[HN Gopher] Dark Matter Alternative Passes Big Test
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Dark Matter Alternative Passes Big Test
Author : gumby
Score : 32 points
Date : 2021-10-19 13:14 UTC (9 hours ago)
(HTM) web link (physics.aps.org)
(TXT) w3m dump (physics.aps.org)
| Svoka wrote:
| Why is Hacker News so fixated on MOND? I mean, I guess general
| relativity is hard for general public, but come on...
|
| At this point MONDs look like really overfit models, failing to
| make any observable predictions, struggling to describe what we
| already observe (by overfitting)
| autokad wrote:
| the biggest point is: you are telling me there is something
| that makes up the majority of matter in the universe (27% dark
| matter vs 5% matter) and we still haven't observed a single
| iota of it, even after spending more than 3 decades and
| billions of dollars?
|
| 'well it fits all of our understanding of the universe and what
| we observe in a few things'
|
| It comes on top of things that cosmologists have been wrong for
| very very very long times on. Look how long they thought it was
| silly to consider the big bang. maybe it would be easier to
| accept if they were like 'well the big bang is a very
| interesting idea but it just doesn't fit evidence' instead of
| 'that's a stupid idea, my idea (static universe) is simpler so
| its right'.
|
| speaking of which, we really need to get off this 'simple idea
| is the right idea' dogma. The universe owes us no such
| obligation to be simple.
|
| cosmology got so many assumptions baked in them its hard to
| figure out where they begin and end. for example, look how much
| relies off the idea that the universe is the same everywhere,
| which we now know as wrong, but cosmologists are only slightly
| admitting to that fact.
| actually_a_dog wrote:
| My thoughts exactly. In particular:
|
| > The idea did not spring from any underlying theory....
|
| It all tends to remind me of epicycles.
|
| https://en.wikipedia.org/wiki/Deferent_and_epicycle
| knzhou wrote:
| One should keep in mind that most dark matter "alternatives",
| including this one, actually _include_ dark matter. It says so
| right on the 2nd page of their paper:
|
| > Consider requirement (iii), that is, successful cosmology. In
| (2) we have a new d.o.f. ph [...] What should the expectation for
| a cosmological evolution of ph be? The MOND law for galaxies is
| silent regarding this matter. There is, however, another
| empirical law which concerns cosmology: the existence of sizable
| amounts of energy density scaling precisely as a^(-3).
|
| In other words, they are saying that to get the cosmology right,
| they need to add stuff that behaves exactly like dark matter --
| that is what they are alluding to with the "sizable amounts of
| energy". They make their ph field play this role. It's just like
| TeVeS, the other major relativistic MOND theory, where the scalar
| "S" field does the same thing.
|
| The popular press likes to frame the debate as "dark matter vs.
| modified gravity", but it's really "dark matter vs. dark matter
| plus modified gravity", which is much less dramatic.
| throwaway894345 wrote:
| > The popular press likes to frame the debate as "dark matter
| vs. modified gravity", but it's really "dark matter vs. dark
| matter plus modified gravity", which is much less dramatic.
|
| Honestly for us lay folks there isn't a perceptible difference
| in the amount of drama between the two. :)
| raattgift wrote:
| > We remark that A_\mu also contains a pure vector mode
| perturbation which is expected to behave similarly as in the
| Einstein-AEther theory [90, 91]"
|
| Their [91] is Jacobson & Mattingly https://arxiv.org/abs/gr-
| qc/0007031 whose SSVII (DISCUSSION) contains this, which I
| struggle to see as helpful for them: "With the action adopted
| in this paper the aether vector generically develops gradient
| singularities even when the metric is perfectly regular. We
| take this as a sign that the theory is unphysical as an
| effective theory". (That doesn't stop Jacobson from
| investigating things like (time-independent) black hole
| solutions https://arxiv.org/abs/gr-qc/0604088 "It is a
| plausible conjecture that nonsingular spherically symmetric
| initial data will evolve to one of the regular black holes
| whose existence has been demonstrated here, but this has
| certainly not been shown", and worse they show that the aether
| does not obey the Raychaudhri equation, so the relativistic
| MOND authors seem to need more ghosts).
