[HN Gopher] Dark Matter: The Situation Has Changed
___________________________________________________________________
Dark Matter: The Situation Has Changed
Author : nsoonhui
Score : 193 points
Date : 2021-05-08 11:04 UTC (11 hours ago)
(HTM) web link (backreaction.blogspot.com)
(TXT) w3m dump (backreaction.blogspot.com)
| caspper69 wrote:
| I prefer the science fiction alternative: black holes are made by
| intelligent beings. The reason none of the numbers make sense
| with respect to dark matter and inflation is because life is
| actually actively altering the Universe.
|
| Supermassive black holes accomplished two things: (i) they
| captured vast quantities of resources for future use; and (ii)
| they acted as gargantuan propulsion systems, thus allowing any
| captured matter (what we would refer to as a "galaxy") to be
| directed far, far, far away from any/all other captured matter.
|
| And thus ended the great intergalactic war, which brought about
| the end of the homogeneous Universe, ensuring that no single
| civilization could ever start such a widespread conflict again.
| The distances would make it so.
|
| Life was the big bang.
|
| (I'll throw the /s and #notserious tags here; Poe's law and all
| that...)
| AnimalMuppet wrote:
| You should write that book. Seriously.
| rossdavidh wrote:
| Actually it's just that the alien TV shows are broadcast from
| there, and the only way you can receive it is via advanced dark
| matter/dark energy technology we don't have. It's basically
| just intellectual property law that is screwing up physics.
| ballenf wrote:
| So what's the R value in terms of insulation of a black hole?
|
| If I'm trying to capture energy in the form of really hot
| things a black seems like a great way to do it. Hawking
| radiation takes eons.
| bcgraham wrote:
| This seems like the kind of thing that got decisively ruled out
| decades ago, but could the estimates of star mass be wrong?
| posix_me_less wrote:
| Yes. And dark matter hypothesis contains this possibility. Just
| add correct amount of dark matter to ever star and you can get
| almost any rotation curve.
| mxcrossb wrote:
| It sounds to me like what the author is really saying is that a
| quantum theory of gravity would look like some of these modified
| gravity equations in the classical limit. Of course it would be
| nice to kill two birds with one stone and knock out quantum
| gravity and dark matter with one discovery. I am curious if some
| of the proposed theories could be made to look like that.
| TrispusAttucks wrote:
| Dark Matter Superfluid [1]
|
| [1] https://arxiv.org/abs/1507.03013
| weeboid wrote:
| Just leaving this here
|
| https://news.ycombinator.com/item?id=23541043
| mleonhard wrote:
| When you post links, please explain why they are relevant to
| the current discussion. Posts saying "just leaving this here
| <link>" are popular on Reddit, but not here on HN. We try to
| put more effort into our comments here.
|
| From the HN Guidelines [0]:
|
| > Comments should get more thoughtful and substantive, not
| less, as a topic gets more divisive.
|
| [0]: https://news.ycombinator.com/newsguidelines.html
| throwaway64643 wrote:
| Regarding the meta of physics posts on here. I'd just like to
| tell my feeling. Reading science related or physics related
| opinions of people on Reddit, I feel like most of them are lay
| people. They express their admiration of science, their surprise
| for new knowledge, etc. What they wrote indicates them being lay.
| Reading physics or science related opinions here on HN, I feel
| like I've fallen into a rabbit hole of personal crackpot ideas of
| pseudo-experts that are ubiquitous on the net. If this is
| cutting-edge science, why wouldn't we non-experts just be a bit
| more conservative with our words? (Alsolutely no offense to any
| real expert out there).
| austincheney wrote:
| The elephant in the room is that you can't find something if it
| isn't there. Dark matter is, so far, a human invention to correct
| for gaps in particle theory. It works great until it doesn't,
| which doesn't imply it's working or even existing. It does imply
| dark matter is a really convenient answer until accounting for it
| breaks something in the math that comes from actual observations.
|
| In other words dark matter must be there, according to the
| evidence, only because otherwise our math is wrong. It could just
| be that our math is wrong.
| 6gvONxR4sf7o wrote:
| Isn't that just a paraphrasing of dark matter? Our math is
| wrong and something is up/missing from the calculations. Call
| it dark matter or an incorrect gravitational field, or whatever
| you like.
|
| I always took dark matter to just refer to the observed
| phenomenon that doesn't line up with the calculations, and that
| gap definitely exists.
| ravi-delia wrote:
| I mean, dark matter specifically refers to mass of some kind
| that interacts gravitationaly but not electromagnetically. It
| posits a real, actual material floating around in space. MOND
| is another attempt to explain the gap, and it isn't dark
| matter at all.
| 6gvONxR4sf7o wrote:
| I'm conflating them the way the article does:
|
| > But more importantly, if you look at the mathematics,
| modified gravity and particle dark matter are actually very
| similar. Dark matter adds new particles, and modified
| gravity adds new fields. But because of quantum mechanics,
| fields are particles and particles are fields, so it's the
| same thing really. The difference is the behavior of these
| fields or particles. It's the behavior that changes from
| the scales of galaxies to clusters to filaments and the
| early universe. So what we need is a kind of phase
| transition that explains why and under which circumstances
| the behavior of these additional fields, or particles,
| changes, so that we need two different sets of equations.
| ravi-delia wrote:
| The article makes the point that any evidence we have
| makes it difficult to disentangle the two, but the
| conclusions specifically calls for a third path. If dark
| matter was just any explanation for the mass gap, that
| third path would just be 'dark matter'.
| austincheney wrote:
| Apparently not, as judged by the downvotes I get when I write
| such opinions. Yet, nobody refutes the opinion in any way,
| which to me suggests either hubris or naivety neither of
| which are science or objective.
| Fooloo wrote:
| How many dyson spheres do we need to account get rid of the dark
| matter and modified gravity?
| forgotpwd16 wrote:
| One Dyson sphere of radius 1 AU (Sun-Earth distance) having
| mass 600 kg/m^2 (8-20 cm thick) will require[1] 10^26 kg of
| material. The Milky Way's dark mater halo is considered[2] to
| have mass 10^12 M_Sun = 10^42 kg. So you will need a total of
| 10^16 (10 quadrillion) spheres.
