We revisit to investigate shadows cast by Kerr-like wormholes. The boundary of the shadow is determined by unstable circular photon orbits. We find that, in certain parameter regions, the orbit is located at the throat of the Kerr-like wormhole, which was not considered in the literature. In these cases, the existence of the throat alters the shape of the shadow significantly, and makes it possible for us to differentiate it from that of a Kerr black hole.

We identify a set of Hertz potentials for solutions to the vector wave equation on black hole spacetimes. The Hertz potentials yield Lorenz gauge electromagnetic vector potentials that represent physical solutions to the Maxwell equations, satisfy the Teukolsky equation, and are related to the Maxwell scalars by straightforward and separable inversion relations. Our construction, based on the Geroch-Held-Penrose formalism, avoids the need for a mode ansatz and leads to potentials that represent both static and nonstatic solutions. As an explicit example, we specialize the procedure to mode-decomposed perturbations of Kerr spacetime and in the process make connections with previous results.

We study the phenomenon of gravitational particle production as applied to a scalar spectator field in the context of $\alpha$-attractor inflation. Assuming that the scalar has a minimal coupling to gravity, we calculate the abundance of gravitationally produced particles as a function of the spectator’s mass $m_{\chi }$ and the inflaton’s $\alpha$ parameter. If the spectator is stable and sufficiently weakly coupled, such that it does not thermalize after reheating, then a population of spin-0 particles is predicted to survive in the Universe today, providing a candidate for dark matter. Inhomogeneities in the spatial distribution of dark matter correspond to an isocurvature component, which can be probed by measurements of the cosmic microwave background anisotropies. We calculate the dark-matter-photon isocurvature power spectrum and by comparing with the upper limits from Planck, we infer constraints on $m_{\chi }$ and $\alpha$. If the scalar spectator makes up all of the dark matter today, then for $\alpha =10$ and $T_{\mathrm{RH}}={10}^4\mathrm{GeV}$ we obtain $m_{\chi }>1.8\times {10}^{13}\mathrm{GeV}\approx 1.2m_{\varphi }$, where $m_{\varphi }$ is the inflaton’s mass.

With the goal of observing a stochastic gravitational-wave background (SGWB) with LISA, the spectral separability of the cosmological and astrophysical backgrounds is important to estimate. We attempt to determine the level with which a cosmological background can be observed given the predicted astrophysical background level. We predict detectable limits for the future LISA measurement of the SGWB. Adaptive Markov chain Monte Carlo methods are used to produce estimates with the simulated data from the LISA Data Challenge. We also calculate the Cramer-Rao lower bound on the variance of the SGWB parameter estimates based on the inverse Fisher information using the Whittle likelihood. The estimation of the parameters is done with the three LISA channels $A$, $E$, and $T$. We simultaneously estimate the noise using a LISA noise model. Assuming the expected astrophysical background around ${\mathrm{\Omega }}_{\mathrm{GW},\mathrm{astro}}(25\mathrm{Hz})=0.355\to 35.5\times {10}^{-9}$, a cosmological SGWB normalized energy density of around ${\mathrm{\Omega }}_{\mathrm{GW},\mathrm{Cosmo}}\approx 1\times {10}^{-12}$ to $1\times {10}^{-13}$ can be detected by LISA after 4 years of observation.

The gravitational coupling of nearby massive bodies to test masses in a gravitational wave (GW) detector cannot be shielded and gives rise to “gravity gradient noise” (GGN) in the detector. In this paper we show that for any GW detector using local test masses in the inner Solar System, the GGN from the motion of the field of $\sim {10}^5$ inner Solar System asteroids presents an irreducible noise floor for the detection of GW that rises exponentially at low frequencies. This severely limits prospects for GW detection using local test masses for frequencies $f_{\mathrm{GW}}\lesssim (\mathrm{few})\times {10}^{-7}\mathrm{Hz}$. At higher frequencies, we find that the asteroid GGN falls rapidly enough that detection may be possible; however, the incompleteness of existing asteroid catalogs with regard to small bodies makes this an open question around $f_{\mathrm{GW}}\sim \mu \mathrm{Hz}$, and further study is warranted. We show that a detector network placed in the outer Solar System would not be overwhelmed by this noise above $\sim 10\mathrm{nHz}$, and make comments on alternative approaches that could overcome the limitations of local test masses for GW detection in the $\sim 10\mathrm{nHz}–\mu \mathrm{Hz}$ band.

Extracting macroscopic properties of a system from microscopic interactions has always been an interesting topic with the most diverse applications. Here, we use the quantum Boltzmann equation to investigate the density matrix evolution of a system of nucleons. Using the quantum field theory tools for constructing the density matrix operators and calculating the interactions is the main advantage of this equation. The right-hand side of this equation involves forward scattering and usual collision terms. As examples of application, we calculate the standard Bloch equations for the nucleon system in the presence of a constant and an oscillating magnetic field from the forward scattering term. We find the longitudinal and transverse (decoherence) relaxation times from the collision term by considering the nucleon-nucleon scattering.

