Dark Energy and Modified Gravity

Dark Energy and Modified Gravity

Our group is significantly involved in connecting data from future surveys with theoretical models that describe the Dark Universe.

The origin of the accelerated expansion of the Universe is still a mystery.  The cosmological constant (a constant contribution to the energy content of the Universe, constant in time and space) was introduced by Einstein in 1917 and is a good fit to observations that we have at present. Nevertheless, its observed value differs by several orders of magnitude with any theoretical expectation; furthermore, a cosmological constant marks our epoch as a very special time in the evolution of the Universe. 

A large effort in theoretical cosmology has led to look for alternative possible explanations, that might effectively behave as a cosmological constant at present but may have been different in the past.  The acceleration is then typically attributed to a fluid (Dark Energy) or to the need to change the laws of gravity at very large scales (Modified Gravity, where 'modified' means different with respect to Einstein Theory of General Relativity) rather than to an ad hoc fitting constant parameter.

Planck Collaboration 2015

A large variety of models of Dark Energy and Modified Gravity is at present viable.  Our group was involved in the coordination of the ESA Planck paper on dark energy and modified gravity for the 2015 release. This includes a systematic analysis of models, codes, data, tested against Planck CMB data and in combination with external data sets. Future observations such as Euclid on large scale structure will test entire classes of models and check whether we can discriminate them from a cosmological constant scenario.

Our group is particularly interested in testing the impact of non-linear scales on testing Modified Gravity theories and how non-linear scales can break the correlation between Modified Gravity parameters.

Casas et al. 2017