New Physics in Heavens: Astrophysical Probes of the Dark Universe

Abstract

Dark matter remains one of the most compelling mysteries in modern physics. Despite decades of effort, its nature and non-gravitational interactions remain unknown. The absence of a definitive direct-detection signal has motivated the exploration of a broad range of dark matter scenarios, including primordial black holes, axion-like particles, and sub-GeV dark matter. At the same time, advances in astrophysical, cosmological, and gravitational-wave observations over the last decade have opened new avenues for probing dark matter. Dark matter may be produced in extreme astrophysical environments, or it may leave observable signatures by modifying, heating, perturbing, or destroying sensitive astrophysical systems. In this thesis, I develop theoretical and computational frameworks to identify and characterize these signatures. I first show that dynamical friction from dark compact objects and axion minihalos can heat the baryonic gas during the dark ages, producing observable modifications to the global 21-cm signal and its power spectrum. I then study three-body interactions between primordial black holes and binary systems. I demonstrate that although perturbations from fast-moving primordial black holes vanish on average, individual encounters can produce stochastic changes in binary orbits, providing a possible probe of asteroid-mass dark compact objects. I also investigate exchange interactions in which a primordial black hole replaces one member of a binary and show that these processes can produce a small but potentially observable population of binaries containing invisible companions, including systems resembling the black hole binaries observed by Gaia. I further explore how long-range dark forces can modify the gravitational-wave emission from extreme mass-ratio inspirals. I show that independent mass measurements of the central black hole can break the degeneracies arising from uncertainties in the binary component masses, thereby improving the sensitivity to dark couplings. Finally, I study dark matter produced in core-collapse supernovae and its detection through electron recoils at large-volume neutrino detectors. Together, these studies demonstrate that astrophysical, cosmological, and gravitational-wave observations can probe dark matter across a wide range of masses and interactions, helping unravel its mysteries one signature at a time.