|
| [1]: https://www.aleph.se/Nada/dysonFAQ.html#ENOUGH
|
| [2]: https://arxiv.org/abs/1306.4013
| Fooloo wrote:
| wouldn't you undercount the stars enclosed?
| forgotpwd16 wrote:
| True. I only considered the spheres (shells) themselves. If
| you include the star then the result is even simpler. M_Sun
| is 10^30 kg so for a back of the envelope calculation you
| don't even have to consider the shell. Therefore you can
| say 10^12.
| paul_f wrote:
| What's truly amazing is that all 10 quadrillion spheres are
| dedicated to cryptocurrency mining.
| samatman wrote:
| A common misconception, up to 18% are in fact running Doom.
| gilbetron wrote:
| Dyson spheres would not qualify as dark matter, as they would
| be made of Baryonic matter.
| cheesejeez wrote:
| Can dark matter be just deformed space ? I am not any good with
| GR but most of the introductions to it assumes that all frame
| references are considered uniform to each others, what if that's
| not the case.
| forgotpwd16 wrote:
| In GR geometry is equivalent to matter and energy. So dark
| matter in GR *is* curved spacetime. Except if you're saying
| it's an intrinsic property of space but then you break that
| equivalence and do not speak for GR anymore. If you put it
| down, this ends up being more complicated.
| pydry wrote:
| Isn't the equivalence already demonstrably broken by space
| expanding faster than the speed of light?
| forgotpwd16 wrote:
| That's why you've dark energy added as the infamous
| cosmological constant in the equations.
| ravi-delia wrote:
| No, you can construct clean solutions to GR that involve
| all sorts of expansions. The faster than the speed of light
| thing governs acceleration, not velocity.
| eloff wrote:
| To me dark matter has always seemed inelegant, like a hack, which
| is exactly why and how it was introduced. Now nature doesn't care
| what's elegant or not, so maybe that's how things actually are. I
| always thought it'd be nicer if we found out our equations for
| gravity were off instead. Something that's a rounding error at
| small scales but that matters at large scales and could explain
| the faster orbits at the edges of galaxies and in galaxy
| clsuters.
|
| However, as Sabine points out, it seems that really, at the
| mathematical level it's the same thing. Add a field, add a
| particle, both ways you're just adding some terms to the
| equations . Fields and particles are two sides of the same coin.
|
| I guess MOND and dark matter aren't as different as I thought.
| candiodari wrote:
| I've always thought it's weird that it isn't the other way
| around. Matter, electrons, even photons ... isn't the building
| block of the university. Fields are. Fields have gravity. I
| think it's really the reverse: why aren't there more fields
| that are simply self-sustaining?
|
| Such fields, if sufficiently reluctant to interact with normal
| matter and/or photons, would be exactly dark matter.
| nicklecompte wrote:
| I am sympathetic to the recently-advocated view that the
| effects of dark matter are really "gravitational magnetism"
| associated with rotating large masses[1].
|
| That said: dark matter is no more a "hack" than the discovery
| of Neptune or Alpha Centauri C: the existence of both were
| inferred from deviations from theoretically expected
| gravitational motion, and only later confirmed by direct
| observation. If Neptune did not exist it might have implied
| Newton's equations of gravity were wrong - but it's existence
| and correctly-predicted mass were instead a major validation of
| those laws. So I think the dark matter hypothesis and the
| subsequent research activity are fully justified areas of
| scientific inquiry.
|
| [1]
| https://link.springer.com/article/10.1140/epjc/s10052-021-08...
| Sniffnoy wrote:
| The gravitomagntesim (note: this effect is more commonly
| known as "frame-dragging") paper is just wrong, lots of other
| people have also done this calculation and shown that it's
| off by multiple orders of magnitude. Robin Hanson claims the
| particular error is the paper's assumption of zero pressure:
| https://www.overcomingbias.com/2021/03/what-holds-up-a-
| north...
| posix_me_less wrote:
| Neptune hypothesis was successful, Vulcan wasn't [1]
|
| [1]
| https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
|
| In both cases, the assumption about another planet was a
| reasonable thing to investigate.
|
| Dark matter is much worse, because we are not introducing
| single planet to explain another planet behaviour, but we are
| introducing new "stuff" that can be distributed in space
| largely arbitrarily and we have great amount of freedom to
| fit the observations. Add a little dark matter here, remove
| little dark matter there, now we can fit a bear.
| Keysh wrote:
| In fact, assuming extra undetected mass to explain the
| anomalous motions of galaxies in clusters (as originally
| noted by Fritz Zwicky in the 1930s) was at least partly
| correct, because we now know that galaxy clusters contain
| rarified, extremely hot intergalactic gas (detectable only
| by its X-ray emission) in amounts equal to five or ten
| times the stellar mass of the galaxies. You still need even
| more mass in some as-yet-unidentified form (i.e., "dark
| matter") to fully explain cluster dynamics, but the basic
| hypothesis was at least partly successful.
|
| (The "undetected mass" hypothesis also turned out to be
| successful in the cases of binary stars, including things
| like Sirius B, where the companion turned out to be a
| bizarre type of star never before seen or theorized.)
|
| And, no, dark matter can't be distributed "arbitrarily":
| its distribution has to be consistent with gravity acting
| on an initially almost-uniform distribution with small
| cosmological perturbations (consistent with those seen in
| the cosmic microwave background).
| posix_me_less wrote:
| That argument with field/particles is not right. MOND isn't
| adding arbitrary new position-dependent field like dark matter
| hypothesis does. MOND changes the universal law of gravity.
| That is a priori much better thing to try.
|
| Yes, there is arbitrary interpolating function there (so also a
| great degree of freedom to fit the observations), but the
| resulting modified law is supposed to be universal, valid for
| all points of space and all material particles. Dark matter
| hypothesis, on the other hand, just adds immense number of new
| obscure quantities(dark matter density at all points of space)
| that can be tuned to fit (almost) anything.
| snarfy wrote:
| In antenna theory there is the concept of the near field and far
| field.
|
| I had an idea that maybe black holes have an additional near
| field component that becomes exposed once the singularity is
| formed, like there is a limit/floor to infinite density. Energy
| in the far field falls of at 1/r^2 but near field is 1/r.
