Cosmology is an exciting field of research. Sometimes it may look like we are looking for clues in the dark but actually the last century has made it possible to speculate about the origins of the universe and to establish the Big Bang theory as the leading cosmological model, now accepted as the basis for further theories and observations.

The Big Bang theory is the cosmological model of the initial conditions and subsequent development of the universe supported by the most comprehensive and accurate explanations from current scientific evidence and observation. The term Big Bang refers to the idea that the universe has expanded from a primordial hot and dense initial condition at some finite time in the past, and continues to expand to this day.

Despite its success, the Big Bang theory also has a few problems. Measurements of the redshift magnitude relation for type Ia supernovae have revealed that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, General Relativity requires that much of the energy in the universe consists of an unkown component with large negative pressure, dubbed "dark energy".

Being quite disappointed with this picture, I started looking in other directions, such as the Modified Gravity theories. These are very promising and interesting classes of models that attribute the observed accelerating expansion of the Universe to modifications and extensions of General Relativity. These convert gravity to a repulsive interaction at late times (z<1) but at early times (z>>1) are indistinguishable from General Relativity. Such theories have particular observational signatures that can be identified by using several observational probes such as the growth of matter perturbations, weak lensing and the Cosmic Microwave Background Radiation.