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
Related Stories
The hypothesis of dark matter has proved incredibly successful in explaining the overall large scale structure of the universe and in interactions on the level of galactic clusters, which competing hypotheses such as modified gravity have failed to adequately explain. However, on the relatively smaller scales of individual galaxies, hypothesising dark matter shows some problems. In a paper recently accepted for publication in Physical Review Letters, astronomers Stacy McGaugh and Federico Lelli of Case Western Reserve University, and Jim Schombert of the University of Oregon, have made observations of 153 different galaxies with a wide variety of shapes, masses, sizes and amounts of gas. They have found a strong relationship between how quickly the galaxy rotates and the presence of normal (baryonic) matter alone. From an article on Case Western Reserve University's Daily:
[...] A team led by Case Western Reserve University researchers has found a significant new relationship in spiral and irregular galaxies: the acceleration observed in rotation curves tightly correlates with the gravitational acceleration expected from the visible mass only.
"If you measure the distribution of star light, you know the rotation curve, and vice versa," said Stacy McGaugh, chair of the Department of Astronomy at Case Western Reserve and lead author of the research.
The finding is consistent among 153 spiral and irregular galaxies, ranging from giant to dwarf, those with massive central bulges or none at all. It is also consistent among those galaxies comprised of mostly stars or mostly gas.
[...] "Galaxy rotation curves have traditionally been explained via an ad hoc hypothesis: that galaxies are surrounded by dark matter," said David Merritt, professor of physics and astronomy at the Rochester Institute of Technology, who was not involved in the research. "The relation discovered by McGaugh et al. is a serious, and possibly fatal, challenge to this hypothesis, since it shows that rotation curves are precisely determined by the distribution of the normal matter alone. Nothing in the standard cosmological model predicts this, and it is almost impossible to imagine how that model could be modified to explain it, without discarding the dark matter hypothesis completely."
[...] Arthur Kosowsky, professor of physics and astronomy at the University of Pittsburgh, was not involved but reviewed the research.
"The standard model of cosmology is remarkably successful at explaining just about everything we observe in the universe," Kosowsky said. "But if there is a single observation which keeps me awake at night worrying that we might have something essentially wrong, this is it."
Additional coverage and commentary by Ethan Siegel and Brian Koberlain. It seems that the universe has just thrown us yet another curve ball. This kind of correlation is just the sort of thing that modified gravity such as MOND and TeVeS predict. However, they fail miserably in explaining the large scale structure and evolution of the universe, which the dark matter explains admirably.
A newly published analysis of Type Ia supernovae calls into question the accelerating expansion of the universe and the existence of dark energy:
Five years ago, the Nobel Prize in Physics was awarded to three astronomers for their discovery, in the late 1990s, that the universe is expanding at an accelerating pace. Their conclusions were based on analysis of Type Ia supernovae – the spectacular thermonuclear explosions of dying stars – picked up by the Hubble space telescope and large ground-based telescopes. It led to the widespread acceptance of the idea that the universe is dominated by a mysterious substance named 'dark energy' that drives this accelerating expansion.
Now, a team of scientists led by Professor Subir Sarkar of Oxford University's Department of Physics has cast doubt on this standard cosmological concept. Making use of a vastly increased data set – a catalogue of 740 Type Ia supernovae, more than ten times the original sample size – the researchers have found that the evidence for acceleration may be flimsier than previously thought, with the data being consistent with a constant rate of expansion.
Marginal evidence for cosmic acceleration from Type Ia supernovae (open, DOI: 10.1038/srep35596) (DX)
A physicist is using a theory he advanced to explain how EmDrive could work to explain how dwarf galaxies can be held together without the requirement of dark matter:
British physicist Dr Mike McCulloch, who previously used quantised inertia to explain how the controversial electromagnetic space propulsion technology EmDrive works, says that he has new evidence showing his theory can also explain galaxy rotation, which is one of physics' biggest mysteries. McCulloch, a lecturer in geomatics at Plymouth University's school of marine science and engineering, says he now has even more evidence that his "new physics theory" about quantised inertia works, and that it makes it possible to explain why galaxies are not ripped apart without using theory of dark matter.
