I Galaxy with no dark matter? (NGC1052-DF2)

[phinds]

Just noticed this article. Wonder if anyone here has further info. Sounds very interesting.

http://www.bbc.com/news/science-environment-43543195

 

[fresh_42]

Here's what I've found:

The research, published in the March 29th issue of the journal Nature, amassed data from the Gemini North and W. M. Keck Observatories, both on Maunakea, Hawai'i, the Hubble Space Telescope, and other telescopes around the world.

https://phys.org/news/2018-03-dark-galaxy.html

Unfortunately the link to the paper: https://www.nature.com/articles/doi:10.1038/nature25676
seems to be broken. But as it happened to me on two different sources (https://www.sciencealert.com/galaxy-ncg1052-df2-no-dark-matter-ultra-diffuse-dragonfly-array being the other one), it might be a local problem. However, I think nature is behind a paywall anyway.

Also interesting in this context is an example of the opposite:

Scientists discover a 'dark' Milky Way: Massive galaxy consists almost entirely of dark matter

https://phys.org/news/2016-08-scientists-dark-milky-massive-galaxy.html

 

[phinds]

I find it interesting that in the article I linked to there is what seems to be a very reasonable statement that the absence of dark matter in the subject galaxy (assuming this proves out) is yet another piece of evidence for the existence of dark matter (not that one is particularly needed). The argument is that the absence of it in this galaxy would imply that its apparent existence in most galaxies cannot be some galaxy-related phenomenon that we do not understand.

 

[kimbyd]

Here's what I've found:

https://phys.org/news/2018-03-dark-galaxy.html

Unfortunately the link to the paper: https://www.nature.com/articles/doi:10.1038/nature25676
seems to be broken. But as it happened to me on two different sources (https://www.sciencealert.com/galaxy-ncg1052-df2-no-dark-matter-ultra-diffuse-dragonfly-array being the other one), it might be a local problem. However, I think nature is behind a paywall anyway.

Also interesting in this context is an example of the opposite:

https://phys.org/news/2016-08-scientists-dark-milky-massive-galaxy.html

I'm seeing this too. If I were to guess, the link is correct but Nature hasn't physically released the paper on their website yet. Probably a website glitch that prevented the paper from being displayed at the same time the press embargo lifted. It'll probably be visible within a day or so, unless there was some publication problem that made Nature want to pull the article at the last minute before release.

 

[kimbyd]

Just noticed this article. Wonder if anyone here has further info. Sounds very interesting.

http://www.bbc.com/news/science-environment-43543195

I do find this interesting, and I would be willing to bet it was the result of some particularly violent events in the galaxy's past which separated matter and dark matter.

One way this could happen, potentially, would be if there was a galaxy cluster collision (similar to the Bullet cluster) which left some cluster gas separated from either cluster, cluster gas which remained dense enough to nevertheless form its own galaxy a long time later. They don't mention this possibility in the article, however, so there's every chance it's been considered and discarded for one reason or another.

One thing I will say is that it would be very, very difficult for any event to remove dark matter from a galaxy, because dark matter interacts so weakly. The process would have had to have been the normal matter being removed from a dark matter halo through some process.

 

[Grinkle]

The process would have had to have been the normal matter being removed from a dark matter halo through some process.

Can you give some example hypothetical processes to show the distinction you have in mind? One uses the same process to separate yolk from white as white from yolk, is why I can't picture it.

 

[DrClaude]

Unfortunately the link to the paper: https://www.nature.com/articles/doi:10.1038/nature25676
seems to be broken. But as it happened to me on two different sources (https://www.sciencealert.com/galaxy-ncg1052-df2-no-dark-matter-ultra-diffuse-dragonfly-array being the other one), it might be a local problem. However, I think nature is behind a paywall anyway.

The article can be found here: https://www.nature.com/articles/nature25767

Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M_halo/M_stars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 10¹⁰ solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052–DF2, which has a stellar mass of approximately 2 × 10⁸ solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 10⁸ solar masses. This implies that the ratio M_halo/M_stars is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052–DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.

It is indeed behind a paywall.

