Starts With A Bang

Dark matter rises to its biggest challenge… and succeeds! (Synopsis)

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).

“Faced with the choice between changing one’s mind and proving that there is no need to do so, almost everybody gets busy on the proof.” -J. K. Galbraith

Last month, tensions between dark matter simulations and galactic rotation observations reached a new high. Despite all the successes of dark matter on the largest scales — for the CMB, for large-scale structure, for gravitational lensing and for galaxy clusters, among others — the simplest dark matter simulations reproduced unrealistic results for how individual galaxies ought to rotate. Moreover, a team of scientists uncovered a surprising relationship: between the normal (baryonic) matter alone and the observed acceleration in galaxies.

The 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

If dark matter were real, then the simulations needed to reproduce that result as well, which appeared to be a tremendous challenge. Yet in a new paper, two scientists from McMaster University, Ben Keller and James Wadsley, did exactly this. Even more impressively, they didn’t make new simulations, they simply took pre-existing ones and showed that this relation does, in fact, get reproduced among all their galaxies.

While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red) can severely impact galactic scales. Image credit: Illustris Collaboration / Illustris Simulation.

If the modified gravity camp through down the gauntlet, the dark matter camp just revealed they had brought a bazooka to a gunfight. Go get the full story!