Dynamical Chern-Simons (dCS) gravity is a promising extension of general relativity (GR), arising naturally from the low-energy limit of some string motivated theories. Even though dCS possesses an additional scalar degree of freedom, interestingly, the Schwarzschild black hole is an exact solution of it. Concerning dynamical phenomena, however, gravitational and scalar perturbations couple with each other, generating possible scenarios to understand the differences between dCS and GR. We study dynamical signatures of dCS considering that the scalar field potential has a mass term. We analyze the influence of the theory’s parameter into the quasi-bound states and in the time evolution of purely gravitational initial profiles. We find that the coupling can make the dipolar modes less stable and that at late times initial gravitational perturbations become contaminated with the scalar sector, presenting an oscillation that depends on the quasibound state frequencies. These results may be interesting in light of superradiant instabilities in rotating systems and to find signatures of alternative theories in gravitational wave detections.

This paper introduces a new effort to study the collision of plane-fronted gravitational waves in four-dimensional, asymptotically flat spacetime, using numerical solutions of the Einstein equations. The pure vacuum problem requires singular, Aichelburg-Sexl-type sources to achieve finite energy solutions, which are problematic to treat both mathematically and numerically. Instead then, we use null (massless) particles to source nontrivial geometry within the initial wave fronts. The main purposes of this paper are to (a) motivate the problem, (b) introduce methods for numerically solving the Einstein equations coupled to distributions of collisionless massless or massive particles, and (c) present a first result on the formation of black holes in the head-on collision of axisymmetric distributions of null particles. Regarding the last-named, initial conditions are chosen so that a black hole forms promptly, with essentially no matter escaping the collision. This can be interpreted as approaching the ultrarelativistic collision problem from within an infinite boost limit, but where the matter distribution is spread out, and thus nonsingular. We find results that are consistent with earlier perturbative calculations of the collision of Aichelburg-Sexl singularities, as well as numerical studies of the high-speed collision of boson stars, black holes, and fluid stars: a black hole is formed containing most of the energy of the spacetime, with the remaining $15\pm 1\%$ of the initial energy radiated away as gravitational waves. The methods developed here could be relevant for other problems in strong-field gravity and cosmology that involve particle distributions of matter.

We consider the general form of correlated worldline (CWL) theories of quantum gravity. We show that one can have two different kinds of CWL theory, in which the generating functional is written as either a sum or a product over multiple copies of the coupled matter and gravitational fields. In both versions, the paths in a functional formulation are correlated via gravity itself, causing a breakdown of the superposition principle; however, the product form survives consistency tests not satisfied by the summed form. To better understand the structure of these two theories, we show how to perform diagrammatic expansions in the gravitational coupling for each version of CWL theory, using particle propagation and scalar fields as examples. We explicitly calculate contributions to two-point and four-point functions, again for each version of the theory, up to second order in the gravitational coupling.

With the help of the parametric solution of the Maxwell equal area law for the Gauss-Bonnet AdS black hole in five dimensions, we find the second analytical solution to the first order phase transition. We analyze the asymptotic behaviors of some characteristic thermodynamic properties for the small and large black holes at the critical and zero temperatures and also calculate the critical exponents and the corresponding critical amplitudes in detail. Moreover, we give the general form of the thermodynamic scalar curvature based on the Ruppeiner geometry and point out that the attractive interaction dominates in both the small and large black hole phases when the first order phase transition occurs in the five dimensional Gauss-Bonnet AdS black hole.

A key goal of the Event Horizon Telescope is to observe the shadow cast by a black hole. Recent simulations have shown that binary black holes, the progenitors of gravitational waves, present shadows with fractal structure. Here we study the binary shadow structure using techniques from nonlinear dynamics, recognizing shadows as exit basins of open Hamiltonian dynamical systems. We apply a recently developed numerical algorithm to demonstrate that parts of the Majumdar-Papapetrou binary black hole shadow exhibit the Wada property: any point of the boundary of one basin is also on the boundary of at least two additional basins. We show that the algorithm successfully distinguishes between the fractal and regular (i.e., nonfractal) parts of the binary shadow.

