Constraining modified gravity scenarios with the 6dFGS and SDSS galaxy peculiar velocity datasets

The detailed nature of dark energy remains a mystery, leaving the possibility that its effects might be explained by changes to the laws of gravity on large scales. The peculiar velocities of galaxies directly trace the strength of gravity on cosmic scales and provide a means to further constrain such models. We generate constraints on different scenarios of gravitational physics by measuring peculiar velocity and galaxy clustering two-point correlations, using redshifts and distances from the 6-degree Field Galaxy Survey and the Sloan Digital Sky Survey Peculiar Velocity samples, and fitting them against models characteristic of different cosmologies. Our best-fitting results are all found to be in statistical agreement with General Relativity, in which context we measure the low-redshift growth of structure to be $f\sigma_8 = 0.329^{+0.081}_{-0.083}$, consistent with the prediction of the standard $\Lambda$CDM model. We also fit the modified gravity scenarios of Dvali-Gabadadze-Porrati (nDGP) and a Hu-Sawicki model of $f(R)$ gravity, finding the $2\sigma$ limit of their characteristic parameters to be $r_cH_0/c>6.987$ and $-\log_{10}(|f_{R0}|)>4.703$, respectively. These constraints are comparable to other literature values, though it should be noted that they are significantly affected by the prior adopted for their characteristic parameters. When applied to much larger upcoming peculiar velocity surveys such as DESI, this method will place rapidly-improving constraints on modified gravity models of cosmic expansion and growth.