Special Theory HEP Seminar

Gravitational entropy and the flatness, homogeneity and isotropy puzzles

Neil Turok

Higgs Centre for Theoretical Physics
University of Edinburgh

We suggest a new explanation for the observed large scale flatness, homogeneity and isotropy of the universe. The basic ingredients are elementary and well-known, namely Einstein’s theory of gravity and Hawking’s method of computing gravitational entropy. The new twist is provided by the boundary conditions we recently proposed for “big bang” type singularities dominated by conformal matter, enforcing CPT symmetry and analyticity. Here, we show that, besides allowing us to describe the big bang, these boundary conditions allow new gravitational instantons, enabling us to calculate the gravitational entropy of cosmologies which include radiation, dark energy and space curvature of either sign. Based on these entropy considerations, we find that the most probable universe is flat. We argue it is also homogeneous and isotropic on large scales.