Speaker: Anna Negro

Curing UV divergences in Primordial Observables — Can N_eff bounds constrain vacuum GWs?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)

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