Einstein’s theory of general relativity has been extensively tested in weak gravitational fields, mainly with experiments in the Solar System and observations of radio pulsars, and current data agree well with the theoretical predictions. Nevertheless, there are a number of scenarios beyond Einstein’s gravity that have the same predictions for weak fields and present deviations only when gravity becomes strong. Here, we try to test general relativity in the strong field regime. We fit the x-ray spectrum of the supermassive black hole in Ark 564 with a disk reflection model beyond Einstein’s gravity, and we are able to constrain the black hole spin $a_{*}$ and the Johannsen deformation parameters ${\alpha }_{13}$ and ${\alpha }_{22}$ separately. For ${\alpha }_{22}=0$, we find $a_{*}>0.96$ and $-1.0<{\alpha }_{13}<0.2$ with a 99% confidence level. For ${\alpha }_{13}=0$, we get $a_{*}>0.96$ and $-0.1<{\alpha }_{22}<0.9$ with a 99% confidence level. Our measurements are, thus, consistent with the hypothesis that the supermassive compact object in Ark 564 can be described by the Kerr metric.

We utilize the chiral kinetic theory in a relaxation-time approximation to investigate the nonlinear anomalous responses of chiral fluids with viscous effects. Unlike the cases in equilibrium, it is found that the chiral magnetic effect and chiral vortical effect are modified by the shear and bulk strengths. Particularly, the shear strength could result in charged Hall currents for chiral magnetic and chiral vortical effects, which propagate perpendicular to applied magnetic fields and vorticity. These quantum corrections stemming from side jumps and anomalies are dissipative and pertinent to interactions. Although the nonequilibrium effects upon charge currents are dissipative, the second law of thermodynamics is still satisfied.

We discuss the experimental prospects for observing processes which violate lepton number ($\mathrm{\Delta }L$) in four units (or more). First, we reconsider neutrinoless quadruple beta decay, deriving a model independent and very conservative lower limit on its half-life of the order of ${10}^{41}\mathrm{ys}$ for ${}^{150}\mathrm{Nd}$. This renders quadruple beta decay unobservable for any feasible experiment. We then turn to a more general discussion of different possible low-energy processes with values of $\mathrm{\Delta }L\ge 4$. A simple operator analysis leads to rather pessimistic conclusions about the observability at low-energy experiments in all cases we study. However, the situation looks much brighter for accelerator experiments. For two example models with $\mathrm{\Delta }L=4$ and another one with $\mathrm{\Delta }L=5$, we show how the LHC or a hypothetical future pp collider, such as the FCC, could probe multilepton number violating operators at the TeV scale.

High-energy positron annihilation is a viable mechanism to produce dark photons ($A^{\prime }$). This reaction plays a significant role in beam-dump experiments using multi-GeV electron beams on thick targets by enhancing the sensitivity to $A^{\prime }$ production. The positrons produced by the electromagnetic shower can produce an $A^{\prime }$ via nonresonant ($e^{+}+e^{-}\to \gamma +A^{\prime }$) and resonant ($e^{+}+e^{-}\to A^{\prime }$) annihilation on atomic electrons. For visible decays, the contribution of resonant annihilation results in a larger sensitivity with respect to limits derived by the commonly used $A^{\prime }$-strahlung in certain kinematic regions. When included in the evaluation of the E137 beam-dump experiment reach, positron annihilation pushes the current limit on $ϵ$ downwards by a factor of 2 in the range $33\mathrm{MeV}/c^2<m_{A^{\prime }}<120\mathrm{MeV}/c^2$.

Spatially nonlocal matrix elements are useful lattice-QCD observables in a variety of contexts, for example in determining hadron structure. To quote credible estimates of the systematic uncertainties in these calculations, one must understand, among other things, the size of the finite-volume effects when such matrix elements are extracted from numerical lattice calculations. In this work, we estimate finite-volume effects for matrix elements of nonlocal operators, composed of two currents displaced in a spatial direction by a distance $\xi$. We find that the finite-volume corrections depend on the details of the matrix element. If the external state is the lightest degree of freedom in the theory, e.g., the pion in QCD, then the volume corrections scale as $e^{-m_{\pi }(L-\xi )}$, where $m_{\pi }$ is the mass of the light state. For heavier external states, the usual $e^{-m_{\pi }L}$ form is recovered, but with a polynomial prefactor of the form $L^m/|L-\xi {|}^n$ that can lead to enhanced volume effects. These observations are potentially relevant to a wide variety of observables being studied using lattice QCD, including parton distribution functions, double-beta-decay and Compton-scattering matrix elements, and long-range weak matrix elements.

