The parameter space in Galileon gravity models

TL;DR

This paper constrains the full parameter space of Galileon gravity models using cosmological data, revealing their potential to fit observations better than standard models but with notable differences in parameters.

Contribution

First comprehensive constraints on the full Galileon model parameter space using combined cosmological datasets and MCMC analysis.

Findings

Galileon models can fit WMAP data better than DM.

Including low-redshift data slightly worsens the fit.

Galileon parameters can differ significantly from DM values.

Abstract

We present the first constraints on the full parameter space of the Galileon modified gravity model, considering both the cosmological parameters and the coefficients which specify the additional terms in the Lagrangian due to the Galileon field, which we call the Galileon parameters. We use the latest cosmic microwave background measurements, along with distance measurements from supernovae and baryonic acoustic oscillations, performing a Monte Carlo Markov Chain exploration of the 9-dimensional parameter space. The integrated Sachs-Wolfe signal can be very different in Galileon models compared to standard gravity, making it essential to use the full CMB data rather than the CMB distance priors. We demonstrate that meaningful constraints are only possible in the Galileon parameter space after taking advantage of a scaling degeneracy. We find that the Galileon model can fit the WMAP 9-year results better than the standard \Lambda-Cold Dark Matter model, but gives a slightly worse fit overall once lower redshift distance measurements are included. The best-fitting cosmological parameters (e.g. matter density, scalar spectral index, fluctuation amplitude) can differ by more than 2\sigma\ in the Galileon model compared with \Lambda CDM. We highlight other potential constraints of the Galileon model using galaxy clustering and weak lensing measurements.