TPPC weekly seminar (in-person): Anna Negro - "Curing UV divergences in Primordial Observables -- Can N_eff bounds constrain vacuum GWs?"

Wednesday 8 May 2024, 12:30 → 14:00 Europe/London

Strand Building, S7.06

Speaker: Anna Negro (Leiden University)

Title: Curing UV divergences in Primordial Observables -- Can N_eff bounds constrain vacuum GWs?

Abstract: In the literature, constraints on primordial vacuum gravitational waves (GWs) -- tensorial counterpart of scalar quantum fluctuations that seed galaxies -- are often based on upper bounds on the effective number of species N_eff derived from CMB data. During my talk, I will examine the validity of this derivation. To that end, I will start by reviewing how "quantum corrections" should be included in computing observables, and show how UV divergences can be consistently renormalized in cosmological settings. Building on the example of a finite inflationary background evolution, I will demonstrate that physical scales corresponding to the beginning and end of inflation do not regulate UV divergences. In addition, I will derive a new formula for the energy density of GWs that does not assume a prior scale separation (as opposed to that of Isaacson). With this final ingredient at hand, I will rederive the contribution of primordial vacuum GWs presenting both foliation-dependent and fully covariant formulations, and address the question: can we use N_eff bounds to constrain primordial vacuum GWs? 

(this talk is based on: arXiv:2402.10008 , arXiv:2403.16806)