Merged Dark Matter vs. Modified Newtonian Dynamics

[JeanTate]

Yes, it could.

And I think you can find the odd paper or two, in arXiv's gr-qc section (General Relativity and Quantum Cosmology), which explore some such ideas. Here, for example, is a recent one (perhaps not directly what you were thinking of): "Gravitational Waves as a New Probe of Bose-Einstein Condensate Dark Matter" (arXiv:1609.03939)

Myself, I'd rate them extremely speculative ...

I should have added that not all papers in that section of arXiv are speculative, far from it.

An excellent recent paper is "General Relativity and Cosmology: Unsolved Questions and Future Directions", by Debono&Smoot (arXiv:1609.09781, link is to abstract). It'll be heavy going in parts for many readers (though PS and W.D.Clinger may find it delightful), but I think it gives a good perspective on GR and why it's such an amazing theory.

 

[JeanTate]

You guys gave me a lot to think about and read up on. Thanks for the replies!

You're very welcome!

This recent paper, by McGaugh, Lelli, and Schombert, may be of interest to you: "The Radial Acceleration Relation in Rotationally Supported Galaxies" (arXiv:1609.05917, link is to abstract). In particular, this, from the Conclusion:

Possible interpretations for the radial acceleration relation fall into three broad categories.

1. It represents the end product of galaxy formation.
2. It represents new dark sector physics that leads to the observed coupling.
3. It is the result of new dynamical laws rather than dark matter.

None of these options are entirely satisfactory.

Holler if you'd like me to try to unpack this ...

 

[Darwin123]

Dark matter continues to evade detection. I've read a few articles lately suggesting theories like Modified Newtonian Dynamics are being taken more seriously. Anyone working in the field have any comments on where things stand/are headed?

The interaction of colliding galaxies supposedly provides evidence against the modified gravity hypothesis. When galaxies collide, the dark matter seems to separate from the light matter. Gravitational lensing shows astronomers that the dark matter separated. Read folowing links.

https://en.wikipedia.org/wiki/Bullet_Cluster
‘The Bullet Cluster (1E 0657-558) consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a co-moving radial distance of 1.141 Gpc (3.7 billion light-years).’

http://phys.org/news/2015-03-galaxy-clusters-collidedark-mystery.html
When galaxy clusters collide, their dark matters pass through each other, with very little interaction. Deepening the mystery, a study by scientists at EPFL and the University of Edinburgh challenges the idea that dark matter is composed of particles.

There is still room for doubt. Read the following link.

http://www.space.com/29115-dark-matter-interactions-galaxy-collisions.html
‘Researchers looking at four colliding galaxies discovered a*discovered a clump of*dark matter apparently**moving more slowly than its host galaxy. This slowing-down would be expected if dark matter was interacting with separate clumps of dark matter through forces other than gravity.’

The article above makes me think this is one of those situations where both ‘conflicting’ hypotheses may be necessary to explain the data. Or maybe they don’t understand the electrodynamic interactions between light matter particles well enough.

The following is an abstract. The rest of the article is behind a paywall. However, I think the abstract by itself provides a reasonably clear answer to ‘how things currently stand.



http://science.sciencemag.org/content/347/6229/1462
‘Collisions between galaxy clusters provide a test of the nongravitational forces acting on dark matter. Dark matter’s lack of deceleration in the “bullet cluster” collision constrained its self-interaction cross section σDM/m < 1.25 square centimeters per gram (cm2/g) [68% confidence limit (CL)] (σDM, self-interaction cross section; m, unit mass of dark matter) for long-ranged forces. Using the Chandra and Hubble Space Telescopes, we have now observed 72 collisions, including both major and minor mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6σ significance. The position of the dark mass has remained closely aligned within 5.8 ± 8.2 kiloparsecs of associated stars, implying a self-interaction cross section σDM/m < 0.47 cm2/g (95% CL) and disfavoring some proposed extensions to the standard model.’

 

[JeanTate]

What if dark matter is not a particle? The second wind of modified gravity. is the title of Bee's most recent BackReAction blog post.

Very pertinent to the discussion in this thread (and quite a few of the comments are good too).

 

[lpetrich]

MOND is Modified Newtonian dynamics^WP. Getting it work theoretically has been difficult.

Jacob Bekenstein and Mordehai Milgrom have proposed AQUAL^WP (A Quadratic Lagrangian), a modification of the Lagrangian formulation of Newtonian gravity.

