Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

General relativity and the bulk-flow puzzle

Christos G. Tsagas

TL;DR

The paper tackles the bulk-flow puzzle by arguing that standard Newtonian analyses overlook the gravitational input of the peculiar flux carried by moving matter. Using a fully relativistic, tilted-FRW framework, it derives that the peculiar flux feeds back into the Einstein equations, yielding a faster linear growth of the peculiar-velocity field with the homogeneous solution $v \propto t$ (the minimal growth rate), in contrast to the Newtonian $v \propto t^{1/3}$. This result strengthens the case that General Relativity can accommodate fast bulk flows within the ΛCDM paradigm, particularly between recombination and the onset of late-time acceleration. The work also stresses the central role of the peculiar flux and urges incorporating flux gravity into simulations to properly test and compare with observations such as CMSS and Wet al, potentially reconciling discrepancies without new physics.

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 $Λ$CDM 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 \textit{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 $Λ$CDM limits to accommodate the reported fast bulk flows.

General relativity and the bulk-flow puzzle

TL;DR

The paper tackles the bulk-flow puzzle by arguing that standard Newtonian analyses overlook the gravitational input of the peculiar flux carried by moving matter. Using a fully relativistic, tilted-FRW framework, it derives that the peculiar flux feeds back into the Einstein equations, yielding a faster linear growth of the peculiar-velocity field with the homogeneous solution (the minimal growth rate), in contrast to the Newtonian . This result strengthens the case that General Relativity can accommodate fast bulk flows within the ΛCDM paradigm, particularly between recombination and the onset of late-time acceleration. The work also stresses the central role of the peculiar flux and urges incorporating flux gravity into simulations to properly test and compare with observations such as CMSS and Wet al, potentially reconciling discrepancies without new physics.

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 CDM 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 \textit{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 CDM limits to accommodate the reported fast bulk flows.
Paper Structure (10 sections, 13 equations, 1 figure)

This paper contains 10 sections, 13 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Peculiar flux and peculiar 4-acceleration
  3. The peculiar flux
  4. The peculiar 4-acceleration
  5. Relativistic vs quasi-Newtonian studies
  6. Linear quasi-Newtonian approach
  7. Linear relativistic approach
  8. Theory vs observations
  9. The fundamental role of the peculiar flux
  10. Discussion

Figures (1)

  • Figure 1: (a) The redshift profile of the bulk flow from the likelihood analysis of CMSS. The peculiar velocity (black dots) systematically exceeds the blue line of the $\Lambda$CDM expectations, but its value also shows signs of decrease at lower redshifts. Note that Fig. \ref{['fig:f1']} shows that 1-D expectation, so it needs to be multiplied by $\sqrt{3}$ to give the 3-D one (see also Fig. 8 in CMSS). (b) The scale dependence of the bulk-flow velocity along the three coordinate axes and of its mean value (green lines), estimated from the CosmicFlows4 catalogue Wetal. The red dashed line is the theoretical expectation for the mean bulk velocity according to the standard cosmological model. Note the profound disagreement between the predicted and the measured bulk-flow velocities, as well as the decline in the magnitude of the latter at lower redshifts (see also Fig. 7 in Wetal).