|
| For the life of me, I can't figure out the relevance of their
| reference [90] which I believe is
| https://www.jstor.org/stable/2414316
|
| I wonder who their Reviewer 2 was.
| ncmncm wrote:
| What happened to the recent work showing that galactic rotation
| curves are consistent with ordinary GR? Last I read, cosmologists
| were choosing to ignore it. The article does not mention it, and
| treats galaxy rotation as if it were still considered anomalous.
|
| It would be amusing if dark matter and MOND turned out to be both
| correct, and both needed. E.g., dark matter is diffuse enough not
| to affect galaxy rotation, but clumped enough to account for
| lensing and cluster adhesion.
|
| Casting MOND as a matter of fields, which are also particles,
| seems to mean just a different sort of dark matter that interacts
| by some other means than gravitation, rather than our model of
| gravity itself being off. Some people who dislike dark matter
| would not like that much.
| knzhou wrote:
| > What happened to the recent work showing that galactic
| rotation curves are consistent with ordinary GR? Last I read,
| cosmologists were choosing to ignore it.
|
| Gravitomagnetism is a well-understood and experimentally
| measured effect. It is also a very small effect, of the order
| v^2 / c^2 where v is the speed of the sources. In the galaxy,
| stars move with v/c ~ 1/1000, which means the gravitomagnetic
| correction is one in a million. So while N-body simulations do
| sometimes account for general relativistic corrections like
| these, they're not nearly large enough to remove the
| requirement for dark matter.
|
| That is the simple reason the paper has been ignored by
| everyone in the scientific community and rejected from decent
| journals. Of course, this hasn't stopped hundreds of fluffy pop
| articles being written on it, or it getting posted every week
| on HN. The blind leading the blind.
| ncmncm wrote:
| What I am hearing is that nobody has found an error in his
| derivation; instead, everybody has chosen to continue skating
| on the v^2/c^2 estimate arrived at without having done the
| detailed maths.
|
| In general, anytime mathematical rigor is at issue, I will
| prefer to bet on the plasma fluid dynamicist over the
| cosmologist.
| tynpeddler wrote:
| > Unlike dark matter models that are often based on fundamental
| symmetry principles--the new model was not conceived with an
| underlying theory in mind. However, such a theoretical basis
| might be uncovered using the new MOND model.
|
| I think this is an important thing to keep in mind. Cosmology has
| a big challenge right now. There is something fundamental in the
| universe that we don't understand. While dark matter is currently
| our best explanation, it is deeply flawed; it predicts the
| existence of an entire class of matter that we've been unable to
| find any physical proof for other than cosmology (and cosmology
| is the thing we want to explain!). Maybe we haven't looked hard
| enough, maybe we haven't look in the right place, or maybe dark
| matter is just wrong.
|
| The CMB has been a huge issue for dark matter alternatives.
| Previously, only dark matter had a good explanation. If this
| result holds, it could be an amazing leap forward is outlining
| the alternative types of theories that could explain cosmology.
| tiborsaas wrote:
| > While dark matter is currently our best explanation, it is
| deeply flawed
|
| Calling it deeply flawed is focusing on just one aspect of it.
| The name just refers to the darkness in our knowledge about
| what it is. We don't know. It might be a form of matter, it
| might be a new factor in our equations or something completely
| different.
| raattgift wrote:
| > There is something fundamental in the universe that we don't
| understand.
|
| Good! I hope there's _lots_ of fundamental things we don 't
| understand well yet. We can then enjoy improving our
| understanding.
|
| "The paper" below is https://arxiv.org/abs/2007.00082 ==
| https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12...
| which is the topic of the fine article linked at the top.
|
| > If this result holds, it could be an amazing leap
|
| I'm not sure if one can use the term result here. In the paper
| there is just a write-down of a theory that is deliberately
| designed to match an extra couple lines of astrophysical
| evidence that previous write-downs incorporating Milgrom's
| function (see https://arxiv.org/abs/1112.3960 for a partial
| catalogue of dozens of families of them) did not. Indeed, the
| paper's abstract says this clearly: "We discuss
| phenomenological requirements leading to [the theory's]
| construction and demonstrate its agreement with the observed
| [CMB & matter]".
