Two recent studies have found galaxies with little or no apparent dark matter, indicating modifying gravity can't work.
Just to be clear, what these galaxies have is a velocity dispersion that is what is expected from the visible matter alone using the standard theory of gravity, correct?Two recent studies have found galaxies with little or no apparent dark matter, indicating modifying gravity can't work.
If observations falsify the theory it means it is indeed a good theory, just not the one that describes reality.Proponents of modified gravity have given this as an advantage of the theory--fewer free parameters means a simpler theory--but this advantage becomes a fatal liability if new data falls outside the theory's predictions.
Possibly. We presume that DM only interacts via gravity (and a further general assumption is "at large scales"), so there would be an interaction with the ejecta, but whether that is sufficient to drag the DM with it - or the counterpoint that the DM inhibits the spread of the ejecta - would have to be an open question given that we still don't understand DM at anything but the "there seems to be something out there" level.Could such ejecta carry 'Dark Matter' along ??
I am not an expert in structure formation, but it seems to me that explaining the existence of ultra-diffuse galaxies without dark matter could be possible in several ways. Since it would be speculation (against forum rules) and I don't want to make a fool of myself (bad karma), I will keep those thoughts to myself.More importantly, understanding why they don't have it might give us the crucial clues we need as to what it is and how it behaves.
Thanks for mentioning this.Many of the odd features go away if the galaxies in question are closer to us. In particular, the unusually bright globulars (which in fact are the mass tracers used in determining M/L) look much more typical if they are closer.
Yes, but there's all sorts of disruption in the Bullet Cluster.Bullet cluster
DF2 and DF4 are examples of low-surface-brightness galaxies. LSB galaxies are dim, gas-rich, and tend to be found disproportionately away from other galaxies. They live quiet lives, because without gravitational interactions with nearby galaxies, star formation is not triggered, so they stay gassy. Wikipedia claims they don't have supernovae, and this isn't entirely true: SN 2009z appears to be from an LSB galaxy. But the rate is way, way down.could you explain this in a bit more detail, pls?
I'm not sure this is the best way to say it. The claim of dark matter is based on more mass being present, as seen in rotation curves, than the visible matter can account for using standard gravitational theory. The claim of no dark matter in these particular galaxies simply means that is not the case: the rotation curves can be accounted for using just the visible matter with standard gravitational theory. So M/L isn't "unusually low" except in comparison to all the other galaxies that are hypothesized to have dark matter; but that really means that all those other galaxies have M/L unusually high--too high to be accounted for by the visible matter alone. These galaxies, by contrast, have M/L "just right"--just right to be accounted for by the visible matter.The claim of "no dark matter" is actually a claim the M/L is unusually low.
What does this mean? I don't know of any free parameter in MOND that corresponds to "star formation history that somehow increases L efficiency relative to the Milky Way".If we say MOND is right, and there is not dark matter, then these galaxies have low M/L due to their weird star formation history that somehow increases L efficiency relative to the Milky Way.
Why not?you still have an unusual type of galaxy that shouldn't have a normal baryonic M/L ratio
It's not an element of the MOND theory, it is a logical ramification of applying MOND to the Milky Way galaxy and the low M/L galaxies at the same time. If all is baryon, then the lower M/L for those special galaxies must mean they are efficient (per baryon) at making L relative to the Milky Way. That is precisely the opposite of what we would expect for diffuse LSB galaxies which should have weak star formation, so does not seem a likely explanation of the situation.What does this mean? I don't know of any free parameter in MOND that corresponds to "star formation history that somehow increases L efficiency relative to the Milky Way".
Same reason-- it's an LSB galaxy, so should not have the same L efficiency per baryon as the Milky Way. The point being, raising the Milky Way M/L with lots of dark matter doesn't explain why an LSB galaxy would have the same M/L as would the Milky Way if you restrict to baryon M. If one holds to the dark matter explanation, then the Milky Way has a deeper gravity well which should create a more rapid star formation rate and lower the M/L compared to a galaxy with no dark matter.Why not?
So again, what free parameter would you vary to change the "efficiency" at making L? You can't just wave your hands and say it can change; what free parameter allows it to change?If all is baryon, then the lower M/L for those special galaxies must mean they are efficient (per baryon) at making L relative to the Milky Way.
Same question as above.it's an LSB galaxy, so should not have the same L efficiency per baryon as the Milky Way
Ok, so the free parameter is basically the fraction of baryonic matter that is in stars (as opposed to gas clouds, dust clouds, etc.). But while that would be expected to affect the overall visible brightness of the galaxy, it would not necessarily affect the rotation curve (since the non-visible baryonic matter would be expected to be distributed similarly to the visible baryonic matter). And it's the rotation curve, not the overall brightness, that is well matched to the visible matter in these new galaxies, whereas it does not match the visible matter well in galaxies which are believed to have large dark matter halos. And dark matter can explain that difference because it does not have the same interactions as baryonic matter, so its distribution can be very different. Whereas MOND says that all galaxies should have rotation curves that do not match the visible matter if you use standard gravity theory, because the whole point is that it's a different gravity theory, that changes the predicted rotation curve from the visible matter.The star formation rate per baryon is the light-creating efficiency per baryon.
Actually not.Two recent studies have found galaxies with little or no apparent dark matter, indicating modifying gravity can't work.
On closer reading, I notice in the details of their methods section that the rms velocity dispersion is 14.3 km/s. It is only after the exclusion of one outlier that the velocity dispersion becomes unusually low. As a statistical exercise rejecting outliers is often OK, but with only 10 objects to start it is worrisome to throw any away. And the outlier is then unbound, making one wonder why it is there at all.
One of the authors of the original DF2 papers addresses a variety of concerns (of the kind that quite honestly should have been addressed at a pre-print/peer review stage rather than post-publication) (hat tip Backreaction).Consider: if they had simply reported the rms velocity dispersion, and done the MOND calculation correctly, they would have found excellent agreement. This certainly could be portrayed as a great success for MOND. Instead, tossing out just one globular cluster makes it look like a falsification. Just one datum, and a choice of how to do the statistics. Not a wrong choice necessarily, but a human choice… not some kind of statistical requirement.
He acknowledges the need for more and better data to get a more accurate measurement, some of which can be done quite easily (and really should have been done prior to publication in Nature).The whole MOND / alternative gravity discussion in the paper rests on a misunderstanding on my part.
Stacy S. McGaugh, "MOND Prediction for the Velocity Dispersion of the `Feeble Giant' Crater II" (November 3, 2016).Crater II is an unusual object among the dwarf satellite galaxies of the Local Group in that it has a very large size for its small luminosity. This provides a strong test of MOND, as Crater II should be in the deep MOND regime (gin≈34km2s−2kpc−1≪a0=3700km2s−2kpc−1). Despite its great distance (≈120 kpc) from the Milky Way, the external field of the host (gex≈282km2s−2kpc−1) comfortably exceeds the internal field. Consequently, Crater II should be subject to the external field effect, a feature unique to MOND. This leads to the prediction of a very low velocity dispersion: σefe=2.1+0.9−0.6kms−1.