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Particle Dynamics in Cosmological Spacetime

Herring, Nathan (2020) Particle Dynamics in Cosmological Spacetime. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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This thesis studies the evolution of quantum fields in the curved spacetime of the expanding early universe, focusing on applications to open questions in cosmology, with special concentration on particle theories of dark matter. Particularly, the processes of particle production and decay are analyzed in detail.

The usual calculation of particle decay rates proceeds by a perturbative approach which supposes global energy conservation, a property not manifest for expanding universes. We demonstrate how the decay law of scalar particles decaying during the radiation dominated epoch of the standard cosmology can be obtained by introducing an adiabatic approximation valid for degrees of freedom with sub-particle horizon wavelengths. The cosmological expansion is treated consistently, through non-perturbative methods borrowed from quantum optics and adapted for cosmology. Both scalar to scalar and scalar to fermion (with Yukawa couplings) decays are studied. The effects of cosmic expansion, leading to salient differences from the usual static spacetime results, are highlighted. We suggest implications for very long-lived particles (such as DM) and baryogenesis.

We also present our study of non-adiabatic cosmological production of dark matter. By stipulating that the dark matter be a spectator field in its vacuum state during inflation and concentrating on super-particle horizon modes immediately after inflation, the particle production for scalar and fermionic dark matter is analytically computed. In both cases, the distribution of produced particles is peaked at low comoving momentum. We obtain the full energy momentum tensor, show explicitly its equivalence with the fluid-kinetic one in the adiabatic regime, and extract the abundance, equation of state and free streaming length for the dark matter. We show how this mechanism yields a cold dark matter particle consistent with astronomical observations, without any coupling to Standard Model species, and with solely gravitational interactions. Thus, these models represent theories of the \emph{darkest} of dark matter. We argue that this abundance yields a lower bound on generic scalar (ULDM) and axion-like particles (ALP) to be included in any assessment of (ULDM)/(ALP) dark matter candidates. For fermions, this production surprisingly leads to a nearly thermal distribution with an emergent temperature, which warrants further analysis.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Herring, Nathannmh48@pitt.edunmh480000-0003-3334-3141
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee CoChairZentner, Andrewzentner@pitt.eduzentner
Committee CoChairBoyanovsky, Danielboyan@pitt.eduboyan
Committee MemberSavinov, Vladimirvps3@pitt.eduvps3
Committee MemberBatell, Brianbatell@pitt.edubatell
Committee MemberWalker,
Date: 16 September 2020
Date Type: Publication
Defense Date: 22 July 2020
Approval Date: 16 September 2020
Submission Date: 1 August 2020
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 367
Institution: University of Pittsburgh
Schools and Programs: Dietrich School of Arts and Sciences > Physics
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: Cosmology, Particle Physics, Dark Matter
Date Deposited: 16 Sep 2020 14:02
Last Modified: 16 Sep 2020 14:02

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