 

[kimbyd]

Can you give some example hypothetical processes to show the distinction you have in mind? One uses the same process to separate yolk from white as white from yolk, is why I can't picture it.

The cluster collision was the one idea I had. This has the nice property of the fact that it can leave some normal matter in a location far from dark matter, and the colllision itself acts to compress the normal matter, potentially kicking off star formation (the formation of bright stars could remove the rest of the gas and dust from the galaxy).

Stars don't interact very much, just like dark matter, so I doubt that you could realistically separate stars from dark matter by any process. But we know gas can be separated. So if it's separated from dark matter while it's gas, and then collapses into a galaxy, that might do it.

 

[Grinkle]

The cluster collision

I don't know how to search for such a paper - I wonder if anyone has tried to model gravitational events that would remove gas from hypothetical rings of dark matter and see if they can get a result that leaves the gas both isolated and in a state that can subsequently form a galaxy that won't spin itself apart.

 

[kimbyd]

I don't know how to search for such a paper - I wonder if anyone has tried to model gravitational events that would remove gas from hypothetical rings of dark matter and see if they can get a result that leaves the gas both isolated and in a state that can subsequently form a galaxy that won't spin itself apart.

I don't know if anybody has tried to model this. I wasn't able to find anything obvious from my small amount of searching.

In any event, I'd be willing to bet that such situations are quite rare in practice. But a cluster collision does, at least on the surface, have two properties that make it nice for the problem:
1) The colliding gas is compressed as a result of the collision.
2) The colliding gas is separated (at least partially) from both the dark matter and stars.

But it also has a third property which will work against the hypothesis:
3) The colliding gas heats up.

Because it's heated up, the resulting gas will tend to want to expand and cool, possibly reversing the density increase that resulted from the collision and preventing the formation of a galaxy.

Regardless of what the actual event was that would have caused a dark matter-poor galaxy, I'm sure that there are a number of theorists who are going to be working on potential solutions over the next months. Ideally there will be a few different proposed solutions, followed by vigorous debate. Hopefully the debate is capable of being resolved through argument and evidence in a relatively short time frame, so that we have a clear understanding of what causes this. We'll see.

 

[JMz]

I don't know if anybody has tried to model this.

I'll bet someone does now, though. ;-)

In any event, I'd be willing to bet that such situations are quite rare in practice. But a cluster collision does, at least on the surface, have two properties that make it nice for the problem:
1) The colliding gas is compressed as a result of the collision.
2) The colliding gas is separated (at least partially) from both the dark matter and stars.

Sensible on its face, at least.

But if, instead, the galaxy already has stars, is it possible to separate the stars from the DM by dynamical friction with the stars of the galaxy it's hypothetically passing through? Stars participate in this... but it's my impression that for most models, DM does not. That's because stars interact with each other in a more point-like manner than DM, with occasional close, high-energy encounters; whereas DM interacts with both itself and stars more diffusely. (I don't mean that as an argument in favor of this scenario, merely an explanation for why the mechanism would distinguish at all between the two types of mass.)

Can we falsify this by what we already know? One drawback is that, without extra complications, this seems like a mechanism to leave the stars trapped in the other galaxy, so the outcome would be a star-light/DM-heavy residual, instead of the observed, reverse result. With the right geometry, you might be able to get the pre-collision observed galaxy's stars to go one way, its DM to go another, and the other galaxy's stars (and whatever DM) to go a third way -- also rare in practice, surely.

 

[Demystifier]

How would scientists living in the galaxy without dark matter explain that all other galaxies have dark matter?
My first guess is that they would try to make up an anthropic explanation, according to which only galaxies without dark matter support intelligent life.

 

[Orodruin]

How would scientists living in the galaxy without dark matter explain that all other galaxies have dark matter?
My first guess is that they would try to make up an antropomorphic explanation, according to which only galaxies without dark matter support intelligent life.

If their scientific history were the same as ours, they would instead be faced with explaining why their galaxy was void of dark matter. They would first observe dark matter in terms of missing mass due to galaxies moving too fast in a nearby cluster. Then by looking at rotational curves of other galaxies. Eventually they would of course try to measure the dark matter density of their own galaxy and conclude it iz zero. As you said, some would likely look for anthropic arguments while others would try to explain it in other ways.

