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

[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.

 

[kimbyd]

Well it falls to Occam's razor. The best theory is the one that predicts the most with minimum amount of assumptions. Modified Gravity don't give a toss about this effects in order to produce the Tully Fischer relation, as gravitational effects are always tied to visible mass. DM is akin to epycyclic models, always in need of more ad-hoc and unecessary tweaking in order to even be able to start to make a rigorous sense of it all.

Except feedback effects are not assumptions. They are a part of reality whether we include them in our models or not. Failing to include them doesn't mean that you've got a simpler theory: it means you're ignoring pieces of the puzzle.

The right way to deal with this is not to say, "Well, MOND doesn't need this!" but rather, "Time to work out the consequences of this feedback effect in the MOND model, to make sure it makes sense."

 

[mfb]

We know antimatter falls down.

Atoms are about 0.03% electrons, 1% quark masses and 99% QCD binding energy, the exact fractions depend on the atom. We know from comparisons of countless atoms that all atoms fall down at the same rate, this is only possible if all three components satisfy the weak equivalent principle.

Anti-atoms are about 0.03% positrons, 1% antiquark masses and 99% QCD binding energy. We already know the last part satisfies the weak equivalence principle. For antimatter to fall up, the other components would have to do something completely crazy, and no matter what they do different antiatoms would fall up at different rates. While this is not yet ruled out by experiment, it doesn't sound plausible at all.

To make antimatter fall up you would have to assign a "matterness" to binding energies. Not just QCD, but also QED which I neglected above. And that doesn't sound plausible either. Which fraction of the QCD binding energy in a pentaquark is matter? ;)

 

[kimbyd]

We know antimatter falls down.

This has yet to be experimentally demonstrated, though it is very true that there would be some pretty extreme theoretical challenges with explaining how anti-matter could possibly interact with gravity differently from normal matter while normal matter still obeys the equivalence principle.

 

[JMz]

The idea that antimatter should fall up seems like an idea based on nothing more than etymology. We call certain particles "anti" because people noted that they have opposite charge, charm, or whatever compared to particles we commonly encounter.

That doesn't prove that they don't have anti-mass as well, but the idea that they do (a) starts with much less plausibility, because they don't even behave that way for momentum, and (b) as @Jonathan Scott notes, photons, which are their own "anti"-particles, are observed to fall down in gravitational lenses.

 

[jerromyjon]

If for instance antimatter falls up consistently in the 3 CERN experiments, then GR is DEAD

What if antimatter and matter are more strongly attracted than either are to themselves? I only ask this naive question because my basic understanding is that matter and antimatter always seem to attract and annihilate so the premise in my mind is that there is something MORE powerful than typical physics involved.

 

[kimbyd]

The idea that antimatter should fall up seems like an idea based on nothing more than etymology. We call certain particles "anti" because people noted that they have opposite charge, charm, or whatever compared to particles we commonly encounter.

I honestly agree. It's highly speculative, and wouldn't match with existing theory in a wide variety of ways. However, it still has some potential value for two reasons:
1) It is a testable prediction. As I understand it, the current best method involves creating neutral anti-Hydrogen and using a photon trap to cool it and then observe its motion. This was attempted a few years ago, but the error bars were still too large (https://www.nature.com/articles/ncomms2787). A refinement of that experiment may produce a definitive answer (I believe they're currently trying for a 1% measurement error on the intertial properties of anti-Hydrogen).
2) However theoretically absurd it may be, there is always some possibility that our theories are incorrect, and so this kind of experiment is worth doing. Any discrepancy in the behavior of anti-matter compared to normal matter would provide tremendously valuable insights into how to improve our current theories.

 

[JMz]

If they are more strongly attracted to each other through electric charge, sure. But that's exactly how they are "anti's" (with respect to each other), and there's no new physics there: Opposite electric charges attract, always.

Being differently gravitationally attracted is the relevant concern here: If it were observed, then GR would be qualitatively wrong. Contrapositively, if GR, or anything like it, is right, then that doesn't happen. (Most of us would bet on the latter.)

 

[JMz]

I honestly agree. It's highly speculative, and wouldn't match with existing theory in a wide variety of ways. However, it still has some potential value for two reasons:
1) It is a testable prediction. As I understand it, the current best method involves creating neutral anti-Hydrogen and using a photon trap to cool it and then observe its motion. This was attempted a few years ago, but the error bars were still too large (https://www.nature.com/articles/ncomms2787). A refinement of that experiment may produce a definitive answer (I believe they're currently trying for a 1% measurement error on the intertial properties of anti-Hydrogen).
2) However theoretically absurd it may be, there is always some possibility that our theories are incorrect, and so this kind of experiment is worth doing. Any discrepancy in the behavior of anti-matter compared to normal matter would provide tremendously valuable insights into how to improve our current theories.

I would not question the value of running the experiments. However, I might make the same remark about measuring the gravitational attraction of a mountain range: a reassuring demonstration, and one that might teach us much about how to conduct experiments of that nature, teaching that might be valuable in other experiments. But this is as nearly settled science as we can expect, not a major uncertainty that keeps people from knowing how to make further progress in gravitation.

The odds are overwhelmingly against this observation -- it seems to me -- but that's not a reason not to look.

 

[kimbyd]

I would not question the value of running the experiments. However, I might make the same remark about measuring the gravitational attraction of a mountain range: a reassuring demonstration, and one that might teach us much about how to conduct experiments of that nature, teaching that might be valuable in other experiments. But this is as nearly settled science as we can expect, not a major uncertainty that keeps people from knowing how to make further progress in gravitation.

The odds are overwhelmingly against this observation -- it seems to me -- but that's not a reason not to look.

Right, if there's a discrepancy at all it will most likely not be of the simple form "anti-matter falls up". That's why it's so valuable that they're trying to get 1% measurement error on the equivalence principle applied to anti-Hydrogen: even a small deviation from equivalence would be a truly dramatic finding.

 

[jerromyjon]

But that's exactly how they are "anti's" (with respect to each other), and there's no new physics there: Opposite electric charges attract, always.

Yes, I understand that. Electrons are repelled from protons and positrons are repelled from antiprotons. But when we are referring to say hydrogen and antihydrogen, there would be no net charge imbalance and therefore no attraction or repulsion due to charge. So what would make all the pieces annihilate unless electrons only attract positrons and not repel antiprotons? Or is there a sequence where fermions and bosons interact in succession?