Self-Gravitating Scalar Field Configurations, Ultra Light Dark Matter and Galactic Scale Observations

Abstract

In this thesis, we investigate the possibility that dark matter consists of ultra light spin-zero particles with mass $m\sim 10^{-22}\text{eV}$. We focus on the role of self-interactions, assuming all other non-gravitational couplings to Standard Model particles are negligible. Such ultra light dark matter (ULDM) is expected to form stable self-gravitating scalar field configurations (solitons), whose properties depend on the particle mass and self-coupling $\lambda$. Using solutions of the Gross-Pitaevskii-Poisson equations, we explore how galactic-scale observations can constrain $m$ and $\lambda$. We show that observational upper limits on the mass enclosed in central galactic regions can probe both attractive and repulsive self-interactions with strengths $\lambda \sim \pm 10^{-96}-10^{-95}$. We further demonstrate that self-interactions can allow ULDM to describe observed rotation curves as well as satisfy an empirical soliton-halo mass relation in low surface brightness galaxies for $m\sim 10^{-22}\text{eV}$ and $\lambda \gtrsim 10^{-90}$. We also study tidal effects in satellite dwarf galaxies and find that attractive self-interactions can extend their lifetimes over cosmological timescales, allowing ULDM to evade recent constraints derived for the non-interacting case. Finally, we explore machine learning based inference of dark matter and baryonic parameters from galaxy rotation curves, showing that neural networks can recover parameters consistent with observations.

Comments: Thesis accepted for the award of PhD degree from Ahmedabad University, September 2025. Based on arXiv:2202.11081, arXiv:2304.04463, arXiv:2310.19664 and arXiv:2412.03547