Primordial black holes generated by the non-minimal spectator field

We improve and generalize the non-minimal curvaton model originally proposed in arXiv:2112.12680 to a model in which a spectator field non-minimally couples to an inflaton field and the power spectrum of the perturbation of spectator field at small scales is dramatically enhanced by the sharp feature in the form of non-minimal coupling.At or after the end of inflation,the perturbation of the spectator field is converted into curvature perturbation and leads to the formation of primordial black holes(PBHs).Furthermore,for example,we consider three phenomenological models for generating PBHs with mass function peaked at~10^(-12) M_(⊙)and representing all the cold dark matter in our universe and find that the scalar induced gravitational waves generated by the curvature perturbation can be detected by the future space-borne gravitational-wave detectors such as Taiji,Tian Qin,and LISA.

Primordial black holes and third order scalar induced gravitational waves

The process of primordial black hole(PBH)formation is inevitably accompanied by scalar induced gravitational waves(SIGWs).The strong correlation between PBH and SIGW signals may offer a promising approach to detecting PBHs in upcoming gravitational wave experiments,such as the Laser Interferometer Space Antenna(LISA).We investigate third order SIGWs during a radiation-dominated era in the case of the monochromatic primordial power spectrum Pζ=Aζkσ(k-k*).For LISA observations,the relationships between the signal-to-noise ratio(SNR)and monochromatic primordial power spectrum are studied systematically,revealing that the effects of third order SIGWs extend the cutoff frequency from 2f+to 3f and lead to an approximately 200%increase in the SNR for the frequency band from 10^(-5)Hz to 1.6×10^(-3)Hz,corresponding to PBHs with masses in the range 4×10^(-12)M⊙~10^(-7)M⊙.We find that there is a critical value,A×=1.76×10^(-2),for the amplitude of the monochromatic primordial power spectra,such that when Aζ>A*,the energy density of third order SIGWs is larger than that of second order SIGWs.

Primordial black holes and secondary gravitational waves from chaotic infiation

Chaotic inflation is inconsistent with the observational constraint at 68% CL. Here, we show that the enhancement mechanism with a peak function in the noncanonical kinetic term not only helps the chaotic model V(φ) = V0φ^(1/3) satisfy the observational constraint at large scales but also enhances the primordial scalar power spectrum by seven orders of magnitude at small scales.The enhanced curvature perturbations can produce primordial black holes of different masses and secondary gravitational waves with different peak frequencies. We also show that the non-Gaussianities of curvature perturbations have little effect on the abundance of primordial black holes and energy density of the scalar-induced secondary gravitational waves.

On primordial black holes and secondary gravitational waves generated from inflation with solo/multi-bumpy potential

It is well known that a primordial black hole(PBH)can be generated in the inflation process of the early universe,especially when the inflation field has a number of non-trivial features that could break the slow-roll condition.In this study,we investigate a toy model of inflation with bumpy potential,which has one or several bumps.We determined that the potential with multi-bump can generate power spectra with multi-peaks in small-scale region,which can in turn predict the generation of primordial black holes in various mass ranges.We also consider the two possibilities of PBH formation by spherical and elliptical collapses.Finally,we discuss the scalar-induced gravitational waves(SIGWs)generated by linear scalar perturbations at second-order.

Primordial black holes and curvature perturbations from false vacuum islands

Recently,much attention has been focused on the false vacuum islands that are flooded by an expanding ocean of true-vacuum bubbles slightly later than most of the other parts of the world.These delayed decay regions will accumulate locally larger vacuum energy density by staying in the false vacuum longer than those already transited into the true vacuum.A false vacuum island with thus acquired density contrast of a super-horizon size will evolve locally from radiation dominance to vacuum dominance,creating a local baby Universe that can be regarded effectively as a local closed Universe.If such density contrasts of super-horizon sizes can ever grow large enough to exceed the threshold of gravitational collapse,primordial black holes will form similar to those collapsing curvature perturbations on super-horizon scales induced by small-scale enhancements during inflation.If not,such density contrasts can still induce curvature perturbations potentially observable today.In this paper,we revisit and elaborate on the generations of primordial black holes and curvature perturbations from delayed-decayed false vacuum islands during asynchronous first-order phase transitions with fitting formulas convenient for future model-independent studies.

