Date of Award

5-8-2024

Author's School

Graduate School of Arts and Sciences

Author's Department

Physics

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Neutron star mergers probe the high-density and high-temperature regime of dense mat- ter. The mergers can reach several times saturation density, and their temperatures can reach up to a hundred MeV. Simulations are essential for a better understanding of dense matter physics and for comparing theoretical predictions with observations. Current merger simulations, based on numerical general relativity and hydrodynamics, lack microphysical inputs. Therefore, the goal of the thesis is to develop models and provide microphysical insights for the theory community and the merger simulation community. Specifically, it includes the state-of-the-art study on flavor equilibration under neutron star merger con- ditions. This includes significant improvements on direct and modified Urca calculations, and the development of a unified approach to Urca processes, etc. As part of the MUSES framework, the physics has been implemented in the flavor equilibration module and is ready for use by the communities. In the first chapter, we will make a short introduction to neutron star mergers and the equation of states that describe the matters in the merger. In the second chapter, we present the state-of-the-art direct Urca calculations and subsequently correct the traditional cold beta-equilibrium. In the third chapter, we estimate the isospin relaxation rates, and find that at temperatures around five MeV, the relaxation timescale is comparable to the timescale of the density oscillations in the mergers, which produces a resonant peak in bulk viscosity. In the fourth chapter, we first provide the state-of-the-art modified Urca calculation, which is the first full phase space, fully relativistic rate calculation. In order to solve an unphysical divergence issue in the modified Urca calculation, we tackle the problem in two ways, and one of them leads to a unified approach that includes all Urca processes. In the fifth chapter, we give a short introduction to MUSES and our flavor equilibration module. Besides, in order to use a better interpolation scheme, we explored and implemented the Gaussian process regression, which outperforms linear interpolation in accuracy.

Language

English (en)

Chair and Committee

Mark Alford

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