In the spirit of effective field theory, the standard-model extension (SME) provides a comprehensive framework to systematically probe the possibility of Lorentz/$CPT$ violation. In the pure gravity sector, operators with mass dimension larger than 4, while in general being advantageous to short-range experiments, are hard to investigate with systems of astronomical size. However, there is exception if the leading-order effects are $CPT$-violating and velocity-dependent. Here we study the lowest-order operators in the pure gravity sector that violate the $CPT$ symmetry with carefully chosen relativistic binary pulsar systems. Applying the existing analytical results to the dynamics of a binary orbit, we put constraints on various coefficients for Lorentz/$CPT$ violation with mass dimension 5. These constraints, being derived from the post-Newtonian dynamics for the first time, are complementary to those obtained from the kinematics in the propagation of gravitational waves.

For various reasons, a number of authors have mooted an “exponential form” for the spacetime metric $d{s}^2=-e^{-2m/r}d{t}^2+e^{+2m/r}\{d{r}^2+r^2(d{\theta }^2+{\sin }^2\theta d{\varphi }^2)\}.$While the weak-field behavior matches nicely with weak-field general relativity, and so also automatically matches nicely with the Newtonian gravity limit, the strong-field behavior is markedly different. Proponents of these exponential metrics have very much focused on the absence of horizons—it is certainly clear that this geometry does not represent a black hole. However, the proponents of these exponential metrics have failed to note that instead one is dealing with a traversable wormhole—with all of the interesting and potentially problematic features that such an observation raises. If one wishes to replace all the black hole candidates astronomers have identified with traversable wormholes, then certainly a careful phenomenological analysis of this quite radical proposal should be carried out.

The statistical cosmological principle states that observables on the celestial sphere are sampled from a rotationally invariant distribution. Previously certain large scale anomalies which violate this principle have been found, e.g., an alignment of the lowest multipoles with the cosmic dipole direction. In this work we continue the search for possible anomalies using multipole vectors which represent a convenient tool for this purpose. In order to study the statistical behavior of multipole vectors, we revisit several construction methods. We investigate all four full-sky foreground-cleaned maps from the Planck 2015 release with respect to four meaningful physical directions using computationally cheap statistics that have a simple geometric interpretation. We find that the full-sky SEVEM map deviates from all the other cleaned maps, as it shows a strong correlation with the Galactic Pole and Galactic Center. The other three maps COMMANDER, NILC and SMICA show a consistent behavior. On the largest angular scales, $l\le 5$, as well as on intermediate scales, $l=20$, 21, 22, 23, 24, all of them are unusually correlated with the cosmic dipole direction. These scales coincide with the scales on which the angular power spectrum deviates from the Planck 2015 best-fit $\mathrm{\Lambda }\mathrm{CDM}$ model. In the range $2\le l\le 50$ as a whole there is no unusual behavior visible globally. We do not find abnormal intramultipole correlation, i.e., correlation of multipole vectors inside a given multipole without reference to any outer direction.

We derive the evolution equation for the density matrix of a UV- and IR- limited band of comoving momentum modes of the canonically normalized scalar degree of freedom in two examples of nearly de Sitter universes. Including the effects of a cubic interaction term from the gravitational action and tracing out a set of longer wavelength modes, we find that the evolution of the system is non-Hamiltonian and non-Markovian. We find linear dissipation terms for a few modes with wavelength near the boundary between system and bath, and nonlinear dissipation terms for all modes. The non-Hamiltonian terms in the evolution equation persist to late times when the scalar field dynamics is such that the curvature perturbation continues to evolve on super-Hubble scales.

Low mass x-ray binaries (LMXBs), with either a neutron star (NS) or a black hole, show in their power spectra quasiperiodic oscillations (QPOs). Those at highest frequencies show up in pairs and are named twin peak high frequency QPOs (HF QPOs). Their central frequencies are typical of the orbital motion timescale close to the compact object. HF QPOs are believed to carry unique information on the matter moving in the extreme gravitational field around the compact object. In previous works we highlighted the work done by strong tides on clumps of plasma orbiting in the accretion disk as a suitable mechanism to produce the HF QPOs. We showed that the upper of the twin peak HF QPOs seen in NS LMXBs could originate from the tidal circularization of the clump’s relativistic orbit, while the lower HF QPO could come from the spiraling clump losing orbital energy. Here we focus on the tidal deformation of a magnetized clump of plasma once tides load energy on it. The likely evolution of the shape of the clump after tidal circularization of the orbit and its subsequent orbital evolution are investigated. In atoll NS LMXBs, a subclass of NS LMXBs less luminous than $Z$ NS LMXBs, the lower HF QPO displays a characteristic behavior of its coherence $Q$ versus its central frequency $\nu$. $Q$ keeps increasing over the range of frequencies $\nu \sim 600–850\mathrm{Hz}$ and then drops abruptly at $\nu \sim 900\mathrm{Hz}$. We note, for the first time, that such behavior is reproduced by magnetized clumps of plasma stretched by strong tides and falling onto the NS on unstable orbits. The increasing part of $Q$ is drawn by the tidal stretching timescale of the clump all over the orbit. The abrupt drop of $Q$ is dictated by the number of turns the clump makes before reaching the innermost stable bound orbit; afterwards the clump would fall onto the NS. We emphasize the overall behavior of the lower HF QPO coherence as a candidate to disclose the innermost stable bound orbit predicted by the general relativity theory in the strong field regime.

