Entanglement in an expanding universe

by Nikolaos Tetradis (National and Kapodistrian University of Athens (GR))

Wednesday 11 Feb 2026, 16:00 → 17:00 Asia/Nicosia

B228 (ΘΕΕ02)

Abstract: The entanglement entropy is a measure of quantum entanglement between two systems. The entanglement entropy of the part of a massless scalar field at its ground state within a spherical region, assuming that the exterior is classically inaccessible, follows an area law in flat space. This is similar to black hole entropy. In the first part of the talk I discuss the evolution of the entanglement entropy in an expanding background. The formalism is applied to the inflationary period and the subsequent era of radiation domination, starting from the Bunch-Davies vacuum. Each field mode evolves towards a squeezed state upon horizon exit during inflation, with additional squeezing when radiation domination sets in. This results in the enhancement of the entropy. A volume term develops during radiation domination, similarly to thermodynamic entropy, and overwhelms the area term at late times. In the second part I discuss the form of the entanglement entropy in various gravitational backgrounds (de Sitter, anti-de Sitter space, the Einstein universe) focusing on the structure of the divergences. Universal coefficients are determined for ultraviolet and infrared divergent terms. In the third part I discuss the use of the finite part of the entropy for the calculation of c- and a-functions.

Speaker:  Nikos Tetradis was born in Greece in 1963. He obtained his BS in physics from the University of Athens in 1986, and his PhD in physics from Stanford University in 1991. He held research posts at DESY in Hamburg (1991-1993), Oxford University, with a Marie Curie Fellowship (1993-1995), CERN (1995-1997), Scuola Normale Superiore in Pisa (1997-1999), University of Crete (2000-2002). Since 2002 he has been a member of the faculty of the Department of Physics of the University of Athens, as Associate Professor (2002-2012) and Professor (2012-present). He has held visiting posts at European and American institutes: Visiting Professor, Stanford University (2008–2009); Scientific Associate, CERN (2013-2014, 2019-2020); Corresponding Associate, CERN (2016, 2017, 2018). He served as Chairman of the Department of Physics of the University of Athens (September 2016 – August 2018). His research areas span a wide range of subject covering beyond the standard model physics, dark matter and dark energy, cosmological perturbations and large scale structure, inflation and supersymmetry, phase transitions and the exact renormalization group, and applications of the AdS/CFT correspondence.