|
| The theory in the paper doesn't match _all_ the troublesome-
| for-relativistic-MOND lines of evidence (choosing to focus on
| CMB polarization) and the paper 's authors are uncertain about
| several. See e.g. the first paragraph (starting at its third
| sentence) in the Discussion section on p. 5 and the subsequent
| paragraph. Note (weak field) _quasistatic approximation_. They
| carry on to an admission that they struggle to suppress
| unlikely imprints on the CMB, and propose to introduce further
| dark fields to do so. These fields will have their own density
| distribution and dynamics. The only really nonstandard thing is
| that they have a strong preference for treating these as
| _gravitational_ degrees of freedom (modified gravity) rather
| than generators of stress-energy (matter).
|
| From a theorist's perspective it's neat that they were able to
| do this, because it's been known for a little more than fifteen
| years that it's _hard_ to write down such a theory and have it
| be apparently self-consistent. It is not, however, apparently
| complete. Indeed, returning to your "there is something
| fundamental ... we don't understand", the paper says: "Absence
| of ghosts to quadratic order signifies a healthy theory that
| could arise as a limit of a more fundamental theory. We do not
| have such a theory at present".
|
| The ghost condensate they go on to discuss is essentially a
| non-zero average correction to the values predicted by their
| theory in the low energy limit. Specifically they appear to be
| trying to control the non-Gaussianities in the CMB associated
| with their scalar's dynamics corresponding to an aether (yes,
| really) having a rest frame that is not the CMB rest frame.
| (Their unit-timelike vector field A^\mu also breaks invariance
| under Lorentz boosts, which probably impacts ultra-high-energy
| cosmic rays if extragalactic, leading to different ghosts.)
|
| > predicts ... an entire class of matter that we've been unable
| to find any physical proof for other than cosmology (and
| cosmology is the thing we want to explain!)
|
| This objection shows up often in online forums, but seems to
| ignore the pattern of discovery of particles that do not
| participate in electromagnetism.
|
| Roll back to ~1930 where we want to explain the statistics of
| beta decay and have Pauli postulating a particle which wasn't
| previously observed in nuclear physics, and defied observation
| for forty-five years. Or to ~1910 when people were scratching
| their head over the atomic masses of heavy water and other
| chemical isotopes leading to the proposal of a proton-massed
| electromagnetically uncharged particle. Neutrinos (non-relic)
| are _hot_ dark matter. If they were heavy enough not to zip
| away from galaxy clusters, they 'd be cold WIMPs. If free
| neutrons didn't decay so quickly, with a suitable distribution
| (bb relic field?) they could also be candidates for WIMP cold
| dark matter, and also their discovery by 1932 (i.e. before
| Bevatron or at least the larger Berkeley deuteron cyclotrons)
| would seem optimistic.
|
| There may be good reasons to doubt particle cold dark matter,
| but that it hasn't been detected in the 40 years since Peebles
| proposed it does not seem like one.
|
| Additionally, particle physicists are looking for a variety of
| new particles to solve _totally non-gravitational_ problems in
| the Standard Model. Several of these could be good candidates
| for particle cold dark matter. Indeed, cold dark matter could
| turn out to be a mix of several of these. Who knows? It 'll be
| fun finding out.
| raattgift wrote:
| This is a surprisingly good write-up.
|
| The Skordis & Zlosnik paper can be found at
| https://arxiv.org/abs/2007.00082
|
| I'll start with some comments for non-experts, and then escalate
| about ten paragraphs further down.
|
| tl;dr: there are lots of things that hang on General Relativity
| being correct practically everywhere in our universe, the only
| "hiding places" that don't break astrophysical objects (or even
| laboratory experiments, including in robotic laboratories we have
| sent to other places in our solar system) is very close to the
| big bang, deep inside black holes, and in large masses brought
| into a quantum superposition of space.
|
| The core of General Relativity is a _mathematically-complete
| relationship_ between matter in a spacetime, and spacetime
| curvature. The key point is that the exact relationship can be
| described correctly at every point in the spacetime.
