In this talk I will show how local measurements of the Hubble expansion rate are affected by structures like galaxy clusters or voids. In particular, I will present a fully relativistic treatment of this effect, studying how clustering modifies the mean distance (modulus)-redshift relation and its dispersion in a standard ΛCDM universe. Our findings is that cosmic variance (i.e. the effects of the local structure) is, for supernova observations at small redshifts (0.01<z<0.1) of the same order of magnitude as the current observational errors.The cosmic variance has to be added to this observational uncertainties and it reduces the tension with the CMB measurement of H0.

## Address

Département de Physique Théorique

Université de Genève

24, quai Ernest Ansermet

1211 Genève 4

Switzerland

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