Disformal transformations on the CMB

TL;DR

This paper investigates how disformal transformations affect the CMB anisotropy power spectrum, revealing observable imprints on acoustic peaks and damping, especially in quartic galileon theories with modified tensor mode speeds.

Contribution

It demonstrates that disformal transformations alter the CMB power spectrum, providing a new way to constrain tensor mode speeds in galileon theories.

Findings

Disformal transformations leave an imprint on the CMB anisotropy spectrum.

The transformation constrains parameters in quartic galileon theories.

Tensor mode speed modifications can be tested via CMB observations.

Abstract

In this work we study the role of disformal transformation on cosmological backgrounds and its relation to the speed of sound for tensor modes. A speed different from one for tensor modes can arise in several contexts, such as Galileons theories or massive gravity, nevertheless the speed is very constrained to be one by observations of gravitational wave emission. It has been shown that in inflation a disformal trans- formation allows to set the speed for tensor modes to one without making changes to the curvature power spectrum. Here we show that this invariance does not hold when considering the CMB anisotropy power spectrum. It turns out that the after doing the transformation there is an imprint on the acoustic peaks and the diffusion damping. This has interesting consequences; here we explore quartic galileon theories which allow a modified speed for tensor modes. For these theories the transformation can be used to constraint the parameter space in different regimes.