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SUMMARY:How Does a Thermal Medium Affect Primordial Gravitational Waves?
DTSTART;VALUE=DATE-TIME:20261108T090000Z
DTEND;VALUE=DATE-TIME:20261108T093000Z
DTSTAMP;VALUE=DATE-TIME:20261023T154727Z
UID:indico-contribution-2970@indico.itp.ac.cn
DESCRIPTION:Speakers: Atsuhisa Ota (Chongqing University)\nA thermal mediu
 m modifies the propagation of gauge fields\, producing collective excitati
 ons with a plasmon gap. Does it affect primordial gravitational waves in t
 he same way? For gravity\, the thermal soft scale coincides with the Hubbl
 e rate\, making cosmic expansion essential to answering this question. In 
 this talk\, I compute the linear response of thermal radiation\, modeled b
 y a conformal scalar\, to metric perturbations on a spatially flat FLRW ba
 ckground. The calculation retains both retarded and contact contributions 
 and satisfies the diffeomorphism Ward identities. Although the adiabatic d
 ispersion relation exhibits a gap\, $\\omega_p^2=8\\mathcal H^2/5$\, with 
 $\\mathcal H$ the conformal Hubble rate\, superhorizon tensor modes remain
  conserved when the initial-time boundary of the retarded response is reta
 ined. The real-time evolution reproduces Weinberg’s gravitational-wave d
 amping\, with amplitude suppression accumulating around horizon reentry. I
  also explain how the superhorizon growth found in our earlier analyses ar
 ose from discarding the response history and adopting a state prescription
  incompatible with the Ward identities. These results clarify how a therma
 l collective gap coexists with conserved primordial perturbations and why 
 cosmological evolution requires a real-time treatment.\n\nhttps://indico.i
 tp.ac.cn/event/417/contributions/2970/
LOCATION:
URL:https://indico.itp.ac.cn/event/417/contributions/2970/
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