|
| The relationship takes on a particular form: the Einstein Field
| Equations (EFE), which can be written (omitting prefactors, the
| cosmological constant, and indices) as G = T, where "G" is the
| Einstein curvature tensor and T is the stress-energy tensor. G
| and T are really tensor _fields_ , which take on a value at every
| point in a spacetime. T encodes the matter content at a given
| point, and can represent incredibly complex "piles" of
| interacting particles or matter field values.
|
| The slogan form of this is, "moving matter generates curvature,
| curvature moves matter". The result is that setting out
| distribution of matter that obeys some dynamical laws generates
| spacetime curvature. But there can additionally be "vacuum-
| generated" curvature around which one might sprinkle some matter,
| which would then be entrained into orbits or other trajectories
| by that curvature. The latter is the approach used in the study
| of theoretical black holes, for example. But also it's a good way
| to understand the expansion of space: there is a "vacuum-
| generated" expanding spacetime curvature, with galaxy clusters
| distributed through it, and entrained into trajectories
| principally by that curvature.
|
| We can also look at trajectories, and with the assumption that
| all matter falls the same way ("the universality of free-fall"),
| recover the curvature, the distribution and dynamics of the
| stress-energy, or both. This is quite common in astrophysical
| settings, where one is trying to determine an equation of state
| for a body like a white dwarf or a neutron star.
|
| One of the problems that particle Dark Matter seeks to solve is
| that gas and stars far from the centres of galaxies of do not
| fall in faster than they do. Something holds them up, keeping
| them on unexpected (non-Newtonian) trajectories. We can model
| this by introducing extra complexity into the stress-energy
| tensor, starting with something simple and abstract and drilling
| down where evidence allows us to do so. One might write this as G
| = f(T), some function on the dark-matter-free matter content
| matches the curvature exactly. We can improve upon an initial
| simple f() over time. But alternatively we can explore f(G) = T.
|
| It's been known since the 1920s that the _relationship_ at the
| core is pretty flexible: we can choose a background curvature
| (flat spacetime, a black hole) and add matter and see what
| happens, or we can start with a distribution of matter (and
| dynamics) and see what it does to the Einstein curvature tensor.
| We can (a) encode more and more complicated representations of
| matter into the stress-energy tensor, and keep curvature simple.
| Or instead, (b) we can adapt the curvature making the
| "background" more and more complicated, and then sprinkle a
| relatively simple distribution of matter on top.
|
| (Particle) Dark Matter is mainly the (a) approach. We assume that
| a galaxy's curvature is fairly simple in the bulk, lay out a
| reasonable model of the bulk visible matter of a galaxy (and
| electromagnetic radiation, and neutrinos), and then ask, "What
| must we do to the stress-energy tensor so that we still generate
| the observed trajectories of outermost matter (gas, dust,
| stars)?".
|
| MOND is mainly the (b) approach. There is an extremely simple
| empirical law (from Mordehai Milgrom in 1981) that adapts
| Newtonian gravitation to generate the non-Newtonian orbits of
| outermost stars observed in most spiral galaxies. This _may_
| translate into a function on the Einstein curvature tensor that
| produces a "background" in which a realistic description of a
| galaxy's _visible_ matter (and electromagnetic radiation and
| neutrinos) is kept from being flung out into intergalactic space.
| The problem is that it turns out one cannot do this while keeping
| General Relativity 's exact relationship between matter and
| curvature, because keeping Milgrom's constant means matter at the
| outsides of galaxies feels gravitation (and other interactions)
| differently from matter in for example the solar circle (the part
| of the Milky Way where we find our sun's orbit, about 8.5
| kiloparsecs from the core), or in the central parsec.
|
| I'll expand on this by quoting [1] (Stacey McGaugh, second
| author, is a MOND proponent quoted in the aps.org article), "The
| heart of GR is the equivalence principle(s), in its weak (WEP),
| Einstein (EEP) and strong (SEP) form. The WEP states the
| universality of free fall, while the EEP states that one recovers
| special relativity in the freely falling frame of the WEP. These
| equivalence principles are obtained by assuming that all known
| matter fields are universally and minimally coupled to one single
| metric tensor, the physical metric. It is perfectly fine to keep
| these principles in MOND, although certain versions can involve
| another type of (dark) matter not following the same geodesics as
| the known matter, and thus effectively violating the WEP.
