[HN Gopher] Rotation curves: still flat after a million light-years
___________________________________________________________________
Rotation curves: still flat after a million light-years
Author : xqcgrek2
Score : 95 points
Date : 2024-06-20 11:04 UTC (11 hours ago)
(HTM) web link (tritonstation.com)
(TXT) w3m dump (tritonstation.com)
| jameskilton wrote:
| For those who aren't sure what Rotation curves are: it's the
| observation that the orbits of stars in galaxies around said
| galaxy's center do not slow down the further away from the center
| they are. In fact some even rotate faster the further out you
| get!
|
| https://en.wikipedia.org/wiki/Galaxy_rotation_curve
| ricksunny wrote:
| I would settle for achieving intuition or even credible
| sources'acknowledgment in popular press on just what point in
| space is being orbited when the satellite is millions of LY (or
| 8 light minutes :) ) out from the massive object (which is
| translating at a measurable fraction of c ) it is orbiting.
| tony69 wrote:
| If it's translating at constant velocity, the satellite is
| orbiting the point in space where the object actually is.
| There is no "lag", gravity points to where the center is now,
| not where it was 8 minutes ago.
| jerf wrote:
| I can't search it up now, but I have seen a derivation that
| if you work all the way through the math, it turns out your
| intuition is wrong and in the case of something like a planet
| orbiting a star, if the star is moving at a reasonably
| constant speed (ignoring all the relativity details around
| how to define that for now) it turns out that the planet
| actually will effectively orbit around the "current" position
| of the star, even though gravity only travels at the speed of
| light [1]. Our naive expectation that it would orbit only
| where it sees the star in the sky right now fails to account
| for some additional correction terms that show up when you
| take the dynamics of the situation into account, and it
| happens to come out in what you may think of as a coincidence
| to "correcting" the point actually being orbited to what is
| also the current position of the star. (Technically I think
| it is still off, but galactic orbits are very, very slow, so
| the errors introduced by them are also very very small.) You
| may recall in physics class how the difficulty of
| understanding situations amped up once you move from statics
| to dynamics, especially if you took a real calculus-based
| version of it. In much the same way that even if you use
| Newtonian gravity, simply knowing the formula may still leave
| you surprised at quite a lot of what can happen in orbital
| mechanics.
|
| If the star suddenly disappeared or zoomed off in another
| direction, it would be a light-speed delay before the planet
| "noticed" anything, but that generally does not happen,
| obviously.
|
| I expect the derivation I saw would not be valid for orbits
| involving speeds close to c, but I _would_ expect the general
| observation that the effective center of the orbit is in fact
| not the time-delayed location would still hold.
|
| [1]: Expected for a long time, but fairly recently
| observationally proved:
| https://www.youtube.com/watch?v=Pa_hLtPIE1s
| AnimalMuppet wrote:
| The same thing happens in electric fields. The electric
| field of a uniformly-moving charge points to where it is,
| not to where it was when the light was emitted from it that
| is now reaching the observer measuring the electric field.
| DonnyV wrote:
| Thank you for this! I was confused what they meant by Rotation
| curve.
| MattPalmer1086 wrote:
| A really interesting result. On it's own, it doesn't destroy dark
| matter as a theory, but it's putting quite a large dent in it.
|
| Obviously will need additional work and review, it's only one
| paper. Maybe there are mistakes or factors not fully considered.
|
| There have been a number of other papers recently on measuring
| wide binaries. Different papers claimed different results on
| these.
|
| Still, it's certainly something that merits a lot more attention.
| We may be looking at needing some new theory of gravity (maybe
| not MOND, but something other than dark matter).
| naasking wrote:
| > A really interesting result. On it's own, it doesn't destroy
| dark matter as a theory
|
| It refutes LCDM though. No DM halos could be responsible for
| this behaviour this far out. And this isn't the first time LCDM
| has been refuted by evidence before they tweaked it with yet
| more parameters/epicycles to make it fit.
|
| It's been clear for awhile now that neither particle dark
| matter nor MOND are adequate explanations for observations.
| MOND clearly matches some data better and with fewer parameters
| (like this), and DM others, like cluster-scale lensing. New
| thinking is needed, and hopefully this paper will surprise
| people into taking MOND-like approaches a little more
| seriously.
| XorNot wrote:
| Presuming the behavior is real and not an artifact of the
| model. This is a statistical technique, needing specially
| selected targets in order to be observed reliably - and also
| assuming those targets themselves are typical.
|
| There's plenty of observations which can accidentally vanish
| because of subtle problems with assumptions, so declaring a
| total refutation is beyond premature.
|
| Like to wit, if MOND is real then you've really got to
| explain how sometimes it also selectively just bails out on
| some galaxies apparently[1].
|
| I'm tired of people trotting out "epicycles" to attack
| theories they don't like: you're gonna be adding a lot of
| those to get a MOND which can explain all the data as well
| (which is to say, it's a trite insult and not useful
| argument).
|
| [1] https://en.wikipedia.org/wiki/NGC_1052-DF2
| throwawaymaths wrote:
| Iirc, the udg makes sense if the distance to it is off by
| something like 25% and the orientation of rotation is off
| by a bit. Those parameters are very hard to measure in
| UDGs.