| 867-5309 wrote:
| infinite density is not physically possible, that's why general
| relativity fails to account for the existence of singularities
| posix_me_less wrote:
| How would near field of black holes explain galaxies' rotation
| curves?
| snarfy wrote:
| It would mean black holes have different gravitational
| properties. Normal matter's field falls off at 1/r^2, but a
| black hole would be more like 1/r^2 + x(1/r). The 1/r term
| would make the outer edges spin faster than expected.
| hwillis wrote:
| Your theory is that at far distances black holes act like
| normal gravity wells, and at near distances the gravity is
| lower (or higher?) than would be expected for the real
| mass.
|
| This is directly contradicted by evidence specifically for
| the case of galactic-scale supermassive black holes, and
| broadly by observations of black holes.
|
| We have directly observed accretion disks, which would look
| very different if the near field of a black hole was
| different from the far field. We have directly observed
| discrete objects falling into black holes with minimal
| accretion disks, ie with no other forces except for
| gravity, and have found that redshift evidence very
| strongly agrees with general relativity. There is just no
| way you are correct for most black holes. They behave the
| same close up as they do from far away.
|
| If supermassive black holes acted differently then we would
| have seen evidence of your theory with Sagittarius A*. It's
| our galaxy's central supermassive black hole. We have
| measured the orbits of objects light years (cluster GCIRS
| 13E) away and light hours (star S2) away from its center.
| Gravity changes as expected.
|
| A near-field linear effect is not sufficient to cause us to
| think that black holes are less massive than they actually
| are, which would be necessary for your theory to explain
| why stars in the near parts of galaxies don't orbit as
| quickly as we'd expect based on the outer stars. If gravity
| stopped increasing as quickly below a certain radius, we
| would have seen it.
| snarfy wrote:
| > Your theory is that at far distances black holes act
| like normal gravity wells, and at near distances the
| gravity is lower (or higher?) than would be expected for
| the real mass.
|
| No that's not right. It's the inverse. At far distances
| gravity is higher than expected for black holes compared
| to regular stars.
| RobertoG wrote:
| I'm not a physicist, so, please take my opinion like you would
| take the opinion of somebody chatting in the groceries shop
| queue, but I find the subject fascinating and I just want to
| share my crazy theory. At least, I think it would make for good
| science fiction.
|
| I "think" that dark matter observations are the effect of
| interference between different Everett branches. The smaller is a
| volume space, the less possible states it has, so gravity in the
| center of a galaxy would be less that in the borders. The borders
| of the galaxy are more entropic because more different states are
| possible, ergo the stars there experience interference with more
| Everett universes. Gravity being so weak, it would only show its
| effects in other branches at huge scales. In this "theory", it
| seems that dark matter effects would be less visible the further
| away in space (time) we look. No idea if that's the case.
|
| Again, I have no idea what I'm talking about, so this is just for
| fun.
| emrah wrote:
| It would be awesome to be able to find people like you in the
| grocery shopping queue to chat with about topics like this!
| RobertoG wrote:
| Sadly, chances are we would talk about the weather instead. I
| bet that none of us want to look crazy.
| [deleted]
| nathias wrote:
| This really is a really good idea (or a start of) for sci-fi.
| DavidSJ wrote:
| This seems sort of like saying a house in California should be
| more entropic than a house in Rhode Island because California's
| bigger than Rhode Island. But the laws of physics don't care
| where we've drawn arbitrary lines on our maps, whether they be
| geographic or galactic.
| RobertoG wrote:
| But California is more entropic than Rhode Island, because,
| being bigger, California can be in more states than Rhode
| Island. In the center of the galaxy a star can be in N
| positions, in the periphery can be in f(N) positions. The
| further from the center the more possible positions (maybe
| the number of positions is related to Newton square law?).
|
| Anyway, as I said, just idle speculation for fun.
| DavidSJ wrote:
| But California [?] a house in California, which was my
| point.
|
| How does the universe know it's supposed to consider the
| positions on the entire circumference of a galaxy, and not
| just the "left" edge, or the left edge plus the center, or
| the right edge plus one quarter of another galaxy's pinky
| finger, when deciding how strong gravity should be in the
| left edge? Only local information should be relevant,
| otherwise you're back to assuming the universe respects our
| arbitrary boundaries.
| RobertoG wrote:
| This is just speculation for fun, but OK, I will bite.
|
| You have a compressed gas, so, low entropy, then it
| starts expanding. The area where it started expanding has
| less possible states that the area around, but just
| because the area around is bigger, nothing to do with the
| relative position itself.
|
| If the number of states are associated to gravity
| somehow, then gravity will be bigger the further you go
| from the center.
|
| Because we are brainstorming in creative mode, I will add
| a bonus: if the number of possible states increase
| gravity, maybe, the older the universe gets, the bigger
| is that influence all around, accelerating its expansion.
|
| I suppose that I will get my Nobel any day now ;-)
| Keysh wrote:
| On the other hand, the center of a star is much hotter than
| the periphery, so there are potentially more accessible
| states in phase space for the particles in the former
| location (it's "can be in N positions in phase space",
| which depends on the available physical space and the
| possible velocities).
|
| Not that this would affect California vs Rhode Island
| analogies, unless you could demonstrate that things in
| Rhode Island were a lot hotter...
| mclightning wrote:
| Interesting. But then we would see effect of dark matter
| correlate with the distance of stars. Right?
|
| I think observations do not show such correlation between
| spacetime location of galaxies and dark matter/energy presence.
| RobertoG wrote:
| Yes, that would be a way of falsifying the theory. No idea
| what the data says.
|
| Dark matter effects should be less or not existing in the
| early universe.
| rossdavidh wrote:
| Wait, what sort of grocery store queues have that conversation?
| AnimalMuppet wrote:
| Bars near universities are where you find this kind of
| conversation.
| vcxy wrote:
| Have you read Neal Stephenson's "Anathem"? I'm not exactly sure
| what to say about it in context of your comment for fear of
| spoilers, but it sounds like you would like it.
| RobertoG wrote:
| I have read 'Snow Crash', 'The diamond age' and
| 'Cryptonomicon', but not that one. Thanks for the
| recommendation.
