# Inter-universal entanglement in a cyclic multiverse

## TL;DR

This paper explores quantum entanglement between parallel cyclic universes, analyzing entanglement entropy and temperature at various cosmological singularities and expansion points, revealing strong quantum effects at certain cosmic events.

## Contribution

It introduces a novel analysis of inter-universal entanglement in cyclic multiverses using thermodynamics, highlighting quantum effects at singularities and turning points.

## Key findings

- Entanglement entropy peaks at big bang, big crunch, and maximum expansion.
- Entanglement temperature is infinite at singularities and maxima, indicating strong quantum effects.
- Entanglement entropy approaches zero at big rip singularities, suggesting universe dissociation.

## Abstract

We study scenarios of parallel cyclic multiverses which allow for a different evolution of the physical constants, while having the same geometry. These universes are classically disconnected, but quantum-mechanically entangled. Applying the thermodynamics of entanglement, we calculate the temperature and the entropy of entanglement. It emerges that the entropy of entanglement is large at big bang and big crunch singularities of the parallel universes as well as at the maxima of the expansion of these universes. The latter seems to confirm earlier studies that quantum effects are strong at turning points of the evolution of the universe performed in the context of the timeless nature of the Wheeler-DeWitt equation and decoherence. On the other hand, the entropy of entanglement at big rip singularities is going to zero despite its presumably quantum nature. This may be an effect of total dissociation of the universe structures into infinitely separated patches violating the null energy condition. However, the temperature of entanglement is large/infinite at every classically singular point and at maximum expansion and seems to be a better measure of quantumness.