We use the interpolating coordinates studied by Hornbostel to investigate a transition from equal-time quantization to light-front quantization, in the context of two-dimensional ${\varphi }^4$ theory. A consistent treatment is found to require careful consideration of vacuum bubbles, in a nonperturbative extension of the analysis by Collins. Numerical calculations of the spectrum at fixed box size are shown to yield results equivalent to those of equal-time quantization, except when the interpolating coordinates are pressed toward the light-front limit. In that regime, a fixed box size is inconsistent with an accurate representation of vacuum-bubble contributions and causes a spurious divergence in the spectrum. The light-front limit instead requires the continuum momentum-space limit of infinite box size. The calculation of the vacuum energy density is then shown to be independent of the interpolation parameter, which implies that the light-front limit yields the same spectrum as an equal-time calculation. This emphasizes the importance of zero modes and near-zero modes in a light-front analysis of any theory with nontrivial vacuum structure.

In this paper, we perform a linear stability analysis of Israel-Stewart theory around a global equilibrium state, including the effects of shear-stress tensor, net-baryon diffusion current, and diffusion-viscous coupling. We find all the relevant modes of this theory and derive necessary conditions that these modes must satisfy in order to be stable and subluminal. With these conditions, we then derive constraints for the shear and diffusion relaxation times and the transport coefficients related to diffusion-viscous coupling.

We revisit the role of the chiral “triangle” anomaly in deeply inelastic scattering of electrons off polarized protons employing a powerful worldline formalism. We demonstrate how the triangle anomaly appears at high energies in the deeply inelastic scattering box diagram for the polarized proton structure function $g_1(x_B,Q^2)$ in both the Bjorken limit of large $Q^2$ and in the Regge limit of small $x_B$. We show that the operator product expansion is not required to extract the anomaly in either asymptotics though it is sufficient in the Bjorken limit. Likewise, the infrared pole in the anomaly arises in both limits. The leading contribution to $g_1$, in both Bjorken and Regge asymptotics, is therefore given by the expectation value of the topological charge density, generalizing a result previously argued by Jaffe and Manohar to hold for the first moment of $g_1$. In follow-up work, we will show how our results motivate the derivation of a helicity-dependent effective action incorporating the physics of the anomaly at small $x_B$ and shall discuss the QCD evolution of $g_1(x_B,Q^2)$ in this framework.

Quantum fluctuations of the vacuum stress-energy tensor are highly non-Gaussian, and can have unexpectedly large effects on spacetime geometry. In this paper, we study a two-dimensional dilaton gravity model coupled to a conformal field, in which the distribution of vacuum fluctuations is well understood. In this model, the fluctuations of the matter field are responsible for the fluctuations of the geometry itself. By analyzing the geodesic deviation in this model, we show that a pencil of massive particles propagating on this fuzzy spacetime eventually converges and collapses. This is consistent with our earlier analysis of null geodesics in [Phys. Rev. Lett. 107, 021303 (2011)].

Given a solution to 4D Einstein gravity with an isometry direction, it is known that the equations of motion are identical to those of a 3D $\sigma$ model with target space geometry $SU(1,1)/U(1)$. Thus, any transformation by $SU(1,1)\cong SL(2,\mathbb{R})$ is a symmetry for the action and allows one to generate new solutions in 4D. Here we clarify and extend recent work on electromagnetic (EM) duality in the context of the classical double copy. In particular, for pure gravity, we identify an explicit map between the Maxwell field of the single copy and the scalars in the target space, allowing us to identify the $U(1)\subset SL(2,\mathbb{R})$ symmetry dual to EM duality in the single copy. Moreover, we extend the analysis to Einstein-Maxwell theory, where we highlight the role of Ehlers-Harrison transformations and, for spherically symmetric charged black hole solutions, we interpret the equations of motion as a truncation of the putative single copy for Einstein-Yang-Mills theory.

We focus on the geometrical reformulation of free higher spin supermultiplets in 4D, $\mathcal{N}=1$ flat superspace. We find that there is a de Wit-Freedman like hierarchy of superconnections with simple gauge transformations. The requirement for sensible free equations of motion imposes constraints on the gauge parameter superfields. Unlike the nonsupersymmetric case, we find several different constraints that can decouple the higher superconnections. By lifting these constraints nongeometrically via compensators we recover all known descriptions of arbitrary integer and half-integer gauge supermultiplets. In the constrained formulation we find a new description of half-integer supermultiplets, generalizing the new-minimal and virial formulations of linearized supergravity to higher spins. However this description can be formulated using compensators. The various descriptions can be labeled as geometrical or nongeometrical if the equations of motion can be expressed purely in terms of superconnections or not.

