Moduli Stars

TL;DR

This paper investigates the formation and properties of long-lived compact objects called moduli stars, arising from scalar fields in string theory, with potential observational signatures like gravitational waves.

Contribution

It introduces new classes of star-like solutions from string moduli, including PQ-balls, and analyzes their stability, formation, and possible detection.

Findings

Heavy microscopic stars with masses depending on extra-dimensional volume.

Ultra-light axion-based macroscopic objects with exponentially large masses.

Growth of density perturbations during moduli-dominated early universe.

Abstract

We explore the possibility that (Bose-Einstein) condensation of scalar fields from string compactifications can lead to long-lived compact objects. Depending on the type of scalar fields we find different realisations of star-like and solitonic objects. We illustrate in the framework of type~IIB string compactifications that closed string moduli can lead to heavy microscopic stars with masses of order $\mathcal{V}^\alpha M_{\rm Planck}$, $\alpha=1,3/2,5/3$ where $\mathcal{V}$ is the volume of the extra dimensions. Macroscopic compact objects from ultra-light string axions are realised with masses of order $e^{\mathcal{V}^{2/3}}M_{\rm Planck}.$ Q-ball configurations can be obtained from open string moduli whereas the closed string sector gives rise to a new class of solutions, named PQ-balls, that arise in the two-field axion-modulus system. The stability, the potential for the formation, and the observability of moduli stars through gravitational waves are discussed. In particular we point out that during the early matter phase given by moduli domination, density perturbations grow by a factor $\mathcal{V}^{\beta}$ with $\beta>2$ and non-linear effects cannot be neglected.