| sega_sai wrote:
| It's a bit of the same from Sabine. Dark matter hypothesis
| allowed us to simulate a vast range of processes in the Universe,
| from large scale cosmological simulations, predicting clustering
| of galaxies, cosmic microwave background to smaller scale
| simulations of galaxy formation and individual galaxies. The
| hypothesis have been extremely successful and predictive. It is
| also true, that some of the systems we've been discovering are a
| bit harder to explain within our Cold Dark matter framework,
| requiring changing some things about how galaxies form (i.e.
| making star formation less efficient in small galaxies,
| introducing feedback effects from accreting supermassive black
| hols and things like that). I (and many astrophysicists) do not
| see that as a problem or evidence that dark matter needs to be
| abandoned, but instead see that as a refinement to the theory.
|
| Regarding modified gravity theories, it's true that you can mimic
| some of the dark matter behaviour by the modification of gravity
| laws. Also if you can add some kind of global scalar field you
| can similarly mimic dark matter behaviour. The problem however is
| that the modified gravity theories (at this point) do not allow
| the same range of simulations that cold dark matter(CDM)
| simulations can do (for various reasons, such as many of those
| theories do not have a general relativity formulation). Because
| these theories can't make as many/as varied simulations as CDM
| ones, they are not as tested as regular CDM and much less
| predictive. Therefore based on that fact alone IMO these theories
| are less useful.
|
| It is also true that if we continue for many years searching for
| the dark matter particle and will still be unable to find it,
| maybe we will need to refocus our attention on alternative
| gravity theories, but the problem is that so far the part of
| parameter space where we looked for the dark matter particle is
| still pretty small and there is no strong reason to think that
| dark matter should have been there.
| posix_me_less wrote:
| > The hypothesis have been extremely successful and predictive.
|
| Epicycles were successful and predictive. But they were later
| found to be the wrong way to understand the celestial motions.
|
| The problem is these calculations with dark matter are just
| fitting observations with zillion parameters. It is busywork,
| yes sometimes it is useful, but it is not discovering new laws,
| just chugging along without having to challenge our concept of
| how star/galaxy motions really work.
| sega_sai wrote:
| Epicycles were a good theory that approximated a ellipses by
| combinations of circles (essentially through a Fourier
| transform). When a simpler theory came that was easier to
| compute, was more predictive and came from a first principles
| calculations it was adopted instead. That's not the case with
| the modified gravity (at least right now).
|
| And please don't BS about zillion of parameters. That's not
| the case. The dark matter only simulations don't have that
| many parameters (other than the basic cosmological ones) .
| When you start putting baryons in, yes that's where the
| parameters start appearing (probably of the order of a 100),
| but it's hardly surprising give the need to describe the
| complexity of star formation, gas dynamics, black hole
| accretion etc. The modified gravity theories don't need as
| many parameters because nobody has ever run any modified
| gravity simulation that is even close in complexity to what
| Cold Dark Matter simulations do.
| posix_me_less wrote:
| I agree MOND results aren't that great now. But Kopernik's
| results weren't that great either, compared to standard
| theory. They gave less precise results initially. It took
| time for the astronomers to accept them.
|
| What kind of initial condition about dark matter
| distribution are you assumming in your simulations?
| sega_sai wrote:
| Copernicus model was not that much of a change from a
| model complexity point of view, as it just removed some
| common epicycles by putting Sun in the center. The key
| changes were Kepler's results and Newton's, where one had
| provided drastically simpler model, and another had a
| theoretical/(first principles) derivation.
|
| Regarding initial conditions, I'll point you towards
| wikipedia. But basically the assumptions is that it's
| collisionless fluid with very small Gaussian density
| perturbations and scale invariant power spectrum. In
| total the specification of that model requires ~ 10
| parameters.
| posix_me_less wrote:
| So I infer you are talking about large scale simulations
| of many galaxies/a region much greater than single
| galaxy, right?
|
| If so, do these kinds of simulations with 10 parameter
| model predict galactical rotation curves in agreement
| with observation?
|
| Or do we need more detailed model of dark matter
| distribution for the rotation curves?
| sega_sai wrote:
| There are different simulations focusing on various
| aspects of the universe. All the CDM simulations give
| (predict) a dark matter density profile close to the so
| called Navarro-Frenk-White (NFW) profile (i.e. here https
| ://ned.ipac.caltech.edu/level5/March01/Battaner/node27...
| . ) . This profile has essentially two parameters, mass
| and concentration. These are the two free parameters you
| work with for individual galaxies if you try to match the
| data.
| posix_me_less wrote:
| Okay I can appreciate the fact that there are DM models
| that work with small number of parameters. But these are
| not working great for some galaxies, which pushes in
| direction of more parameters, or something different than
| DM.
|
| Quote from from the page you've linked [1]:
|
| > However, either the observations do not constitute a
| proof of the CDM models, or dynamic ingredients other
| than halo and disk density profiles are necessary to
| study the rotation of spirals.
|
| [1] https://ned.ipac.caltech.edu/level5/March01/Battaner/
| node27....
| dnautics wrote:
| The simulations themselves don't have zillions of
| parameters, but the field itself does. When you say
| something like "the bullet cluster blah blah" you are
| talking about a custom parameterization of the lambda-cdm
| _field_. You 're not talking about simulation, because
| simulations predict that bullet cluster speed collisions
| should not happen in our universe's lifetime with the
| number of galaxies observed.
| sega_sai wrote:
| I'll point you here on the topic of the cluster speed htt
| ps://ui.adsabs.harvard.edu/abs/2007MNRAS.380..911S/abstra
| ... I.e. it's not a problem.
| dnautics wrote:
| ok that's fine but it still doesn't change the fact that
| there is a distinction between parameterizing field vs
| parameterizing the simulation. You shouldn't have
| conflated the two in your defense of LCDM.
| dTal wrote:
| Epicycles weren't really predictive, in the sense that every
| time measurements were refined, the new data didn't fit and
| more epicycles had to be added - and this was only possible
| because combinations of epicycles can represent anything at
| all.
|
| So by analogy, you're saying that modern dark matter theories
| are so general that they a) could account for any conceivable
| observation, and b) fail to predict novel phenomena? I don't
| have the physics knowledge to evaluate that assertion, but it
| doesn't seem to be shared by the broader physics community.
