Observational effects of varying speed of light in quadratic gravity cosmological models

Abstract

We study different manifestations of the speed of light in theories of gravity where metric and connection are regarded as independent fields. We find that for a generic gravity theory in a frame with locally vanishing affine connection, the usual degeneracy between different manifestations of the speed of light is broken. In particular, the space-time causal structure constant ($c_{ST}$) may become variable in that local frame. For theories of the form $f(R,R^{\mu\nu} R_{\mu\nu})$, this variation in $c_{ST}$ has an impact on the definition of the luminosity distance (and distance modulus), which can be used to confront the predictions of particular models against Supernovae type Ia (SN Ia) data. We carry out this test for a quadratic gravity model without cosmological constant assuming i) a constant speed of light and ii) a varying speed of light, and find that the latter scenario is favored by the data.