Cosmology in a Time-Crystal Background

Abstract

We investigate the effects of a Time Crystal-like Condensate on cosmological dynamics. It is well known that quadratic gravity reduces to Einstein gravity along with a decoupled higher derivative dynamical scalar \cite{Alvarez-Gaume:2015rwa}. According to \cite{Chakraborty:2020ktp}, the above scalar sector can sustain a Time Crystal-like minimum energy state, with non-trivial time dependence. In the present work we treat the Time Crystal-like state as the background (that replaces the classical Minkowski vacuum) and study cosmic evolution on this ``dynamic'' ground state. In the first part we re-derive \cite{Chakraborty:2020ktp}, in a covariant and more systematic way, the frequencies that characterize the oscillator like Time crystalline condensate and interpret it as a background energy-momentum tensor simulating a matter-like effect. Importantly, no external matter is introduced here and the condensate, consists of a combination of the metric field $g_{\mu\nu}$ and is generated due to the $R^2$-term ($R$ is the Ricci scalar) in quadratic gravity \cite{Alvarez-Gaume:2015rwa}. In a way the spurious degrees of freedom of $R^2$-gravity turns into a useful component. The second part comprises of new effects where the cosmology in Friedmann-Lem\^{i}atre-Robertson-Walker (FLRW) universe is studied in presence of the energy-momentum tensor characterizing the Time Crystal Condensate. Under certain approximations, the scale factor of the FLRW universe is analytically obtained for any spatial geometry. We also find that the Time Crystal Condensate contributes as a new matter candidate having radiation-like behavior in the universe. Additionally, irrespective of the spatial geometry of the universe, the Time Crystal condensate generates a decelerating phase before the early acceleration starts. This is an indication of a contracting phase of the universe before its accelerated expansion.Comment: 11 pages, 4 compound figures. Comments are welcom