| Keysh wrote:
| That's not how epicycles were used. Ptolemy set up the
| standard geocentric model with epicycles (and deferents and
| equants), and these were used more or less unchanged for
| over a thousand years. When, in the late medieval period,
| some Arab astronomers began tinkering with modifications,
| these were motivated by a desire for better agreement with
| the Aristotelian ideal of uniform circular motion (i.e., to
| somehow replace Ptolemy's non-uniform equant device with
| some combination of uniform circular motion), not any
| desire to match the measurements better.
| dnautics wrote:
| Yes, and yes. Not "any conceivable" but "any conceivable
| gravitational observation".
|
| The trouble with dark matter is that it's unimodal: It
| exists to fix a problem where our theory of gravity
| disagrees with observation, but at the same time the only
| way to measure dark matter is by observing gravitational
| effects.
|
| The only true predictions made have been "if dark matter is
| _actually_ x particle then ... " Or (weaker) "if dark
| matter has non-gravitational property X, then..."
|
| And thus far all prediction of those sort have been
| refuted.
| edflsafoiewq wrote:
| Feel free to propose a Keplerian model. Until then people
| will continue on with their epicycles.
| nosianu wrote:
| > _I (and many astrophysicists) do not see that as a problem or
| evidence that dark matter needs to be abandoned_
|
| As I read it, and I think it was pretty straightforward,
| neither does she? She wants to keep it where it seems to be an
| appropriate model.
| slowmovintarget wrote:
| Modified gravity would not cover the main case for dark matter,
| though, which is not galaxy formation or patterns of motion we
| observe.
|
| Rather the main case is the ratio of protons and neutrons to
| photons produced during Big Bang nucleosythesis. We know the
| ratio of the number of protons to number of photons. We know
| the number of photons in the universe (most of them are in the
| CMB), and we therefore know the number of protons. That means
| we know the amount of "normal" matter in the universe, and it
| is far less than the total amount of matter in the universe
| (that survived from baryogenesis). This requires there to be
| "dark matter" in the universe.
|
| The other effects like galaxy formation and stellar motion in
| galaxies are just things that might be caused by dark matter
| (though that's come into doubt, as Sabine points out). The real
| reason to believe in dark matter is Big Bang nucleosynthesis.
|
| Sean Carroll discusses it here:
| https://youtu.be/tZQadPmTd84?t=4642
| Sniffnoy wrote:
| But Hossenfelder _doesn 't_ propose abandoning dark matter;
| this comment seems like a mischaracterization of her position
| as stated in the original post.
|
| Rather Hossenfelder suggests that both dark matter _and_
| modified gravity may be necessary, with each being relevant in
| different situations due to some sort of phase change (?),
| noting in the comments Khoury 's idea of superfluid dark matter
| (although I'm unclear as to whether that includes modified
| gravity?) as one example of such a theory.
|
| Edit: Oops, misattributed & misdescribed superfluid dark matter
| eutectic wrote:
| Seems like an excuse for introducing even more free
| parameters.
| Fellshard wrote:
| So the obvious problem is that the unobservable, untestable,
| unrepeatable current hypothesis of origins is clung to, then, in
| flagrant disregard for the observable, testable, repeatable
| theories regarding gravity and basic mechanics. I wonder what
| would incentivize such irrational, anti-scientific behavior.
| gus_massa wrote:
| > _I wonder what would incentivize such irrational, anti-
| scientific behavior._
|
| A) We have a very good model of gravity, tested in a lot of
| systems (like the lab, planets orbits, ...)
|
| B) We have some other systems (like galaxies, ...) were the
| model fail.
|
| So the possibilities are:
|
| 1a) The model of gravity we have is wrong. [We know it's wrong
| in other cases and we need a Quantum Gravity theory, but this
| correction is necessary for other king of systems, like very
| small and with huge gravity fields, so it's probably not
| relevant.]
|
| 1b) The model is correct, but we are making bad approximations.
| [Other comments are about gravitomagnetism. I doubt that's the
| problem, but this is not my specialty.]
|
| 2) We have a bad understanding of galaxies and other systems
| where the current model of gravity appears to fail. Perhaps we
| are only considering the matter we can see. Perhaps there is
| more matter that is somewhat "invisible". We can guess where
| that "invisible" things are and see if we can fix the
| predictions of the gravity model. We only need a catchy name
| for this "invisible" thing because it may not be actually
| invisible [1]. What about "Dark Matter"?
|
| 3) As this article explain, perhaps both are correct and we
| need to fix gravity at a galactic scale and also find some part
| of the thing that are inside a galaxy.
|
| What is the correct rational scientific behavior?
|
| [1] They may be invisible from here using a telescope, but they
| many be visible if you are nearby. Nobody is sure. The names of
| the models are weird.
| https://en.wikipedia.org/wiki/Massive_compact_halo_object
| https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...
| https://en.wikipedia.org/wiki/Robust_associations_of_massive...
| chris1993 wrote:
| In the comments why does everyone refer to the author as
| 'Sabine'? It seems overly familiar and derisory. Men are granted
| the respectful use of their full name or their surnames.
| TrispusAttucks wrote:
| The conclusion drawn in the article, but not mentioned by name,
| is a reference to Dark Matter Superfluid [1]
|
| [1] https://arxiv.org/abs/1507.03013
| nxpnsv wrote:
| Does this really add any thing to the current status quo? This
| nor theory or hypothesis yields no new predictions, no ground
| breaking insights, and actually nothing has changed.
| optimalsolver wrote:
| You don't need to introduce anything new in order to point out
| flaws in the current status quo.
| nxpnsv wrote:
| If she pointed out a flaw other than a lack of discovery then
| that should be no surprise to anybody. It is the whole reason
| so many people are looking for so many different signals in
| so many experiments...
| dooglius wrote:
| An article on HN a couple months ago
| https://news.ycombinator.com/item?id=26442021 claims that our
| theory of gravity is perfectly fine, we just haven't been taking
| into account the proper relativistic corrections at galactic
| scales. This seems much more plausible and elegant of an
| explanation to me.
| Sniffnoy wrote:
| This paper is just wrong, lots of other people have also done
| this calculation and shown that it's off by multiple orders of
| magnitude. Robin Hanson claims the particular error is the
| paper's assumption of zero pressure:
| https://www.overcomingbias.com/2021/03/what-holds-up-a-north...