Direct detection experiments would be ... difficult.

 

[anorlunda]

What a fun find.

This is not my field, but here's a novice question. Is there any scenario where radiation pressure could be large enough to separate ordinary matter from DM?

 

[Demystifier]

What a fun find.

This is not my field, but here's a novice question. Is there any scenario where radiation pressure could be large enough to separate ordinary matter from DM?

Note that EM radiation does not interact with DM, after all that's why DM is dark.

 

[anorlunda]

Note that EM radiation does not interact with DM, after all that's why DM is dark.

Yes, that's the origin of my novice question. Radiation pressure would push ordinary matter away but it would not interact with the DM. Therefore radiation pressure if strong enough, should tend to separate the OM from the DM.

 

[phinds]

Yes, that's the origin of my novice question. Radiation pressure would push ordinary matter away but it would not interact with the DM. Therefore radiation pressure if strong enough, should tend to separate the OM from the DM.

Yeah, but it would tend to spread it out in all directions, not move it off in a cluster.

 

[atyy]

https://doi.org/10.1038/nature25767
https://arxiv.org/abs/1803.10237 (free version)
A galaxy lacking dark matter
Pieter van Dokkum, Shany Danieli, Yotam Cohen, Allison Merritt, Aaron J. Romanowsky, Roberto Abraham, Jean Brodie, Charlie Conroy, Deborah Lokhorst, Lamiya Mowla, Ewan O'Sullivan, Jielai Zhang
(Submitted on 27 Mar 2018)
Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5x10^{10} solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5 kilometers per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4x10^8 solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.

 

[Stephenk53]

How would scientists living in the galaxy without dark matter explain that all other galaxies have dark matter?
My first guess is that they would try to make up an anthropic explanation, according to which only galaxies without dark matter support intelligent life.

I would think they would think dark matter does not exist then they find dark matter in other galaxies and assume that dark matter may limit the growth of life altogether. Frankly I am unsure of how they could detect other life at all, thus they may come to that conclusion

 

[JMz]

How would scientists living in the galaxy without dark matter explain that all other galaxies have dark matter?
My first guess is that they would try to make up an anthropic explanation, according to which only galaxies without dark matter support intelligent life.

My first guess is that they would use the particular facts that were well known in their galactic neighborhood -- say, the presence of an equivalent of M31. Galaxy collisions and the size & nature of M31 have been known here longer than DM has been known.

I admit that the difference is measured in mere decades. For another civilization, the order of learning could, I suppose, be reversed. In that case, people there might speculate about DM & intelligent life (speculations that would, of course, get banned from their version of PF ;-).

 

[fresh_42]

speculations that would, of course, get banned from their version of PF ;-).

Maybe not banned, but it appeal to the participants of this thread to stop speculations about an imaginary civilization in another galaxy. I've read the post which started this as a critic on the anthroposophical principle, which is certainly worth a discussion, but not here. It is even highly speculative with regard to the fact, that we have absolutely no idea what dark matter actually is, and less its possible cosmological interactions - except for one.

 

[kimbyd]

But if, instead, the galaxy already has stars, is it possible to separate the stars from the DM by dynamical friction with the stars of the galaxy it's hypothetically passing through?

That's difficult to do, I think. Stars experience very little dynamical friction. The effect you'd be looking for here is a galaxy moving through a cloud of gas that is so large that the extremely tiny amount of dynamical friction they do experience is enough to separate them from the dark matter. I don't have a clear handle on the magnitude of dynamical friction for stars, however, so I don't know what would be required to observe this effect. It may be utterly infeasible.

 

[JMz]

That's difficult to do, I think. Stars experience very little dynamical friction. The effect you'd be looking for here is a galaxy moving through a cloud of gas that is so large that the extremely tiny amount of dynamical friction they do experience is enough to separate them from the dark matter. I don't have a clear handle on the magnitude of dynamical friction for stars, however, so I don't know what would be required to observe this effect. It may be utterly infeasible.

