I am Joshua Boyd Hill,
A theoretical astrophysicist and
a PhD Student in the
Physics and Astronomy Department
at the University of California, Irvine.

Research

My research focuses on constraining the microscopic properties of dark matter through their macroscopic effects on structure formation and galaxy dynamics. While the standard ΛCDM paradigm provides an excellent description of the Universe on large scales, persistent discrepancies on galactic and sub-galactic scales motivate the exploration of well-motivated extensions, particularly self-interacting dark matter (SIDM).

I study how dark matter self-interactions modify the internal structure and evolution of dark matter halos, and how these modifications manifest in observable systems. My work combines theoretical modeling, cosmological simulations, and statistical inference to connect dark matter microphysics to astrophysical data in a physically grounded and testable way.

Self-Interacting Dark Matter and Halo Structure

A central focus of my research is the study of self-interacting dark matter using cosmological simulations and analytical methods. In SIDM models, elastic scattering between dark matter particles redistributes energy and momentum within halos, leading to characteristic signatures such as central density cores, altered subhalo populations, and gravothermal evolution in dense environments.

I investigate how these effects depend on the interaction cross section and its possible velocity dependence, and how they vary across halo mass and environment. By systematically comparing simulation predictions to observational probes including galaxy rotation curves, stellar kinematics, and strong gravitational lensing I aim to place robust constraints on viable SIDM parameter space while identifying observational signatures that can distinguish SIDM from ΛCDM.

Future Research Directions

My long-term research goal is to establish robust, physically grounded constraints on dark matter microphysics using a unified, multi-probe approach. In the near term, I plan to extend my work on SIDM by jointly analyzing constraints from galaxy dynamics and strong gravitational lensing across a wide range of halo masses.

Looking ahead, I aim to leverage next-generation datasets from surveys such as Rubin and Roman to move beyond individual systems toward population-level tests of dark matter models. By integrating simulations, inference techniques, and observations within a coherent framework, my research seeks to clarify the role of dark matter self-interactions in shaping cosmic structure.

Past Research: The Galaxy–Halo Connection

In previous work, I studied the galaxy–halo connection as a framework for linking dark matter halo properties to observable galaxy populations. This research examined how changes in halo internal structure and assembly history impact galaxy observables, and developed flexible, physically motivated mappings between galaxies and halos.

This work provided foundational experience in connecting theoretical models and simulations to data, and informs my current research by clarifying how uncertainties in galaxy formation propagate into constraints on dark matter physics.

Selected Projects

• Constraining Self-Interacting Dark Matter with Cosmological Simulations
  Using cosmological and zoom-in simulations to study how dark matter self-interactions modify halo structure and dynamical observables.
• Multi-Probe Tests of Dark Matter Microphysics
  Developing a unified inference framework that combines galaxy dynamics, stellar kinematics, and gravitational lensing.
• Galaxy–Halo Connection Modeling (Past Work)
  Developing galaxy–halo connection models to consistently compare ΛCDM and non-standard dark matter scenarios.

I am always interested in collaborations related to dark matter phenomenology, numerical simulations, and small-scale cosmology.