Kerr–Vaidya metrics are the simplest nonstationary extensions of the Kerr metric. We explore their properties and compare them with the near-horizon limits of the spherically symmetric self-consistent solutions (the ingoing Vaidya metric with decreasing mass and the outgoing Vaidya metric with increasing mass) for the evaporating and accreting physical black holes. The Newman–Janis transformation relates the corresponding Vaidya and Kerr-Vaidya metrics. For nonzero angular momentum, the energy-momentum tensor violates the null energy condition (NEC). However, we show that its structure differs from the standard form of the NEC-violating tensors. The apparent horizon in the outgoing Kerr–Vaidya metric coincides with that of the Kerr black hole. For the ingoing metric, its location is different. We derive the ordinary differential equation for this surface and locate it numerically. A spherically symmetric accreting black hole leads to a firewall—a divergent energy density, pressure, and flux as perceived by an infalling observer. We show that this is also true for the outgoing Kerr–Vaidya metric.

We revisit Weyl’s unified field theory, which arose in 1918, shortly after general relativity was discovered. As is well known, in order to extend the program of geometrization of physics started by Einstein to include the electromagnetic field, H. Weyl developed a new geometry which constitutes a kind of generalization of Riemannian geometry. However, despite its mathematical elegance and beauty, a serious objection was made by Einstein, who considered Weyl’s theory not suitable as a physical theory since it seemed to lead to the prediction of a not yet observed effect, the so-called “second clock effect”. In this paper, our aim is to discuss Weyl’s proposal anew and examine its consistency and completeness as a physical theory. Finally, we propose new directions and possible conceptual changes in the original work. As an application, we solve the field equations assuming a Friedmann-Robertson-Walker universe and a perfect fluid as its source. Although we have entirely abandoned Weyl’s attempt to identify the vector field with the 4-dimensional electromagnetic potentials, which here must be simply viewed as part of the space-time geometry, we believe that in this way we could perhaps be led to a rich and interesting new modified gravity theory.

We conduct a thorough Bayesian analysis of the possibility that the black hole merger events seen in gravitational waves are primordial black hole (PBH) mergers. Using the latest merger rate models for PBH binaries drawn from a log-normal mass function, we compute posterior parameter constraints and Bayesian evidences using data from the first two observing runs of LIGO-Virgo. We account for theoretical uncertainty due to possible disruption of the binary by surrounding PBHs, which can suppress the merger rate significantly. We also consider simple astrophysically motivated models and find that these are favored decisively over the PBH scenario, quantified by the Bayesian evidence ratio. Paying careful attention to the influence of the parameter priors and the quality of the model fits, we show that the evidence ratios can be understood by comparing the predicted chirp mass distribution to that of the data. We identify the posterior predictive distribution of chirp mass as a vital tool for discriminating between models. A model in which all mergers are PBH binaries is strongly disfavored compared with astrophysical models, in part due to the overprediction of heavy systems having ${\mathcal{M}}_{\text{chirp}}\gtrsim 40M_{\odot }$ and positive skewness over the range of observed masses which does not match the observations. We find that the fit is not significantly improved by adding a maximum mass cutoff or a bimodal mass function or imposing that PBH binaries form at late times. We argue that a successful PBH model must either modify the log-normal shape of the initial mass function significantly or abandon the hypothesis that all observed merging binaries are primordial. We develop and apply techniques for analyzing PBH models with gravitational wave data, which will be necessary for robust statistical inference as the gravitational wave source sample size increases.

Black hole binaries formed dynamically in globular clusters are believed to be one of the main sources of gravitational waves in the Universe. Here, we use our new population synthesis code, cbhbd, to determine the redshift evolution of the merger rate density and masses of black hole binaries formed in globular clusters. We simulate $\sim 2\mathrm{million}$ models to explore the parameter space that is relevant to real globular clusters and overall mass scales. We show that when uncertainties on the initial cluster mass function and their initial half-mass density are properly taken into account, they become the two dominant factors in setting the theoretical error bars on merger rates. Uncertainties in other model parameters (e.g., natal kicks, black hole masses, and metallicity) have virtually no effect on the local merger rate density, although they affect the masses of the merging black holes. Modeling the merger rate density as a function of redshift as $\mathcal{R}(z)={\mathcal{R}}_0{(1+z)}^{\kappa }$ at $z<2$, and marginalizing over uncertainties, we find: ${\mathcal{R}}_0=7.2_{-5.5}^{+21.5}{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ and $\kappa =1.6_{-0.6}^{+0.4}$ (90% credibility). The rate parameters for binaries that merge inside the clusters are ${\mathcal{R}}_{0,\mathrm{in}}=1.6_{-1.0}^{+1.9}{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ and ${\kappa }_{\mathrm{in}}=2.3_{-1.0}^{+1.3}$; $\sim 20\%$ of these form as the result of a gravitational-wave capture, implying that eccentric mergers from globular clusters contribute $\lesssim 0.4{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ to the local rate. A comparison to the merger rate reported by Laser Interferometer Gravitational Wave Observatory-Virgo shows that a scenario in which most of the detected black hole mergers are formed in globular clusters is consistent with current constraints and requires initial cluster half-mass densities $\gtrsim {10}^4{M}_{\odot }{\mathrm{pc}}^{-3}$. Interestingly, these models also reproduce the inferred black hole mass function in the range $13–30M_{\odot }$. However, all models underpredict the data outside this range, suggesting that other mechanisms might be responsible for the formation of these sources.