The differential cross section $d\sigma /d{q}^2$ of diffractive electroproduction of heavy quarkonia on protons is a sensitive study tool for the interaction dynamics within the dipole representation. Knowledge of the transverse momentum transfer $\overrightarrow{q}$ provides a unique opportunity to identify the reaction plane, due to a strong correlation between the directions of $\overrightarrow{q}$ and impact parameter $\overrightarrow{b}$. On top of that, the elastic dipole-proton amplitude is subject to a strong correlation between $\overrightarrow{b}$ and dipole orientation $\overrightarrow{r}$. Most of models for $b$-dependent dipole cross sections either completely miss this information, or make unjustified assumptions. We perform calculations basing on a realistic model for $\overrightarrow{r}\text{−}\overrightarrow{b}$ correlation, which significantly affect the $q$ dependence of the cross section, in particular the ratio of ${\psi }^{\prime }(2S)$ to $J/\psi (1S)$ yields. We rely on realistic potential models for the heavy quarkonium wave function and the Lorentz-boosted Schrödinger equation.

We study coherent diffractive photon and vector meson production in electron-proton and electron-nucleus collisions within the Color Glass Condensate effective field theory. We show that electron-photon and electron-vector meson azimuthal angle correlations are sensitive to nontrivial spatial correlations in the gluon distribution of the target, and perform explicit calculations using spatially dependent McLerran-Venugopalan initial color charge configurations coupled to the numerical solution of small $x$ JIMWLK evolution equations. We compute the cross-section differentially in $Q^2$ and $|t|$ and find sizeable anisotropies in the electron-photon and electron-$\mathrm{J}/\psi$ azimuthal correlations ($v_{1,2}\approx 2–10\%$) in electron-proton collisions for the kinematics of the future Electron-Ion Collider. In electron-gold collisions these modulations are found to be significantly smaller ($v_{1,2}<0.1\%$). We also compute incoherent diffractive production where we find that the azimuthal correlations are sensitive to fluctuations of the gluon distribution in the target.

In this paper we describe how a spectrum of entropic perturbations generated during a period of slow contraction can source a nearly scale-invariant spectrum of curvature perturbations on length scales larger than the Hubble radius during the transition from slow contraction to a classical nonsingular bounce (the “graceful exit” phase). The sourcing occurs naturally through higher-order scalar field kinetic terms common to classical (nonsingular) bounce mechanisms. We present a concrete example in which, by the end of the graceful exit phase, the initial entropic fluctuations have become negligible and the curvature fluctuations have a nearly scale-invariant spectrum with an amplitude consistent with observations.

Inspired by the evidence of the odderon exchange recently observed by the D0 and TOTEM Collaborations, a quantum chromodynamics (QCD) sum rule investigation is performed to study the odderon as a three-gluon bound state. There may exist six lowest-lying three-gluon odderons with the quantum numbers $J^{PC}=1/2/3^{\pm -}$. We systematically construct their interpolating currents, and calculate their mass spectra. To verify their existence, we propose to search for the spin-3 odderons in their $VVV$ and $VVP$ decay channels directly at LHC, with $V$ and $P$ light vector and pseudoscalar mesons respectively.

The null energy condition is considered the most fundamental of the energy conditions, on which several key results, such as the singularity theorems, are based. The Casimir effect is one of the rare equilibrium mechanisms by which it is breached without invoking modified gravity or nonminimal couplings to exotic matter. In this work we propose an independent dynamical mechanism by which it is violated, with the only ingredients being standard (but nonperturbative) quantum field theory and a minimally coupled scalar field in a double-well potential. As for the Casimir effect, we explain why the averaged null energy condition is not violated by this mechanism. Nevertheless, the transient behavior could have profound impacts in early universe cosmology.

Affine gravity is a connection-based formulation of gravity that does not involve a metric. After a review of basic properties of affine gravity, we compute the tree-level scattering amplitude of scalar particles interacting gravitationally via the connection in a curved spacetime. We find that, while classically equivalent to general relativity, affine gravity differs from metric quantum gravity.

The postmerger signal in binary black hole merger is described by linear, black hole perturbation theory. Historically, this has been modeled using the dominant positive-oscillation frequency (corotating), fundamental mode. Recently, there has been a renewed effort in modeling the postmerger waveform using higher, positive-oscillation frequency overtones in an attempt to achieve greater accuracy in describing the waveform at earlier times using linear perturbation theory. It has been shown that the inclusion of higher overtones can shift the linear regime to the peak of $(l,m)=(2,2)$ spherical harmonic mode. In this work, we show that the inclusion of negative-oscillation frequency (counterrotating) modes, called “mirror” modes, extends the validity of linear perturbation theory to even earlier times, with far lower systematic uncertainties in the model in recovering remnant parameters at these early times. A good description of the signal at early times also enables for a greater signal-to-noise ratio to be accumulated in the ringdown phase, thereby allowing for a more accurate measurement of remnant parameters and tests of general relativity.

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes.

We estimate the chirality of the cosmological plasma due to parity violating decays of Standard Model particles, focusing on the example of tau leptons. The nontrivial chirality is however too small to make a significant contribution to the cosmological magnetic field via the chiral-magnetic effect.

A haloscope of the QUAX–$a\gamma$ experiment composed of an oxygen-free high thermal conductivity-Cu cavity inside an 8.1 T magnet and cooled to $\sim 200\mathrm{mK}$ is put in operation for the search of galactic axion with mass $m_a\simeq 43\mu \mathrm{eV}$. The power emitted by the resonant cavity is amplified with a Josephson parametric amplifier whose noise fluctuations are at the standard quantum limit. With the data collected in about 1 h at the cavity frequency ${\nu }_c=10.40176\mathrm{GHz}$, the experiment reaches the sensitivity necessary for the detection of galactic QCD-axion, setting the 90% confidence level limit to the axion-photon coupling $g_{a\gamma \gamma }<0.766\times {10}^{-13}{\mathrm{GeV}}^{-1}$.