Gravity is everywhere. This is good, because it means we can test it in so many different environments. For centuries, gravity has been tested with rolling balls (Galileo), orbits of planets (Newton, Lagrange, and many others), precise laboratory instruments (Eotvos etc.), satellite experiments within the solar system. Now the challenge moved to distances that only astrophysics and cosmology can approach.
One particularly interesting enviroment in which to test gravity are galaxy clusters. These are concentrations of hundred or thousand galaxies within a radius of approximately 1 Megaparsec. Some of the closest and best studied ones are the Virgo cluster (distance 15.5 Mpc) and the Coma clusters (99 Mpc). Galaxy clusters are gravitationally bound, which means they will stay forever together, without dispersing themselves in the open space like a gas would do. This also means that they should be approximately in equilibrium and, since they rotate very slowly, also should be approximately spherical.
Neither of these two criteria are in general satisifed, since galaxy clusters often undergo merging events, and their quiet internal life might be disturbed. One particular cluster, however, MACS J1206.2-0847 (located at redshiftt z=0.44), seems really an ideal “smooth” cluster, sufficiently spherical, sufficiently “relaxed” (i.e., in equilibrium, without major bumps here and there). About this cluster, we have lots of data: we know the temperature of its X-ray emitting regions, we know the velocity of hundred of galaxy members, we have good lensing imaging. This means we can use this galaxy as a sort of Galilean “rolling ball” and see if it obeys standard Einstein gravity or something else. Comparing lensing and galaxy velocities, in particular, one can test whether gravity is the same for light rays and for galaxies, as it should in General Relativity. This is what we did in a recent paper published by JCAP, the Journal of Cosmology and Astroparticle Physics, using data from the CLASH-VLT collaboration.
Ok, we didn’t find anything special. Gravity seems to go on as expected in MACSJ1206. But this is not the interesting part of the story. The interesting part is that even with a single galaxy cluster, we have been able to put limits on how much gravity can deviate from Einstein’s law in a new scale range, that one typical of clusters, i.e. 1 Mpc, that are stronger than available so far. Moreover, this has been done in an almost “model independent” way, that is, without using a particular alternative to standard gravity, but parametrizing in a general way the possible deviation. This means that when we will have dozens or hundreds of similar well-behaved clusters, we will be able to dramatically improve our constraints on gravity and, possibly, find traces of something beyond Einstein.