| naasking wrote:
| > I'm tired of people trotting out "epicycles" to attack
| theories they don't like
|
| That's disingenuous. LCDM has a long history of failing to
| successfully predict later observations and adding
| parameters to fit the data, where MOND has made many
| successful _a priori_ predictions without any added
| parameters since the 1980s. This is not just a matter of
| not liking something, successful predictions vs. post-hoc
| curve fitting strikes at the very core of what it means to
| be a good scientific theory. See:
|
| From Galactic Bars to the Hubble Tension: Weighing Up the
| Astrophysical Evidence for Milgromian Gravity,
| https://www.mdpi.com/2073-8994/14/7/1331
|
| As I said, neither approach is fully satisfactory, and we
| need new thinking about this problem.
| l33tman wrote:
| Consider also that the author of the linked blog post seems to
| be a (pretty strong) advocate for MOND, it did read as somewhat
| biased
| MattPalmer1086 wrote:
| He is a bit more nuanced than that, I've been reading the
| blog for quite some time.
|
| He has pointed out areas where MOND falls short in other
| posts, but it is true that he thinks that LCDM has some
| serious flaws, and that MOND like theories have a better
| predictive record.
| Maro wrote:
| Quick explanation:
|
| A galaxy (elliptic, or spiral) is made out of billions of stars,
| like our sun.
|
| These stars rotate around the center of the galaxy (very slowly,
| think millions of years for 1 rotation).
|
| A rotation curve measures the velocity of stars as a function of
| distance from the center of the galaxy.
|
| Newtonian physics (or Einstein's GR) says that the rotation curve
| should decay with distance, ie. with greater distance the stars'
| velocity should go down --- assuming the matter in the galaxy is
| the visible matter that we see, ie. the stars (which shine
| light).
|
| The problem is, there is a rich set of observational data, from
| many different experiments, telescopes, and methodologies that
| show that the rotational curve is in fact flat, it does not
| decay.
|
| There are 2 big competing theories to explain this discrepancy:
|
| 1. Assume that there is a lot of unseen, non-shining, ie. Dark
| Matter (DM) in the galaxies (also ours). If you put the
| appropriate amount of dark matter in there, with the right
| distribution, you can reproduce the observed rotational curve.
| There are also other places is astrophysics/cosmology where
| having dark matter (specifically Cold Dark Matter, CDM, where
| cold just means "slowly moving") is useful. The biggest example
| is to explain the history of the Universe and the observed
| Hubble-constant. In fact the standard model of cosmology is
| called lCDM, CDM for Cold Dark Matter (l for the cosmological
| constant, currently modeled as Dark Energy, not relevant for this
| discussion).
|
| 2. Assume that Newton was wrong and gravity is not exactly 1/r^2
| --- this is called MOND, Modified Newtonian Dynamics. This way
| you can also reproduce the observed rotation curves. This is much
| less popular, because: (i) physicsts don't want to give up the
| beautiful and geometric simplicity of 1/r^2 (ii) Dark Matter is
| also useful for solving other discrepancies in
| astrophysics/cosmology.
|
| What this article is saying is that, even in the first Dark
| Matter model, per the model DM distributions inside galaxies that
| also work with all the other places where DM is used to explain
| something (eg. in cosmology), at some distance from the center,
| the dark matter bubble has an edge and stops --- and then the
| velocities should finally break down. However, these latest
| observations are showing that the velocities remain constant even
| beyond the modeled/assumed DM bubbles. This is an additional e
| argument in favor of MOND, and science proceeds.
| merek wrote:
| Fantastic explanation, thank you.
|
| For a more detailed description of the Milky Way's rotation
| curve, this is a brief segment from David Butler's How Far Away
| Is It video series (which I highly recommend):
|
| https://youtu.be/uVxrsJ5lZlQ?si=ZwpBDpAvTV8AALke&t=1890
| xeonmc wrote:
| What falloff function will create an exactly flat curve? G ~
| 1/r instead of 1/r^2 ?
|
| What if gravity has non-scalar components?
| canjobear wrote:
| https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%.
| ..
|
| The extra components would have energy and function as Dark
| Matter.
| jprete wrote:
| I'm not a physicist but 1/r^2 strikes me as conceptually very
| important, because it's the relative contribution of any fixed
| area of spherical surface to the total area of that surface. So
| the total strength of gravitational field emanating from a
| particular object, at a given distance from that object, is a
| constant.
|
| It's somewhat weird to think of the total gravitational "force
| field" _increasing_ in magnitude with distance. Decreasing,
| sure. Increasing? That makes no sense. Certainly not at a large
| enough function of distance for the rotational curve to be
| _flat_. That's got to be some kind of wonky power term over
| distance which implies potential energy from the field goes up
| with distance as well.
|
| As above, I'm not a physicist, but a linear rotational curve
| breaks every intuition I've ever gleaned from physics about the
| nature of what's really going on with relativity, particle
| mediation of forces, or even the concept of a field.
|
| Maybe it means spacetime curvature is way higher than we think.
| throwawaymaths wrote:
| > spacetime curvature is way higher than we think.
|
| You mean way weirder.
|
| Remember 1/r^2 does not work quite right for, e.g. mercury.
