Symmetry and Primordial Black Hole

Dear Colleagues,

Primordial black holes (PBHs) were formed in the early universe due to their large curvature perturbation on small scales when they re-enter the Hubble horizon. The phenomena related to PBHs are very fruitful. They involve the entirety of dark matter, some of the LIGO detections, seeds for galaxy and structure formation, hot spots for baryogenesis, etc. The related physics includes ultra-slow-roll inflation, multifield inflation, stochastic inflation, nonlinearity of the curvature perturbation, pairing and clustering of PBHs in the early universe, phase transition and the change of equation of state, microlensing, Hawking radiation and its remnant, profile of dark matter halo, induced gravitational waves, stochastic gravitational waves from the binary PBHs, etc.

The study of symmetry and primordial black holes continues to play an important role in our ongoing efforts to better understand the fundamental physics of our universe. The aim of this Special Issue is to encourage the publication of original research papers in a wide variety of fields related to PBHs in which theoretical or practical results of symmetry can be applied. Relevant topics include, but are not limited to:

1. Formation mechanisms: There are several proposed mechanisms for the formation of PBHs, including the collapse of density perturbations in the early universe, the collapse of cosmic strings, and the collision of bubbles during a phase transition;
2. Mass spectrum: The mass spectrum of PBHs can provide insights into the small-scale power spectrum of the curvature perturbation, as well as into the physics of the early universe. Additionally, the calculation of the PBH mass spectrum has some uncertainties, depending on the methods employed;
3. Observational constraints: Various observational techniques have been proposed for probing PBHs, including microlensing, extra-galactic gamma rays, CMB μ- and y-distorsion, dynamical constraints, and indirect probe by scalar-induced gravitational waves;
4. Induced gravitational waves: The stochastic gravitational wave background induced by the scalar perturbation, which is usually accompanied by the PBH formation by such large scalar perturbations, provides an indirect probe of PBHs. Additionally, such waves are the important scientific goals of the space-borne interferometers and pulsar timing arrays;
5. Early universe implications: PBHs were generated in the very early universe, of which the thermal history as well as the formation mechanisms can be probed by the use of PBHs. For instance, the softening of the equation of state in the QCD era can enhance the PBH formation, thus generating a peaked PBH mass function around 1 solar mass, which could be detectable for some inflation models. Besides, the PBHs generated by bubble nucleation reflect the property of first-order phase transition;
6. Distinguish with astrophysical black holes: Primordial black holes have some characteristic properties which can be used to distinguish with astrophysical black holes, such as high eccentricity and low spins. Besides, the mass functions of PBHs are also different from those of the astrophysical black holes. Recent LIGO result shows that the mass distribution of black holes which are LIGO-detected deviate from astrophysical predictions, which implies that a portion of such detections might be primordial.

Dr. Shi Pi
Guest Editor

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