What Is The Nature Of Dark Matter’s Self-interaction, And Could It Explain The Unexpected Diversity In Galaxy Rotation Curves Across Different Mass Scales?

Abstract

The rise in rotational speed of galaxies, despite increasing radii away from the center, hints toward non-luminous, dark matter. The standard CDM model, predicting cuspy density profiles, was proposed to explain the nature of dark matter and, initially, aligned with the observations; however, later observations of dwarf galaxies induced complications, because these systems were found to have shallower, core-like density profiles. This issue, characterized as the ‘core-cusp’ problem, indicates diversity in galactic rotation curves at large. The Self-interacting Dark Matter (SIDM) model emerged as an alternative explanation for dark matter’s nature, where self-interactions between dark matter particles naturally produce cores. The intensity of these self-interactions is judged by the merit of cross-section per unit mass: σ/m_dm ∼ 1 cm²/g. This paper takes into account distinct behaviors of dark matter at three galactic mass scales: dwarf galaxies, mid-sized galaxies, and clusters. Although SIDM simulations were steadily in line with observations at lighter mass scales, it raised the requirement of velocity-dependent self-interaction cross-section in order to remain applicable at heavier mass scales. Not inclining toward SIDM only, this paper scrutinizes other proposed paradigms for explaining dark matter behaviour as well. Finally, it discusses what upcoming observatory missions may entail and proposes future research directions in four domains (observations, simulations, data analysis, and laboratory experiments) to vindicate or disprove SIDM framework.