| Additionally, note that the local Lorentz invariance of special
| relativity could be spontaneously violated in MOND theories. The
| SEP, on the other hand, states that all laws of physics,
| including gravitation itself, are fully independent of velocity
| and location in spacetime [...] This principle _has_ to be broken
| in MOND. "
|
| SEP also means WEP holds, and WEP requires that gravitational
| mass and inertial mass are identical for all bodies including
| self-gravitating ones like planets, stars, neutron stars, and so
| on.
|
| A few pages along [2], Famaey & McGaugh write, "It is perhaps
| more important that, if MOND is correct in the sense of the
| acceleration _a_0_ [Milgrom 's law's constant] being a _truly_
| fundamental quantity, the strong equivalence principle cannot
| hold anymore, and local Lorentz invariance could perhaps be
| spontaneously violated too. "
|
| That is, relativistic MOND generally means you lose the guarantee
| of Special Relativity holding in a small neighbourhood around
| every point in spacetime, which is liable to affect tests of the
| Standard Model of Particle Physics (which has that guarantee
| fundamentally baked in). Worse than that, with SEP violation, in
| general laws of physics _must_ vary depending on a probe 's
| proximity to mass, particularly probed bodies' response to
| acceleration. This is awkward given recent astrophysical evidence
| supporting the SEP (e.g. [3]).
|
| In order to accord with evidence in favour of the SEP, one has to
| do some handstands, adding complexity to the function on
| curvature.
|
| Of note to experts is Skordis & Zlosnik p. 5: "The vector in (5)
| does not seem to obey gauge invariance but in the quadratic
| action (13) it does so through mixing with diffeomorphisms of
| h_munu" and "The resulting action is that of the gauged ghost
| condensate (GGC) [122] or bumblebee field [123, 124] which has
| been proposed as a healthy gauge-invariant theory of spontaneous
| Lorentz violation."
|
| from which one can jump into
| https://en.wikipedia.org/wiki/Bumblebee_models#Nambu%E2%80%9...
| (which is decently encyclopedic) and relate that to the quote at
| the top's argument that in a relativistic MOND, either Special
| Relativity isn't a guarantee in the small neighbourhood around
| every point (it _is_ guaranteed by General Relativity, and is
| highly tested) or we recover it by adding more fields to the
| replacement of the \Lambda-equipped Einstein-Hilbert action.
|
| I gather the idea is to suppress significant violations of the
| Strong Equivalence Principle, and to do so by adding yet more
| degrees of freedom.
|
| From Famaey & McGaugh again [at [2]], "it is true that it would
| be more elegant to avoid too many additional degrees of freedom",
| which we can relate to K Freese's quote in the aps.org article.
|
| Finally, quoting Skordis & Zlosnik again, "Studies of MOND with
| galaxy clusters [...] report that either _a_0_ is larger in
| clusters and /or an additional dark component is necessary _even
| when the MOND prescription is used_ [...] the theory presented
| here has additional features warranting its separate testing with
| clusters. " [Emphasis mine]
|
| - --
|
| [1] Famaey & McGaugh https://arxiv.org/abs/1112.3960 SS7 p. 88.
|
| [2] ibid., SS10 p. 122
|
| [3] Scott Ransom's 2014 slides on PSR J0337+1715
| https://websites.utdallas.edu/nsm/texas2013/proceedings/1/2/...
|
| later detailed observations (Ransom, Stairs, Archibald et al
| 2014) https://doi.org/10.1038%2Fnature12917 ==
| https://arxiv.org/abs/1401.0535
|
| test of Strong Equivalence Principle (Archibald et al 2018)
| https://doi.org/10.1038%2Fs41586-018-0265-1 ==
| https://arxiv.org/abs/1807.02059
| AnimalMuppet wrote:
| Wow. Impressive work. Thank you!
|
| One nit:
|
| > "moving matter generates curvature, curvature moves matter".
|
| Doesn't stationary matter generate curvature also? (Though
| moving matter can generate _additional_ curvature...)
|
| Or am I missing something?
| tragictrash wrote:
| You are a scholar and gentleman, thank you.
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