Colloquium on May. 21, 2025

New cosmological probes of ultralight dark matter

Speaker: Bohua Li (Guangxi University)

Venue: SWIFAR Building 2111

Time: 16:00 PM, Wednesday, May. 21, 2025

Abstract:

Ultralight scalar field dark matter (SFDM) has become one of the most studied dark matter candidates over the last decade. As a viable alternative to cold dark matter (CDM), ultralight SFDM mimics CDM on large scales, while resolving some of the small-scale challenges to the CDM model potentially due to its quantum nature. The propotypical ultralight SFDM—ultralight axions—modeled by a free real scalar field has been subject to various observational tests and the Lyman-alpha forest data provide the strongest constraint so far. In this talk, I will present several new cosmological probes of ultralight SFDM and an extended ultralight SFDM model described by a complex scalar field. First, a possible early kination phase from the scalar field dynamics can amplify the inflationary stochastic gravitational-wave background (SGWB) and explain the recent pulsar timing array results. I will demonstrate why a first-principle calculation of the inflationary SGWB is necessary while presenting our dynamical-system-based code, stiffGWpy. I will further show that this code can be accelerated by deep learning methods and present our neural network emulator, SageNet, which achieves a speed-up of more than 10,000 times. Next, I will describe how ultralight SFDM delays the formation of first dwarf halos and thus affects the 21-cm signal from Cosmic Dawn and the Epoch of Reionization. Finally, I will show that the complex SFDM model acquires two novel elements compared with ultralight axions: the imperfect-fluid nature and the Noether current associated with the U(1) symmetry. I will discuss their dynamical implications for large-scale structure of the Universe.

Report PPT:  SWIFAR_Bohua Li.pdf