Is Dark Matter Really a Cold, Collisionless Fluid?

[ohwilleke]

I didn't see the Tully-Fisher relationship as similar to a galaxy rotation curve as you seem to be implying. Of course that is the underlying problem that both dark matter and MOND are trying to solve. As I am not a supporter of MOND, it is always with care that I reference Stacy McGaugh but non the less his paper from 2016 entitled ‘The Radial Acceleration Relation in Rotationally Supported Galaxies’ is an observational paper and as such can be read independently of Stacy McGaugh's preferred solution of MOND. I understood the observations to show a tight relation between baryonic mass and rotational velocity. It is surely meant to question the validity of dark matter as in order for dark matter theories to be compatible with these observations one has to invoke complex 'feed back' mechanisms. To my way of thinking any theory that can explain galaxy rotation velocities without dark matter will be compatible with these observations.

This is true. But, general relativity without dark matter is inconsistent with the Tully-Fischer relationship.

would you not agree that within the theoretical framework of SM a field requires a boson (I don't know if a future theory of quantum gravity can dispense with this requirement) but look how desperate the physics community was in its search for the Higg's boson.

All quantum gravity theories except the loop quantum gravity class of theories (e.g. causal dynamic triangles) and entropic theories, require a bosonic graviton at a minimum. LQG like GR focuses on space-time as the mechanism in lieu of gravitons. Entropic theories, IIRC, rely in part on quantum entanglement as a mechanism.

However, returning to an earlier post in this thread, you alluded to being a supporter of a version of MG. I don't know if you are prepared to say which and why if it is not too off topic for this thread.

I think that A. Deur's efforts to explain dark matter phenomena and dark energy are the closest to the truth (whether or not they are perfectly correct). His insights as a primarily QCD physicists inform his analysis in ways that other MG and GR researchers don't benefit from that are likely to be valid, he is quantum from the bottom up, his approach of starting from the non-abelian self-interacting scalar graviton static case and generalizing from that case is a smart one, and his back of napkin analysis suggests that he is in the right ball back to be explaining all of the dark matter and dark energy phenomena (solar system, galaxies, galaxy clusters, coincidence problem, dark energy, other cosmology issues) with no non-SM particles except a massless self-interacting graviton, but without a separate scalar or vector field. He made a successful prediction regarding apparent dark matter halo size in elliptical galaxies unique to his theory. It actually has one less free experimentally determined parameter than GR as it doesn't have a cosmological constant, in principle at least (he hasn't derived some constants that could be calculated from first principles). The fact that is provides a mechanism from which GR and MOND phenomena arise that is consistent with GR to the same extent of any vanilla attempt to do a quantum gravity theory that replicates GR, except one subtle issue regarding how self-interaction terms in non-spherically symmetric systems are treated, is big too

I didn't want to fixate too much on a particular theory that is not widely accepted and get into a debate over that particular theory, however. There are links summing up the key papers and a good powerpoint explanation above. None of the papers are terribly challenging for a physics grad student although the powerpoint presentation provides good motivation and context.

 

[kimbyd]

Also potentially relevant to this thread: the bTFR apparently evolved over time:
http://iopscience.iop.org/article/10.3847/1538-4357/aa7558/meta

They measure a decrease in the bTFR from z=2.3 to z=0.9. I'm not sure that it's possible for such an evolution in the bTFR in a modified gravity model.

 

[ohwilleke]

Thanks for the paper. Good catch.

FWIW, the statistical significance of the result is not great: 2.65 sigma tops, and that is before their final error estimate which they never neatly sum up in relation to their hypothesis. They also note a host of other studies that have attempted to make the same determination and come up with wildly different answers from each other than this study.

 

[kimbyd]

Thanks for the paper. Good catch.

FWIW, the statistical significance of the result is not great: 2.65 sigma tops, and that is before their final error estimate which they never neatly sum up in relation to their hypothesis. They also note a host of other studies that have attempted to make the same determination and come up with wildly different answers from each other than this study.

Which is fine. More data will help with that.

 

[Adrian59]

3. The simplest non-trivial function of R is A+BRA+BRA + BR, with A and B being constants (by convention, A and B are often expressed in terms of ΛΛ\Lambda, GGG, and ccc).

I agree with your equation and even if the variables are tensors, the underlying equation looks linear. Also, I can appreciate the difficulty with baryons in that one is dealing with potentially all four fundamental forces and not just the one.

 

[Adrian59]

I think that A. Deur's efforts to explain dark matter phenomena and dark energy are the closest to the truth (whether or not they are perfectly correct). His insights as a primarily QCD physicists inform his analysis in ways that other MG and GR researchers don't benefit from that are likely to be valid, he is quantum from the bottom up,

I'll have a look at your suggestion. Incidentally, I have updated my table in the light of points raised in this thread though my initial motivation for producing the summary table was to challenge the view that dark matter was an almost complete theory. There are several major weaknesses in the dark matter paradigm which makes consideration of the alternatives completely necessary.

