Fredagskollokvium: Claudio Llinares: Testing modified gravity theories on large scales

In spite that general relativity shows itself as a very succesful theory, there is an increasing part of the community that accepts the possibility that it can be improved on the scales of galaxies and beyond. Many alternatives were proposed since the very early days. MOND is a dramatic example of such proposals since involves changes in the non-relativistic limit, and if valid, will require a complete redefinition of the cosmological model. In other cases, as for instance certain types of scalar-tensor theories, the modifications act only as a perturbation on the original GR description; nevertheles they still include non-linearities that could lead very complex and interesting phenomenology. The philosophical discussion about GR being valid or not is open, but in any case, the only way to know which of these theories is closer to reality is to make the necessary predictions and compare them with observations. I will discuss the basics of this two types of modifications of gravity and give clarifications to common misconceptions that exist in the MONDian case. I will also show few results of my own work related to the search for testable predictions.