The celebrated singularity theorems of Penrose and Hawking imply that classically our universe had a singularity in its past.
This breakdown of general relativity is a clear sign that we need a more complete theory in order to understand the big bang and the purported beginning of our universe – in particular it is expected that we will need a theory of quantum gravity. Finding a full theory of quantum gravity has however proven rather difficult.
In this talk, Dr. Jean‐Luc Lehners will illustrate what we can learn about the early universe, and the big bang in particular, by using the tools already at our disposal, namely by using semi‐classical gravity in the path integral formalism.
This allows one to assess theories of initial conditions, such as the no‐boundary proposal of Hartle and Hawking, which was proposed as a replacement of the big bang.
It also allows us to investigate under what circumstances one can, and more interestingly when one cannot, describe the universe using quantum field theory in curved spacetime. But most promisingly, the semiclassical approach hints at the existence of quantum transitions across the big bang, into an earlier phase of evolution.
Brief biography
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 and the Perimeter Institute, 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 effects in cosmology.