Viability of complex self-interacting scalar field as dark matter

TL;DR

This paper investigates a complex scalar field with self-interaction as a dark matter candidate, demonstrating its cosmological viability, formation of a Bose-Einstein condensate, and compatibility with observational constraints.

Contribution

It introduces a self-interacting scalar field model for dark matter, analyzing its cosmological evolution and showing it naturally leads to dark matter domination without fine tuning.

Findings

Model admits dark matter domination at late times.

Compatible with observed dark matter halo sizes and cosmological bounds.

Predicts dark matter particle mass around 1-10 meV for certain parameters.

Abstract

We study the viability of a complex scalar field $\chi$ with self-interacting potential $ V = m^\chi_0/2 \, |\chi|^2 + h \, |\chi|^4$ as dark matter. The scalar field is produced at reheating through the decay of the inflaton field and then, due to the self-interaction, a Bose-Einstein condensate of $\chi$ particles forms. The condensate represents dark matter in that model. We analyze the cosmological evolution of the model, stressing how, due to the presence of the self-interaction, the model naturally admits dark matter domination at late times, thus avoiding any fine tuning on the energy density of the scalar field at early times. Finally we give a lower bound for the size of dark matter halos at present time and we show that our model is compatible with dark matter halos greater than $0.1 \, Kpc$ and with BBN and CMB bounds on the effective number of extra neutrinos $\Delta_\nu^{eff}$. Therefore, the model is viable and for $h\simeq 10^{-4}-10^{-12}$ one obtains a mass $m^\chi \simeq m^{\chi}_0 \simeq 1-10^{-2} \, eV$ for dark matter particles from radiation-matter equality epoch to present time, but at temperatures $T_\gamma \gg 10 \, eV$, where $T_\gamma$ is the photons temperature, thermal corrections to $m^\chi_0$ due to the self-coupling $h$ are dominant.