Stable Cosmic Time Crystals

TL;DR

This paper introduces a new class of stable cosmological time crystals that overcome previous gradient instability issues, potentially serving as a viable scalar matter source in cosmology.

Contribution

The authors present the first stable cosmological time crystals, addressing gradient instability problems in prior models and proposing a physically viable scalar matter candidate.

Findings

Demonstration of stable scalar field configurations in cosmology.

Resolution of gradient instability issues in time crystal models.

Potential application as a cosmological matter source.

Abstract

Cosmological time crystals are created when a scalar field moves periodically through phase space in a spatially flat Friedmann-Robertson-Walker spacetime due to the presence of a limit cycle. All such cosmological time crystals in the literature suffer from gradient instabilities occurring at Null Energy Condition violating phases where the square sound speed for cosmological perturbations becomes negative. Here we present stable cosmological time crystals. Our analysis suggests this new form of scalar matter--cosmic time crystals--may be considered as a physically viable cosmological matter source.