| Avshalom wrote:
| Well the general solution is to invent a field with force
| mediating particles that have the exact same properties as
| dark matter but insist it's not dark matter.
| empath75 wrote:
| The strong force increases with distance, fwiw.
| PuffinBlue wrote:
| Maybe not so weird if gravity isn't the curvature of
| spacetime but a symptom of there being either more or less of
| it, and mass _creates_ spacetime.
|
| Replace the highly curved spacetime region close to a
| blackhole with the idea that huge amount of spacetime is
| being created by the mass of the blackhole, so there is more
| spacetime near the blackhole.
|
| The more spacetime being created and 'flowing outwards' away
| from the mass, the faster the apparent 'velocity' of an
| object through that region of spacetime ner the blackhole
| (and have this work out that the spacial component handles
| the physical motion and time slows down to compensate - just
| like it does in highly curved spacetime), and consequently
| the slower it moves relative to an external observer.
|
| Areas further from mass see much more 'dilute' spacetime
| (whatever the heck that means) and travel with relative
| slower spacial velocity but faster in time, so it _appears_
| to be travelling faster up. This would be doubly obvious at
| the scale of galaxies.
|
| I think this ridiculousness would rely on the relativity of
| simultaneity in rather a large way!
|
| The other interesting thing is, if mass does create spacetime
| then pockets of mass like galaxies should move away from each
| other faster and faster as they make more of it in between
| themselves.
|
| (NOTE - this is just a silly thought experiment, don't take
| it seriously)
| MilStdJunkie wrote:
| I don't think this sounds silly at all. Virtually
| everywhere we look in cosmology the last decade or so, you
| get a sensation of _things unseen_. Like we 're looking at
| one of those optical illusions that change shape when you
| cover your left eye, or like the McGurk Effect, when your
| audio perception shifts completely because you can see
| someone's lips.
|
| Another element that's being discussed is, like with mass
| and spacetime, that the spatial dimensions themselves are
| emergent phenomenon arising from bulk entanglement. Sean
| Carrol has talked about it a fair amount, and it's been
| surfing around maybe harder than it would ordinarily,
| because it provides some edge cases that are, at least
| conceivably, testable without solar-system-sized
| accelerators or a DeLorean to the beginning of the cosmos.
| It's an evocative thought. In one interpretation of this,
| Double Slit restricts many of the spatial dimensions,
| resulting in a particle that might seem to be in different
| places, but which is, in some respects, the same particle.
| Another interesting notion is that singularities, in some
| dimensions, might be the same place.
|
| Combined with your notion, it almost re-frames mass as -
| forgive me for getting poetic here - a _measure of fate_.
| How much does _this_ resist doing _that_?
| jiggawatts wrote:
| It's not at all silly. There's some nice visualisations[1]
| of GR in YouTube that _look_ like space is being swallowed
| up by matter.
|
| A toy model I like to use in my mind is that matter absorbs
| spacetime. It is _literally_ sucked in!
|
| A possible extension of this model is that the tension
| introduced in the vacuum causes it to stretch out. That
| could potentially explain the non-r^2 terms in galactic
| rotation curves.
|
| [1] https://youtu.be/DYq774z4dws?si=6vDWZ8jPzgjxSBb1
| ryandrake wrote:
| > (i) physicsts don't want to give up the beautiful and
| geometric simplicity of 1/r^2
|
| Not a physicist here, so maybe this is a naive question: but is
| this really something that they care about? Why does a formula
| describing some physical principle have to beautiful and
| simple? Aren't we supposed to observe reality and _then_ come
| up with the math? Rather than start with a "known true"
| equation and add factors and parameters it as more and more
| observations call the equation into question? Who's in charge
| of the direction physics proceeds? The observing scientists or
| the mathematicians?
| xeonmc wrote:
| 1/r^2 directly comes from how much area an object spans your
| field of view vs how far away it is. To a paraxial
| approximation, of course.
| wyager wrote:
| A general principle of science is that simpler explanations
| are more likely. Rule-of-thumb described by Occam's razor,
| formalized by concepts like Solomonoff induction. Rules like
| 1/r^2, being simple, are assigned a higher prior probability.
|
| This makes sense because otherwise you waste a bunch of time
| on overfitted theories.
| jlokier wrote:
| It's not about the formula. The formula comes from the
| geometry.
|
| 1/r^2 falls out as the formula, starting from the geometry of
| "flux" and "field lines", along with "conservation of flux",
| in 3d space.
|
| That's the idea that the force acts like something that's
| radiated in all directions, that _doesn 't fade_ with
| distance, instead it just _spreads out_ so it seems weaker at
| individual points. The amount of spreading out, if it 's
| uniform in 3d, turns out to be exactly 1/r^2.
|
| You get the same 1/r^2 if you measure the flow of water in a
| 3d volume with a point source of water in the centre, or
| electric current in a 3d block of metal with a point source
| of electric current in the centre. (In both cases, presumably
| through a thin pipe or cable to the centre).
|
| In 2d space, you get a different formula from the geometry,
| 1/r. If you see a force, or flow, reducing by 1/r^2 in a
| system you thought was 2d, you might ask "is there a third
| dimension involved here which I haven't accounted for?"
