modified einstein

10

Galaxy Clusters Prove Dark Matter’s Existence

“The key to understanding gravitational observations arises from gravitational lensing, where mass bends the background starlight. Under serendipitous configurations, background galaxies are deformed into arcs and multiple, distorted images. This phenomenon – strong lensing – allows us to determine the total cluster mass. Even without optimal configurations, weak gravitational lensing causes a well-defined distortion in the shape of background galaxies.”

If all we had were galactic rotation curves – like those measured by Vera Rubin – we would know that something was wrong with our picture of the Universe, but we wouldn’t know how. Two equally good explanations, that there was either a flaw in the law of gravity or there was the existence of some unseen mass, could account for what we saw. But observations of galaxy clusters point to dark matter in a dramatic fashion. Both dark matter and the modifications one can make to gravity to explain galactic rotation make specific predictions for other phenomena. The motions of individual galaxies within a cluster, the bending of background light by strong lensing, the distortion of galaxy shape from weak lensing and the separation of effective mass from normal matter are four independent ways clusters can discriminate. On all four counts, they point to dark matter, not modified gravity.

Come get the full story in pictures, video and no more than 200 words on today’s Mostly Mute Monday!

8

Why are dark matter and modified gravity in such conflict?

“Most in the dark matter camp are convinced that the full suite of their successes to date mean that a better understanding of the nature of dark matter and improved computational power will lead to galaxy rotation falling into line. Similarly, most in the modified gravity camp are equally convinced that the failure of dark matter on these small scales is a catastrophe, and that the correlations they’ve discovered are a natural law that’s a precursor to a revolution even bigger than Einstein’s was 100 years ago. The great challenge for modified gravity is to reproduce the successes on large-scales of modern cosmology; the challenge for dark matter is to reproduce the details of the smallest scales correctly.”

The past month has seen a slew of papers out highlighting the tension between modified gravity and dark matter. Both recognize the same puzzles and problems with the Universe, and both ideas recognize that either one could be valid. In fact, if you look at the two greatest “crises” in gravity in the 19th century, it’s arguable that dark matter (Neptune) solved one, the Uranus problem, while modifying gravity (with Einstein’s general relativity) solved the other. Now in the 21st century, we have a whole Universe to explain, and while dark matter is definitely the leading theory, the idea of modifying gravity isn’t crazy. Moreover, it has a success that dark matter can’t match: on galaxy-scales and below. In the end, it will take a big step forward for a true victor to emerge, but here’s where the science stands right now.