Cosmological phase transitions proceed via the nucleation of bubbles that subsequently expand and collide. The resulting gravitational wave spectrum depends crucially on the bubble wall velocity. We use holography to compute the wall velocity from first principles in a strongly coupled, non-Abelian, four-dimensional gauge theory. The wall velocity is determined dynamically in terms of the nucleation temperature. We verify that ideal hydrodynamics provides a good description of the system everywhere except near the wall.

We propose a new broadband search strategy for ultralight axion dark matter that interacts with electromagnetism. An oscillating axion field induces transitions between two quasidegenerate resonant modes of a superconducting cavity. In two broadband runs optimized for high and low masses, this setup can probe unexplored parameter space for axionlike particles covering 15 orders of magnitude in mass, including astrophysically long-ranged fuzzy dark matter.

Flat-space cosmologies (FSCs) are solutions to three-dimensional theories of gravity without cosmological constants that have cosmological horizons. A detector located near the timelike singularity of the spacetime can absorb Hawking modes that are created near the horizon. Continuation of this process will eventually cause the entropy of the radiation to be larger than the entropy of the FSC, which leads to the information paradox. In this paper, we resolve this paradox for the FSC using the island proposal. To do this, we couple an auxiliary flat bath system to this spacetime in timelike singularity so that Hawking modes are allowed to enter the bath, and the entropy of radiation can be measured in its asymptotic region where gravity is also weak. We show that adding island regions that receive the partners of Hawking modes cause the entropy of radiation to follow a Page curve, which leads to resolving the information paradox. Moreover, we design a quantum teleportation protocol by which one can extract the information residing in islands.

Decay rates in quantum field theory (QFT) are typically calculated assuming the particles are represented by momentum eigenstates (i.e., plane waves). However, strictly speaking, localized free particles should be represented by wave packets. This yields width corrections to the decay rate and to the clock behavior under Lorentz boosts. We calculate the decay rate of a particle of mass $M$ modeled as a Gaussian wave packet of width $a$ and centered at zero momentum. We find the decay rate to be ${\mathrm{\Gamma }}_0[1-\frac{3a^2}{4M^2}+\mathcal{O}(\frac{a^4}{M^4})]$, where ${\mathrm{\Gamma }}_0$ is the decay rate of the particle at rest treated as a plane wave. The leading correction is then of order $\frac{a^2}{M^2}$. We then perform a Lorentz boost of velocity $v$ on the above Gaussian and find that its decay rate does not decrease exactly by the Lorentz factor $\sqrt{1-v^2}$. There is a correction of order $\frac{a^2{v}^2}{M^2}$. Therefore, the decaying wave packet does not act exactly like a typical clock under Lorentz boosts, and we refer to it is a “WP clock” (wave packet clock). A WP clock does not move with a single velocity relative to an observer but has a spread in velocities (more specifically, a spread in momenta). Nonetheless, it is best viewed as a single clock as the wave packet represents a one-particle state in QFT. WP clocks do not violate Lorentz symmetry and are not based on new physics; they are a consequence of the combined requirements of special relativity, quantum mechanics, and localized free particles.

Recently, Blázquez-Salcedo, Knoll, and Radu (BSKR) have given a class of static, spherically symmetric traversable wormhole spacetimes with Dirac and Maxwell fields. The BSKR wormholes are obtained by joining a classical solution to the Einstein-Dirac-Maxwell (EDM) equations on the “up” side of the wormhole ($r\ge 0$) to a corresponding solution on the “down” side of the wormhole ($r\le 0$). However, it can be seen that the BSKR metric fails to be $C^3$ on the wormhole throat at $r=0$. We prove that if the matching were done in such a way that the resulting spacetime metric, Dirac field, and Maxwell field composed a solution to the EDM equations in a neighborhood of $r=0$, then all of the fields would be smooth at $r=0$ in a suitable gauge. Thus, the BSKR wormholes cannot be solutions to the EDM equations. The failure of the BSKR wormholes to solve the EDM equations arises both from the failure of the Maxwell field to satisfy the required matching conditions (which implies the presence of an additional shell of charged matter at $r=0$) and, more significantly, from the failure of the Dirac field to satisfy required matching conditions (which implies the presence of a spurious source term for the Dirac field at $r=0$).