|
| In 1d, the force or flow doesn't reduce with distance. For
| example, current in an electrical wire is the same all along
| the wire.
|
| And if you see 1/r^3 in 3d, you might speculate about a
| hidden fourth dimension to explain it.
| empath75 wrote:
| They will 100% replace 1/r^2 in an instant if they find
| something else that matches results better.
| naasking wrote:
| I don't think history bears this out. "Science progresses
| one funeral at a time", as they say.
| dameyawn wrote:
| Are you aware of any visuals that show what the density
| distribution of DM looks like to fix the expected the rotation
| curves for some example galaxies?
| Maro wrote:
| Google images: dark matter halo
| naasking wrote:
| DM distribution is different for every single galaxy, and so
| can only be fit to the galaxy's rotation curve after
| observing it. In contrast, MOND successfully predicts
| rotation curves a priori using only the visible matter.
| joshjje wrote:
| For 1/r^2, wouldn't curved spacetime mess that up? Not sure how
| you would calculate say a moon or other things in between.
| hwc wrote:
| it _does_ mess it up, but near the gravitational source, not
| way out like the problems with the galactic gravity.
| yongjik wrote:
| > physicsts don't want to give up the beautiful and geometric
| simplicity of 1/r^2
|
| Eh, just like MOND proponents don't want to give up the
| beautiful simplicity of "If something is attracting me
| gravitationally I'd better see it!"
|
| When you think about it, there's no _a priori_ reason why a
| particle with mass should interact with any other force. We 'd
| just like to assume it because it seems "simpler" that way.
| Filligree wrote:
| Nor that there should be just one family of interconnected
| fields. We've got, what, two dozen or so?
|
| Some affect each other, some don't. You can create a graph
| from that, and you get one that's dense in places but have
| some nearly disconnected regions. Why not a graph with actual
| disjoint subgraphs? We'd only be able to tell through
| gravity.
| infogulch wrote:
| It has to drop off at some point otherwise it would affect
| other galaxies...
| Supermancho wrote:
| > If you put the appropriate amount of dark matter in there,
| with the right distribution, you can reproduce the observed
| rotational curve.
|
| What is the "right distribution"? If it's not roughly uniform,
| it's unlikely to result in a uniform flatness of rotation
| curves, across the distances in the galaxy. This seems almost
| impossible when accounting for clustering within the galaxy. I
| would believe such a uniform distribution of DM would be
| possible, if Dark Matter is something that exists/acts
| differently than matter. For example, if there were space-time
| bumps that form. Small bubbled/hilled spacetime is created in
| reaction to masses traversing it? ie the classic ball on a
| sheet, except it behaves more like a liquid than a sheet.
|
| This feels like a blow to Dark Matter theories, regardless.
| burnte wrote:
| "MOND predictions keep being corroborated, yet the community
| persists in ignoring its implications, even in terms of dark
| matter. It's gotta be telling us something."
|
| No, it doesn't. MOND falls apart in every single theory they put
| forth. The fact that this happens without fail should lead one to
| understand the answer probably lies elsewhere than MOND.
| kurthr wrote:
| It's not like MOND (or even dark matter) are even single
| theories with a fixed set of predictions. They are groups of
| competing ever evolving theories. I don't understand the need
| to believe one over the other as a predictive tool without some
| conclusive evidence. If you're not the astronomer who makes
| that measurement (or explains it), it just feels like galactic
| sports betting.
| MattPalmer1086 wrote:
| MOND successfully predicts many things we observe that DM fails
| to, or that requires special tuning for DM to fit observations
| (which is not prediction). DM actually has quite a poor
| predictive record, even if it can be made to fit observation.
|
| MOND as a theory is probably not correct. It's not relativistic
| for a start, although I believe there are other modified
| gravity theories that are.
| throwawaymaths wrote:
| MOND explains (discovered before Mond, but the Mond equations
| unexpectedly fit):
|
| Tully fisher relation
|
| Renzos rule
|
| MOND predicted (these were observed _after_ mond predicted it):
|
| External field effect
|
| Keplerian descent in milky way
|
| Early galaxies after the big bang
|
| Consistently "No dark matter found" in elliptical and
| lenticular galaxies
| shiandow wrote:
| I'm getting interested now, the comments have alleged several
| predictions MOND made. On the other hand I'm pretty sure the
| evidence for dark matter is pretty strong as well (I believe
| there are several ways to calculate the amount of dark matter,
| all of which agree).
|
| Can you give an example of MOND falling apart? One that
| requires so much fine-tuning that it cannot be adequately
| explained, or one requiring a violation of one of the more
| fundamental laws of physics?
| Filligree wrote:
| The Bullet galaxy.
| XorNot wrote:
| For one thing MOND doesn't actually appear to eliminate the
| need for dark matter[1]. It gets rid of ~80% of the missing
| mass requirement, but not all of it. Whereas pure DM can just
| eliminate MOND.
|
| Then you've got the "Bullet Cluster"[2] - where two colliding
| galaxies have had their observable and dark matter masses
| apparently separated. MOND can't explain this one without a
| lot of tweaking, but it's pretty trivial for DM:
| electromagnetically interacting matter is "sticky" where as
| gravity only matter isn't. The Bullet Cluster shows a galaxy
| shaped blob of gravitational lensing exactly where you'd
| expect it to be if a bunch of non-interacting matter had
| flown through each other, whereas the electromagnetic matter
| has interacted and re-shaped.