The subleading power of the scattering amplitude for deeply-virtual Compton scattering (DVCS) off the nucleon contains leading-twist and twist-3 generalized parton distributions (GPDs). We point out that in DVCS, at twist-3 accuracy, one cannot address any individual twist-3 GPD. This complication appears on top of the deconvolution issues familiar from the twist-2 DVCS amplitude. Accessible are exclusively linear combinations involving both vector and axial-vector twist-3 GPDs. This implies, in particular, that the (kinetic) orbital angular momentum of quarks can hardly be constrained by twist-3 DVCS observables. Moreover, using the quark-target model, we find that twist-3 GPDs can be discontinuous. The discontinuities however cancel in the DVCS amplitude, which further supports the hypothesis of factorization at twist-3 accuracy.

The anomalous decays $f_1(1285)\to {\rho }^0{\pi }^{+}{\pi }^{-}$ and $a_1(1260)\to \omega {\pi }^{+}{\pi }^{-}$ violating natural parity for vectors and axial-vectors are studied in the framework of the Nambu–Jona-Lasinio model. We consider the Lagrangian with $U(2{)}_L\times U(2{)}_R$ chiral symmetric four-quark interactions. The theory is bosonized, and corresponding effective meson vertices are obtained in the leading order of $1/N_c$ and derivative expansions. The uncertainties related with the surface terms of anomalous quark triangle diagrams are fixed by the corresponding symmetry requirements. We make a numerical estimate of the decay widths $\mathrm{\Gamma }(f_1(1285)\to {\rho }^0{\pi }^{+}{\pi }^{-})=2.74\mathrm{MeV}$ and $\mathrm{\Gamma }(a_1(1260)\to \omega {\pi }^{+}{\pi }^{-})=293\mathrm{keV}$. Our result on the $f_1(1285)\to {\rho }^0{\pi }^{+}{\pi }^{-}$ decay rate is in good agreement with experiment. It is shown that a strong suppression of the $a_1(1260)\to \omega \pi \pi$ decay is a direct consequence of destructive interference between box and triangle anomalies.

We explore the infrared limit of quantum gravity in the presence of a cosmological constant or effective potential for scalar fields. For a positive effective scalar potential, one-loop perturbation theory around flat space is divergent due to an instability of the graviton propagator. Functional renormalization solves this problem by a flow of couplings avoiding instabilities. This leads to a graviton barrier limiting the maximal growth of the effective potential for large values of scalar fields. In the presence of this barrier, variable gravity with a field dependent Planck mass can solve the cosmological constant problem by a cosmological runaway solution. We discuss the naturalness of tiny values of the cosmological constant and cosmon mass due to a strong attraction towards an infrared fixed point.

We study the absorption of monochromatic electromagnetic plane waves impinging upon a Kerr black hole, in the general case that the direction of incidence is not aligned with the black hole spin axis. We present numerical results that are in accord with low- and high-frequency approximations. We find that circularly polarized waves are distinguished by the black hole spin, with counter-rotating polarizations being more absorbed than co-rotating polarizations. At low frequencies and moderate incidence angles, there exists a narrow parameter window in which superradiant emission in the dipole mode can exceed absorption in the non-superradiant modes, allowing a planar electromagnetic wave to stimulate net emission from a black hole.

We study the superradiance effect in a class of scalar-tensor Horndeski theory. We first study the dynamics of a massive charged scalar wave scattered off the horizon of a Reissner-Nordström black hole which, in addition to its canonical coupling to gravity, is also coupled kinetically to curvature. We find that a trapping potential is formed outside the horizon of a Reissner-Nordström black hole, due to this coupling, and as the strength of the new coupling is increased, the scattered wave is superradiantly amplified, resulting in the instability of the Reissner-Nordström spacetime. We then consider the backreacting effect of the scalar field coupled to curvature interacting with the background metric, and we study the superradiant effect and find the superradiance conditions of a massive charged wave scattered off the horizon of a Horndeski black hole.

