Sailing through the big crunch-big bang transition

Understand

In a recent series of papers, we have shown that theories with scalar fields coupled to gravity (e.g., the standard model) can be lifted to a Weyl-invariant equivalent theory in which it is possible to unambiguously trace the classical cosmological evolution through the transition from big crunch to big bang.

• The key was identifying a sufficient number of finite, Weyl-invariant conserved quantities to uniquely match the fundamental cosmological degrees of freedom across the transition.
• In so doing we had to account for the well-known fact that many Weyl-invariant quantities diverge at the crunch and bang.
• Recently, some authors rediscovered a few of these divergences and concluded based on their existence alone that the theories cannot be geodesically complete.