|
| NGC 1052-DF2[3] and NGC 1052-DF4 are both ultra-diffuse
| galaxies which have no, or _very_ little dark matter. That
| is, they appear to have normal galactic rotation curves fully
| explained by their observed visible mass. This works totally
| fine for DM existing (it 's a problem for lambda-CDM though
| because it's not clear how they could've formed without dark
| matter, but I mean - we also don't yet know how black holes
| actually manage to ever merge either yet we do observe them
| too). This one always seems like a problem to me: MOND
| proposes a new universal principle of matter, then suddenly
| we have some matter where it's not doing that.
|
| The theoretical problems[4] are somewhat beyond me, but they
| get well into issues with violating relativity and that's a
| big one: relativity is stupidly, reliably accurate under
| every single test we put it through, to absurd levels of
| precision. Build a better instrument, you can just dial in
| your precision and get the answer out ahead of time before
| you launch the satellite which is testing it. Also without
| careful adjustment you get violations of conservation of
| momentum (conversely, if MOND is real this would be handy
| because maybe it means we can reactionless spacedrives).
|
| It's worth noting that none of this is implicitly fatal.
| lambda-CDM could be wrong, a MOND variant could be right. But
| a list of convenient things MOND explains easily doesn't
| escape the need to also include the things it _can 't_ - and
| appeals to the idea that DM is being "tweaked" to match
| observations unnaturally ignores the fact that MOND has to
| have the same thing done to it to fix within cosmology.
|
| [1] https://arxiv.org/abs/1404.7525
|
| [2] https://en.wikipedia.org/wiki/Bullet_Cluster
|
| [3] https://en.wikipedia.org/wiki/NGC_1052-DF2
|
| [4] https://arxiv.org/pdf/astro-ph/0104435
| naasking wrote:
| > Then you've got the "Bullet Cluster"[2] - where two
| colliding galaxies have had their observable and dark
| matter masses apparently separated. MOND can't explain this
| one without a lot of tweaking
|
| The bullet cluster is so over-played as a refutation of
| MOND. "A lot of tweaking" basically reduces to adding
| sterile neutrinos, as one possible solution. All galactic
| clusters have issues in both MOND and LCDM, the bullet
| cluster was nothing new when it was discovered, it was just
| visually dramatic because they could image the
| gravitational lensing.
|
| > but it's pretty trivial for DM: electromagnetically
| interacting matter is "sticky" where as gravity only matter
| isn't.
|
| Actually LCDM can't explain the bullet cluster either:
|
| https://tritonstation.com/2024/02/06/clusters-of-galaxies-
| ru...
|
| As always, MOND and LCDM appear to just trade off one set
| of issues for other equally problematic set of issues.
| Neither is favoured very strongly by the sum of evidence.
| Physicists have just gotten in the habit of ignoring all of
| the problems with LCDM and consider even trivial problems
| with MOND to be fatal.
|
| > relativity is stupidly, reliably accurate under every
| single test we put it through, to absurd levels of
| precision
|
| Those precise tests do not extend to galactic scales, which
| is exactly where the problems appear. It would be nice if
| our existing theory worked across all scales, but that
| doesn't mean it must.
| isthatafact wrote:
| > "The bullet cluster is so over-played as a refutation
| of MOND. "A lot of tweaking" basically reduces to adding
| sterile neutrinos, as one possible solution."
|
| If MOND requires "dark matter" to explain the bullet
| cluster, then what is the appeal of MOND?
| naasking wrote:
| Read the article this thread is about. Particle DM is not
| sufficient to explain all observations without extreme
| contortions that make MOND seem more reasonable. Also,
| MOND predictions made decades ago keep being validated.
| Why does this keep happening if there's nothing to MOND?
|
| Finally, as I said, particle DM can't fully explain the
| Bullet Cluster either. The evidence is screaming in our
| faces that we need better thinking here.
| isthatafact wrote:
| > Read the article this thread is about.
|
| It is one brand new paper. I am skeptical. Rotation
| curves are perfectly flat out to whatever arbitrary
| distance that they happen to be able to measure? I am
| very skeptical.
|
| > "particle DM can't fully explain the Bullet Cluster
| either."
|
| According to one scientist, who happens to be the same
| scientist claiming that particle DM cannot explain
| rotation curves. I will not check every claim, but the
| bullet cluster collision speed "problem" is readily
| explained in the reference in the wikipedia article:
| https://arxiv.org/abs/1410.7438
|
| The broader point is that every time MOND has claimed to
| refute dark matter so far, the refutation has been
| refuted, so I will wait to see the outcome of this new
| claim.
| naasking wrote:
| > The broader point is that every time MOND has claimed
| to refute dark matter so far, the refutation has been
| refuted,
|
| By adding more epicycles to LCDM. I'll leave it to you to
| decide when to call shenanigans on that, but it's been
| going on for 30+ years now.
| isthatafact wrote:
| I am no expert, but I do not follow. Nothing was added to
| the LCDM model there, so using the epicycle fallacy does
| not help the discussion.
