[HN Gopher] James Webb Space Telescope finds evidence for altern...
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James Webb Space Telescope finds evidence for alternate theory of
gravity
Author : jchanimal
Score : 156 points
Date : 2024-11-16 18:33 UTC (4 hours ago)
(HTM) web link (thedebrief.org)
(TXT) w3m dump (thedebrief.org)
| jchanimal wrote:
| What's MOND really mean? Here's the Wikipedia entry
| https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
| uoaei wrote:
| I follow the lead author, Stacy McGaugh, via his blog where he
| posts discussions and musings about the latest research into the
| dark matter vs MOND debate: https://tritonstation.com/new-blog-
| page/
|
| His arguments are very convincing and relatively clear. I am not
| an astrophysicist but I have two degrees in physics and have
| always found the dark matter theory to be lacking -- in absence
| of any evidence of causation whatsoever, dark matter can only be
| described trivially as "where we would put matter if we could to
| make our theory of gravity make sense," which is totally
| backwards from a basic scientific perspective.
|
| Predictions based on modern MOND postulates are shown to be more
| and more accurate as our observational instruments continue to
| improve in sensitivity.
| simonh wrote:
| I don't think that's quite fair. That approach is exactly how
| we find planets. Here's an unexpected variance in the motion of
| a planet or star. It could be explained by a planet over there.
| Oh look, there's a planet over there.
| TheOtherHobbes wrote:
| Planets are visible when you look for them.
|
| Dark matter - so far - isn't.
| drdeca wrote:
| What do you mean by "visible when you look for them"? Like,
| with light?
|
| Does gravitational lensing count as "visible" to you?
| MarkusQ wrote:
| Right, which is why it quickly led to the detection of dark
| matter...hmm.
|
| I think a better analogy would be "that approach is exactly
| how we explain failing to find planets like Vulcan; we
| hypothesize that they are made of as-yet-unknown stuff that
| you can't see, touch, hear, smell, or in fact detect at all.
| But we know they're there because our calculations say they
| are."
| solid_fuel wrote:
| Hypothesizing that a planet might be over there is a testable
| hypothesis.
|
| Have we found a way to verify the presence of dark matter
| yet? Or is it still an untestable hypothesis sprinkled around
| distant galaxies so their acceleration curves look right?
| User23 wrote:
| I'm particularly amused by the hypothesis that spacetime
| can be bent without the presence of matter. We can't detect
| dark matter because there's no such thing, it's just a
| brute topological fact.
| mr_mitm wrote:
| Dark matter predicted lensing effect which were
| successfully tested. Same for the baryonic acoustic
| oscillations in the CMB.
| MattPalmer1086 wrote:
| That's not quite true. General relativity predicts
| gravitational lensing, not dark matter. Lensing has been
| used as an experimental probe for the presence of dark
| matter.
| elashri wrote:
| MOND is an alternative theory of gravity competing with
| GR. People usually forget that while MOND started to
| present a different explanation for Dark Matter, it is a
| theory of gravity. Dark Matter is not a theory of gravity
| and is compatible with GR.
| zeroonetwothree wrote:
| Dark matter isn't much of a theory in the first place.
| russdill wrote:
| It's actually a better example than you think. This exact
| theory led to long and protracted searches for the planet
| Vulcan, which would explain Mercury's strange behavior.
| griffzhowl wrote:
| > which is totally backwards from a basic scientific
| perspective
|
| This is not right, because if we have a situation where our
| theories and observations don't cohere, it's not given whether
| the theory requires modification or we're missing something in
| our observations (or both). A classical illustration is the
| orbit of Uranus being observed in the nineteenth century to be
| contrary to the predictions of Newtonian theory. Calculations
| were made assuming the truth of the Newtonian theory and that
| we were missing something in our observations - the position of
| Neptune was predicted and it was subsequently discovered.
|
| On the other hand, the orbit of Mercury diverged from the
| prediction of Newton's theory. Again, a previously unobserved
| planet closer to the sun was postulated as being responsible,
| but in this case it really did require a modification to the
| theory of gravity: general relativity, which accurately
| predicted the 43 arcseconds per century of perihelion
| precession by which Mercury's orbit diverges from Newtonian
| predicitions.
|
| GR has obviously made many other predictions, such as the
| gravitational bending of light, black holes, and gravitational
| waves, which have been vindicated.
|
| So there's obviously a problem of the theory and observations
| not cohering, but whether the solution is a modification of the
| theory or a new form of matter is not clear in advance, and the
| latter is not unreasonable and certainly it's not unscientific
| to make as a hypothesis, to see where it leads.