[...] There are 20 dwarf galaxies in existence from Segue-1 (the smallest) to Canes Venatici-1 (the largest), and dark matter is only meant to work by spreading out across a wide distance, but it is still used to explain dwarf galaxies, even though this requires dark matter to be concentrated within these systems, which is implausible. Instead, McCulloch asserts that quantised inertia can be used to explain how galaxies rotate without using dark matter, and he has written a paper that has been accepted by the bi-monthly peer reviewed journal Astrophysics and Space Science.
Reprint of the IBT link here.
From the abstract of Low-acceleration dwarf galaxies as tests of quantised inertia (DOI not yet published):
Dwarf satellite galaxies of the Milky Way appear to be gravitationally bound, but their stars' orbital motion seems too fast to allow this given their visible mass. This is akin to the larger-scale galaxy rotation problem. In this paper, a modification of inertia called quantised inertia or MiHsC (Modied inertia due to a Hubble-scale Casimir effect) which correctly predicts larger galaxy rotations without dark matter is tested on eleven dwarf satellite galaxies of the Milky Way, for which mass and velocity data are available. Quantised inertia slightly outperforms MoND (Modied Newtonian Dynamics) in predicting the velocity dispersion of these systems, and has the fundamental advantage over MoND that it does not need an adjustable parameter.
Previously: Study Casts Doubt on Cosmic Acceleration and Dark Energy
Dark Matter Beats its Latest Challenge
Emergent Gravity and the Dark Universe
Space Race 2.0: China May Already be Testing an EmDrive in Orbit
Milky Way is Not Only Being Pulled—It's Also "Pushed" by a Void
(Score: 0) by Anonymous Coward on Saturday October 29 2016, @12:18AM
(Score: 0) by Anonymous Coward on Saturday October 29 2016, @12:29AM
It's how science works. Observers and experimenters see stuff in nature, and theoreticians build models and simulations that try to explain what is observed. As Ethan Siegel writes:
(Score: 0) by Anonymous Coward on Saturday October 29 2016, @12:38AM
I was just kidding...
However, the point of that Mcgaugh finding was that you didn't need "a sophisticated [simulation] 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" to get the distribution of dark matter. You needed a simple equation with no free parameters and only the amount of visible light*.
Really the comparison is between simple equation using observed light vs sophisticated simulation using observed light plus a bunch of other stuff.
*Note: I am sure there were some other assumptions that went into converting this to mass.
(Score: 0) by Anonymous Coward on Saturday October 29 2016, @01:21AM
(Score: 2) by Bot on Saturday October 29 2016, @10:20AM
Indeed, we do not have observation, hypothesis on interpretation, experiment to prove the hypothesis' predictive power, new theory, repeat.
Here we have Theory, simulation of theory, OK if match with observation.
This way of working is equivalent to
1. the hypothesis of planet to explain some gravitational interference
2. epicycles
in the first case you are OK, in the last case you are not. It is inconclusive.
Account abandoned.
(Score: 3, Interesting) by stormwyrm on Saturday October 29 2016, @02:53PM
There are plenty of branches of science where we cannot perform experiments. Heck, the mechanics of planets in the Solar System is not amenable to controlled experimentation any more than the dynamics of a galaxy is. So since antiquity all astronomers could do was watch and study the planets as they moved across the heavens, and try to figure out how it all worked. They made what amount to simulations of planetary motion by calculating what are called ephemerides. Claudius Ptolemy made such a model of solar system dynamics based on the notions of deferents and epicycles and it took so long for it to be discarded because the Ptolemaic ephemerides matched the crude observations possible with naked eye astronomy closely enough. It was only after the development of the telescope that folks like Galileo and Kepler realised that the Ptolemaic system wasn't good enough, and a better model needed to be made. Kepler made his own laws of planetary motion based on observations, and later Newton came up with a theoretical framework in his laws of motion and universal gravitation that tried to explain why Kepler's laws held.
