Quantum Signatures of Gravity from Superpositions of Primordial Massive Particles

TL;DR

This paper investigates the quantum superpositions of primordial massive particles, analyzing their decoherence times and potential observational signatures, to explore quantum gravity effects in the early universe.

Contribution

It provides a detailed analysis of decoherence times for primordial particles and proposes observable signatures of their quantum superpositions in cosmological data.

Findings

Primordial particles up to 10^7 kg remain in pure quantum states over the universe's age.

Heavier particles' quantum states are limited by photon wavelength uncertainties.

Three observational signatures of quantum superpositions are identified: metric interference, gravitational wave lines, and quantum entanglement.

Abstract

We study the superposition of primordial massive particles and compute the associated decoherence time scale in the radiation dominated universe. We observe that for lighter primordial particles with masses up to $10^7\,\rm{kg}$, the corresponding decoherence time scale is significantly larger than the age of the observable universe, demonstrating that a primordial particle would persist in a pure quantum state, with its wavefunction spreading freely. For heavier particles, they can still be in a quantum state while their position uncertainties are limited by the wavelength of background photons. We then discuss three observational signatures that may arise from a quantum superposition of primordial particles such as primordial black holes and other heavy dark matter candidates, namely, interference effects due to superpositions of the metric, transition lines in the gravitational wave spectrum due to gravitationally bound states indicating the existence of gravitons, and witnesses of quantum entanglement between massive particles and of the gravitational field.