|
| The difficulty is in coming up with a theoretical framework
| that retains all the successful predictions of GR while also
| accounting for the galactic rotation curves.
| bbor wrote:
| Well put, thanks for sharing! Never saw it phrased in such a
| clear narrative. As a novice, it seems like there's one big
| difference between those anecdotes and the current situation,
| though: sample size. Sure, if we were observing Andromeda
| spinning too slowly I'd be open to our instruments not
| capturing some massive objects/clouds, but we're actively
| observing, what, ~1E5-6 galaxies? In the case of a missing
| planet there were accidents of history/solar system makeup
| that led to our otherwise solid frameworks missing a key
| piece of information. But that clearly couldn't happen
| millions of times; whatever explains the inconsistencies
| we're seeing _has_ to be a fundamental misunderstanding.
|
| Once we've arrived at this point, we can compare the two
| theoretical re-workings on their own terms: one is that we're
| glossing over some important detail of how gravitational
| relations in spacetime work, and the other is that we're
| failing to observe some new class of matter. I mean, right?
| There's no way this conundrum will be solved by "whoops turns
| out there was more plain ol' dust than we thought" at this
| point, right?
|
| In those terms, I feel parsimony clearly favors one
| possibility over the other. Every hypothesis is worth
| exploring (I mean, QM and GR are dumb as hell, yet
| nonetheless turned out to be correct), but when funding is on
| the line it's also not out of line to favor one explanation
| explicitly. That's already happening anyway, just in the
| other direction.
|
| But also I'm just some kid who's awed and grateful to be
| living in times of such profound mystery and discovery. Could
| be totally off base -- I barely passed physics I!
| necovek wrote:
| > ...turned out to be correct
|
| What we have learned so far is that our theories and models
| are only correct up to our ability to precisely observe and
| measure.
|
| In that sense, Newtonian physics is still very much correct
| under a very wide set of circumstances, and as such
| amazingly useful.
|
| GR improves on that (adds precision) on what would be
| extreme cases for NP, but it is likely as correct as
| Newtonian laws are: up to a point.
|
| All this to say that "correct" is not the right term to
| use: many of the theories are simultaneously "correct" with
| sufficient constraints and a particular error range. What
| matters more is if they are useful in predicting behaviour,
| and that's where I like using "correct" instead (as above).
| njtransit wrote:
| One difference between dark matter and Neptune is that the
| existence of Neptune is falsifiable. The formulation of dark
| matter inherently is not. Falsifiable hypotheses is the
| cornerstone of science.
| renewiltord wrote:
| Surely the idea of it being a new kind of matter that
| interacts gravitationally but not electromagnetically
| yields some testable result? Does it actually yield nothing
| testable with today's experimental methods?
| MattPalmer1086 wrote:
| There is a lot of indirect evidence for dark matter. All
| the direct tests for dark matter particles we have
| performed have found nothing so far - but since we have
| no idea what it might be, there's a lot of possibilities
| to test.
| LegionMammal978 wrote:
| Is the existence of a planet so easily falsifiable? It
| hasn't been so long since the Planet Nine hypothesis
| started going around, and while we've observationally ruled
| out a big chunk of the original parameter space, there's
| still lots of room for a big dark dwarf planet to be
| floating around out there. It doesn't seem so different
| from how we've gradually been ruling out the parameter
| space for dark-matter observations.
| pixl97 wrote:
| I mean, dark matter may be discoverable, we just don't know
| how if it exists. There was time between the irregularities
| that were noticed in the orbit and the discovery of a new
| planet.
| antognini wrote:
| > where we would put matter if we could to make our theory of
| gravity make sense
|
| Dark matter behaves in a fundamentally different way from
| baryonic matter. We can constrain the total amount of matter in
| the universe (both dark and baryonic) from the observed
| abundances of baryogenesis. But dark matter has a different
| effect on the relative amplitudes of peaks in the CMB.
|
| As far as I can tell, MOND has never really had any success
| outside of modeling galaxy rotation curves.
|
| The skepticism I've seen towards dark matter vs. MOND has
| always been strange to me. Dark matter doesn't really require
| much in the way of new physics --- there's just a new particle
| to add to the standard model. But most MOND theories violate
| Lorentz invariance which is a vastly more radical departure
| from standard physics. (And in my mind, the more sophisticated
| MOND theories that maintain Lorentz invariance like TeVeS are
| really a theory of dark matter dressed up in the language of
| MOND.)
| MattPalmer1086 wrote:
| There are more successful predictions than just rotation
| curves. For example, see:
|
| http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html
| antognini wrote:
| These successful predictions are all generally variants on
| modeling galactic dynamics, though. The trouble is that
| galaxies and galaxy clusters are very messy places, so it's
| hard to make sure you've incorporated all the relevant
| physics.