Supermassive black holes triggered by QCD axion bubbles

Supermassive black holes(SMBHs)are ubiquitous in the center of galaxies,although the origin of their massive seeds is still unknown.In this paper,we investigate the formation of SMBHs from the quantum chromodynamics(QCD)axion bubbles.In this case,primordial black holes(PBHs)are considered as the seeds of SMBHs,which are generated from the QCD axion bubbles due to an explicit Peccei–Quinn(PQ)symmetry breaking after inflation.The QCD axion bubbles are formed when the QCD axion starts to oscillate during the QCD phase transition.We consider a general case in which the axion bubbles are formed with the bubble effective angle θ_(eff)∈(0,π],leading to the minimum PBH mass∼■(10^(4)−10^(7))M⊙for the axion decay constant f_(a)∼■(10^(16))GeV.The PBHs at this mass region may account for the seeds of SMBHs.

Gravitational waves and primordial black hole productions from gluodynamics by holography

Understanding the nature of quantum chromodynamics(QCD)matter is important but challenging due to the presence of nonperturbative dynamics under extreme conditions.We construct a holographic model describing the gluon sector of QCD at finite temperatures in the non-perturbative regime.The equation of state as a function of temperature is in good accordance with the lattice QCD data.Moreover,the Polyakov loop and the gluon condensation,which are proper order parameters to capture the deconfinement phase transition,also agree quantitatively well with the lattice QCD data.We obtain a strong first-order confinement/deconfinement phase transition at Tc=276.5 Me V that is consistent with the lattice QCD prediction.Based on our model for a pure gluon hidden sector,we compute the stochastic gravitational waves and primordial black hole(PBH)productions from this confinement/deconfinement phase transition in the early Universe.The resulting stochastic gravitational-wave backgrounds are found to be within detectability in the International Pulsar Timing Array and Square Kilometre Array in the near future when the associated productions of PBHs saturate the current observational bounds on the PBH abundances from the LIGO-Virgo-Collaboration O3 data.

Reconstruction of power spectrum of primordial curvature perturbations on small scales from primordial black hole binaries scenario of LIGO/VIRGO detection

As a candidate bound for the Binary Black Hole(BBH) merger events detected by LIGO/Virgo, Primordial Black Holes(PBHs)provide a useful tool to investigate the primordial curvature perturbations on small scales. Using the GWTC-1 to GWTC-3 catalogs, under the scenario that PBHs originate from large primordial curvature perturbations on small scales during infiationary epoch, we for the first time reconstruct the power spectrum of primordial curvature perturbations on small scales. It is found that the value of the amplitude of the primordial power spectrum is enhanced to O(10-2)on scales O(1) pc. This may imply the validity of PBH as a possible BBH merger candidate.

Primordial black hole from the running curvaton

In light of our previous study [Chin. Phys. C 44(8), 085103(2020)], we investigate the possibility of the formation of a primordial black hole in the second inflationary process induced by the oscillation of the curvaton. By adopting the instability of the Mathieu equation, one can utilize the δ function to fully describe the power spectrum.Owing to the running of the curvaton mass, we can simulate the value of the abundance of primordial black holes covering almost all of the mass ranges. Three special cases are given. One case may account for dark matter because the abundance of a primordial black hole is approximately 75%. As late times, the relic of exponential potential may be approximated to a constant of the order of a cosmological constant, which is dubbed as the role of dark energy.Thus, our model could unify dark energy and dark matter from the perspective of phenomenology. Finally, it sheds new light on exploring Higgs physics.

