An anthropic solution to the cosmological moduli problem

TL;DR

This paper proposes an anthropic solution to the cosmological moduli problem by selecting initial conditions that naturally produce the observed dark matter and baryon densities, avoiding the need for unnaturally high modulus masses.

Contribution

It introduces an anthropic mechanism that sets the initial modulus field strength to address the cosmological moduli problem without requiring high modulus masses.

Findings

The initial modulus field strength is anthropically selected to match observed matter ratios.

Neutralinos inherit reduced density, aligning with observed dark matter relic density.

The solution alleviates the need for unnatural SUSY breaking scales.

Abstract

Light moduli fields, gravitationally coupled scalar fields with no classical potential and which are expected to emerge as remnants from string theory compactification, are dangerous to cosmology in that 1. their late-time decays may disrupt successful Big Bang Nucleosynthesis (BBN), 2. they may decay into gravitino pairs which result in violation of BBN constraints or overproduction of lightest SUSY particles (LSPs, assumed to constitute at least a portion of the dark matter in the universe) and 3. they may decay directly into LSPs, resulting in gross DM overproduction. Together, these constitute the cosmological moduli problem (CMP). The combined effects require lightest modulus mass m_\phi >~10^4 TeV, and if the lightest modulus mass m_\phi is correlated with the SUSY breaking scale m_{3/2}, then the underlying SUSY model would be highly unnatural. We present a solution to the CMP wherein the lightest modulus initial field strength \phi_0 is anthropically selected to be \phi_0\sim 10^{-7}m_P by the requirement that the dark matter-to-baryonic matter ratio be not-too-far removed from its present value so that sufficient baryons are present in the universe to create observers. In this case, instead of dark matter overproduction via neutralino reannihilation at the modulus decay temperature, the neutralinos inherit the reduced moduli number density, thereby gaining accord with the measured dark matter relic density.