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CATEGORIES:Defenses
DESCRIPTION:Title: Non-equilibrium Evolution of Scalar Fields in the Early 
 Universe\n\nAbstract:  This dissertation studies the non-equilibrium evolut
 ion of scalar fields in the early universe. It mainly covers three aspects 
 of the topic. The first is the evolution of multiple fields in a thermal ba
 ckground\, where their coupling to common decay channels in the background 
 induces effective couplings among them\, similar to $K^0-\bar{K}^0$ mixing.
  Within this framework\, we described the decay\, decoherence and thermaliz
 ation of axion condensates in a connected and uniform way\, discussed the C
 hern-Simons condensate induced in the medium by a coherent axion field and 
 its possible observation method. We also generalized the Wigner-Weisskopf T
 heory to a multi-particle description for field mixing in thermal backgroun
 d and between particles with different masses\, which complements and exten
 ds Lee-Oehme-Yang's theory of meson mixng used in the study of CP vilation.
  This method shows the quantum beats and asymptotic bath-induced coherence 
 of the system\, which are resonantly amplified when the masses of two parti
 cle species are close\, leading to potential observational consequences. In
  the second aspect we revisit the effective potential method for the dynami
 cs of a initially coherent and self-interacting scalar field. Our analysis 
 shows that due to the profuse particle production\, spinodal instability an
 d parametric amplification\, the valid regime of the effective potential is
  narrow. We then proposed a energy-conserving framework for this dynamics\,
  from which we learnt that particles are produced in highly entangled two-m
 ode squeezed states and coarse-graining of the density matrix in this basis
  leads to decoherence of the closed quantum systems. In the third aspect we
  did an ab initio calculation up to one loop for the relaxational dynamics 
 of a scalar condensate in a radiation  dominated era. A main conclusion is 
 that a phenomenological friction term is inadequate to describe the decay i
 n the super-Hubble regime\, during which (relevant for ultralight dark matt
 er) the condensate amplitude decays as $(mt)^{\frac{g^2}{10} t^2}$\, and in
  the sub-Hubble regime\, a local friction term always underestimates the ti
 mescale of the condensate decay as a consequence of the cosmological expans
 ion.
DTEND:20250508T170000Z
DTSTAMP:20260418T013204Z
DTSTART:20250508T150000Z
GEO:40.444273;-79.959034
LOCATION:Thaw Hall\, 102
SEQUENCE:0
SUMMARY:Shuyang Cao Dissertation Defense
UID:tag:localist.com\,2008:EventInstance_49587584311812
URL:https://calendar.pitt.edu/event/shuyang-cao-dissertation-defense
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