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SUMMARY:An open system approach to cosmology - Enrico Pajer\, University o
 f Cambridge
DTSTART:20251009T120000Z
DTEND:20251009T130000Z
UID:TALK237244@talks.cam.ac.uk
CONTACT:122734
DESCRIPTION:Cosmological models and predictions rely extensively on the we
 ll-established field theory framework of particle physics. However\, a qua
 litatively new challenge arises: cosmological systems inherently contain s
 ubstances with poorly constrained macroscopic properties and entirely unkn
 own microphysics\, such as the inflaton sector\, dark matter\, and dark en
 ergy. This results in a rich array of novel phenomena\, including dissipat
 ion\, stochastic fluctuations\, out-of-equilibrium dynamics\, and non-unit
 ary macroscopic evolution. Moreover\, since gravitational observables are 
 of primary interest\, and gravity universally couples to all forms of matt
 er\, a closed-system approach would require precise description of all cos
 mic constituents—something feasible only in the simplest toy models. To 
 address these challenges\, I propose an open system approach to cosmology.
 \n\nI begin with a pedagogical introduction to open quantum system techniq
 ues\, formulated within the Schwinger-Keldysh path integral framework. The
 n\, I present the open effective field theory of inflation as a general cl
 ass of theories of single-field inflation in the presence of an unknown me
 dium. This local dissipative single-field effective theory yields a new cl
 ass of predictions for cosmological correlators\, generalizing existing mo
 dels. I then tackle the challenge of formulating general relativity in the
  presence of an unspecified medium. As a warmup\, I present a Schwinger-Ke
 ldysh formulation of electromagnetism in a medium\, incorporating dissipat
 ion and fluctuations while ensuring a consistent treatment of gauge symmet
 ries within an open system framework. Building on these results\, I introd
 uce the general and systematic construction of dissipative extensions of g
 eneral relativity and explore their implications for modeling open dark en
 ergy and the late-time evolution of the universe. Finally\, I study the im
 plications for the dissipative propagation of gravitational waves through 
 the dark sector medium.
LOCATION:Potter room
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