Dark matter faces its biggest challenge of all (Synopsis)
“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.” -David Merritt
Dark matter is a hugely successful theory for explaining a whole slew of observations about the Universe. Just by adding this one ingredient to the mix, we can successfully simulate and reproduce the large-scale structure, CMB fluctuations, galaxy clustering and cluster collision properties observed in our Universe. Without dark matter, there’s no other way known to make the Universe work in line with what we see.
A clumpy dark matter halo with varying densities and a very large, diffuse structure, as predicted by simulations, with the luminous part of the galaxy shown for scale. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).
And yet, if you go down to the small scales of individual galaxies, dark matter predicts a dark matter halo of a specific profile with specific rotation properties. When we look at the actual galaxies, those rotation properties don’t match! Even worse, they appear to be correlated solely with the normal matter content of the galaxies, and have no dependence on whether the galaxy is rich-or-poor in dark matter.
he correlation between gravitational acceleration (y-axis) and the normal, baryonic matter (x-axis) visible in an assembly of 153 galaxies. The blue points show each individual galaxy, while the red show binned data. Image credit: The Radial Acceleration Relation in Rotationally Supported Galaxies, Stacy McGaugh, Federico Lelli and Jim Schombert, 2016. From https://arxiv.org/pdf/1609.05917v1.pdf.