All nonlinear extensions of the source-free Maxwell equations preserving both $SO(2)$ electromagnetic duality invariance and conformal invariance are found, and shown to be limits of a one-parameter generalization of Born-Infeld electrodynamics. The strong-field limit is the same as that found by Bialynicki-Birula from Born-Infeld theory but the weak-field limit is a new one-parameter extension of Maxwell electrodynamics, which is interacting but admits exact light-velocity plane-wave solutions of arbitrary polarization. Small-amplitude waves on a constant uniform electromagnetic background exhibit birefringence, but one polarization mode remains lightlike.

We study the photon trident process, where an initial photon turns into an electron-positron pair and a final photon under a nonlinear interaction with a strong plane-wave background field. We show that this process is very similar to double Compton scattering, where an electron interacts with the background field and emits two photons. We also show how the one-step terms can be obtained by resumming the small- and large-$\chi$ expansions. We consider a couple of different resummation methods and also propose new resummations (involving Meijer-G functions) which have the correct type of expansions at both small and large $\chi$. These new resummations require relatively few terms to give good precision.

Based on lattice nonrelativistic QCD (NRQCD) studies, we present results for Bethe-Salpeter amplitudes for $\mathrm{\Upsilon }(1S)$, $\mathrm{\Upsilon }(2S)$, and $\mathrm{\Upsilon }(3S)$ in vacuum as well as in quark-gluon plasma. Our study is based on $2+1$ flavor ${48}^3\times 12$ lattices generated using the Highly Improved Staggered Quark action and with a pion mass of 161 MeV. At zero temperature, the Bethe-Salpeter amplitudes follow the expectations based on nonrelativistic potential models. At nonzero temperatures, the interpretation of Bethe-Salpeter amplitudes turns out to be more nuanced but consistent with our previous lattice QCD study of excited Upsilons in quark-gluon plasma.

The low $x$ behavior of the gluon density $xg(x,{\mu }^2)$ at scale ${\mu }^2=2.4{\mathrm{GeV}}^2$ is determined using exclusive $J/\psi$ production data from HERA and LHCb within the framework of collinear factorization at next-to-leading order (NLO). It is shown that in the interval $3\times {10}^{-6}<x<{10}^{-3}$ the gluon distribution function grows as $xg(x,{\mu }^2)\propto x^{-\lambda }$ with $\lambda =0.135\pm 0.006$. The impact this experimental data will have for the global parton distribution function (PDF) analyses in this low $x$ domain is quantified. No indication in favour of parton density saturation is observed.

In this paper we systematically study a recently proposed model of spherically symmetric polymer black/white holes by Bodendorfer, Mele, and Münch (BMM), which generically possesses five free parameters. However, we find that, out of these five parameters, only three independent combinations of them are physical and uniquely determine the local and global properties of the spacetimes. After exploring the whole 3-dimensional (3D) parameter space, we show that the model has very rich physics, and depending on the choice of these parameters, various possibilities exist, including: (i) spacetimes that have the standard black/white hole structures, that is, spacetimes that are free of spacetime curvature singularities and possess two asymptotically flat regions, which are connected by a transition surface (throat) with a finite and nonzero geometric radius. The black/white hole masses measured by observers in the two asymptotically flat regions are all positive, and the surface gravity of the black (white) hole is positive (negative). In this case, there also exist possibilities in which the two horizons coincide, and the corresponding surface gravity vanishes identically. (ii) Spacetimes that have wormholelike structures, in which the two masses measured in the two asymptotically flat regions are all positive, but no horizons exist, neither a trapped (black hole) horizon nor an anti-trapped (white hole) horizon. (iii) Spacetimes that still possess curvature singularities, which can be either hidden inside trapped regions or naked. However, such spacetimes correspond to only some limit cases. In particular, the necessary (but not sufficient) condition is that at least one of the two “polymerization” parameters vanishes. These results are not in conflict to the Hawking-Penrose singularity theorems, as the effective energy-momentum tensor, purely geometric and resulted from the “polymerization” quantization, satisfies none of the three (weak, strong or dominant) energy conditions in any of the two asymptotically flat regions for any choice of the three independent free parameters, although they can hold at the throat and/or at the two horizons for some particular choices of them. In addition, it is true that quantum gravitational effects are mainly concentrated in the region near the throat, however, in this model even for solar mass black/white holes, such effects can be still very large at the black/white hole horizons, again depending on the choice of the parameters. Moreover, in principle the ratio of the two masses (for both of the black/white hole and wormhole spacetimes) can be arbitrarily large.