We study shadows cast by a certain class of rotating wormholes and point out the crucial role of a rotating wormhole throat in the formation of a shadow. Overlooking this crucial role of a wormhole throat has resulted in incomplete results in the previous studies on shadows of the same class of rotating wormholes. We explore the dependence of the shadows on the spin of the wormholes. We compare our results with that of the Kerr black hole. With increasing values of the spin, the shapes of the wormhole shadows start deviating considerably from that of the black hole. Such considerable deviation, if detected in future observations, may possibly indicate the presence of a wormhole. In other words, the results obtained here indicate that, through the observations of their shadows, the wormholes which are considered in this work and have reasonable spin can be distinguished from a black hole.

Twisted gravitational waves (TGWs) are nonplanar waves with twisted rays that move along a fixed direction in space. We study further the physical characteristics of a recent class of Ricci-flat solutions of general relativity representing TGWs with wave fronts that have negative Gaussian curvature. In particular, we investigate the influence of TGWs on the polarization of test electromagnetic waves and on the motion of classical spinning test particles in such radiation fields. To distinguish the polarization effects of twisted waves from plane waves, we examine the theoretical possibility of existence of spin-twist coupling and show that this interaction is generally consistent with our results.

In this paper, we explore the signatures of nonrotating and rotating black hole mergers in the matter-free modified gravity. First, we solve the unstable circular null orbits and the innermost stable circular timelike orbits via the geodesic motion. The characteristic quantities of these orbits are systematically analyzed by varying the black hole spin and the scalar field parameter of the gravity. Then based on it, we study the ringdown modes from the light ring/quasinormal modes correspondence. The final spins of the merged black holes are also estimated with the Buonanno-Kidder-Lehner recipe. Several black hole merging cases are investigated in detail. All these results show that the black hole mergers are closely dependent on the scalar field parameter of the gravity.

We study inflation in a supersymmetric Pati-Salam model driven by a potential generated in the context of no-scale supergravity. The Pati-Salam gauge group $SU(4{)}_C\times SU(2{)}_L\times SU(2{)}_R$ is supplemented with a $Z_2$ symmetry. Spontaneous breaking via the $SU(4)$ adjoint leads to the left-right symmetric group. Then the $SU(2{)}_R$ breaks at an intermediate scale and the inflaton is a combination of the neutral components of the $SU(2{)}_R$ doublets. We discuss various limits of the parameter space, and we show that consistent solutions with the cosmological data for the spectral index $n_s$ and the tensor-to-scalar ratio $r$ are found for a wide range of the parameter space of the model. Regarding the latter, which is a canonical measure of primordial gravity waves, we find that $r\sim {10}^{-3}-{10}^{-2}$. An alternative possibility where the adjoint scalar field $S$ has the rôle of the inflaton is also discussed.

The current $3.8\sigma$ tension between local [A. G. Riess et al., arXiv:1804.10655.] and global [N. Aghanim et al. (Planck Collaboration), Astron. Astrophys. 596, A107 (2016).] measurements of $H_0$ cannot be fully explained by the concordance $\mathrm{\Lambda }\mathrm{CDM}$ model. It could be produced by unknown systematics or by physics beyond the standard model. In particular, nonstandard dark energy models were shown to be able to alleviate this tension. On the other hand, it is well known that linear perturbation theory predicts a cosmic variance on the Hubble parameter $H_0$, which leads to systematic errors on its local determination. Here, we study how including in the likelihood the cosmic variance on $H_0$ affects statistical inference. In particular we consider the $\gamma \mathrm{CDM}$, $w\mathrm{CDM}$ and $\gamma w\mathrm{CDM}$ parametric extensions of the standard model, which we constrain with the latest CMB, BAO, SNe Ia, RSD and $H_0$ data. We learn two important lessons. First, the systematic error from cosmic variance is—independently of the model—approximately ${\sigma }_{\mathrm{cv}}\approx 0.88\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$ (1.2% $H_0^{\mathrm{loc}}$) when considering the redshift range $0.0233\le z\le 0.15$, which is relative to the main analysis of [A. G. Riess et al., arXiv:1804.10655.], and ${\sigma }_{\mathrm{cv}}\approx 1.5\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$ (2.1% $H_0^{\mathrm{loc}}$) when considering the wider redshift range $0.01\le z\le 0.15$. Although ${\sigma }_{\mathrm{cv}}$ affects the total error budget on local $H_0$, it does not significantly alleviate the tension which remains at $\approx 3\sigma$. Second, cosmic variance, besides shifting the constraints, can change the results of model selection: much of the statistical advantage of nonstandard models is to alleviate the now-reduced tension. We conclude that, when constraining nonstandard models it is important to include the cosmic variance on $H_0$ if one wants to use the local determination of the Hubble constant by Riess et al. [arXiv:1804.10655]. Doing the contrary could potentially bias the conclusions.