Actually, I was asking about star/star interactions, in the absence of all gas. Yes, friction is small, but I don't think we've hypothesized a pre-collision size for the galaxy, so it could be a small fraction of something large.

 

[JMz]

Maybe not banned, but it appeal to the participants of this thread to stop speculations about an imaginary civilization in another galaxy. I've read the post which started this as a critic on the anthroposophical principle, which is certainly worth a discussion, but not here. It is even highly speculative with regard to the fact, that we have absolutely no idea what dark matter actually is, and less its possible cosmological interactions - except for one.

I took @Demystifier's suggestion as being tongue-in-cheek, and my comment about "their" PF was in the same spirit.

 

[kimbyd]

Actually, I was asking about star/star interactions, in the absence of all gas. Yes, friction is small, but I don't think we've hypothesized a pre-collision size for the galaxy, so it could be a small fraction of something large.

Star-star interactions have effectively zero friction. Direct collisions are far too rare to be a significant friction component. Gravitational-only interactions are something that dark matter also experiences, and won't be capable of separating stars from dark matter.

 

[JMz]

Star-star interactions have effectively zero friction. Direct collisions are far too rare to be a significant friction component. Gravitational-only interactions are something that dark matter also experiences, and won't be capable of separating stars from dark matter.

I'm referring to https://en.wikipedia.org/wiki/Dynamical_friction, a la Chandrasekhar. My intuition -- and this is really the heart of my question -- is that DM will experience far less of it than stars, due to stars being "lumpier". That is, star/star interactions will be less frequent but, when they do occur, very much larger; DM interactions with stars or with DM will be frequent but tiny, exactly as if the star were moving through gas. The former would be capable of large angular change for a small fraction of the stars, the latter would not. (Just a form of the central limit theorem, or at least the law of large numbers.)

As I think about this, I am more convinced that this mechanism can separate a small fraction of stars from DM, but less convinced that it can explain the OP.

 

[mitchell porter]

"In MOND, violation of Newton's laws occurs at extremely small accelerations" (wikipedia). Some commenters are treating this as a falsification of MOND because it's a very low mass galaxy, so the gravitational force should be in the MONDian regime.

However, in MOND, the presence of gravitational fields from neighboring massive objects can impose a Newtonian or quasi-Newtonian regime on a light object. Ironically, this is something dark matter theorists want too, in order to explain the lack of dark matter in NGC1052-DF2 - "the larger gravitational field from adjacent galaxies could have pulled dark matter away from it" (Quanta).

 

[JMz]

"In MOND, violation of Newton's laws occurs at extremely small accelerations" (wikipedia). Some comenters are treating this as a falsification of MOND because it's a very low mass galaxy, so the gravitational force should be in the MONDian regime.

However, in MOND, the presence of gravitational fields from neighboring massive objects can impose a Newtonian or quasi-Newtonian regime on a light object. Ironically, this is something dark matter theorists want too, in order to explain the lack of dark matter in NGC1052-DF2 - "the larger gravitational field from adjacent galaxies could have pulled dark matter away from it" (Quanta).

I did not follow the point about MOND. As for the Quanta quote, that seems to be an error in reasoning, right?

 

[Grinkle]

To whoever may know - is there any evidence that dark matter has the same G (gravitational constant) as visible matter? I think the reasoning in this thread all assumes that it does, I'm wondering if that is a default assumption or if there some way to draw that conclusion from cosmological observations.

I am admittedly over my head in asking this question - I intend to be asking if the ratio of inertia to gravitational attraction for dark matter is the same as for visible matter, or if we have any evidence to say one way or the other. I think we just assume that it does, and that drives our calculation of halo's etc of dark matter.

 

[phinds]

To whoever may know - is there any evidence that dark matter has the same G (gravitational constant) as visible matter? I think the reasoning in this thread all assumes that it does, I'm wondering if that is a default assumption or if there some way to draw that conclusion from cosmological observations.

I am admittedly over my head in asking this question - I intend to be asking if the ratio of inertia to gravitational attraction for dark matter is the same as for visible matter, or if we have any evidence to say one way or the other. I think we just assume that it does, and that drives our calculation of halo's etc of dark matter.

