Sweeping vacuum gravitational waves under the rug

Negro, A. (2024)

Doctoral Thesis

One of the most important correlation functions in physics, especially in cosmology, is the energy density, which describes how much energy is present at each point in spacetime due to matter fields. A key contribution to the energy density of the primordial universe comes from gravitational waves (GWs), as they offer a way to study epochs inaccessible with electromagnetic signals. This thesis focuses on vacuum GWs, the tensorial counterpart of scalar quantum fluctuations that seeded galaxies. Their renormalized energy density connects to the observed abundance of the first light elements, providing a pathway to constrain vacuum GWs without direct detection. Careful renormalization of the divergences arising in computing the energy density of vacuum GWs reveals that they only renormalize background quantities and do not act as an additional radiation-like species. We stress that a fundamental role in reaching this conclusion is played by properly following through with the renormalization procedure, which is not a way to "sweep divergences under the rug" by introducing a hard cutoff in divergent integrals. Instead, it is a well-defined method to absorb UV-divergent contributions and extract meaningful physical predictions.

• Early Universe Cosmology
• Quantum Field theory in Curved Spacetime
• Primordial Vacuum Fluctuations
• Vacuum Gravitational Waves
• Renormalization in Curved Spacetime
• Stress Energy Tensor Gravitational Waves
• Neff bounds

• All authors: Negro, A.
• Supervisor: Achúcarro, A.
• Co-supervisor: Patil, S.P.
• Committee: Akrami, Y.; Burgess, C.; Schalm, K.E.; Silvestri, A.; Molen, S.J. van der; Vulpen, I.B. van
• Qualification: Doctor (dr.)
• Awarding Institution: Leiden Institute of Physics (LION), Faculty of Science, Leiden University
• Date: 2024-10-01