With the successes of $f(R)$ theory as a neutral modification of Einstein’s general relativity (GR), we continue our study in this field and attempt to find general neutral and charged black hole (BH) solutions. In the previous papers [Phys. Rev. D 102, 124022 (2020); Phys. Lett. B 820, 136475 (2021)], we applied the field equation of the $f(R)$ gravity to a spherically symmetric space-time $d{s}^2=-U(r)d{t}^2+\frac{d{r}^2}{V(r)}+r^2(d{\theta }^2+{\sin }^2\theta d{\varphi }^2)$ with unequal metric potentials $U(r)$ and $V(r)$ and with and without electric charge. Then we have obtained equations which include all the possible static solutions with spherical symmetry. To ensure the closed form of system of the resulting differential equations in order to obtain specific solutions, we assumed the derivative of the $f(R)$ with respect to the scalar curvature $R$ to have a form $F_1(r)=\frac{df(R(r))}{dR(r)}=1-\frac{F_0-(n-3)}{r^n}$ with a constant $F_0$ and show that we can generate asymptotically GR BH solutions for $n>2$ but we show that the $n=2$ case is not allowed. This form of $F_1(r)$ could be the most acceptable physical form that we can generate from it physical metric potentials that can have a well-known asymptotic form and we obtain the metric of the Einstein general relativity in the limit of $F_0\to n-3$. We show that the form of the electric charge depends on $n$ and that $n\ne 2$. Our study shows that the power $n$ is sensitive and why we should exclude the case $n=2$ for the choice of $F_1(r)$ presented in this study. We also study the physics of these black hole solutions by calculating their thermodynamical quantities, like entropy, the Hawking temperature and Gibb’s free energy, and derive the stability conditions by using geodesic deviations. In the standard Reissner-Nordström space-time which is the charged black hole solution in GR, there appear two black hole horizons, that is, inner horizon and outer horizon. When the radii of the two horizons coincide with each other, which is called the extremal limit, the absolute value of the charge equals to the mass and the Hawking temperature vanishes. In our model, however, the absolute value of the charge is not equal to the mass in the limit although the Hawking temperature vanishes.

Within the context of general relativity we study in a fully covariant way the so-called Euler-Maxwell system of equations. In particular, on decomposing the aforementioned system into its 1 temporal and $1+2$ spatial components at the ideal magnetohydrodynamic limit, we bring it in a simplified form that favors physical insight to the problem of a self-gravitating, magnetized fluid. Of special interest is the decomposition of Faraday’s law which leads to a general relation governing the evolution of the magnetic field during the motion of the highly conducting fluid. According to the latter relation, the magnetic field generally grows or decays in proportion to the inverse cube of the scale factor (not generally implying a cosmological setting in the first place)—associated with the continuous contraction or expansion of the fluid, respectively. The result in question, which has remarkable implications for the motion of the whole fluid, is subsequently applied to homogeneous (anisotropic-magnetized) cosmological models—especially to the Bianchi I case—as well as to the study of homogeneous and anisotropic gravitational collapse in a magnetized environment. Concerning the cosmological application, we derive the evolution equations of Bianchi I spacetime permeated by large-scale magnetic fields. As for the application in astrophysics, our results point out the crucial role of the electric Weyl curvature (associated with tidal forces) and the magnetic energy density in determining the fate of gravitational implosion.

