Speaker
Description
Hints of dynamical dark energy have recently emerged from distance measurements from type Ia supernovae and galaxies through the Baryonic Acoustic Oscillation feature, challenging the current paradigm in cosmology. This new perspective has encouraged the proposal of physical mechanisms able to consistently explain the observations, such as modified gravity and interactions between dark matter and dark energy. While the equation of state is an important property of dark energy, in order to understand its nature and distinguish different candidate models, we must probe its other characteristics. One such quantity is the sound speed, which dictates the evolution of dark energy inhomogeneities and can affect the formation of the large-scale structure of the Universe. In standard analyses, the sound speed is assumed to be equal to one, which suppresses dark energy perturbations inside the horizon. In this talk, I explore the consequences of a non-standard dark energy sound speed, both within General Relativity and in Modified Gravity theories. In particular, I show that Modified Gravity can make a phantom equation of state physically viable and absent of instabilities. When analyzing data from CMB, BAO, supernovae, and cosmic shear, we observe hints for deviations from General Relativity. This work ties dynamical dark energy with modified gravity, two extensions to $\Lambda$CDM that can work together to provide a consistent picture of the Universe.