|
| The research was simply done again with better accuracy.
| naasking wrote:
| I wasn't referring to the bullet cluster specifically,
| but this obsession with the bullet cluster is typical of
| the confirmation bias in this field: hyperfocus on what
| confirms bias and ignore the countervailing evidence. The
| past 30+ years have seen many "corrections" to get LCDM
| to fit observations it did not predict [1]. Clusters in
| general pose challenges to both MOND and LCDM for
| different reasons [2,3], but LCDM's typically get ignored
| and MOND's treated as a fatal blow. As I said, neither
| theory is fully satisfactory, but it's clear that
| research on these questions is fairly one-sided.
|
| [1] https://www.mdpi.com/2073-8994/14/7/1331
|
| [2] https://iopscience.iop.org/article/10.3847/1538-4357/
| ace62a
|
| [3] https://www.sciencedirect.com/science/article/pii/S13
| 8764732...
| tyn_peddler wrote:
| Wikipedia's bullet cluster article is written in a very
| misleading way. The bullet cluster is not a hard problem
| for MOND since regular baryonic gas easily explains the
| lensing. Working out the bullet cluster with LambdaCDM
| actually took quite a bit longer because there were
| problems in the initial data set that took astronomers 10
| years to work out. In short, the bullet cluster doesn't
| "disprove" MOND anymore than it "proves" LambdaCDM.
|
| NGC 1052-DF2 is as big a problem for LambdaCDM as it is for
| MOND. LambdaCDM requires dark matter for galaxy formation.
| Which means either LambdaCDM is wrong about how galaxies
| form, or there was some event (of which we currently lack
| evidence) that removed it. As for MOND, it's possible that
| actually doing the math may show that this galaxy is not
| particularly weird. There's aren't many folks fluent in
| MOND, so work on these kinds of issues tends to lag their
| discovery.
|
| As for MOND and cosmology, yeah, MOND sucks at cosmology.
| The fact that it can so easily explain so many galactic
| dynamics (far better than LambdaCDM) is really weird
| though. MOND isn't a great theory, but it's a really
| interesting model because it really highlights how poorly
| LambdaCDM predicts (not explains!) the galaxies we see
| today.
| burnte wrote:
| > I'm getting interested now, the comments have alleged
| several predictions MOND made. On the other hand I'm pretty
| sure the evidence for dark matter is pretty strong as well (I
| believe there are several ways to calculate the amount of
| dark matter, all of which agree). > Can you give an example
| of MOND falling apart? One that requires so much fine-tuning
| that it cannot be adequately explained, or one requiring a
| violation of one of the more fundamental laws of physics?
|
| Pick any MOND theory you like, it all fails when it hits
| relativity, which is a theory with an embarrassment of riches
| of evidence in its favor, and can't be reconciled with
| relativity either. So while it explains galactic rotational
| speeds, it then fails to explain lots of other things, so
| it's a huge step backwards. It's the equivalent of saying
| Newtonian physics is wrong because it can't explain Mercury's
| precession, so let's go back to epicycles. To favor MOND we
| give up tremendously more than we gain.
|
| MOND CAN explain things, if I have been interpreted to say it
| never predicts anything, that was never my intent. The
| problem is that MOND can't explain much else, so rather than
| making our theories simpler (which usually means we're in the
| right direction) it complicates things.
|
| To make relativity work for the things MOND looks at, we only
| add 1 thing, WIMPs, particles we theorize but haven't seen.
| We've predicted lots of particle and found them, so this
| isn't a problem. chances we have missed a particle that turns
| out to be highly non-interactive? High. It took us ages to
| really solve the missing neutrino problem by discovering
| them, and we predicted the Higgs boson with high accuracy
| too. So this is a road we've been down before.
|
| To make MOND work, we throw out a lot of theory, and we have
| nothing to replace it with under MOND. Chances all those
| other theories are wrong even though they work great? Low.
|
| Dr Becky is amazing with her MOND analyses. Here's one of the
| latest: https://www.youtube.com/watch?v=HlNSvrYygRc
| firebot wrote:
| The latest is, afaik, is called RelMOND, short for relativistic
| MOND. It describes the cosmos very similarly to the standard
| model, but with MOND as well. So slightly better matching
| observations than lambda CDM, but still imperfect. They plan to
| incorporate electromagnetism in future models.
| xanzi2048 wrote:
| Why does MOND keep showing up on HN? Pretty odd considering it's
| not particularly popular elsewhere and fails to explain a lot of
| observations of dark matter (bullet cluster, CMB). It's worth
| noting these rotation curves are not all the same curve, and
| we've discovered galaxies with varying quantities of implied dark
| matter (eg
| https://www.aanda.org/articles/aa/full_html/2023/07/aa46291-...)
| - so is MOND different for every galaxy?
| MattPalmer1086 wrote:
| No. MOND is the same everywhere, with basically no free
| parameters.
| xanzi2048 wrote:
| but... how is that compatible with the article I linked here?
| Some galaxies look like they have very little dark matter, a
| MOND with no free parameters can't explain why different
| galaxies have different inconsistencies in their rotation
| curves.
| throwawaymaths wrote:
| You actually have to do the math, it's not just a "no dark
| matter hurr durr" thing.
