The Three Faces of Omega_m: Testing Gravity with Low and High Redshift SN Ia Surveys

TL;DR

This paper demonstrates how peculiar velocities in Type Ia supernova data can be used to test cosmological models, specifically the matter density and gravity theories, by analyzing low and high redshift observations.

Contribution

It introduces a method to turn peculiar velocity effects from a systematic error into a tool for constraining cosmological parameters using SN Ia data.

Findings

All three estimates of Omega_m agree with 0.25 within 1 sigma.

Current data cannot distinguish between Einstein gravity and modified gravity models.

Future surveys could improve constraints significantly.

Abstract

Peculiar velocities of galaxies hosting Type Ia supernovae generate a significant systematic effect in deriving the dark energy equation of state w, at level of a few percent. Here we illustrate how the peculiar velocity effect in SN Ia data can be turned from a 'systematic' into a probe of cosmological parameters. We assume a flat Lambda-Cold Dark Matter model (w=-1) and use low and high redshift SN Ia data to derive simultaneously three distinct estimates of the matter density Omega_m which appear in the problem: from the geometry, from the dynamics and from the shape of the matter power spectrum. We find that each of the three Omega_m's agree with the canonical value Omega_m=0.25 to within 1 sigma, for reasonably assumed fluctuation amplitude and Hubble parameter. This is consistent with the standard cosmological scenario for both the geometry and the growth of structure. For fixed Omega_m = 0.25 for all three Omega_m's, we constrain gamma = 0.72 +/- 0.21 in the growth factor Omega_m(z)^gamma, so we cannot currently distinguish between standard Einstein gravity and predictions from some modified gravity models. Future surveys of thousands of SN Ia, or inclusion of peculiar velocity data, could significantly improve the above tests.