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.
Highlights
Dynamical systems can display motion even in their lowest energy states
An exciting field theory extension explored scalar field oscillatory solutions in a cosmological context [3]; it was shown that the phase space of such solutions was plagued with regions containing ghost degrees of freedom or imaginary sound speed c2s < 0, referred to as a gradient instability [4]
We organize this note as follows: after a short description of the background and its instability, we briefly describe the effective field theory (EFT) of cosmological perturbations
Summary
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. We present a method to cure gradient instabilities in such solutions and provide the first examples of stable cosmological time crystals These novel fields are the first of their kind and have many different applications in cosmology.
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