L(Newt) = L(potential) + ρ*V + L(matter)
ρ = mass density, V = gravitational potential

where L(potential) = (1/(8*pi*G)) * |D(V)|²G = gravitational constant, D(V) = gradient of potential: acceleration of gravity = - D(V)

The AQUAL version of L(potential) is (1/(8*pi*G)) * F(|D(V)|²)

where F is a new function. For Poisson's equation, D²(V) = 4*pi*G*ρ it gives

D(F'(|D(V)|²)*D(V)) = 4*pi*G*ρ

In the Newtonian limit, F(x) = x, while for making MOND effects, F(x) ~ x^1/2 when x is small. That is theoretically rather ugly.

TeVeS, Jacob Bekenstein's Tensor–vector–scalar gravity^WP, is even worse. General relativity's Einstein-Hilbert Lagrangian can be given very succinctly:

(1/(16*pi*G))*R*sqrt(-g)
R = Ricci curvature scalar, g = space-time metric

At the same level of abstraction, the TeVeS Lagrangian is a nightmare that I don't wish to repeat here.

 

[JeanTate]

You're very welcome!

This recent paper, by McGaugh, Lelli, and Schombert, may be of interest to you: "The Radial Acceleration Relation in Rotationally Supported Galaxies" (arXiv:1609.05917, link is to abstract). In particular, this, from the Conclusion:

Possible interpretations for the radial acceleration relation fall into three broad categories.

1. It represents the end product of galaxy formation.
2. It represents new dark sector physics that leads to the observed coupling.
3. It is the result of new dynamical laws rather than dark matter.

None of these options are entirely satisfactory.

Holler if you'd like me to try to unpack this ...

Well, that didn't take long!

Check out "La Fin du MOND? Λ CDM is Fully Consistent with SPARC Acceleration Law" (arXiv:1610.06183, link is to abstract):

Recent analysis (McGaugh et al. 2016) of the SPARC galaxy sample found a surprisingly tight relation between the radial acceleration inferred from the rotation curves, and the acceleration due to the baryonic components of the disc. It has been suggested that this relation may be evidence for new physics, beyond {\Lambda}CDM . In this letter we show that the 18 galaxies from the MUGS2 match the SPARC acceleration relation. These cosmological simulations of star forming, rotationally supported discs were simulated with a WMAP3 {\Lambda}CDM cosmology, and match the SPARC acceleration relation with less scatter than the observational data. These results show that this acceleration law is a consequence of dissipative collapse of baryons, rather than being evidence for exotic dark-sector physics or new dynamical laws.

(my bold)

Looks like option 1, or some variant of it, might work ...

 

[Reality Check]

Starts with a Bang has a good description of he new discovery and fast response: Physicists Beat Yet Another Challenge To Dark Matter's Existence
Older computer simulations of galaxy formation with dark matter cannot match the discovery that the rotational properties of galaxies appears to depend only on the normal, observable matter within them. However modern computer simulations include a lot more physics, e.g. the effects of the existence and formation of massive stars and black holes on star formation and dark matter density. Ben Keller and James Wadsley ran a computer simulation for a sample of 18 galaxies and matched the relationship. However not for the entire range of galaxy masses in the McGaugh, Lelli and Schombert paper so despite the blogs language this is not yet a complete victory for dark matter as particles. What is needed is running an ensemble of simulations on a larger number and mass range of galaxies.

The response of Stacy McGaugh, one of the authors of the first paper: La Fin de Quoi? which has a couple of comments from Ben Keller.

 

[JeanTate]

Several preprints directly relevant to this topic have appeared on arXiv in the last few weeks (actually, more like days).

Bee, in her blog Backreaction, has a new post up today, "Modified Gravity vs Particle Dark Matter. The Plot Thickens." It's a good discussion, I think. And some of the comments are excellent too.

If you're interested in some of the recent preprints, holler and I'll try to compile a list ...

 

[MaartenVergu]

A new posted article this night 2 am (European time): Dark matter is an illusion. About 'emergent gravity' by Erik Verlinde (Dutch physicist):
https://arxiv.org/pdf/1611.02269v1.pdf

 

[MaartenVergu]

Physicist Erik Verlinde seems to talk about 'emergent gravity', but I think he means what I call 'relativistic gravity'. Dark matter and dark energy is actually both relativistic/emergent gravitaty.

And I hope physicists will say one day: the observer is a fundamental constant which determines all these emergent observations of gravity.

(an observer= a measuring device, a detector, a scanner, a human eye, an animal eye...)

