General Relativity and the Bulk-flow Puzzle

Abstract Bulk peculiar flows are commonplace in the Universe, with many surveys reporting their presence on scales spanning between few hundred and several hundred Mpc. However, the sizes and the speeds of some of these bulk flows are well in excess of those theoretically anticipated, which has made them a potentially serious problem for the ΛCDM model. Having said that, essentially all the available theoretical studies are Newtonian, or quasi-Newtonian, in nature, and both bypass a key feature of peculiar motions, namely the gravitational contribution of the peculiar flux . To begin with, recall that bulk flows are matter in motion and that moving matter means nonzero energy flux. In relativity energy fluxes gravitate, but the gravitational input of the peculiar flux has been largely bypassed. As we will show here, when the flux contribution to the gravitational field is accounted for, linear peculiar velocities grow considerably faster than in the Newtonian/quasi-Newtonian studies. Therefore, general relativity could naturally relax the current ΛCDM limits to accommodate the reported fast bulk flows.