The spacetime surrounding compact objects such as neutron stars and black holes provides an excellent place to study gravity in the strong, nonlinear, dynamical regime. Here, the effects of strong curvature can leave their imprint on observables which we may use to study gravity. Recently, NICER provided a mass and radius measurement of an isolated neutron star using x-rays, while LIGO/Virgo measured the tidal deformability of neutron stars through gravitational waves. These measurements can be used to test the relation between the tidal deformability and compactness of neutron stars that are known to be universal in general relativity. Here, we take (shift-symmetric) scalar-Gauss-Bonnet gravity (motivated by a low-energy effective theory of a string theory) as an example and study whether one can apply the NICER and LIGO/Virgo measurements to the universal relation to test the theory. To do so, this paper is mostly devoted on theoretically constructing tidally deformed neutron star solutions in this theory perturbatively and calculate the tidal deformability for the first time. We find that the relation between the tidal deformability and compactness remains to be mostly universal for a fixed dimensionless coupling constant of the theory though the relation is different from the one in general relativity. We also present a universal relation between the tidal deformability of one neutron star and the compactness for another neutron star that can be directly applied to observations by LIGO/Virgo and NICER. For the equations of state considered in this paper, it is still inconclusive whether one can place a meaningful bounds on scalar Gauss-Bonnet gravity with the new universal relations. However, we found a new bound from the mass measurement of the pulsar $\mathrm{J}0740+6620$ that is comparable to other existing bounds from black hole observations.

The standard cosmological model assumes that more than 85% of matter is in the form of collisionless and pressureless dark matter. Unstable decaying dark matter has been proposed in the literature as an extension to the standard cold dark matter model. In this paper, we investigate a scenario when dark matter decays, and the resultant particle moves with respect to the dark matter. A covariant hydrodynamical model is developed in which the decay is modeled by the transfer of energy-momentum between two dark dust fluid components. We parametrize the model in terms of the decay rate $\mathrm{\Gamma }$ and injection velocity $v_i$ of the resultant dark matter particles. We apply the framework to study the evolution of cosmic voids which are environments with low content of baryonic matter. Thus, unlike baryon-rich environments, voids provide an opportunity to measure dark matter signals that are less contaminated by complex baryonic processes. We find that the growth of S-type voids is modified by the dark matter decay, leading to imprints at the present day. This paper serves as a proof of concept that cosmic voids can be used to study dark mater physics. We argue that future cosmological observations of voids should focus on signs of reported features to either confirm or rule out the decaying dark matter scenario. Lack of presence of reported features could put constraints of the decay of dark matter in terms of $\mathrm{\Gamma }>H_0^{-1}$ and $v_i<10\mathrm{km}/\mathrm{s}$.

Making use of both the stochastic approach to the tunneling phenomenon and the threshold statistics, we offer a simple argument to show that critical bubbles may be correlated in first-order phase transitions and biased compared to the underlying scalar field spatial distribution. This happens though only if the typical energy scale of the phase transition is sufficiently high. We briefly discuss possible implications of this result, e.g., the formation of primordial black holes through bubble collisions.

The increasing interest in studying the role of holographic dark energy in the evolution of the very early Universe motivates us to study it for the scenario of warm inflation. Because of this scenario, the holographic dark energy, which now drives inflation, has an interaction with the radiation. The case of interacting dark energy also has received increasing interest in studying the late time cosmology. The infrared cutoff is taken as the Hubble length, and all corrections are assumed to be exhibited by the parameter $c$, which appears in the holographic dark energy. By comparing the predictions of the model with observational data, the free constants of the model could be determined. Then, by using these values of the constants, the energy density of inflation is estimated. Next, we consider the validity of the fundamental assumptions of the warm inflation, e.g., $T/H>1$, which is necessary to be held during inflation, for the obtained values of the constant. Gathering all outcomes, the model could be counted as a suitable candidate for warm inflation.

We study the effect of supergravity corrections due to a linear and a squared term in the Kähler potential in the context of a supersymmetric hybrid inflation model. By appropriate choice of the parameters associated to these terms, we are able to satisfy the main cosmological constraints for the spectral index $n_s$ and the tensor-to-scalar ratio $r$. In addition, this model predicts primordial black hole abundance enough to account for the whole dark matter of the Universe and gravitational wave spectra within the reach of future detection experiments. The predictions of the model can be made compatible to the NANOGrav reported signal at the cost of significantly lower primordial black hole abundance.

