General M a t h ema t i c s Seminar (GMS)

��������� of the University of Luxembourgin cooperation with the Luxembourg Mathematical Society

Tuesday, 26th of June 2018, 4:30 pm

Campus Belval, Maison du Nombre, room MNO 1.040

Dr. Jean-Luc Lehners (Max-Planck-Institute for Gravitational Physics

(Albert-Einstein-Institute), Potsdam)

Jean-Luc Lehners (born 1978 in Luxembourg) studied physics

and mathematics at Imperial College London and at the

University of Cambridge. He obtained his Ph.D. in 2005 at

Imperial College for his research on braneworlds in supergravity.

After holding postdoctoral positions at the University of

Cambridge, Princeton University (USA) and the Perimeter

Institute (Canada), he established the Theoretical Cosmology

group at the Max Planck Institute for Gravitational Physics

(Albert Einstein Institute) in Potsdam in 2010. Dr. Lehners is

the recipient of both an ERC Starting Grant (2010) and a

Consolidator Grant (2017). He works on early universe

cosmology, with a particular emphasis on the big bang as well as

on quantum e�ects in cosmology.

Path integrals and the early universe

The prevailing theory for the origin of all structures in the universe is that they arose out

of ampli�ed quantum perturbations in the early universe. Beyond this crucial use of quantum

theory, it seems clear that if we want to understand the big bang, and the beginning of space

and time, we will need to apply quantum theory to the entire universe. We can make progress on

this front by applying semi-classical path integral techniques to gravity. In the presence of gravity,

there were however long-standing complications such as the conformal mode problem. We will

explain how a key development, namely the use of Picard-Lefschetz theory, allows us to overcome

these di�culties, and allows us to assess theories of the initial conditions of the universe such as

the no-boundary proposal of Hartle and Hawking. The gravitational path integral also hints at

the existence of quantum transitions across the big bang, into an earlier phase of cosmological

evolution.

Co�ee and cookies: 16:10 on the 6th �oor of the MNO, in the kitchen corner of maximal distance to the elevator.

Time and place of the talk: 16:30 (4:30 p.m.) in the Maison du Nombre, MNO 1.040.

University of Luxembourg contact : Jean-Marc Schlenker

Coordinator: Alexander D. Rahm