Reconstructing the peculiar velocity of the Local Group with modified gravity and 2MASS

Abstract

The peculiar velocity of the Local Group (LG), reconstructed from inhomogeneities in the local density field, differs in direction and magnitude from the velocity inferred from the cosmic microwave background (CMB) dipole. We investigate whether generalized theories of gravity, which predict a modified growth of perturbations, are able to alleviate this discrepancy. We introduce a general formalism for calculating the real-space peculiar-velocity field for modified gravity and theories with interactions in the dark sector. For different classes of theories - scalar-tensor and higher dimensional gravity - we reconstruct the LG peculiar velocity using groups of galaxies identified in the 2MASS Redshift Survey. We show that, for realistic parameters, the change in direction, with respect to the standard general relativity result, is less than 2°; this is significantly smaller than the ∼40° angular discrepancy between the reconstructed LG velocity and the dipole in the CMB. © 2009 RAS.