The detection of ultralight dark matter through interactions with nucleons, electrons, and photons has been explored in depth. In this work we propose to use precision muon experiments, specifically muon g-2 and electric dipole moment measurements, to detect ultralight dark matter (DM) that couples predominantly to muons. We set direct, terrestrial limits on DM-muon interactions using existing g-2 data, and show that a time-resolved reanalysis of ongoing and upcoming precession experiments will be sensitive to dark matter signals. Intriguingly, we also find that the current muon g-2 anomaly can be explained by a spin torque applied to muons from a pseudoscalar dark matter background that induces an oscillating electric dipole moment for the muon. This explanation may be verified by a time-resolved reanalysis.

The new round of experiments, MEG II, COMET/Mu2e, and Mu3e, would soon start to push the $\mu \to e\gamma$, $\mu N\to eN$ conversion, and $\mu \to 3e$ frontier, while Belle II would probe $\tau \to \mu \gamma$ and $\tau \to 3\mu$. In the general two Higgs doublet model with extra Yukawa couplings, we show that all these processes probe the lepton flavor violating (LFV) dipole transition that arises from the two loop mechanism, with scalar-induced contact terms subdominant. This is because existing data suggest the extra Yukawa couplings ${\rho }_{\mu e},{\rho }_{ee}\lesssim {\lambda }_e$, while ${\rho }_{\tau \mu },{\rho }_{\tau \tau }\lesssim {\lambda }_{\tau }$ and ${\rho }_{tt}\lesssim {\lambda }_t$, with ${\lambda }_i$ the usual Yukawa coupling of the Standard Model (SM), where ${\rho }_{\mu e}{\rho }_{tt}$ and ${\rho }_{\tau \mu }{\rho }_{tt}$ enter the $\mu e\gamma$ and $\tau \mu \gamma$ two loop amplitudes, respectively. With the $B_s\to \mu \mu$ decay rate basically consistent with SM expectation, together with the $B_s$ mixing constraint, we show that $B_s\to \tau \tau$ would also be consistent with SM, while $B_s\to \tau \mu$ and $B\to K\tau \mu$ decays would be out of reach of projected sensitivities, in strong contrast with some models motivated by the B anomalies.

We revisit the scenario of having a new interaction between neutrinos and a Higgs-like scalar suggested by short-baseline neutrino anomalies. We find that there are two possible attractive channels in the resulting effective 4-Fermi theory which lead to a neutrino condensation in cosmic neutrinos and the creation of a neutrino superfluid at low temperatures and finite chemical potential. We find that at the minimum of the effective potential $V$ the condensates are mostly made up of pairs of left-left $+$ right-right composites, with a tiny admixture of left-right $+$ right-left composites which differs from the scenario of Kapusta which focused on the energetically unfavored left-right $+$ right-left scenario. We obtain an approximate equation for the gap ${\mathrm{\Delta }}^2=4m_H(\mu -m)\mathrm{Exp}[-\frac{{\pi }^2}{G_{\mathrm{eff}}\mu \sqrt{{\mu }^2-m^2}}]$, where $m_H$ is the mass of the Higgs scalar, and $G_{\mathrm{eff}}$ is the effective (renormalized) 4- Fermi interaction. Recent constraints on cosmological neutrinos suggest the possibility that ${\log }_{10}({\mathrm{G}}_{\text{eff}}{\mathrm{MeV}}^2)=-1.72$ which would lead to an exponentially small gap.

We discuss an exact false vacuum decay rate at one loop for a real and complex scalar field in a quartic-quartic potential with two tree-level minima. The bounce solution is used to compute the functional determinant from both fluctuations. We obtain the finite product of eigenvalues and remove translational zero modes. The orbital modes are regularized with the zeta function and we end up with a complete renormalized decay rate. We derive simple expansions in the thin and thick wall limits and determine their validity.

Cosmography is an ideal tool to investigate the cosmic expansion history of the Universe in a model-independent way. The equations of motion in modified theories of gravity are usually very complicated; cosmography may select practical models without imposing arbitrary choices a priori. We use the model-independent way to derive $f(z)$ and its derivatives up to fourth order in terms of measurable cosmographic parameters. We then fit those functions into the luminosity distance directly. We perform the MCMC analysis by considering three different sets of cosmographic functions. Using the largest supernovae Ia Pantheon sample, we derive the constraints on the Hubble constant $H_0$ and the cosmographic functions, and find that the former two terms in Taylor expansion of luminosity distance work dominantly in $f(Q)$ gravity.