APEC Seminar (Astronomy - Particle Physics - Experimental Physics - Cosmology)

Speaker: Borna Salehian (IPM, Tehran, Iran)
Title: Effective theories for a nonrelativistic field in an expanding universe
Date (JST): Thu, Jun 04, 2020, 15:30 - 16:30
Place: Zoom Seminar
Related File: 2545.pdf
Abstract: A massive, nonrelativistic scalar field in an expanding spacetime is usually approximated by a pressureless perfect fluid, which leads to the standard conclusion that such a field can play the role of cold dark matter. In this talk I will try to systematically study subleading corrections. I will talk about two equivalent effective descriptions of the system: (i) A nonrelativistic effective field theory (EFT) with which we show that the relativistic corrections induce an effective self-interaction for the nonrelativistic field. (ii) An effective (imperfect) fluid description, with which we demonstrate that, for a perturbed
Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) universe: (a) The pressure is small but nonzero (and positive), even for a free theory with no tree-level self interactions. (b) The sound speed of small fluctuations is also nonzero (and positive), reproducing already known leading-order results, correcting a subdominant term, and identifying a new contribution that had been omitted in previous analyses. (c) The fluctuations experience a negative effective bulk viscosity. This is based on arXiv: 2005.05388