Clocks and singularities in quantum gravity and quantum cosmology

by Steffen Gielen (University Sheffield)

Thursday 4 Jul 2024, 14:15 → 15:45 Europe/Berlin

Abbeanum/Ground floor-HS 2 - Hörsaal 2 (TPI, FSU Jena)

If quantum gravity is to respect the symmetries of classical relativity, there can be no absolute notion of evolution with the respect to an external time parameter, unlike in usual quantum mechanics. The issue is particularly pressing in cosmology where we try to give meaning to a quantum theory of the entire Universe. A long used strategy to resolve the issue is to define dynamics in relational terms, as the evolution of one degree of freedom with respect to another degree of freedom seen as a clock. The most common approach (appearing, e.g., in loop quantum cosmology and group field theory) is to use a massless scalar field as a clock, but other options are available, leading in general to inequivalent theories. After a general review, I will discuss a particular example in Wheeler-DeWitt quantum cosmology showing that key questions such as whether the theory respects unitarity and whether it resolves cosmological singularities depend on the choice of clock. I will also discuss extensions to black hole dynamics as well as clock choice issues in loop quantum cosmology and group field theory.