of a
| sphere centered in Laniakea with radius r [?] 110 Mpc (i.e. the
| average distance of the boundary of the ellipsoid from the
| center) using the CF4 reconstruction we found d ~ -0.06, within
| the prediction of the concordance model (see for example Fig. 6
| of Ref. [55]). However, this sphere is not centered in the
| Milky Way, which might explain why the result differs from the
| aforementioned ones. Indeed, overdensities such as Laniakea are
| surrounded by voids (from which they have collected matter),
| and therefore any sufficiently spherical average will include
| these under-dense regions. On the other hand, Refs. [55, 79,
| 97] also found no evidence of any large void or overdensity,
| thus disfavoring a local resolution of the Hubble tension. Our
| analysis corroborates these results, suggesting instead that
| the tension is likely to be (slightly) worsened by Laniakea's
| backreaction. An important caveat, however, is that our
| analysis does not exclude the possibility that large voids in
| the annular region between 110 - 400 Mpc outside Laniakea could
| balance and overcome the backreaction from Laniakea, like a
| rather picturesque Matryoshka doll. Alternative modelling
| choices accounting for the impact of these voids are therefore
| required to fully understand the impact our cosmic
| environment's gravitational backreaction, which will be the
| focus of forthcoming studies.
| ReptileMan wrote:
| Question - what are the conditions in the denser parts of the
| universe for earth style life?
| layer8 wrote:
| At the mentioned 25% higher density, probably no significant
| difference.
| jacobwilliamroy wrote:
| Sometimes I think about how everything Ive ever known is a tiny
| invisible spot on a little wet rock hurtling through an empty
| nightmare abyss.
| ptsneves wrote:
| I read that sentence as poetry. Beautiful.
| wholinator2 wrote:
| I do disagree with nightmare, but it's all in perspective. More
| like an infinitely deep well of incomprehensible potential to
| me. Deep water is scary but a well is life giving. It's a
| little bit of both
| mojomark wrote:
| While we're all throwing out our whacky idea laundry in front of
| people I presume actually know what they're talking about:
|
| I've wondered for many years if our universe isn't like a
| supersaturated (SS) solution (1), where mass/energy and the
| fabric of space-time itself can eventually combine and settle
| into a balanced but very precarious state, like an SS solution
| comprised of Mass-Energy-Space-Time [MEST] as a single "fabric"
| (which perhaps was the state of the universe prior to the big
| bang, and will be again after the universe' heat death).
|
| Say this large swath of peaceful-yet-highly unstable MEST fabric
| is perturbed slightly (like dropping a seed crystal in a SS
| solution) - then matter, energy space and time precipitate
| outward in a rapidly evolving chain reaction. The horizon of this
| chain reaction would be a place where matter and energy particles
| are continually being generated (liberated) from the serene METS
| fabric, which generates a gravitational pull in all directions.
|
| So, all of the mass in our universe is actually being pulled
| outward, and because this precipitation horizon is expanding
| (growing in surface area), it generates an increasing amount of
| matter and hence an increasing amount of gravity that ultimately
| pulls matter within this horizon outward - at an accelerated
| rate. Perhaps the big bang was really, instead, a 'big fizz' -
| chain reaction of precipitated matter and energy from an initial
| supersaturated MEST fabric.
|
| Or maybe I'm just full of silly nonsense. Either way, would love
| to hear an actual astrophysicist 's take on that idea. Maybe
| gently tear it to shreds:)
|
| 1. https://youtu.be/G8nHu-IOpTg?feature=shared
| ganzuul wrote:
| Seems like you are picking up on the intuition of cosmic
| inflation of quark gluon plasma.
|
| The degenerate matter state is like a partial order of time.
| https://en.wikipedia.org/wiki/Causal_sets
| Finnucane wrote:
| Because of galactic-scale NIMBY zoning.
| immersible wrote:
| This article starts from an infinity of axioms that all have to
| be true for that single statement of the title to have even a
| small chance of making sense.
|
| It is a purely speculative article.
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