Abstract

Cosmological models and predictions rely extensively on the well-established field theory framework of particle physics. However, a qualitatively new challenge arises: cosmological systems inherently contain substances with poorly constrained macroscopic properties and entirely unknown microphysics, such as the inflaton sector, dark matter, and dark energy. This results in a rich array of novel phenomena, including dissipation, stochastic fluctuations, out-of-equilibrium dynamics, and non-unitary macroscopic evolution. Moreover, since gravitational observables are of primary interest, and gravity universally couples to all forms of matter, a closed-system approach would require precise description of all cosmic constituents—something feasible only in the simplest toy models. To address these challenges, I propose an open system approach to cosmology.

I begin with a pedagogical introduction to open quantum system techniques, formulated within the Schwinger-Keldysh path integral framework. Then, I present the open effective field theory of inflation as a general class of theories of single-field inflation in the presence of an unknown medium. This local dissipative single-field effective theory yields a new class of predictions for cosmological correlators, generalizing existing models. I then tackle the challenge of formulating general relativity in the presence of an unspecified medium. As a warmup, I present a Schwinger-Keldysh formulation of electromagnetism in a medium, incorporating dissipation and fluctuations while ensuring a consistent treatment of gauge symmetries within an open system framework. Building on these results, I introduce the general and systematic construction of dissipative extensions of general relativity and explore their implications for modeling open dark energy and the late-time evolution of the universe. Finally, I study the implications for the dissipative propagation of gravitational waves through the dark sector medium.