|
| By contrast something like baryon acoustic oscillations are
| very simple to model, so you can be quite confident that
| you've incorporated all the relevant processes. And in that
| regime LCDM performs beautifully and MOND completely fails.
| So it's reasonable to suspect that in more complicated
| environments the problem is that we're not modeling the
| systems correctly rather than that there's new physics
| going on.
| MattPalmer1086 wrote:
| There are other predictions MOND makes. For example, it
| predicts higher collision velocities than LCDM, for
| example, see:
|
| https://ieeexplore.ieee.org/document/8193356
|
| And, of course, it predicted that the early universe
| would have bigger and more structured galaxies (which is
| what the posted article is about).
|
| Dark matter has a slew of problems of its own; it's not
| the case that LCDM is problem free, despite good success
| in some areas.
| kelseyfrog wrote:
| MOND doesn't cover the existence of CBM, distribution of
| galaxies, non-metallic abundance - things all covered by
| LCDM.
|
| What MOND has going for it is that galactic rotation
| curves are readily consumed by popsci readers and the
| story of the "little guy" vs the scientific establishment
| is an easily available frame story popsci authors can
| sell clicks for.
|
| The proportion of lay people who think MOND could be true
| greatly outnumbers the proportion of MOND researchers and
| doesn't reflect the veracity of the theory.
| MattPalmer1086 wrote:
| MOND is not a cosmological theory unlike LCDM, and it
| isn't relativistic. So we should not expect it to cover
| the range of things that LCDM tries to.
|
| It's just a tweak to Newtonian gravity, which
| surprisingly matches observation very well, and has
| accurately predicted quite a few things in the regime it
| operates in, before they were observed.
|
| The fact it works so well in the areas it does apply to
| is the reason that science hasn't given up on it yet
| (regardless of what pop science or lay people think).
| gus_massa wrote:
| Very interesting. Do you know an article that ELI25 this?
| antognini wrote:
| For a more non-technical overview, Sean Carroll had a
| nice episode on his podcast where he talked about the
| evidence for dark matter among other things: https://www.
| preposterousuniverse.com/podcast/2023/07/31/245-...
|
| For something more technical, this article just came out
| as an overview of the evidence for dark matter:
| https://arxiv.org/abs/2411.05062
| russdill wrote:
| The mond theories that add a factor that behaves like dark
| matter do a rather good job of matching observational data.
| halgir wrote:
| I usually understand "dark matter" to be shorthand for the
| discrepancy between theory and observation. The explanation
| might indeed be matter that is dark, or it might be solved by
| entirely unexpected observations and/or changes to theory.
| mr_mitm wrote:
| Not really. You might think this after watching Angela
| Coulliers video, but when you read something like "25% of the
| universe's energy content is made of dark matter", they do
| not mean changes to some theory. They literally mean non-
| baryonic matter.
| OutOfHere wrote:
| Nope. It can mean change to some theory, without a need for
| matter. It is the difference between relativistic gravity
| and the corresponding observed mass.
| zeroonetwothree wrote:
| Energy content not only comes from matter but also from
| fields.
| samsartor wrote:
| My hangup with MOND is still general relativity. We know for a
| fact that gravity is _not_ Newtonian, that the inverse square law
| does not hold. Any model of gravity based on an inverse law is
| simply wrong.
|
| Another comment linked to https://tritonstation.com/new-blog-
| page/, which is an excellent read. It makes the case that GR has
| never been tested at low accelerations, that is might be wrong.
| But we know for a fact MOND is wrong at high accelerations.
| Unless your theory can cover both, I don't see how it can be
| pitched as an improvement to GR.
|
| Edit: this sounds a bit hostile. to be clear, I think modified
| gravity is absolutely worth researching. but it isn't a silver
| bullet
| MattPalmer1086 wrote:
| MOND isn't pitched as an improvement to GR. It was always a
| Newtonian theory - it's in its name!
|
| There are relativistic versions of MOND, for example, TeVeS
| [1], but they all still have some problems.
|
| [1]
| https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%...
| samsartor wrote:
| TeVeS is definitely interesting, but it still has problems
| like you said. AFAICT gravitational wave observations are
| particularly bad for TeVeS theories. TeVeS isn't dead, but if
| dark matter theories are criticized for being patched up
| post-hoc, that standard should also apply to modified
| gravity.
| gliptic wrote:
| The weirdest thing about TeVeS IMO is that it adds
| additional fields that warp spacetime, so how is it not a
| dark matter theory?