| escape_goat wrote:
| I have no doubt that Robin Hanson is a brilliant man, and I
| am aware that he has an extremely strong physics background
| for a professor of economics, but it would seem prudent at
| this point for those of us who are not domain experts to have
| somewhat Bayesian anticipations regarding the cross-
| disciplinary adventurism of economics professors. People read
| his blog. If this thing is indeed a ball, there will be
| plenty of astrophysicists happy to pick it up and run with
| it.
|
| This doesn't mean that you're wrong that the paper is wrong,
| but Dr. Hanson should absolutely not be our referent for
| certitude on the matter.
| 542458 wrote:
| My understanding is that while MOND seems simpler on the
| surface, it a) is hard to make match observed data (you end up
| needing dark matter anyways to make it work, and it still falls
| apart on things like the bullet cluster) and b) breaks all
| sorts of other things in physics that don't look like they need
| to be broken (you have to staple new stuff to the side of
| relativity to keep it all consistent, in some cases breaking
| conservation).
|
| Dark matter looks/sounds hacky and broken, but in general
| matches real-world data better than the competing theories.
| Blammar wrote:
| The OP presentation is saying that dark matter matches some
| things and MOND matches other things. Both together seem to
| explain everything.
| Maursault wrote:
| > we just haven't been taking into account the proper
| relativistic corrections at galactic scales
|
| Dark Matter was first postulated because galaxy rotation curves
| strongly suggested if there was not unseen matter, galaxies
| would fly apart. But these observations of galaxies were each
| of a galaxy in isolation. So in essence, Dark Matter is a fudge
| to explain the observation that galaxies are not flying apart.
| Since, other observations that can't be explained have been
| lumped into Dark Matter. But it turns out, Dark Matter is
| unnecessary to explain galaxy rotation curves.[0] Galaxies are
| never isolated. They come in clusters and superclusters.
|
| [0] https://www.youtube.com/watch?v=PL0ewiwqoTw
| ravi-delia wrote:
| The scope of observations goes a little further than
| rotational curves. The Bullet Cluster, for instance, clearly
| shows mass where there is no visible matter. Plus, there are
| lots of smaller galaxies that behave just as our normal
| theories prescribe, which dark matter (or the lack thereof)
| can explain and other theories have trouble with.
| bjornsing wrote:
| So? The relativistic effects of mass currents (that the
| parent is talking about) could put "apparent mass" where
| there is none, no?
| ravi-delia wrote:
| In principle, yes. In practice, these proposals match the
| spin curve very well and the bullet cluster not so much.
| That isn't a knock down argument; the bullet cluster
| offers oddities for dark matter too. However OP implied
| that because galactic rotation works without dark matter,
| and dark matter was first posited to deal with galactic
| rotation, we don't need dark matter. That's not right.
| Keysh wrote:
| I kind of assumed that a YouTube video on that subject would
| be a waste of time, and boy was it. That was amazingly
| ignorant.
|
| Galaxies are _often_ isolated. They come in clusters, and
| also in groups, and in isolation (including isolated galaxies
| inside cosmic voids). The isolated galaxies have the same
| "dark matter" phenomena as galaxies in clusters (which, by
| the way, do _not_ have perfectly circular orbits, just to
| start with).
| rich_sasha wrote:
| I meant to comment there but never got round to it: we have now
| found galaxies apparently without dark matter. They behave
| exactly as you'd expect from direct measurements.
|
| So either somehow that explanation doesn't apply there (but
| why?) or dark matter is real, and just somehow absent from some
| galaxies.
|
| Link: https://www.space.com/19-galaxies-missing-dark-
| matter.html
| cjohansson wrote:
| Sounds like the DM theory is great for postdictions but not
| for predictions, like you can't know in advance whether it is
| applicable or not
| devoutsalsa wrote:
| Dr. Becky does a good job of describing how this could be
| possible in this video =>
| https://www.youtube.com/watch?v=gMDKDs9C4lo&t=742s
| XorNot wrote:
| Also specifically artifacts like the bullet nebula, where
| observationally we can see gravitational lensing totally
| removed from the colliding gases in the X-ray spectrum -
| exactly what would be expected if dark matter had been
| separated from the object by it's inability to collide with
| itself via normal mechanisms.
| TrispusAttucks wrote:
| FTA:
|
| Then there are collisions of galaxy clusters at high
| velocity, like the bullet cluster or the el gordo cluster.
| These are difficult to explain with particle dark matter,
| because dark matter creates friction and that makes such
| high relative velocities incredibly unlikely. Yes, you
| heard that correctly, the Bullet cluster is a PROBLEM for
| dark matter, not evidence for it. Yes,
| you heard that correctly, the Bullet cluster is a PROBLEM
| for dark matter, not evidence for it.
| Keysh wrote:
| This is rather overstated (especially the "incredibly
| unlikely" bit).
|
| The best attempt I've seen to summarize this is in this
| paper https://arxiv.org/abs/1412.7719 which argues that
| there's roughly a 10% chance of getting something as
| extreme as the Bullet Cluster under Lambda-CDM. So,
| mildly unlikely, but not even approaching the
| (problematic) P < 0.05 threshold that's emblematic of the
| "replication crisis" in fields like experimental
| psychology.
| XorNot wrote:
| Yes and I'm straight up not buying that argument because
| it being improbable that the bullet cluster might arise
| doesn't explain why it did: remove particle dark matter
| and now you're left with gravitational effects floating
| around in space totally divorced from the shape and
| location of regular matter which should be producing
| them.
| pavel_lishin wrote:
| Does "friction" refer to something different here? I
| thought that Dark Matter didn't even interact with itself
| in any meaningful sense, and therefore had no friction.
| Keysh wrote:
| Friction in this context refers to "dynamical friction",
| where massive objects -- e.g., dense cluster cores --
| moving through a medium of many small masses -- e.g., DM
| particles -- will experience gravitational drag and slow
| down.
|
| https://en.wikipedia.org/wiki/Dynamical_friction
| SAI_Peregrinus wrote:
| Except that the core thing that makes dark matter dark is
| that it doesn't interact via the electromagnetic force,
| and friction is a result of the electromagnetic
| interaction. So that statement is nonsense.
