报告题目：Tracing out massive fields in cosmology

报 告 人：Guanhao Sun 孙冠豪 (Chongqing University)

报告时间：2026年11月19日14：00

报告地点：Physics Building 物理楼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.

报告人简介：孙冠豪，重庆大学弘深青年教师。2022年博士毕业于哥伦比亚大学物理系，2022-25年任加州大学圣迭戈分校博士后研究员。主要研究方向为宇宙学和有效场论。发表文章涵盖暴胀宇宙学，基础量子场论，黑洞超辐射等主题。

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.