| MathMonkeyMan wrote:
| For the fields to be considered particles, they have to
| be freely propagating in space. TeVeS adds a vector
| field, a scalar field, and some lagrange fields that are
| part of their coupling. The degrees of freedom aren't
| consistent with one or more particles.
| ajross wrote:
| To be fair, there are relativistic generalizations of MOND, in
| the sense of relativistic theories that simplify to MOND
| dynamics in the low energy case. My understanding (this not
| being my field) is that they're sort of kludgey and non-
| calculable and that no one takes them very seriously. All the
| "real work" on MOND is just done using the classical stuff.
|
| And yeah, that seems like pretty terrible cheating. It's one
| thing to hang a big theory on a single conjecture, but you
| still need to be trying to prove the conjecture.
| meindnoch wrote:
| >We know for a fact that gravity is _not_ Newtonian, that the
| inverse square law does not hold
|
| [citation needed]
|
| The consensus is that gravity - outside of extreme mass/energy
| environments - works just as Newton described it _to many many
| decimal places_.
|
| _Emphasized part added because people in the replies thought
| that I literally think that General Relativity is somehow
| wrong. Don 't be dense. All I'm saying is that gravity at
| galactic scales works as Newton described it. General
| Relativity has extremely tiny effect at those scales._
| hobs wrote:
| When you say "outside of" - that's the thing where it doesn't
| hold. It's interesting and not even wrong to say "these rules
| work in these contexts" but as far as I can tell we're
| looking for the scenario invariant rules.
| samsartor wrote:
| https://en.wikipedia.org/wiki/Tests_of_general_relativity#Pe.
| .. is the example I learned in school. You don't need to be
| around a black hole for GR to suddenly switch on.
|
| Newtonian gravity is an approximation. A perfectly acceptable
| one in many contexts, but still measurably incorrect.
| meindnoch wrote:
| Nobody said that general relativity is "switched on" around
| black holes.
|
| But ok, let me put it this way: outside of extreme
| energy/mass environments, gravity is described by Newton's
| law of gravitation with very high precision. If you look
| very hard, you may notice differences on the order of
| 10e-MANY. But for all intents and purposes, gravity is
| Newtonian in 99.99999% of the universe.
| exe34 wrote:
| that's like saying the visible mass of the universe is
| 99% hydrogen and helium, so we don't need to learn about
| chemistry.
| meindnoch wrote:
| So you're saying we should model galaxies down to the
| level of individual protons? Lol.
|
| Galactic dynamics is governed by gravity, which is
| Newtonian at those scales.
| samsartor wrote:
| Not for all intents and purposes.
|
| If we are asking whether MOND is useful, then the answer
| is probably yes. You might use it for simulations of
| galaxy formation where Newtonian gravity is considered a
| reasonable approximation today. But MOND is not a correct
| model of the universe. There is no place in the universe
| that Newtonian gravity applies, only places where the
| error is an acceptable trade-off for simpler calculation.
| meindnoch wrote:
| By the same logic, there's no place in the universe that
| general relativity applies either, since it breaks down
| at the quantum level. There's no place in the universe
| where any theory other than the one true grand unified
| theory applies, because everything else is just an
| approximation. At which point we're just arguing about
| semantics, and I don't see a reason for continuing it on
| my part.
| DiogenesKynikos wrote:
| The inaccuracy of the Newtonian theory of gravity is
| large enough that it was already noticed by astronomers
| in the mid-1800s.
| radishingr wrote:
| So spacetime (interactions between mass, space, and time)
| are required for any sort of precision explanation. If
| "extreme" means planet size masses, I guess, but I
| generally consider our solar system pretty normal.
| However we cannot explain the planetary motion of mercury
| without relativity, so define your extreme.
|
| But sure, newton is good enough to handle most ground
| based scenarios where we only care about forces at low
| precision.
| bobmcnamara wrote:
| My first thought was that we only know Cavendish's
| constant to a little over 4 significant figures, so how
| could this be right? The relativistic effects at Earth's
| surface would change this by only ~10^-8, so I think the
| challenge in refining the Cavendish gravitic constant lie
| elsewhere.
| superjan wrote:
| These extremes exist, and GR predictions are better than
| Newton's in those cases. Closest to home is mercury's
| perihelion drift. We have observed black hole mergers,
| gravitational lensing, and GR is also an essential component
| in understanding the universe's expansion(that we know from
| redshift and the CMB). Likely MOND will address these, but
| Newtonian mechanics will not get you there.
| auntienomen wrote:
| Citation needed? That's ridiculous. The empirical evidence is
| well over century old at this point. Start with the anomalous
| precession of Mercury's perihelion. That already can't be
| accounted for by Newtonian gravity.