| dnautics wrote:
| No that's not right. Dark matter itself causes an
| entirely gravitational "friction". If you have a
| noninteracting particle over time a bunch of them get
| slingshotted to very high velocity on accident which
| bleeds the net momentum of the rest of the system. If
| you've ever written n-body simulators you would see this.
| gus_massa wrote:
| I think you are correct, but I like freebees, ... Do you
| have an online demo?
|
| Is the maximal speed of the slingshotted particle
| something like twice the speed of the big objects? Do
| they escape or they just form a cloud that gets hotter?
|
| For this subject, it's important that the simulation
| conserves energy. Most naive numeric simulations don't do
| that, and even symplectic simulations increase/decrease
| the total energy slowly.
| dnautics wrote:
| > Do they escape or they just form a cloud that gets
| hotter?
|
| They escape.
|
| yeah, having done this before, I am 100% sure that the
| simulation will be inexact due to numerics, and that the
| inexactness will be worse around these phenomena, but we
| are probably talking, gut feeling say, under 20% in most
| scenarios where you see an escape (I think the observable
| error is effectively unbounded because you could in
| theory get two particles within one ULP or even two
| particles that collide and cause a NaN error)... But in
| general I don't think that changes the qualitative nature
| of the phenomenon. There's probably a reasonably easily
| derived "starting from three particles at rest" where you
| can see one of them escape from the other two; if not 3
| then four.
| Keysh wrote:
| This seems to suffer from the usual problem of non-astronomers
| not understanding the breadth of the problem they think they're
| trying to solve. That is, they've vaguely heard that the
| problem is in the rotation of disk galaxies, and so they
| propose solutions that depend on the rotation (and maybe also
| on the flattening).
|
| But the "dark matter" problem occurs for basically _all_
| galaxies, including things like elliptical and dwarf spheroidal
| galaxies that have no bulk rotation at all. (And also disk
| galaxies where a significant fraction of the stars and gas are
| _counter-rotating_.) The same applies to groups and clusters of
| galaxies.
| bjornsing wrote:
| > And also disk galaxies where a significant fraction of the
| stars and gas are counter-rotating.
|
| So? Have you calculated the relativistic effects of those
| incredibly complex mass currents and come to the conclusion
| that GR is not a sufficient explanation for the motion?
|
| I'm sure there's a high risk that GR can't explain
| everything, but it sure is frustrating that people tend to
| grasp for these adhoc hypotheses without exhausting GR first.
| abnry wrote:
| But surely someone would have worked out the math and people
| would agree that the relativistic explanation fits the data?
| fpoling wrote:
| The third explanation for the problem is that we use wrong
| approximations of equations of the General Relativity. Even
| Wikipedia entry on the rotational curves site a paper that shows
| that properly accounting for General Relativity is enough to
| explain the rotational curves of our Galaxy.
|
| Then there is very interesting take on the equations in [1] that
| also explains the rotational curves and few more observed effects
| again without any extra particles or fields.
|
| [1]
| https://link.springer.com/article/10.1140/epjc/s10052-021-08...
| Sniffnoy wrote:
| This paper is just wrong, lots of other people have also done
| this calculation and shown that it's off by multiple orders of
| magnitude. Robin Hanson claims the particular error is the
| paper's assumption of zero pressure:
| https://www.overcomingbias.com/2021/03/what-holds-up-a-north...
| forgotpwd16 wrote:
| The galactic rotation curves isn't the only reason for
| requiring dark matter. And gravitomagnetism is a known effect.
| I don't think for decades being studied everyone was doing it
| wrong.
| fpoling wrote:
| The problem of rotational curves and few other effects
| attributed to the dark matter appears if one applies
| Newtonian mechanics on a galaxy or intergalactic scales. Yet
| to this day there is no proof that this is a sound
| approximation even for small galaxies. It is just assumed
| that is a valid approximation without justification. And the
| reason for that is that accounting for General Relativity is
| hard even if one consider just next order approximation after
| Newtonian gravity. So yes, it could be that almost everybody
| was doing it wrong.
| nicklecompte wrote:
| Has there been any updates from the astrophysics community
| about this "gravitational magnetism" explanation[1]?
| GR/cosmology is not my area of expertise, but if
| astrophysicists were indeed neglecting the impact of rotating
| masses on gravitation that seems like a major oversight that
| needs to be investigated.
|
| [1] A static charge generates an electric field and a moving
| charge generates a magnetic field. There is a similar effect
| for gravity: the Einstein equations have a sort of symmetry to
| the Maxwell equations.
| Sniffnoy wrote:
| Yes -- the paper is just wrong, lots of other people have
| also done this calculation and shown that it's off by
| multiple orders of magnitude. Robin Hanson claims the
| particular error is the paper's assumption of zero pressure:
| https://www.overcomingbias.com/2021/03/what-holds-up-a-
| north...
|
| Note that what the paper refers to as "gravitomagnetism" is
| more commonly referred to as "frame-dragging":
| https://en.wikipedia.org/wiki/Frame-dragging
| fpoling wrote:
| The article cited from Wikipedia,
| https://arxiv.org/abs/1810.04445 , also pointed out that
| accounting for terms in the metric tensor related to rotation
| was essential to explain the rotational curves and not using
| them is simply unsound approximation.
| [deleted]
| Diti wrote:
| Is it possible that what we call dark matter might be the
| positive equivalent of gravity?
| meowface wrote:
| If you mean something like an inverted form of gravity, that
| would be the hypothetical dark energy, I think. Dark energy is
| hypothesized to be a repulsive force, while gravity is an
| attractive force.
| prox wrote:
| Really? First time I read this. I always thought it was more
| gravity.
| Svoka wrote:
| Dark Matter is something producing regular gravity (bending
| spacetime in a way regular matter would) except we don't see
| the source, hence the name.
| dvfjsdhgfv wrote:
| I always thought the name is quite sinister for this kind of
| mostly neutral (or even positive in the large scheme of
| things) phenomenon that we don't understand the nature of. We
| could give it a more interesting name like Barbelo.
