New Insights into Primordial Black Hole Spins from Early Universe Conditions

Recent research has focused on the spins of primordial black holes (PBHs) that formed during an early matter-dominated era of the Universe. The paper titled "Revisiting spins of primordial black holes in a matter-dominated era based on peak theory" by Daiki Saito, Tomohiro Harada, Yasutaka Koga, and Chul-Moon Yoo estimates the probability distribution for these spins using the Zel'dovich approximation. This approach emphasizes the linear-order effects of cosmological perturbations that generate tidal torque.

The authors assume that fluctuations follow Gaussian statistics and apply peak theory to calculate the root mean square (RMS) and the probability distribution of the non-dimensional Kerr parameter, denoted as $a_{}$, at the time of formation. They find that the RMS value decreases with the amplitude of fluctuations, allowing for the establishment of a threshold value based on the under-extremal condition, where $a_{} < 1$.

Additionally, the study discusses the influence of spin and anisotropic collapse on the production rate of PBHs. It concludes that for certain variance conditions, the suppression from spin effects is significant, while anisotropic effects become more relevant under different circumstances. This research suggests that substantial values of PBH spins are expected, contrasting with the formation of PBHs in a radiation-dominated universe.

The findings have implications for understanding the formation and characteristics of primordial black holes, which are believed to play a role in the evolution of the Universe. The paper can be accessed at arXiv:2409.00435.