We compare the predictions of aafrag for the spectra of secondary photons, neutrinos, electrons, and positrons produced in proton-proton collisions to those of the parametrizations of Kamae et al., Kelner et al. and Kafexhiu et al. We find that the differences in the normalization of the photon energy spectra reach 20%–50% at intermediate values of the transferred energy fraction $x$, growing up to a factor of two for $x\to 1$, while the differences in the neutrino spectra are even larger. We argue that LHCf results on the forward production of photons and neutral pions favor the use of the qgsjet-II-04m model on which aafrag is based. The differences in the normalization have important implications in the context of multimessenger astronomy, in particular, for the prediction of neutrino fluxes, based on gamma-ray flux measurements, or regarding the inference of the cosmic ray spectrum, based on gamma-ray data. We note also that the positron-electron ratio from hadronic interactions increases with energy toward the cutoff, an effect which is missed using the average electron-positron spectrum from Kelner et al. Finally, we describe the publicly available python package aafragpy, which provides the secondary spectra of photons, neutrinos, electrons, and positrons. This package complements the aafrag results for protons with energies above 4 GeV with previous analytical parameterizations of particle spectra for lower energy protons.

We analyze the quasi-two-body charmed $B$ decays $B_{(s)}^{+,0}\to D_{(s)}^{*}{P}_2\to D_{(s)}{P}_1{P}_2$ with $P_{1,2}$ as a pion or kaon. The intermediate processes $B_{(s)}\to D_{(s)}^{*}{P}_2$ are calculated with the factorization-assisted topological-amplitude approach, and the resonant effects are calculated with the Breit-Wigner formalism. Taking all $P$-wave resonance states ${\overline{D}}_{(s)}^{*}$ into consideration, we present the related branching fractions, calculate the Breit-Wigner-tail effects, and investigate the flavor $SU(3)$ breaking effects. Most of our branching fractions are consistent with the perturbative QCD approach’s predictions as well as the current experimental data. With more precision calculation of the intermediate two-body charmed $B$-meson decays, our quasi-two-body $B$ decays calculation has significantly less theoretical uncertainty than the perturbative QCD approach. Many of those channels without any experimental data will be confronted with the future more accurate experiment measurements. Our results of the Breit-Wigner-tail effects also agree with the experimental data very well. In $B^0$ decays, this effect can reach approximately to 5%. It is also found that the Breit-Wigner tail effects are not sensitive to the widths of their corresponding resonances.

For a complete development of quantum electrodynamics in the presence of a strong external field, proper understanding of the resonant processes and all their peculiarities is essential. We present our attempt to analytically investigate the resonant case of laser-assisted ultrarelativistic electron-positron pair photoproduction on a nucleus. The initial gamma quantum with energy $\hslash {\omega }_i\lesssim 100\mathrm{GeV}$ and external field with intensity up to $I\sim {10}^{24}\mathrm{W}/{\mathrm{cm}}^2$ are considered. Due to the presence of the external field, the intermediate virtual particle may become real, herewith the second order process in the fine structure constant effectively reduces into the two successive first order processes. All inherent kinematics features were discussed in details and the resonant differential cross sections with simultaneous registration of the produced particle (positron for channel A or electron for channel B) energy and its outgoing angle were obtained. We established that the resonant energies of produced particles acquire two different values with respect to the positron (channel A) or electron (channel B) outgoing angle. Additionally, the certain minimal amount of absorbed wave photons is required for resonance to happen. Wherein, the resonant differential cross section significantly exceeds the corresponding one without the external field within the particular kinematic regions. Consequently, the considered process can be used as a marker for probing theoretical predictions of quantum electrodynamics with strong background electromagnetic field.

