Primordial black holes from a cosmic phase transition: The collapse of Fermi-balls (2106.00111v3)

Abstract: We propose a novel primordial black hole (PBH) formation mechanism based on a first-order phase transition (FOPT). If a fermion species gains a huge mass in the true vacuum, the corresponding particles get trapped in the false vacuum as they do not have sufficient energy to penetrate the bubble wall. After the FOPT, the fermions are compressed into the false vacuum remnants to form non-topological solitons called Fermi-balls, and then collapse to PBHs due to the Yukawa attractive force. We derive the PBH mass and abundance, showing that for a $\mathcal{O}({\rm GeV})$ FOPT the PBHs could be $\sim10^{17}$ g and explain all of dark matter. If the FOPT happens at higher scale, PBHs are typically overproduced and extra dilution mechanism is necessary to satisfy current constraints.

• The paper introduces a novel mechanism where trapped fermions during a first-order phase transition form Fermi-balls that collapse into primordial black holes.
• Methodology involves detailed calculations of vacuum dynamics, fermion mass scales, and thermal effects to predict PBH masses and abundances.
• Implications suggest these PBHs could serve as dark matter candidates and offer new targets for gravitational wave and cosmic background observations.

Primordial Black Hole Formation from Cosmic Phase Transition: An Analysis of Fermi-Ball Collapse

This paper introduces a compelling mechanism for the formation of primordial black holes (PBHs) linked to a unique process involving a first-order phase transition (FOPT) in the early universe. The theoretical framework presented focuses on the interaction of fermions trapped during the FOPT, leading to the creation and subsequent collapse of non-topological solitons, termed Fermi-balls, into PBHs. The work by Kiyoharu Kawana and Ke-Pan Xie offers a new perspective on the genesis of these cosmological structures.

Mechanism and Methodology

The authors propose that if a fermion species gains significant mass in the true vacuum during a FOPT, these fermions become trapped in the false vacuum due to insufficient energy to pass through the bubble walls of the new phase. This entrapment leads to the compression of fermions into remnants, which form non-topological solitons named Fermi-balls. The collapse into PBHs occurs under the influence of the attractive Yukawa potential once the universe cools sufficiently, allowing the PBHs to form.

Central to this approach are several key variables: the mass of the fermion species, the parameters of the potential dictating vacuum transitions, and the thermal dynamics of the phase transition. The authors provide a robust mathematical treatment of the conditions necessary for Fermi-balls' formation and subsequent collapse, detailing how these entities can evolve to become PBHs. This includes calculating the PBH mass and abundance using the characteristics of the first-order phase transition and particle physics models.

Key Findings and Numerical Results

The paper derives several notable results. It demonstrates that for a FOPT occurring at a scale on the order of GeV, PBHs formed could have a mass around $\sim 10^{17}$ g. The derived parameters show these PBHs could potentially account for all dark matter, an intriguing possibility for resolving one of cosmology's major mysteries.

Moreover, the research indicates that if the FOPT occurs at even higher scales, an overproduction of PBHs is likely, necessitating additional universe expansion models such as entropy injection or secondary phase transitions to remain consistent with observational constraints. These constraints mainly arise from impacts on cosmic microwave background, BBN, and other astrophysical observations.

Theoretical and Practical Implications

The theoretical implications of this work are substantial, extending the potential origins of PBHs beyond traditional inflationary models. By introducing a linkage between cosmic phase transitions and PBH formation, the research bridges concepts from particle physics, cosmology, and astrophysical phenomena, such as dark matter.

Practically, the ability to interpret PBHs as a dark matter candidate opens up several paths for future research and exploration in both theoretical and observational astrophysics and cosmology. Addressing the scenarios where PBHs fit seamlessly into the current constraints on dark matter provides exciting avenues for investigating PBH signatures in gravitational wave data and cosmic background radiation.

Future Directions

Looking forward, experimental approaches to detect the consequences of such PBH formation scenarios would be impactful. Specifically, they would involve searching for variability in gravitational wave signals that Fermi-ball collapse might produce. Additionally, cosmologists could further refine large-scale simulations of early universe dynamics to predict the collapse of Fermi-balls more accurately, extending this approach to other candidate particles or interactions.

In conclusion, the paper presents a novel mechanism of PBH formation driven by particle physics processes and cosmic phase transitions. The research establishes a significant theoretical framework, providing predictions that invite both rigorous examination and sophisticated testing within the burgeoning field of cosmological physics and dark matter studies. This work lays the groundwork for converting theoretical predictions into observational insights in the near future.