Title: Tracing out massive fields in cosmology

Speaker: Guanhao Sun 孙冠豪 (Chongqing University)

Time: 2:00 pm, Nov 19 (Thursday), 2026

Venue: Physics Building 物理楼（新）W260

Abstract: Cosmological correlators are calculated through the in-in formalism in (quasi-)de Sitter background. Their effective description is best represented by a reduced density matrix. In this talk, I will first review the differences between the usual EFT used in scattering amplitude calculations and the Schwinger-Keldysh EFT for correlators. I will then discuss the reduced density matrix for a light field at finite time, obtained by tracing out a massive field in a quasi-single-field model with non-linear couplings. The trace generates branch-mixing terms which are generally present in the Schwinger-Keldysh formalism. Explicit calculations of the effective description and the bispectrum will be shown for a conformally coupled massive field. I will then discuss how a local effective field theory is obtained through a large mass expansion of the exact non-local effective description. Finally, I will study the symmetry and Schwinger-Keldysh constraints on the reduced density matrix.

Bio:Guanhao Sun is a member of the Hongshen Young Faculty at Chongqing University. He received his Ph.D. in Physics from Columbia University in 2022 and was a postdoctoral researcher at the University of California, San Diego, from 2022 to 2025. His primary research interests are cosmology and effective field theory. His publications cover topics including inflationary cosmology, fundamental quantum field theory, and black hole superradiance.