| EPWN3D wrote:
| You're simply wrong. There's no other way to put it. The GPS
| system would have been simply impossible to deploy without
| the general theory of relativity. There's no extreme energy
| or mass involved, just precision requirements that are
| influenced by the minuscule differences in time experienced
| by the surface of the earth and orbiting satellites.
|
| Also Newton's laws famously could not account for Mercury's
| orbit. Mercury is just an ordinary planet orbiting an
| ordinary star. Nothing extreme is involved. He knew his laws
| were incomplete. But they were so dead-on in basically every
| other scenario that could be physically observed at the time
| that he figured there was some small tweak missing (or maybe
| another planetary body that hadn't been spotted yet).
| meindnoch wrote:
| Easy there champ. Noone is shitting on general relativity.
|
| All I'm saying is that the effect of general relativity at
| galactic scales is so minuscule, that galactic dynamics is
| - for all intents and purposes - governed by the Newtonian
| _limit_ of gravity.
|
| If you propose that gravity doesn't behave like the
| Newtonian limit at those scales, then you're contradicting
| general relativity as well, since the far-field limit of
| the Schwartzschild metric is literally Newton's inverse
| square law.
|
| In layman terms, modified Newtonian gravity, that the
| article talks about, is an attempt to explain why galaxies
| don't rotate the way they should according to Newton (and
| Einstein, because at those distances the two are the
| same!!!).
| wbl wrote:
| We can see gravitational redshift on Harvard's campus thanks
| to gamma ray Mossbauer spectroscopy.
| Bengalilol wrote:
| << Stunning evidence >> ... then later on: << Instead, the
| readings _seem_ to support a basis for MOND, which _would_ force
| astronomers and cosmologists to reconsider this alternative and
| long-controversial theory of gravity. >> What's conditional
| evidence? I may be missing the overall picture, but I view such
| writing as non precise at its best.
| MattPalmer1086 wrote:
| It's just typical pop sci journalism, with a click baity
| headline. Read the paper instead.
| Bengalilol wrote:
| Thanks, I will.
| https://iopscience.iop.org/article/10.3847/1538-4357/ad834d
| joe_the_user wrote:
| Not entirely typical. MOND proponents seem to be trying more
| and more sell their approach to the public.
|
| It annoys me but I suppose every theory has to do that now,
| "the mouse trap must go to market now" and all.
| bbor wrote:
| Well, it's evidence that a) must be verified on a mathematical
| and empirical level, and b) (arguably) fits _better_ with a
| currently unpopular theory than the dominant one. There's so
| many unknowns in physics that opponents can easily reply "well
| your theory doesn't explain XYZ yet, so we likely just need to
| tweak our theory".
|
| In other words, reasonable minds do disagree. AFAIU as an
| amateur.
| yieldcrv wrote:
| There is no consensus yet, there is no repeatable metric
|
| It is perfectly valid to say "hey look over there for further
| review"
| astroH wrote:
| In my opinion, this article is misleading at best. "...scans of
| ancient galaxies gathered by the JWST seem to contradict the
| commonly accepted predictions of the most widely accepted Cold
| Dark Matter theory, Lambda-CDM." --> LCDM doesn't predict what
| galaxies should look like, it simply predicts how much mass is in
| collapsed structures and that dark matter haloes grow
| hierarchically. In contrast, with JWST we see light and need to
| infer what the underlying properties of the system are. It was
| shown very early on that the theoretical upper limit (i.e. taking
| all of the gas that is available in collapsed structures and
| turning it into stars) predicts a luminosity function (i.e.
| number of galaxies per unit luminosity) that is orders of above
| what JWST has observed (e.g.
| https://ui.adsabs.harvard.edu/abs/2023MNRAS.521..497M/abstra...).
| This means that there is plenty of space within the context of
| LCDM to have bright and seemingly large and massive galaxies
| early on. Based on current JWST data at these early epochs, there
| are really no convincing arguments for or against LCDM because
| it's highly sensitive to the galaxy formation model that's
| adopted.
| RicoElectrico wrote:
| Waiting for Angela Collier to make a video on this, I'm sure many
| people will forward her this article. MOND is actually a niche in
| cosmology despite its PR.
| mgraczyk wrote:
| Are any of the MOND theories consistent with this new data also
| consistent with recent gravitational wave observations? My
| understanding is that gravitational wave detectors have recently
| ruled out most plausible MOND theories. The linked paper doesn't
| seem to discuss this.
| verzali wrote:
| Why why why do people share articles with sensational headlines
| like this? Its no wonder science journalism gets a bad rap. This
| kind of thing really undermines all the people who are actually
| trying to communicate science properly.
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