 

[MaartenVergu]

double

 

[steenkh]

Physicist Erik Verlinde seems to talk about 'emergent gravity', but I think he means what I call 'relativistic gravity'.

I am sure he is not thinking about your "relativistic" gravity. He is trying to unify quantum physics and spacetime. Gravity and dark matter emerges from this. There nothing about observers.

 

[MaartenVergu]

I must honestly say that I do not have the mathematical skills to understand his theory. I wait until I hear his conclusions and the philosophical implications. (in words).

He is talking about 'information theory' and 'the holographic universe'. Everything is information, as far as I understand him.

But 'information' is platonic. It has no physical substance.
And you need observers to have space-time-coordinates for this information.

 

[Reality Check]

Physicist Erik Verlinde seems to talk about 'emergent gravity', but I think he means what I call 'relativistic gravity'.

'Emergent gravity' or entropic gravity is science. It is theoretical physics and currently speculation because there are no predictions that make it different from existing theories. Entropic gravity is basically an interesting, alternative way to derive GR .

This nothing to do with a vague fantasy of 'relativistic gravity', Maartenn100 or ignorance about observers. Or your later fantasy of "philosophical implications".

Emergent Gravity and the Dark Universe is nice theoretical physics. The central result is on page 38

[Equation 7.40]
This is the main formula and central result of our paper, since it allows one to make a direct comparison with observations. It describes the amount of apparent dark matter M_D(r) in terms of the amount of baryonic matter M_B(r) for (approximately) spherically symmetric and isolated astronomical systems in non-dynamical situations.

Explaining the success of MOND in reproducing galaxy rotation curves by assuming M_B(r) is constant is probably a bad approximation. Astronomers do not do this when calculating galaxy rotation curves since spiral galaxies are not spherically symmetric.

The bottom of page 39 goes onto galaxies in galaxy clusters with "These discrepancies can be significantly reduced and perhaps completely explained away in our theoretical description". The best that he can do is give a range of values of a parameter that has an upper limit that may account for the dark matter needed in galaxy clusters.

There is a strange comment in the Discussion and Outlook section on page 42: "The real reason why most physicists believe in the existence of particle dark matter is not the observations, but because there was no theoretical evidence nor a conceptual argument for the breakdown of these laws at the scales where the new phenomena are being observed".
The real reason "why most physicists believe in the existence of particle dark matter" is observations that cannot be explained otherwise (yet). The observations of colliding galaxy clusters starting from 2005 where colliding gas particles separate into normal matter and something else are convincing evidence for dark matter as particles.

 

[MaartenVergu]

First test of rival to Einstein’s gravity kills off dark matter (New Scientist)

 

[steenkh]

First test of rival to Einstein’s gravity kills off dark matter (New Scientist)

Splendid! But there is still a long way to go ...

MOND is out of favour because although it does very well with some tests, it does not pass a number of other tests. Verlinde's model could have the same problem.

Einstein's model + dark matter passes more tests, so that one will still be the favourite until more data comes in.

 

[GodMark2]

“The dark matter model actually fits slightly better with the data than Verlinde’s prediction,” says Brouwer. “But then if you mathematically factor in the fact that Verlinde’s prediction doesn’t have any free parameters, whereas the dark matter prediction does, then you find Verlinde’s model is actually performing slightly better.”

In other words: It's objectively worse at making describing the observations, but it's more 'elegant', so he counts it as better anyway. Also, it only accounts for one of the types of observation (Extreme distance galaxy magnification), but not as well as Dark matter; Wake me when it comes close to explaining the bullet cluster observations.

 

[Roboramma]

One of the things to remember regarding dark matter is that given that we know the standard model is incomplete we pretty much need some particles that haven't yet been discovered. And surprise, observations of gravitation acting on large scales suggest that some undiscovered particles exist.

While we might or might not expect so much dark matter, none would actually be hard to understand from a particle physics perspective.

 

[Samson]

Is dark matter redundant?

http://phys.org/news/2016-12-verlinde-theory-gravity.html

Verlinde now claims that he not only explains the mechanism behind gravity with his alternative to Einstein's theory, but also the origin of the mysterious extra gravity, which astronomers currently attribute to dark matter.


I don't like dark matter so will prefer anything, but scientists do build dark matter detecting instruments, which should have a serious enterprise underpinning the funding.
This subject bugs me, and is way beyond my pay grade.
Any help here would be greatly appreciated.

 

[Aepervius]

Betteridge law of headline : No.