UC Riverside

The Standard Model of particle physics and standard cosmology are two widely accepted frameworks that describe the observable universe, from elementary particles to large-scale cosmological structures. Despite their success, they cannot explain the nature of dark matter, which constitutes approximately 27 percent of the energy content and 85 percent of the mass content of the universe. Terrestrial experiments, including direct detection and collider searches, have so far been unsuccessful in detecting any dark matter signals. While these experiments have placed strong constraints on the particle nature of dark matter, the parameter space they probe remains limited. This is where astrophysical observations become important, as they can explore a much wider parameter space for particle dark matter. In this study, we use compact astrophysical objects including neutron stars, supermassive black holes, and exoplanets to test predictions of several well-motivated dark matter models, including inelastic dark matter, light-mediator dark matter, and superheavy dark matter.The dense environment of neutron stars makes them an excellent target for probing dark matter interactions with the Standard Model. We study neutron star heating from capture of inelastic dark matter, which can evade direct detection constraints. We investigate the kinematics of inelastic scattering between quasirelativistic dark matter particles and both ultrarelativistic and nonrelativistic targets in neutron stars, and derive analytical expressions for the maximum mass gap that allows the scattering to occur. We implement them into a fully relativistic formalism for calculating the capture rate and apply it to various scenarios of inelastic dark matter. The projected constraints from neutron stars can systematically surpass those from terrestrial searches, including direct detection and collider experiments. Our results indicate that extreme astrophysical environments, such as neutron stars, are an important target for searching dark matter.The supermassive black hole at the center of M87 could redistribute dark matter particles within its sphere of influence, creating a high-density region known as a density spike. This spike can significantly enhance dark matter annihilation signals, making M87 a critical target for deriving stringent constraints on annihilation cross sections. In this work, we demonstrate that these constraints are highly sensitive to the choice of the halo density profile for M87. Motivated by recent kinematic studies of M87, we adopt a cored halo model and find that the constraints on dark matter annihilation are significantly relaxed. Specifically, in the cored halo scenario, the smooth part of the halo overwhelmingly dominates the annihilation signals, whereas the commonly assumed cuspy halo model attributes a major contribution to the spike. We demonstrate this effect using a dark matter model with a light mediator.Exoplanets, with their large volumes and low temperatures, are ideal celestial detectors for probing dark matter interactions. Dark matter particles can lose energy through scattering with the planetary interior and become gravitationally captured if their interaction with the visible sector is sufficiently strong. In the absence of annihilation, the captured dark matter thermalizes and accumulates at the planet's center, eventually collapsing into black holes. Using gaseous exoplanets as an example, we demonstrate that black hole formation can occur within an observable timescale for superheavy dark matter with masses greater than 106 rm GeV and nuclear scattering cross sections. The black holes may either accrete the planetary medium or evaporate via Hawking radiation, depending on the mass of the dark matter that formed them. We explore the possibility of periodic black hole formation within the unconstrained dark matter parameter space and discuss potential detection methods, including observations of planetary-mass objects, pulsed high-energy cosmic rays, and variations in exoplanet temperatures. Our findings suggest that future extensive exoplanet observations could provide complementary opportunities to terrestrial and cosmological searches for superheavy dark matter.