In a recent study, researchers Tianning Wang, Evan Grohs, and Laura Mersini-Houghton from the University of North Carolina at Chapel Hill have explored the intriguing relationship between primordial black holes (PBHs) and Big Bang Nucleosynthesis (BBN). Their work, published in the journal Physical Review D, sheds light on how these enigmatic cosmic objects might have influenced the early universe’s thermal history and the formation of light elements.
Primordial black holes, which are hypothesized to have formed in the early universe, can evaporate over time through a process known as Hawking radiation. This study focuses on PBHs with masses ranging from 10^8 to 10^13 grams and investigates how their evaporation affects BBN, the process that created the light elements in the universe shortly after the Big Bang.
The researchers employed a bottom-up approach, incorporating PBH evaporation into a reaction-network code to assess its impact on the abundances of light elements. They found that PBH evaporation acts as an entropy injection mechanism, increasing the comoving entropy density. This means that to match the observed comoving entropy density per baryon from the cosmic microwave background (CMB), BBN simulations must start with a smaller initial entropy than in standard scenarios without PBHs.
The study also revealed a critical threshold near a PBH mass of approximately 10^10 grams. Above this threshold, the mass fraction of helium-4 (Y_P) increases steadily with the PBH mass fraction (β), driven by the enhanced Hubble expansion from the PBH energy density. Below this threshold, the behavior of Y_P becomes non-monotonic, influenced by the timing of PBH evaporation and its effect on nuclear reaction rates.
These findings underscore the sensitivity of BBN to PBH evaporation and provide a framework for understanding how populations of PBHs might have shaped the thermal history of the early universe. While this research is primarily theoretical, it offers valuable insights into the potential cosmic impacts of PBHs and could inform future studies on the early universe and the nature of dark matter.
Source: Wang, T., Grohs, E., & Mersini-Houghton, L. (2023). How Primordial Black Holes Change BBN. Physical Review D, 107(6), 063525.
This article is based on research available at arXiv.