Towards resolving the sigma8 and H0 cosmic tensions

by Vivian Poulin (Montpellier)

Europe/Berlin (HS2 (5101.EG.502) and online)

HS2 (5101.EG.502) and online


In recent years, high precision cosmological data have revealed a number of `cosmic tensions’, i.e. a mismatch between the prediction of certain quantities in the `Lambda Cold Dark Matter’ model calibrated onto cosmic microwave background data (probing early times) and their direct measurement at low redshift (probing late times). In particular, tensions in the determination of `H0’ (the expansion rate today) and `sigma8’ (the local amplitude of fluctuation) have attracted a lot of theoretical (and experimental) attention, as they could be the first signs of the nature of a rich dark sector. In this talk, I will review promising theoretical efforts to resolve these tensions. I will show that resolving the H0 tension most likely require new physics in the early universe, but that current models generally exacerbate the sigma8 tension. I will then discuss guidelines to resolving the sigma8 tension. I will argue that the low-sigma8 values could indicate that cold Dark Matter is not stable on cosmological time-scales, decaying with a lifetime of ~15Gyrs, and leading to the creation of a warm dark matter component (representing roughly 30% of the total Dark Matter today). Future DESI and Euclid data will be able to detect — or exclude — this model.