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posted by takyon on Saturday October 29 2016, @12:05AM   Printer-friendly
from the standard-model dept.

Last month, a team of scientists led by Stacy McGaugh at Case Western Reserve University determined from observations of 153 galaxies that the dynamics of galaxy rotation seems to depend solely on the normal, visible matter in it (SN coverage here). It was a strong argument that rather than hypothesising dark matter to explain the oddities in galactic rotation, it may instead be necessary to modify the laws of gravity.

However, two scientists from McMaster University, Ben Keller and James Wadsley, have just recently examined the results of a detailed simulation of dark matter in galaxy formation previously done known as the McMaster Unbiased Galaxy Simulations 2 (MUGS2). The simulation was a sophisticated one that took into account various other factors such as gas dynamics, star formation, and stellar feedback, but incorporated no new physics beyond that of the standard Lambda-Cold Dark Matter (ΛCDM) cosmological model. They found that the relation that McGaugh et. al. discovered from observations of real galaxies was reproduced just about exactly by the simulation. Their paper is here. Their abstract states:

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 ΛCDM. 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 ΛCDM 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.

So now it seems that the earlier troubles with dark matter were actually the result of too naïve a simulation, and by taking into account additional known, relevant physics, the troubles disappear.

Further coverage and commentary by astrophysicist Ethan Siegel here (archive.is).

Related: Study Casts Doubt on Cosmic Acceleration and Dark Energy


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  • (Score: 5, Interesting) by stormwyrm on Saturday October 29 2016, @12:25AM

    by stormwyrm (717) Subscriber Badge on Saturday October 29 2016, @12:25AM (#419998) Journal

    It seems that MUGS2 is not the only ΛCDM-based galaxy modelling simulation that exhibits the SPARC acceleration law. Shortly after submitting the article I was made aware of a second group that saw the same thing in their models: The Mass-Discrepancy Acceleration Relation: a Natural Outcome of Galaxy Formation in CDM halos [arxiv.org].

    One other thing that these models predict is that the SPARC acceleration law discovered by the McGaugh team should become less and less accurate the younger the galaxy is, i.e. a very distant galaxy which we would be seeing as it looked when it was still very young should not have a very close fit to the acceleration relation. We can't yet measure the rotation curves of galaxies so far away that the deviation should be clear, but it may be possible with better telescopes such as the JWST that will become available in the near future.

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  • (Score: 0) by Anonymous Coward on Saturday October 29 2016, @12:42AM

    by Anonymous Coward on Saturday October 29 2016, @12:42AM (#420002)

    a very distant galaxy which we would be seeing as it looked when it was still very young should not have a very close fit to the acceleration relation.

    Good stuff.

  • (Score: 2) by RedBear on Saturday October 29 2016, @07:04AM

    by RedBear (1734) Subscriber Badge on Saturday October 29 2016, @07:04AM (#420065)

    It seems that MUGS2 is not the only ΛCDM-based galaxy modelling simulation that exhibits the SPARC acceleration law. Shortly after submitting the article I was made aware of a second group that saw the same thing in their models: The Mass-Discrepancy Acceleration Relation: a Natural Outcome of Galaxy Formation in CDM halos.
    One other thing that these models predict is that the SPARC acceleration law discovered by the McGaugh team should become less and less accurate the younger the galaxy is, i.e. a very distant galaxy which we would be seeing as it looked when it was still very young should not have a very close fit to the acceleration relation. We can't yet measure the rotation curves of galaxies so far away that the deviation should be clear, but it may be possible with better telescopes such as the JWST that will become available in the near future.

    Well this is fascinating. I thought everything in the summary seemed strangely familiar (strange because I typically don't read physics papers). It's because not two days ago I was pointing people here to this blog [blogspot.com], by Mike McCulloch, where in the latest (as of now) October 18 post [blogspot.com] McCulloch was discussing the data sent to him by "Prof Stacy McGaugh" and how it fits quite well with his own much simpler MiHsC theory [blogspot.co.uk] that doesn't require dark matter. A commenter pointed Mike to the exact article linked in the summary ("La Fin du MOND? Λ CDM is Fully Consistent with SPARC Acceleration Law"). As highlighted above, said paper seems to fail to match the data for younger, more red-shifted galaxies, as appears to be shown in figure 2. Yet, in McCulloch's MiHsC theory, one of the terms is (I'm totally going to screw this up) an astrophysical "horizon" just larger than the diameter of the observable universe(?). Thus as you go backwards in time, this term shrinks with the shrinking diameter of the universe, and MiHsC supposedly easily lines up with the shifting acceleration curves. Without dark matter, or "baryon depletion" or what have you. But, that's just a brief blog conversation from a few days ago.

    I'm probably not making too much sense, but this is all laid out much more clearly in many different posts in the blog. Before you immediately dismiss this theory as crackpottery you should really look through the blog for a few minutes. McCulloch provides specifically testable, and thus scientifically falsifiable, ways of proving or disproving the theory. Like, testable right now, not like "it will be impossible to test this until 50 years from now".

    I wonder if it will be helpful or counter-productive to mention that MiHsC also purports to provide the only explanation I've ever seen that makes any sense for why the EmDrive inexplicably seems to work. Meh, anybody interested will click the links.
    .

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