Research

Ultralight Dark Matter

Wave nature-induced relaxation & DM cores

While DF is suppressed in ULDM halos, I demonstrated that the wave nature of ULDM induces a distinct relaxation process–gravitational cooling–capable of efficiently dissipating kinetic energy. I performed comparative simulations of head-on subhalo collisions using a pseudo-spectral code (PyUltraLight) for ULDM and an SPH-based N-body code (GADGET4) for CDM. I established that ULDM subhalos undergo significantly greater kinetic energy dissipation via scalar wave emission compared to their CDM counterparts. These findings offer a novel resolution to the anomalous collisional dynamics observed in galaxy clusters like Abell 520, where DM cores exhibit collisional behavior contradictory to standard collisionless CDM predictions.

Resolution of the final parsec problem

Addressing the long-standing final parsec problem of supermassive black hole binary (SMBHB) mergers, we proposed a mechanism driven by the interplay between DF and ULDM wave dynamics. I simulated the evolution of SMBHBs within a solitonic ULDM core using a modified N-body integrated PyUltraLight code. My simulations revealed that the binary’s orbital decay excites density oscillations and quasi-normal modes in the scalar field, which efficiently radiate energy away from the system. This sustained DF, enhanced by gravitational cooling, drives the binary to merge rapidly, overcoming the stalling expected in stellar environments. These results directly impact the prediction of GW backgrounds, suggesting that ULDM environments e=could enhance merger rates detectable by LISA and PTA.

Repulsive self-interaction & the timing problem for Fornax GCs

Motivated by tightening constraints on the ULDM particle mass, I extended the framework to include repulsive self-interactions (SI, \(\sim \lambda \phi^4\)) and investigated their impact on galactic scale. I derived the analytical DF force for circular orbits within a repulsive SI-ULDM halo. By applying this formalism to the Fornax dwarf spheroidal galaxy (dSph), I demonstrated that a sufficiently strong SI extends the infall timescales of GCs beyond 10 Gyr, thereby resolving the classical ``timing problem’’. Crucially, I identified a cosmologically consistent parameter space (\(m_\phi/\lambda^{1/4} \sim \mathcal{O}(\mathrm{1~eV})\)) that satisfies these local constraints while remaining compatible with large-scale structure bounds. This work positions SI-ULDM as a compelling, observationally verifiable alternative to the standard fuzzy DM model.