Did time have a beginning, and what came before the hot Big Bang?
In plain words
A theorem shows that an expanding universe like ours cannot be extended infinitely into the past using ordinary spacetime. Whether quantum gravity removes that edge or describes a true beginning is unknown.
Precise statement
The Borde-Guth-Vilenkin theorem (2003) shows that any spacetime with averaged expansion rate $H_{\mathrm{avg}} > 0$ along past-directed geodesics is past-incomplete, so eternal inflation alone cannot be past-eternal. Determine what quantum gravity puts at that boundary (no-boundary state, bounce, emergent or quantum creation) in a framework that yields predictions for observable spectra or spatial curvature.
What would settle it
A quantum-gravity model of the initial state that predicts a measurable signature (spatial curvature, low-multipole CMB features, tensor tilt) later confirmed.
Related problems
- More general than Is the Lorentzian no-boundary wave function well defined and stable?