| posix_me_less wrote:
| The name "dark matter" is bad, because matter that is dark
| absorbs a lot of light. Dark matter does not interact with
| light, light passes right through. It should have been
| called invisible matter, or non-electromagnetic matter.
| samatman wrote:
| I like "subtle matter", personally.
|
| It isn't just light, as far as we know baryonic matter
| also passes through without interacting in any measurable
| way.
|
| It has certain... resonances with mystic traditions such
| as Theosophy, and I could see that being unwelcome in
| some circles. To my taste it's perfect.
| Sniffnoy wrote:
| Non-mobile link: http://backreaction.blogspot.com/2021/05/dark-
| matter-situati...
| slowmovintarget wrote:
| Even on desktop, the mobile link is better (opinion). :)
| ...Unless you like a narrow column of text on your 1080p or
| wider display.
| Sniffnoy wrote:
| My main point is doing this is to have the canonical link,
| not the best-looking link.
| Svoka wrote:
| I don't get why people are so attracted to idea of "modified
| gravity". So far it is a body of knowledge without predictive
| power struggling to adjust itself to things predicted by GR.
| joeberon wrote:
| One huge motivation is the disturbingly fierce incompatibility
| between the Standard Model and GR
| bunje wrote:
| How are the theories incompatible? My basic understanding is
| that GR describes how the space looks like and Standard Model
| describes fields defined in this space.
| joeberon wrote:
| That's a very complicated question really and kind of
| beyond my specialisation, but it mostly comes down to
| something called "renormalization", which is a technique
| used to construct a Quantum Field Theory. Unfortunately it
| doesn't seem to be possible to construct a renormalizable
| field theory for gravity, which makes it seem incompatible
| with the approach for the Standard Model.
|
| In their basic forms, GR and the Standard Model are
| basically entirely different languages, as you said, one
| using a model of motion in a curved spacetime, and the
| other using a formalism of quantum fields. However this
| brings up a big question: why are the electromagnetic,
| weak, and strong interactions described by a totally
| different formalism to gravity?
|
| You can do quantum field theory in a curved spacetime, but
| that is more of a band-aid approach to the problem than
| anything and doesn't really help to unify the two
| approaches.
|
| It could be that the answer is that two different Gods
| designed gravity and the other forces, they certainly seem
| that disparate. But as a Physicist is it quite hard to
| accept that gravity and the other interactions are
| fundamentally separate, we would like a full theory of all
| four of the interactions.
|
| EDIT: I should point out that QFT is hardly very nice
| anyway. We don't have evidence for a specific
| ontology/interpretation for Quantum Mechanics yet, we don't
| know what Quantum Mechanics is or means. We may come closer
| in the years coming though.
| AnimalMuppet wrote:
| In addition to what others have said: They have very
| different definitions of time.
| posix_me_less wrote:
| Quantum theory is about linear reversible evolution based
| on some Hamiltonian operator. GR is non-linear non-
| Hamiltonian theory (a naive Hamiltonian is zero); it has
| strange one-way solutions such as irreversible collapse of
| stars into black holes.
|
| These two theories have very different mathematics and
| different concepts of state. Nobody has been able to
| connect them in a way that would be a "success" - a unified
| theory that explains both GR and QT in a consistent way.
| enkid wrote:
| Is that a swipe against the article? I think she would argue
| Dark Matter is so over parametrized it no longer makes
| meaningful predictions. And the argument that any new particle
| requires a new quantum field makes total sense. So Dark Matter
| is Modified Gravity, there's not clear distinction because of
| the need for new fields anyways. Of course, IANAP, so I could
| be totally misreading it.
| akvadrako wrote:
| The difference between dark matter and modified gravity is
| how it varies throughout space.
|
| Modified gravity is a formula that applies exactly at every
| point based on other mass, like Newtonian gravity _F=G(m1m2)
| /r^2_.
|
| The density of dark matter varies throughout space and it
| isn't based on a simple formula, but all the complicated
| dynamics of the past.
| Svoka wrote:
| How exactly Dark Matter is overparameterized? So far there
| was just constraints on what it could be from actual physical
| representation, not from what effects it has in terms of
| spacetime curvature.
| enkid wrote:
| I'm not a physicist, I'm just trying to represent what's in
| the article.
| idlewords wrote:
| The appeal of modified gravity lies in all the observations
| Sabine outlines here, that don't fit our understanding.
|
| You need something to explain them, and the choice is either
| positing lots of invisible stuff, or modifying the laws of
| gravity. It seems reasonable to try both approaches,
| particularly since positing lots of invisible stuff has a
| pretty checkered history in science.
| ww520 wrote:
| Could gravity in higher dimensional space be a way to explain
| dark matter?
| idlewords wrote:
| Not really. See Penrose for good rants about why higher
| dimensions introduce insuperable problems. Basically, if you
| start adding new dimensions, almost everything will leak into
| those higher dimensions and then you need to start adding on
| kluges for why we don't notice them except for the one desired
| effect.
|
| There's a good discussion of this in his book "Fashion, Faith,
| and Fantasy in the New Physics of the Universe".
| irjustin wrote:
| As an arm-chair/youtube scientist, I admit I don't like MOND, but
| apparently I shouldn't like Dark Matter either. I
|
| I hope in my life time this question is solved.
|
| Probably one of my favorite areas to follow - that and the
| quantization of gravity.
| nolroz wrote:
| What's "MOND"? Is "Dark Matter" referring to the show or the
| substance?
| mmusson wrote:
| Dark Matter is an unfortunate term. The intent was simply to
| say that observations did not match predictions in a way that
| seemed to imply that a galaxy had more mass than it seemed
| from existing measurements.
|
| That wasn't even so controversial at first because measuring
| the visible mass of distant galaxies is a hard problem.
| idatum wrote:
| There is something about this scientist's style of writing I
| like. She conveys complex ideas, gives details, and keeps the
| reader engaged. Bookmarked.
|
| EDIT: And looking at the non-mobile link I realize now it is a
| transcript. So that crispness in writing came out from
| essentially a lecture. Nice.
| idlewords wrote:
| Her blog includes both videos like this and regular posts, and
| is just a wonderful internet rabbit hole to fall into.
| tanin wrote:
| Big fan of her YouTube videos. Watched a lot of them. Then
| randomly there's her singing.. I was so confused lol
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