The formation of astrophysical and primordial black holes influences the distribution of dark matter surrounding them. Black holes are thus expected to carry a dark matter “dress” whose properties depend on their formation mechanism and on the properties of the environment. Here we carry out a numerical and analytical study of the merger of dressed black holes, and show that the distribution of dark matter around them dramatically affects the dynamical evolution of the binaries. Although the final impact on the merger rate of primordial black holes is rather small with respect to the case of “naked” black holes, we argue that our analysis places the calculation of this rate on more solid ground, with LIGO-Virgo observations ruling out a dark matter fraction of ${10}^{-3}$ for primordial black holes of 100 solar masses, and it paves the way to more detailed analyses of environmental effects induced by dark matter on the gravitational wave emission of binary black holes.

Due to late-time nonlinearities, the location of the acoustic peak in the two-point galaxy correlation function is a redshift-dependent quantity, and thus it cannot be simply employed as a cosmological standard ruler. This has motivated the recent proposal of a novel ruler, also located in the baryon acoustic oscillation range of scales of the correlation function, dubbed the linear point. Unlike the peak, it is insensitive at the 0.5% level to many of the nonlinear effects that distort the clustering correlation function and shift the peak. However, this is not enough to make the linear point a useful standard ruler. In addition, we require a model-independent method to estimate its value from real data, avoiding the need to deploy a poorly known nonlinear model of the correlation function. In this manuscript, we precisely validate a procedure for model-independent estimation of the linear point. We also identify the optimal setup to estimate the linear point from the correlation function using galaxy catalogs. The methodology developed here is of general validity and can be applied to any galaxy correlation-function data. As a working example, we apply this procedure to the LOWZ and CMASS galaxy samples of the Twelfth Data Release of the Baryon Oscillation Spectroscopic Survey, for which the estimates of cosmic distances using the linear point have been presented by Anselmi et al. [Phys. Rev. Lett. 121, 021302 (2018)].

We present a new measurement of the energy spectrum of iron nuclei in cosmic rays from 20 TeV to 500 TeV. The measurement makes use of a template-based analysis method, which, for the first time, is applied to the energy reconstruction of iron-induced air showers recorded by the VERITAS array of imaging atmospheric Cherenkov telescopes. The event selection makes use of the direct Cherenkov light which is emitted by charged particles before the first interaction, as well as other parameters related to the shape of the recorded air shower images. The measured spectrum is well described by a power law $\frac{\mathrm{d}F}{\mathrm{d}E}=f_0·{(\frac{E}{E_0})}^{-\gamma }$ over the full energy range, with $\gamma =2.82\pm 0.30{(\text{stat})}_{-0.27}^{+0.24}(\text{syst})$ and $f_0=(4.82\pm 0.98{(\text{stat})}_{-2.70}^{+2.12}(\text{syst}))\times {10}^{-7}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{TeV}}^{-1}{\mathrm{sr}}^{-1}$ at $E_0=50\mathrm{TeV}$, with no indication of a cutoff or spectral break. The measured differential flux is compatible with previous results, with improved statistical uncertainty at the highest energies.

We consider a light dark matter candidate that is produced by the freeze-out mechanism with $3\to 2$ annihilations, the so-called strongly interacting massive particles (SIMPs). SIMPs are identified as dark pions in dark chiral perturbation theory (ChPT) in which both light mass and strong coupling needed for SIMPs can be realized by strong dynamics. In QCD-like theories with $SU(3{)}_L\times SU(3{)}_R/SU(3{)}_V$ flavor symmetry, including dark vector mesons in the hidden local symmetry scheme, we illustrate that dark vector mesons unitarize the dark ChPT efficiently, thus determining the correct relic density condition within the validity of the dark ChPT.