I will give an overview about some of the cosmology and tests of gravity that we can do with galaxy and weak lensing surveys like Euclid, some of the challenges (theoretical and simulations) that we face to take full advantage of the data once it comes and how studies of big underdense regions in our Universe - cosmic voids - can help to significantly improve the parameter constraints that we will be able to extract from such surveys on our cosmological model.
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