|
| MOND predicts "no dark matter" if the acceleration regime
| is high. Most galaxies that havd "very little dark matter"
| are in the high acceleration regime.
|
| Most of the others seem to be susceptible to observational
| error (e.g. ultra diffuses). Galaxy rotation curves
| measurements are highly sensitive to orientation of the
| rotational axis and distance to us (you're gonna have a
| hard time measuring it if the galaxy is face on)
| MattPalmer1086 wrote:
| Correct. MOND cannot explain why some galaxies don't fit
| it's predicted rotation curve.
|
| However, the vast majority do, and the linked paper says
| that the ones they are describing are extremely rare.
| ttpphd wrote:
| Because HN loves Sabine H.
| stoperaticless wrote:
| I do. Just please don't mention that to my wife.
|
| Worth noting that recently (when couple of months is
| considered recent), she mentioned that she had falling out
| with MOND (due to some new study coming to ger attention).
| mrguyorama wrote:
| And they specifically love her because she is extremely
| contrarian. HN has a massive problem with contrarian bias.
| ansible wrote:
| I also don't know why MOND remains so popular.
|
| Angela Collier talks about Dark Matter here:
|
| https://www.youtube.com/watch?v=PbmJkMhmrVI
|
| And talks more about how often MOND is brought up in comments
| on her own video:
|
| https://www.youtube.com/watch?v=qS34oV-jv_A
| roywiggins wrote:
| It's easy to be a MOND enthusiast without a physics PhD, it's
| a lot harder to be an axion or WIMP enthusiast.
|
| "Just tweak the laws of physics, bozos!" is a really easy
| idea for laymen to latch on to, which is why you see so much
| interest in it among science enthusiasts.
| mrguyorama wrote:
| "The experts are wrong because they are overcomplicating
| things" is also just a rhetorical trick that has pretty
| much always worked on some segment of the population.
| naasking wrote:
| MOND remains popular because it's made so many successful a
| priori predictions that DM failed to do. At the risk of being
| too reductive: DM is popular among maths-oriented
| astrophysicists because as physicists they like particles and
| deep theories, and MOND by contrast, doesn't have a full
| theory justifying the adjustments to gravity and they don't
| like that; MOND tends to be more popular among astronomers
| than astrophysicists because they like effective theories
| that make successful predictions using few parameters, and
| MOND is somehow better at that (as with this paper).
| Sevii wrote:
| It's because people don't like the idea of matter that you
| can't interact with especially if it makes up a double digit
| percentage of all mass in the universe.
| notfed wrote:
| Maybe we need both?
| nonameiguess wrote:
| Hacker News is just a part of the larger web. It's the same
| reason you see so many people arguing over lab leak, ketogenic
| diets, barefoot running. Anything whatsoever that is vaguely
| heterodox and hints that you might have some knowledge that
| either the "establishment" doesn't know or is lying about is a
| tremendous ego boost. If your default position epistemically is
| you don't really have the expertise or time to investigate
| everything out there, which is more or less true for everyone
| no matter how intelligent you are or how much expertise you
| have on one specific topic, then if you're going to throw darts
| anyway, you may as well throw them at the target that, if
| correct, makes you look and feel just a little bit more special
| than the unwashed masses being spoonfed that food pyramid.
|
| I can't remember where at this point, but decades ago I heard
| this kind of thing called "insight porn" and have myself been
| guilty of it for much of my life. It's endemic to web
| communities composed of relatively smart people, at least if we
| take "smart" to mean something like intellectually curious and
| having right tail levels of raw cognitive ability, rather than
| meaning anything you believe is more likely to be true.
| roywiggins wrote:
| MOND is also just a lot easier to feel like you understand
| than the zoo of proposed dark matter particles, so it sticks
| in the layman mind.
| TheBlight wrote:
| Which best satisfies Occam's Razor?
|
| 1) The vast majority of matter in the Universe is
| invisible. We've tried looking for it in a variety of ways
| for decades but we can't find it but we're sure it's there.
|
| 2) Our model of the Universe is slightly incorrect. It
| works in many many cases but not in interesting outlier
| situations. eg. at very low accelerations.
| roywiggins wrote:
| MOND doesn't explain _all_ of the missing mass, it just
| reduces it to a fifth of what it was. So you need MOND
| and _something else_ , which is two things, which Occam's
| Razor disfavors anyway.
| TheBlight wrote:
| I don't necessarily believe canonical MOND is the answer,
| just that it seems more reasonable to me to assume we
| don't have the Universe completely figured out and
| accurately modeled just yet vs. being emphatically sure
| most matter never interacts with light but only with
| gravity despite continued failure in finding direct
| evidence of it because it would satisfy our current model
| without the need to adjust it.
| naasking wrote:
| > So you need MOND and something else, which is two
| things, which Occam's Razor disfavors anyway.
|
| Misapplication of Occam's razor. LCDM also needs multiple
| things: non-interacting dark matter + a fine-tuned
| distribution of DM that cannot be a priori predicted from
| any observations, but only post-hoc fitted after
| observation. By contrast, MOND has successfully predicted
| rotation curves (and lots more) from the visible matter
| alone.