 

[George Jones]

IAs I am not a supporter of MOND, it is always with care that I reference Stacy McGaugh but non the less his paper from 2016 entitled ‘The Radial Acceleration Relation in Rotationally Supported Galaxies’ is an observational paper and as such can be read independently of Stacy McGaugh's preferred solution of MOND. I understood the observations to show a tight relation between baryonic mass and rotational velocity. It is surely meant to question the validity of dark matter as in order for dark matter theories to be compatible with these observations one has to invoke complex 'feed back' mechanisms. To my way of thinking any theory that can explain galaxy rotation velocities without dark matter will be compatible with these observations.

The feedback mechanisms are either there or they aren't. They shouldn't be dependent upon any unknown physics. Every aspect of these feedbacks should be measurable through a combination of observations and simulations. The only aspect of the feedbacks that relies upon unknown physics is the degree to which dark matter interacts (both with itself and with normal matter). Everything else just depends upon understanding in detail the normal matter within galaxies.

Research on feedeback reported in a paper newly submitted to the Monthly Notices of the Royal Astronomical Society has generated some interest,
https://arxiv.org/abs/1808.06634

The authors claim that dark matter is "heated up" by repeated bursts of star formation.These bursts produce a time-fluctuating gravitational potential to which the dark matter responds, giving dwarf galaxy core dark matter densities more in line with observations. No other types interactions are necessary, so the authors take is as evidence for collisionless cold dark matter.

A talk on this by one of the authors, Justin Read:
http://online.kitp.ucsb.edu/online/cdm-c18/read/rm/jwvideo.html

Also, Justin Read has a nice discussion twitterverse discussion,
https://twitter.com/ReadDark/status/1032176578808168448

 

[kimbyd]

Research on feedeback reported in a paper newly submitted to the Monthly Notices of the Royal Astronomical Society has generated some interest,
https://arxiv.org/abs/1808.06634

The authors claim that dark matter is "heated up" by repeated bursts of star formation.These bursts produce a time-fluctuating gravitational potential to which the dark matter responds, giving dwarf galaxy core dark matter densities more in line with observations. No other types interactions are necessary, so the authors take is as evidence for collisionless cold dark matter.

A talk on this by one of the authors, Justin Read:
http://online.kitp.ucsb.edu/online/cdm-c18/read/rm/jwvideo.html

Also, Justin Read has a nice discussion twitterverse discussion,
https://twitter.com/ReadDark/status/1032176578808168448

That's extremely fascinating!

 

[ohwilleke]

Research on feedeback reported in a paper newly submitted to the Monthly Notices of the Royal Astronomical Society has generated some interest,
https://arxiv.org/abs/1808.06634

The abstract concludes with the sentence: "Our results suggest that, to leading order, dark matter is a cold, collisionless, fluid that can be kinematically 'heated up' and moved around."

My intuition is that a "cold, collisionless, fluid" that can be kinemtically "heated" and moved around.", is almost trivially a contradiction of terms.

 

[George Jones]

The abstract concludes with the sentence: "Our results suggest that, to leading order, dark matter is a cold, collisionless, fluid that can be kinematically 'heated up' and moved around."

My intuition is that a "cold, collisionless, fluid" that can be kinemtically "heated" and moved around.", is almost trivially a contradiction of terms.

Did you read what I wrote? Did you at least read the twitter verse thread?

Note that I put "heated up" in scare quotes in my post. The "heating up" is caused by a time-varying gravitational field, which in turn is caused by repeated bursts of star formation, not by other interaction (i.e., not by collisions).

 

[kimbyd]

The abstract concludes with the sentence: "Our results suggest that, to leading order, dark matter is a cold, collisionless, fluid that can be kinematically 'heated up' and moved around."

My intuition is that a "cold, collisionless, fluid" that can be kinemtically "heated" and moved around.", is almost trivially a contradiction of terms.

At first, when I read this, I thought they were talking about some kind of new interaction between matter and dark matter. But as George Jones mentioned, they're not: only gravitational interactions are considered. So the models are still considering dark matter to be a collisionless fluid.

As for cold, that means that the intergalactic medium of dark matter is cold. Within galaxies dark matter will always be warmer because it gains energy as it falls into gravitational potential wells. The temperature of dark matter in galaxy clusters, for instance, is likely to be similar to the temperature of the hot cluster gas, which emits radiation in the x-ray range. In fact, the dark matter is probably hotter if anything, because normal matter has lots of mechanisms to cool down that dark matter lacks.

This paper is based upon earlier simulations which demonstrate that when a burst of star formation occurs and changes gravitational potentials, dark matter in the area gets a tiny kick in energy. Each star formation burst imparts a small amount of energy to the dark matter, but repeated star formation bursts over an extended period of time have an incredibly significant impact on the entire shape of the density of dark matter near the center of the galaxy, making for an entirely different shape of the galaxy's dark matter profile (having a less dense core).

What the paper George Jones linked does is applies the earlier models to galaxy observations, and shows that galaxies which have evidence of a history of recent star formation are a fundamentally distinct population in terms of their mass profiles than are galaxies which do not have evidence of recent star formation. And while the error bars are pretty large, they broadly match the simulations which are based on cold, collisionless dark matter.