It would make no sense at all to believe that gravity acts one way for normal matter and another way for dark matter.

 

[Keith_McClary]

only galaxies without dark matter support intelligent life.

They could be right .

 

[Keith_McClary]

Interesting, we just had the LIGO detection of neutron star merger with gamma burst, which eliminated some alternate gravity theories because they predicted different travel times for light and GW. This new observation may eliminate more.

 

[kimbyd]

I did not follow the point about MOND. As for the Quanta quote, that seems to be an error in reasoning, right?

I don't buy for an instant that MOND can explain the variation in observed dark matter between different galaxies. At least not in anything approaching a reasonable manner (that is, no parameters that are tuned per-galaxy). MOND is basically dead now anyway. Has been for a long time.

I agree that the Quanta note is just incorrect. Pure gravitational attraction would pull normal matter just as much as it pulls dark matter, so it won't separate them. The only possible way to separate normal matter and dark matter would be through friction which the dark matter doesn't experience, but the normal matter does. It would be easiest to separate normal matter and dark matter while the normal matter is a diffuse gas, but then the diffuse gas will have a harder time collapsing into stars due to the lack of dark matter.

 

[JMz]

Quite so. A fascinating conundrum.

 

[JMz]

asking if the ratio of inertia to gravitational attraction for dark matter is the same as for visible matter, or if we have any evidence to say one way or the other. I think we just assume that it does, and that drives our calculation of halo's etc of dark matter.

Questioning that ratio is the appropriate way to ask about this assumption.

At the moment, the equality [more precisely, the proportionality] of the two is assumed, because people have looked in many ways for discrepancies and failed to find them -- in ordinary matter. Moreover, Einstein "baked it in" when he developed GR, and, from what we can tell, that's the one domain in which DM behaves understandably.

So at a minimum, we would need to posit a specific alternative that has some very special properties. That's not an attractive choice at the moment: Creative thinking is probably best directed elsewhere. There are several deep principles of physics, they are deep for a reason, and overthrowing anyone of them is a recipe for perhaps decades of development that, most likely, will NOT yield a successful result. (OTOH, if it did, there would be several Nobel prizes along the way.)

 

[mfb]

Direct detection experiments would be ... difficult.

Not that they would be easy here...

To whoever may know - is there any evidence that dark matter has the same G (gravitational constant) as visible matter? I think the reasoning in this thread all assumes that it does, I'm wondering if that is a default assumption or if there some way to draw that conclusion from cosmological observations.

I am admittedly over my head in asking this question - I intend to be asking if the ratio of inertia to gravitational attraction for dark matter is the same as for visible matter, or if we have any evidence to say one way or the other. I think we just assume that it does, and that drives our calculation of halo's etc of dark matter.

Which G would you use for the attraction between dark matter and regular matter?
No, you can't make a reasonable theory out of that. Especially as normally regular matter and dark matter stay together.

 

[Jonathan Scott]

I don't buy for an instant that MOND can explain the variation in observed dark matter between different galaxies. At least not in anything approaching a reasonable manner (that is, no parameters that are tuned per-galaxy). MOND is basically dead now anyway. Has been for a long time.

This seems to be the wrong way round. MOND in its original form has only a single universal acceleration parameter which applies to all galaxies and is amazingly successful in explaining or predicting individual galaxy rotation curves with no additional parameters, whereas in contrast different galaxies seem to require distinctly different distributions of dark matter to explain their rotation curves, so this is the area where MOND excels. MOND however is extremely unsatisfactory as a "theory" as it violates basic principles such as conservation of momentum, and has difficulty explaining motion above the scale of individual galaxies. There are more sophisticated modified gravity theories which approximate MOND but they have far more parameters and seem quite arbitrary especially compared with the neatness of General Relativity.