|
| You can't naively apply Occam's razor to two theories
| that both fail some set of observations. However, as a
| scientific theory, MOND has a better track record of
| successful predictions.
| mrguyorama wrote:
| It took us until the 20th century to discover radiation,
| despite it having extremely direct and powerful
| interactions. Gravity is a shitty, weak interaction that
| we largely don't have a full understanding of, so why is
| it so unlikely we haven't properly discovered matter that
| only interacts weakly with gravity? How the hell else
| would we "discover" it?
| ljsprague wrote:
| Great comment. (It was clearly a lab leak though.)
| naasking wrote:
| > Why does MOND keep showing up on HN? Pretty odd considering
| it's not particularly popular elsewhere and fails to explain a
| lot of observations of dark matter (bullet cluster, CMB).
|
| And DM fails to explain flat rotation curves out to 1000
| parsecs, or how MOND was able to make so many successful a
| priori predictions where DM has to be curve fitted after the
| fact. This paper is just the latest to refute LCDM, but no
| doubt proponents will add yet more parameters to correct for
| this failure, as they have done many times in the past when
| observations refuted DM predictions.
|
| Honestly, you and many others have fallen for the DM
| propaganda. Both DM and MOND are problematic and fail in
| various ways. DM is not nearly as successful a scientific
| theory as most think, and MOND is not nearly as problematic:
|
| From Galactic Bars to the Hubble Tension: Weighing Up the
| Astrophysical Evidence for Milgromian Gravity,
| https://www.mdpi.com/2073-8994/14/7/1331
| canjobear wrote:
| What would cause the large majority of a scientific field to
| fall for propaganda? If MOND is predictively superior then
| why is it not accepted?
| naasking wrote:
| The simple answer is 1) because physicists don't want to
| muck with general relativity which has had many successes
| across sub-galactic scales, so they are trying as hard as
| possible to jam a particle into the hole that's been
| observed when extending GR/Newton to galactic scales, 2)
| MOND is just an "effective theory" that seems to fit
| observations but doesn't really have a fully fleshed out
| theory explaining why this change to gravity happens; this
| "inelegance" rubs math-oriented physicsts the wrong way.
|
| I've mentioned elsewhere here that astrophysicsts tend to
| prefer DM because they like particles and mathematical
| elegance, and astronomers are more open to MOND because
| they like theories with few parameters that make successful
| predictions (a reductive generalization, but broadly true I
| think). JWST has validated many MOND predictions, so I hope
| people open their minds a little more now, but as I said,
| neither theory is fully satisfactory in the end.
|
| Edit: the author of this post is an astronomer that has
| been working in this field for a long time and he posted
| another article that reviews MOND and LCDM with some
| history:
|
| https://tritonstation.com/2023/01/05/question-of-the-year-
| an...
| TheBlight wrote:
| There are also tremendous social/professional/monetary
| incentives to stick to Lambda-CDM (or whatever the
| consensus happens to be.)
| naasking wrote:
| There's also outside support due to converging interests.
| If DM is a particle that gives particle physicists an
| excuse to fund numerous experiments in an attempt to
| detect it. There are just more physicists motivated
| overall to prefer particle DM.
| TheBlight wrote:
| It's also safe. There is no reputational risk because
| "everyone" (who matters to you professionally) already
| agrees with you. There isn't much immediate upside to
| sticking your neck out. Especially in the era of social
| media.
| cratermoon wrote:
| Marketing. https://www.youtube.com/watch?v=qS34oV-jv_A
| thriftwy wrote:
| Are these rotation flats variable in magnitude? Would one galaxy
| have a different curve height than another one? What does it
| depend on?
|
| Update: https://en.m.wikipedia.org/wiki/Tully-Fisher_relation But
| the linear regression looks unconvincing
| mikewarot wrote:
| Nobody wants to give up 1/r^2, including me.
|
| However, what if it holds, but if _inertia_ is quantized, then
| you get less gravitational effect at 90 degrees to the path of
| motion at astronomic distances as it recedes into the quantum
| noise.
|
| Which fits observations with inventing dark matter, or tweaking
| gravity.
| _nalply wrote:
| I just watched Sabine Hossenfelder's video (Gravity without Mass,
| https://youtu.be/Q0fwRMvNkoA) where she talks about hollow
| spheres. Inspired by this video I have this hypothesis:
|
| There's quantum fluctuation. Particles appear out of nowhere and
| disappear again. Hossenfelder talked about negative mass, so
| allow me to do this as well: What if a pair of two particles, one
| of negative and one of positive mass can very rarely appear?
|
| They disappear immediately again but for a short moment we have
| acceleration (more about that in Hossenfelder's video). Could
| this be enough to explain "dark matter"?
| Razengan wrote:
| As told in certain science fiction, I like to imagine that dark
| matter/energy is the "pollution" or other side effects caused by
| some alien civilizations [over]use of faster-than-light travel.
|
| Which is actually a terrifying existential threat to think about:
| The more an earlier civilization uses FTL, the more space will
| expand, and eventually it will become all but impossible for
| younger civilizations to traverse space without becoming
| dependent on the elder races.
| hwc wrote:
| that is very surprising.
___________________________________________________________________
(page generated 2024-06-20 23:02 UTC)