The stress tensor for gravitational waves and N_{eff} bounds revisited

by Subodh Patil

Tuesday May 19, 2026, 12:00 PM → 1:30 PM Europe/Madrid

IFAE Seminar Room (In-Person)

We critically re-examine the derivation for the stress tensor for gravitational waves, and the need for an averaging prescription in its definition. The latter, for instance, introduces a non-covariant scale into the problem, and leads to avoidable scheme dependences and inconsistencies when attempting renormalization. We attempt to disentangle several confusions in the literature (and textbooks), and argue for use of an unadulterated, unaveraged stress tensor in any computation that necessitates renormalization, or on cosmological spacetimes more generally, and conclude with a presentation of how vacuum fluctuations of the gravitational field do not constitute a physical gas of gravitons, even if its vacuum polarization effects leave real physical imprints. We revisit N_{eff} bounds in this context, and comment on the relevance of our findings to efforts to detect high frequency gravitational waves. The work presented in this talk is based on the following articles -- Phys.Rev.D 111 (2025) 4, 045009, Riv.Nuovo Cim. 47 (2024) 3, 179-228