 

[PAllen]

This seems to be the wrong way round. MOND in its original form has only a single universal acceleration parameter which applies to all galaxies and is amazingly successful in explaining or predicting individual galaxy rotation curves with no additional parameters, whereas in contrast different galaxies seem to require distinctly different distributions of dark matter to explain their rotation curves, so this is the area where MOND excels. MOND however is extremely unsatisfactory as a "theory" as it violates basic principles such as conservation of momentum, and has difficulty explaining motion above the scale of individual galaxies. There are more sophisticated modified gravity theories which approximate MOND but they have far more parameters and seem quite arbitrary especially compared with the neatness of General Relativity.

I think the issue for this galaxy with MOND is that it simply fails to explain the rotation curve for this galaxy. This galaxy is different from most so you can choose:

1) To rescue MOND, assume there is an unknown counter effect for this galaxy. Since MOND is a gravity law, changing the law for one galaxy doesn’t make sense, so you are left with ... repulsive dark matter ?? that so far exists for only one known galaxy??

2) To rescue dark matter models, just assume little or no dark matter for this galaxy, leaving the problem of how the separation might have occurred. Such separation would be expected to be rare, consistent with observation.

To me, this galaxy finding clearly works against MOND due to implausibility of what is needed to explain this galaxy.

 

[Jonathan Scott]

I think the issue for this galaxy with MOND is that it simply fails to explain the rotation curve for this galaxy. This galaxy is different from most so you can choose:

1) To rescue MOND, assume there is an unknown counter effect for this galaxy. Since MOND is a gravity law, changing the law for one galaxy doesn’t make sense, so you are left with ... repulsive dark matter ?? that so far exists for only one known galaxy??

2) To rescue dark matter models, just assume little or no dark matter for this galaxy, leaving the problem of how the separation might have occurred. Such separation would be expected to be rare, consistent with observation.

To me, this galaxy finding clearly works against MOND due to implausibility of what is needed to explain this galaxy.

I agree that if the interpretation of the observations is correct in this case, this particular galaxy leads to something like the above options. There are probably other possible explanations too, perhaps about a very unusual line of sight giving misleading results.
But the curious success of MOND in the vast majority of cases suggests that something systematic that we don't understand is going on to make the results fit the MOND pattern, even if it somehow involves dark matter.
And my main point was simply that MOND doesn't need extra parameters to match different dark matter distributions for different galaxies.

 

[kimbyd]

This seems to be the wrong way round. MOND in its original form has only a single universal acceleration parameter which applies to all galaxies and is amazingly successful in explaining or predicting individual galaxy rotation curves with no additional parameters, whereas in contrast different galaxies seem to require distinctly different distributions of dark matter to explain their rotation curves, so this is the area where MOND excels. MOND however is extremely unsatisfactory as a "theory" as it violates basic principles such as conservation of momentum, and has difficulty explaining motion above the scale of individual galaxies. There are more sophisticated modified gravity theories which approximate MOND but they have far more parameters and seem quite arbitrary especially compared with the neatness of General Relativity.

I have a hard time believing that MOND can accurately describe the rotation curves of this galaxy. My understanding is that it has had problems with the diversity of rotation curves in visible galaxies ever since we started measuring a large number of them in detail. And it's never satisfactorily explained the behavior of galaxy clusters.

 

[nikkkom]

A medium-velocity galaxy collision with particularly suitable geometry might do it. Say, two spiral galaxies colliding edge-on would leave most of their gas and dust piled up at the site of the collision, while DM and stars would pass through and fly away.

 

[phinds]

A medium-velocity galaxy collision with particularly suitable geometry might do it. Say, two spiral galaxies colliding edge-on would leave most of their gas and dust piled up at the site of the collision, while DM and stars would pass through and fly away.

But would they not remain in the neighborhood and swirl back and collide again? You seem to imply that they would not remain gravitationally bound. Seems unlikely

 

[nikkkom]

But would they not remain in the neighborhood and swirl back and collide again? You seem to imply that they would not remain gravitationally bound. Seems unlikely

Obviously, depends on the velocity of the collision.

 

[JMz]

To one point there:

gravitational properties of antimatter, specifically if antimatter is repelled gravitationally by matter.

This will contradict GR, won't it? That is, positrons (for an example of antimatter) are as much concentrations of energy as electrons, and GR would therefore treat them identically. And, of course, both have the same momentum per unit velocity, so even the gravitational-mass/inertial-mass ratio would be different. So we would even need to give up the Equivalence Principle. Right?

