Browsing by Author "Ling, Siyang"

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    An analytic evaluation of gravitational particle production of fermions via Stokes phenomenon
    (Springer Nature, 2022) Hashiba, Soichiro; Ling, Siyang; Long, Andrew J.
    The phenomenon of gravitational particle production can take place for quantum fields in curved spacetime. The abundance and energy spectrum of gravitationally produced particles is typically calculated by solving the field’s mode equations on a time-dependent background metric. For purposes of studying dark matter production in an inflationary cosmology, these mode equations are often solved numerically, which is computationally intensive, especially for the rapidly-oscillating high-momentum modes. However, these same modes are amenable to analytic evaluation via the Exact Wentzel-Kramers-Brillouin (EWKB) method, where gravitational particle production is a manifestation of the Stokes phenomenon. These analytic techniques have been used in the past to study gravitational particle production for spin-0 bosons. We extend the earlier work to study gravitational production of spin-1/2 and spin-3/2 fermions. We derive an analytic expression for the connection matrix (valid to all orders in an adiabatic parameter ħ) that relates Bogoliubov coefficients across a Stokes line connecting a merged pair of simple turning points. By comparing the analytic approximation with a direct numerical integration of the mode equations, we demonstrate an excellent agreement and highlight the utility of the Stokes phenomenon formalism applied to fermions. We discuss the implications for an analytic understanding of catastrophic particle production due to vanishing sound speed, which can occur for a spin-3/2 Rarita-Schwinger field.
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    Cosmological gravitational particle production of dark matter during inflation and beyond
    (2024-08-05) Ling, Siyang; Long, Andrew J
    Cosmological gravitational particle production (CGPP) is a viable production mechanism for dark matter. Spin-0, spin-1/2, spin-3/2, spin-2 fields can be produced via CGPP during inflation and reheating, and the relics can be dark matter candidates. Chapter 1 of this thesis contains the framework of CGPP, which is based on quantum field theory in curved spacetime and inflationary cosmology. Chapter 2 contains a study of CGPP in the context of alpha-attractor inflationary models. Chapter 3 contains a method for analytically evaluating CGPP of fermions, namely the Exact WKB (EWKB) method, which is based on Stokes phenomenon. Chapter 4 contains a study of CGPP of massive spin-2 fields. Chapter 5 contains a study of phenomenology of ultralight vector dark matter with nonminimal interactions with gravity.
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    Cosmological gravitational particle production of massive spin-2 particles
    (Springer Nature, 2023) Kolb, Edward W.; Ling, Siyang; Long, Andrew J.; Rosen, Rachel A.
    The phenomenon of cosmological gravitational particle production (CGPP) is expected to occur during the period of inflation and the transition into a hot big bang cosmology. Particles may be produced even if they only couple directly to gravity, and so CGPP provides a natural explanation for the origin of dark matter. In this work we study the gravitational production of massive spin-2 particles assuming two different couplings to matter. We evaluate the full system of mode equations, including the helicity-0 modes, and by solving them numerically we calculate the spectrum and abundance of massive spin-2 particles that results from inflation on a hilltop potential. We conclude that CGPP might provide a viable mechanism for the generation of massive spin-2 particle dark matter during inflation, and we identify the favorable region of parameter space in terms of the spin-2 particle’s mass and the reheating temperature. As a secondary product of our work, we identify the conditions under which such theories admit ghost or gradient instabilities, and we thereby derive a generalization of the Higuchi bound to Friedmann-Robertson-Walker (FRW) spacetimes.