Open Effective Field Theory and the Physics of Cosmological Collider Signals

TL;DR

This paper uses open effective field theory to analyze cosmological collider signals, revealing how local and non-local signals originate from different parts of the effective theory and linking non-local signals to entropy production.

Contribution

It introduces a framework for understanding cosmological collider signals through open EFT, distinguishing local and non-local contributions and connecting signals to quantum state properties.

Findings

Local signals arise from unitary operators in the EFT

Non-local signals are linked to stochastic noise

Non-analytic coefficients can be softened by scale hierarchy

Abstract

We examine the origin of the cosmological collider signal using the framework of open effective field theories. Focusing on the single exchange of a massive scalar field, we demonstrate that the trispectrum splits cleanly into its local and non-local components once the heavy-field propagators are decomposed in the Keldysh basis. Integrating out the massive degree of freedom yields a single-field effective field theory for the light scalar that necessarily contains both unitary operators and non-unitary contributions associated with dissipation and stochastic noise. We show that the leading local signal in parity-preserving theories arises from the unitary part of this effective field theory, whereas the non-local signal is intrinsically associated with its stochastic sector. The effective field theory coefficients themselves are a priori non-analytic in the external kinematics; however, this non-analyticity can be softened when a scale hierarchy - such as the heavy-mass expansion - is imposed, up to spurious contributions that ultimately cancel in observables. Finally, we establish a connection between the cosmological collider signal and entropy production, linking the observable non-local signal to intrinsic properties of the quantum state, including its degree of mixedness.