No problem, if CERN shows it's truly necessary. But that's a very high hurdle.

 

[PeterDonis]

So called External Field Effects (EFE) can come to the rescue of MOND in the case of galaxy clusters.

MOND fits all galactic systems perfectly with its one universal parameter a0.

Please give references (textbooks or peer-reviewed papers) for these statements.

 

[Vanadium 50]

Please give references (textbooks or peer-reviewed papers) for these statements.

The 2nd one is: McGaugh et al. Phys. Rev. Lett. 117, 201101 (2016)

The first one will be harder to find that exact thing, but it certainly stands to reason: MOND's mechanics assumes that what matters is the total force on the object, not just the force from the galaxy of interest. In that regard it is identical to Newton and Einstein.

 

[Olorin]

Please give references (textbooks or peer-reviewed papers) for these statements.

Regarding the EFE here are two links for explanations: http://astroweb.case.edu/ssm/mond/EFE.html and http://astroweb.case.edu/ssm/mond/milgromonefe.html.
For peer rewiewed papers there are a few actually, just putting one here, but please, feel free to do your own research on the subject: https://arxiv.org/pdf/1404.2202.pdf.

 

[JMz]

Yes, that's correct. I guess it is fair to assume that GR won't survive a direct experimental violation of the weak equivalence principle. If antimatter falls up, that's the end of "space-time geometry" as a valid theory of gravity. My gut guess is that the quantum vacuum as a gravitational and electric dipolar medium is a much more profound and sound starting point to rethink the way gravity works. It actually naturally allows sweet coupling effects between electromagnetic and gravitational phenomena!
Not necessarily if that explains a lot of other things we fail to grasp while willing to keep GR as a viable theory of gravity at all costs, i.e. dark matter, dark energy, inflation, black hole and big bang singularities, information paradox etc...if breaking the weak equivalence principle has the power to explain all of it, which it seems to do when you delve into the consequences of anti-gravitational antimatter, so be it. But we must not wait till CERN results are published to develop the full consequences of the theory, which can have rather large implications for our understanding of the universe. Mark my words: I bet that GR won't survive the next decade of observational and experimental evidence, and depending on the cunning and openness of our best minds, we might have a new and better theory of gravity by then.

Fair enough. My own bet, though, would be that, if GR doesn't "survive" the next decade, it will only be because something came along that fully agrees except where quantum effects become important: more properly an extension of GR than a contradiction of it. That's not entirely a foundational statement (i.e., that it embodies all the correct non-quantum insights, such as special relativity), but partly an expectation that, if anything else about it is amiss, we won't have the right equipment or do the right experiments to recognize it until long after that. Of course, I recognize that people are willing to live with some current problems with GR partly because there isn't an alternative that both agrees better with experiments and has foundations that are at least as simple and appealing.

 

[kimbyd]

The 2nd one is: McGaugh et al. Phys. Rev. Lett. 117, 201101 (2016)

The first one will be harder to find that exact thing, but it certainly stands to reason: MOND's mechanics assumes that what matters is the total force on the object, not just the force from the galaxy of interest. In that regard it is identical to Newton and Einstein.

Arxiv link to that article:
https://arxiv.org/abs/1609.05917

Note that in a response, these authors argue that the relation described above is not something new, but rather a function of the well-known baryonic Tully-Fisher relation:
https://arxiv.org/abs/1803.01849

As for MOND explaining these galaxies "perfectly", that's a matter open to interpretation. There's substantial scatter.

Regardless, MOND still fails to explain galaxy cluster behavior.

 

[Vanadium 50]

As for MOND explaining these galaxies "perfectly", that's a matter open to interpretation. There's substantial scatter.

I would say that scatter is no better and no worse than many other astronomical measurements, e.g. SNe as standard candles.

Regardless, MOND still fails to explain galaxy cluster behavior.

Agreed. MOND works on galactic scales and nowhere else. I believe that when the dust settles, the outcome will be MOND tells us little about gravity and more about galaxy formation.