# Clock Time in Quantum Cosmology

## TL;DR

This paper explores how a matter clock can induce a notion of time in quantum cosmology, showing that under certain conditions, the Wheeler-DeWitt equation yields a Schrödinger-like evolution for the universe's geometry.

## Contribution

It introduces a framework linking matter clocks to geometric evolution in quantum cosmology using Berry connections, with explicit models demonstrating the emergence of clock time.

## Key findings

- Clock time aligns with cosmic time when environment-induced decoherence occurs.
- Explicit wave functions for minimal quantum clocks are constructed.
- The Berry connection governs the transition from timeless to time-dependent descriptions.

## Abstract

We consider the conditioning of the timeless solution to the Wheeler-DeWitt equation by a predefined matter clock state in the simple scenario of de Sitter universe. The resulting evolution of the geometrodynamical degree of freedom with respect to clock time is characterized by the "Berry connection" of the reduced geometrodynamical space, which relies on the coupling of the clock with the geometry. When the connection vanishes, the standard Schr\"odinger equation is obtained for the geometry with respect to clock time. When one considers environment-induced decoherence in the semi-classical limit, this condition is satisfied and clock time coincides with cosmic time. Explicit results for the conditioned wave functions for minimal clocks made up of two quantum harmonic oscillator eigen-states are shown.