Multifrequency Gravitational Wave Background from Continuous Sources

Gravitational waves have been detected in the past few years from several transient events such as merging stellar mass black holes, binary neutron stars, etc. These waves have frequencies in a band ranging from a few hundred hertz to around a kilohertz to which LIGO type instruments are sensitive. LISA would be sensitive to much lower range of frequencies from SMBH mergers. Apart from these cataclysmic burst events, there are innumerable sources of radiation which are continuously emitting gravitational waves of all frequencies. These include a whole mass range of compact binary and isolated compact objects as well as close planetary stellar entities. In this work, quantitative estimates are made of the gravitational wave background produced in typical frequency ranges from such sources emitting over a Hubble time and the fluctuations in the h values measured in the usual devices. Also estimates are made of the high frequency thermal background gravitational radiation from hot stellar interiors and newly formed compact objects.

About the Observed Asymmetry between Matter and Antimatter

Some consequences, due to the existence of a pair of decoupled Schrödinger-like but relativistic quantum mechanics wave equations, are explored. It is shown that one equation directly describes the quantum states of a single spin-0 particle, and the other one indirectly describes the quantum states of the corresponding antiparticle. In correspondence with the matter-antimatter symmetry, for a Coulomb potential, a charge conjugation operation transforms the second equation in the first one. However, if a particle could interact with itself (gravitationally or electrically) due to the spread of its wavefunction, the C-symmetry could be broken;therefore, matter and antimatter could be distinguished. Under these assumptions, it is deducted the impossibility of the existence of particles and antiparticles with a mass larger than the Plank mass (mP), or with the absolute value of the charge larger than the Plank charge (qP). It is proposed the existence of primordial antimatter electrical sinks. It is also suggested that all macroscopic matter objects with a mass m > mP, and all macroscopic antimatter bodies with a charge |q| > qP should not be quantum but classical objects. It is argued that these findings could explain the absence of antimatter with a complicated structure and partially explain the excess of charged matter in the known Universe.

Dark sector production and baryogenesis from not quite black holes

Primordial black holes have been considered attractive dark matter candidates,whereas some of the predictions rely heavily on the near-horizon physics that remains to be tested experimentally.As a concrete alternative,thermal 2-2-holes closely resemble black holes without event horizons.Being a probable endpoint of gravitational collapse,they provide a solution to the information loss problem but also naturally result in stable remnants.Previously,we have considered primordial 2-2-hole remnants as dark matter.Owing to the strong constraints from a novel phenomenon associated with remnant mergers,only small remnants with mass approximate to the Planck mass can constitute all dark matter.In this paper,we examine the scenario in which the majority of dark matter consists of particles produced by the evaporation of primordial 2-2-holes,whereas the remnant contribution is secondary.The products with sufficiently light mass may contribute to the number of relativistic degrees of freedom in the early universe,which we also calculate.Moreover,2-2-hole evaporation can produce particles that are responsible for the baryon asymmetry.We observe that baryogenesis through direct B-violating decays or through leptogenesis can both be realized.Overall,the viable parameter space for the Planck remnant scenario is similar to that of primordial black holes with Planck remnants.However,heavier remnants result in different predictions,and the viable parameter space remains large even when the remnant abundance is small.

Stochastic ravitational wave background from PBH-ABH mergers

The measurement of gravitational waves produced by binary black-hole mergers at the Advanced LIGO has encouraged extensive studies on the stochastic gravitational wave background. Recent studies have focused on gravitational wave sources made of the same species, such as mergers from binary primordial black holes or those from binary astrophysical black holes. In this paper, we study a new possibility - the stochastic gravitational wave background produced by mergers of one primordial black hole and one astrophysical black hole. Such systems are necessarily present if primordial black holes exist. We study the isotropic gravitational wave background produced through the history of the universe. We find it is very challenging to detect such a signal. We also demonstrate that it is improper to treat the gravitational waves produced by such binaries in the Milky Way as a directional stochastic background due to a very low binary formation rate.