Primordial gravitational waves and the H0-tension problem

We analyze the H0-tension problem in the context of models of the early universe that predict a blue tilted spectrum of primordial gravitational waves (GWs), which is a positive value of the tensor tilt nT. By considering the GW’s contribution, NeffGW, to the effective number of relativistic degrees of freedom, Neff, and assuming standard particle physics, we discuss the effects of NeffGW on the background expansion, especially the constraints on the Hubble parameter H0. We analyze three scenarios that take into account the contribution of NeffGW using recent data of cosmic microwave background, baryon acoustic oscillation, the latest measurement of the local expansion rate, along with the LIGO constraints on the tensor to scalar ratio, r, and the tensor index. For the models explored, we show that an additional contribution from the primordial GW’s background to Neff does not solve but alleviates the current H0-tension problem.