This dissertation studies the aspects of dark matter and gravitational waves in dense and compact environments. We consider gravitational waves from supermassive black hole formation, dark photon production and decay in big bang nucleosynthesis, and millicharged dark matter acceleration in supernova shock. We begin with a general overview of the three topics studied in this dissertation.

In Chapter 2, we investigate the formation of supermassive black holes (SMBHs) from the collapse of high-redshift supermassive stars and study the neutrino burst-generated gravitational waves (GW). We investigated the GW signatures driven by the neutrino bursts, which create unique memory GW signals. Our result may provide an intriguing hint toward solving the problem of the formation of SMBHs in the high redshift universe.

In Chapter 3, we calculate the freeze-in abundance of dark photons and use the code BURST to trace the evolution of nucleosynthesis numerically from the beginning of weak decoupling with the presence of late-decay dark photons. Using the 1%-level precision in the primordial deuterium abundance measurements from quasar absorption lines, our result not only excludes a range of dark photon model parameters but also identify ranges of dark photon mass and couplings accessible in future Stage-4 cosmic microwave background experiments.

In Chapter 4, we provide a mechanism for millicharged dark matter (mDM) to scatter efficiently with Standard Model particles in supernova remnants. Our work reveals that the supernova shocks could sweep up and thermalize the ambient mDM via plasma instability. We also address the difficulty of getting Fermi-accelerated mDM owing to an ultra-slow instability upstream of the shock. Our result implies that the sensitivity for detection of terrestrial experiments for charged DM is, in fact, not strongly affected by supernova shocks, despite prior claims.