Disformal transformations on the CMB
  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.
