Cosmology in theories with spontaneous scalarization of neutron stars

Abstract

In a model of spontaneous scalarization of neutron stars proposed by Damour and Esposite-Farese, a general relativistic branch becomes unstable to trigger tachyonic growth of a scalar field $\varphi$ toward a scalarized branch. Applying this scenario to cosmology, there is fatal tachyonic instability of $\varphi$ during inflation and matter dominance being incompatible with solar-system constraints on today's field value ${\varphi }_0$. In the presence of a four-point coupling $g^2{\varphi }^2{\chi }^2/2$ between $\varphi$ and an inflaton field $\chi$, it was argued by Anson et al. that a positive mass squared heavier than the square of a Hubble expansion rate leads to the exponential suppression of $\varphi$ during inflation and that ${\varphi }_0$ can remain small even with the growth of $\varphi$ after the radiation-dominated epoch. For several inflaton potentials approximated as $V(\chi )=m^2{\chi }^2/2$ about the potential minimum, we study the dynamics of $\varphi$ during reheating as well as other cosmological epochs in detail. For certain ranges of the coupling $g$, the homogeneous field $\varphi$ can be amplified by parametric resonance during a coherent oscillation of the inflaton. Incorporating the backreaction of created particles under a Hartree approximation, the maximum values of $\varphi$ reached during preheating are significantly smaller than those obtained without the backreaction. We also find that the minimum values of $g$ consistent with solar system bounds on $\varphi$ at the end of reheating are of order $10^{-5}$ and hence there is a wide range of acceptable values of $g$. Thus, the scenario proposed by Anson et al. naturally leads to the viable cosmological evolution of $\varphi$ consistent with local gravity constraints, without modifying the property of scalarized neutron stars.

Comments: 15 pages, 8 figures