In the early 19th century astronomers comparing the calculated ephemerides of Uranus with actual observations of the planet realised that its orbit around the sun was perturbed and deviated from the way it should have been moving if it obeyed Newton's laws. They realised that there had to be another planet ("dark matter") beyond Uranus that was causing the perturbations, and soon enough Neptune was discovered. Later that century, better observations of Mercury showed that it too was moving in ways different from what Newtonian mechanics predicted. That led some astronomers (one of them was, in fact the very same astronomer who is credited with discovering Neptune, Urbain le Verrier) to hypothesise yet another planet even closer to the sun, named Vulcan (no relation to Star Trek) that was likewise causing the perturbations. No such planet was ever found, so it took a modification to the theory of gravity in the form of Einstein's theory of General Relativity to explain all the observed features of Mercury's orbit.
I think the parallels to the present-day observations of galactic dynamics and problems in cosmology are rather clear. We have known since the 1930s that the motions of stars in galaxies do not seem to obey the laws of Newtonian dynamics. We hypothesise, just as 19th century astronomers did in hypothesising an unknown planet, that there might be some matter out there surrounding the galaxies which we still cannot see directly that is affecting the motion of the galaxy such that it moves the way it does. Since we can't do an experiment on a galactic scale, the best we can do is make a simulation based on our theories and see how well it matches up with the real stuff out there. Galaxies of course are far more complicated beasts than planets in our solar system, and so observers like Stacy McGaugh discover new properties about them every now and then. When that happens the theorists like Ben Keller and James Wadsley must look at their simulations and see if they are accurate enough to exhibit the features that have been observed. If they do, as has been shown in this case, the theory gets to live a little longer. I don't think it's fair to accuse them of metaphorically "adding more epicycles" in this case: their additions to the simulation were known physics that they already knew would have a measurable effect on the dynamics of galaxies.
The second case though, is a more interesting question. When do we decide that it's time to stop chasing Vulcan and start looking for a new theory of gravitation? Perhaps when attempting to shore up a theory in the face of contrary observation or experiment we should ask if what we are doing is the equivalent of adding more epicycles, i.e. complicating the theory without an observational or experimental basis. By hypothesising dark matter, we have added precisely one such epicycle, and we are trying our damnedest to make that go away. Modifying gravity à la TeVeS though... you should see what their equations look like, and how they try to explain phenomena such as the Bullet Cluster and the peaks in the CMB that dark matter is able to explain so well, and then tell me who is adding more epicycles.
Numquam ponenda est pluralitas sine necessitate.
(Score: 5, Interesting) by stormwyrm on Saturday October 29 2016, @12:25AM
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.
Numquam ponenda est pluralitas sine necessitate.
(Score: 0) by Anonymous Coward on Saturday October 29 2016, @12:42AM
Good stuff.
(Score: 2) by RedBear on Saturday October 29 2016, @07:04AM
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.
.
¯\_ʕ◔.◔ʔ_/¯ LOL. I dunno. I'm just a bear.
... Peace out. Got bear stuff to do. 彡ʕ⌐■.■ʔ
(Score: -1, Flamebait) by Anonymous Coward on Saturday October 29 2016, @02:43AM
Right. And fuck you.
(Score: 0, Flamebait) by andersjm on Saturday October 29 2016, @07:02AM
I'm going to to out on a limb here and say that the kind of pretentious mathematician who would even think to use a greek letter mixed into a latin lettered acronym, probably doesn't have good intuition about the nature of reality.
Based on that and that alone, I'm calling it: There is no such thing as dark matter.
You heard it here first, folks!
(Score: 2) by lgw on Saturday October 29 2016, @08:50AM
It's actually the name of the "standard model" of cosmology. Three theories stuck together with duct tape and bailing wire: big bang + CDM for dark matter + cosmological constant (lambda) for dark energy. Amusingly, it's Lambda-CDM - the big bang is silent.
The CDM part is reasonably uncontroversial at this point, though not yet settled obviously. The cosmological constant is entirely a guess, one of many dark energy theories, but it's the simplest so it's the starting point. The big bang part includes "inflation", but no particular inflationary model is standard, and there's a lot of work around whether inflation and dark energy are two unrelated effects, or one effect that evolved over time.
(Score: 3, Touché) by stormwyrm on Saturday October 29 2016, @09:45AM
Numquam ponenda est pluralitas sine necessitate.