The radiative energy shift of an electron in a constant magnetic field has been considered in the framework of the ($2+1$)-dimensional quantum electrodynamics with four-component fermions as well as in reduced ${\mathrm{QED}}_{3+1}$ in which photons propagate in a ($3+1$)-dimensional bulk and fermions are localized on a 2-brane. Analytical expressions are obtained for the energy of interaction of the electron spin with an external field and the radiative shift of the electron mass after averaging over the spin states of the electron. For ultrarelativistic energies of an electron and relatively weak magnetic fields, the total probability of a photon emission by a massive electron and the anomalous magnetic moment of an electron in a reduced ${\mathrm{QED}}_{3+1}$ are calculated. The dependence of the obtained values on the invariant dynamic parameter of synchrotron radiation is investigated. The total probabilities of synchrotron radiation of a charged massless fermion and the production of a pair of charged massless fermions by a photon in an external magnetic field are calculated in ($2+1$)-dimensional models of quantum electrodynamics.

We propose a Standard Model extension by a $U(1{)}_R$ gauge symmetry where only right-handed chiral fermions can carry a nontrivial charge. Here, we show that the simplest anomaly-free solution to accommodate the proton charge radius discrepancy takes right-handed muons, ${\mu }_R$, and first generation quarks, $u_R$ and $d_R$. Consistency with the latest muon’s ($g-2$) measurements is achieved through an extra light scalar, which itself must lie in the tens of MeV mass range to be viable.

We consider the photoexcitation of charm and bottom pentaquarks with the holographic assignments $[\frac{1}{2}{\frac{1}{2}}^{-}{]}_{S=0,1}$ and $[\frac{1}{2}{\frac{3}{2}}^{-}{]}_{S=1}$, in the photoproduction of heavy vector mesons such as charmonia and bottomonia near threshold. We use a Witten diagram to combine the s-channel photoexcitation of holographic pentaquarks with a massive t-channel graviton or tensor glueball exchange, to extract the scattering amplitude for heavy meson photoproduction in the threshold region. The pentaquark signal is too weak to be detected at current electron facilities.

We analyze the decay modes of the three $[\frac{1}{2}{\frac{1}{2}}^{-}{]}_{S=0,1}$ and $[\frac{1}{2}{\frac{3}{2}}^{-}{]}_{S=1}$ nonstrange pentaquarks with hidden charm and bottom, predicted by holographic QCD in the heavy quark limit. In leading order, the pentaquarks are degenerate and stable by heavy quark symmetry. At next to leading order, the spin interactions lift the degeneracy and cause the pentaquarks to decay. We show that the open charm (bottom) decay modes dwarf the hidden charm (bottom) ones, with total widths that are consistent with those recently reported by LHCb for charm pentaquarks. Predictions for bottom pentaquarks are given. The result of the current paper will be used in the part III of the current series of paper [Y. Liu, K. A. Mamo, M. A. Nowak, and I. Zahed, following paper, Holographic charm and bottom pentaquarks. III. Excitations through photo-production of heavy mesons, Phys. Rev. D 104, 114023 (2021).].

We revisit the three nonstrange pentaquarks $[\frac{1}{2}{\frac{1}{2}}^{-}{]}_{S=0,1}$ and $[\frac{1}{2}{\frac{3}{2}}^{-}{]}_{S=1}$ predicted using the holographic dual description, where chiral and heavy quark symmetry are manifest in the triple limit of a large number of colors, large quark mass and strong ’t Hooft gauge coupling. In the heavy quark limit, the pentaquarks with internal heavy quark spin $S$ are all degenerate. The holographic pentaquarks are dual to an instanton bound to heavy mesons in bulk, without the shortcomings related to the nature of the interaction and the choice of the hard core inherent to the molecular constructions. We explicitly derive the spin-spin and spin-orbit couplings arising from next to leading order in the heavy quark mass, and lift totally the internal spin degeneracy, in fair agreement with the newly reported charmed pentaquarks from LHCb. New charm and bottom pentaquark states are predicted.