We compute Schwinger pair production rates at finite temperature, in the presence of homogeneous, concurrent electric, and magnetic fields. Expressions are obtained using the semiclassical worldline instanton formalism, to leading order, for spin-0 and spin-$\frac{1}{2}$ particles. The derived results are valid for weak coupling and fields. We thereby extend previous seminal results in the literature, to coexistent electric and magnetic fields, as well as fermions.

We propose the first experimental test of the inelastic boosted dark matter hypothesis, capitalizing on the new physics potential with the imminent data taking of the ProtoDUNE detectors. More specifically, we explore various experimental signatures at the cosmic frontier, arising in boosted dark matter scenarios, i.e., relativistic, inelastic scattering of boosted dark matter often created by the annihilation of its heavier component which usually comprises of the dominant relic abundance. Although features are unique enough to isolate signal events from potential backgrounds, vetoing a vast amount of cosmic background is rather challenging as the detectors are located on the ground. We argue, with a careful estimate, that such backgrounds nevertheless can be well under control by performing dedicated analyses after data acquisition. We then discuss some phenomenological studies which can be achieved with ProtoDUNE, employing a dark photon scenario as our benchmark dark-sector model.

Dark matter that participates in baryon-number violating interactions can annihilate with baryons if the dark matter particle is not protected under discrete symmetries. In this paper we investigate the dark matter–baryon annihilation in color-triplet extensions of the standard model, in which a fermionic dark matter can become kinematically stable within a small mass range near the proton mass. We demonstrate that the DM’s annihilation with nucleons can be probed to stringent limits at large-volume water Cherenkov detectors like the Super-Kamiokande experiment, with the mediator scale $m_{\mathrm{\Phi }}$ constrained up to $10^7\mathrm{GeV}$. In case of a Majorana light dark matter, this constraint is weaker than, yet close in magnitude to that from neutron-antineutron oscillation. In the Dirac DM case, the dark matter–nucleon annihilation gives stronger bounds than that from the uncertainties of the neutron decay lifetime. In a limited range of the DM mass above $m_p+m_e$, the DM-nucleon annihilation bound can be higher than the requirement from the DM’s stability in the Universe. Given the strong limits from Super-Kamiokande, we find it below the current experimental capabilities to detect indirectly the dark matter- nucleon annihilation signal in diffuse Galactic gamma rays or from neutron star heating.

We study spin and spin-flavor oscillations of Dirac neutrinos in a plane electromagnetic wave with circular polarization. The evolution of massive neutrinos with nonzero magnetic moments in the field of an electromagnetic wave is based on the exact solution of the Dirac-Pauli equation. We formulate the initial condition problem to describe spin-flavor oscillations in an electromagnetic wave. The transition probabilities for spin and spin-flavor oscillations are obtained. In case of spin-flavor oscillations, we analyze the transition and survival probabilities for different neutrino magnetic moments and various channels of neutrino oscillations. As an application of the obtained results, we study the possibility of existence of ${\nu }_{e\mathrm{L}}\to {\nu }_{\mu \mathrm{R}}$ oscillations in an electromagnetic wave emitted by a highly magnetized neutron star. Our results are compared with findings of other authors.

We investigate an interesting correlation among dark matter phenomenology, neutrino mass generation and grand unified theory (GUT) baryogenesis, based on the scotogenic model. The model contains additional right-handed neutrinos $N$ and a second Higgs doublet $\mathrm{\Phi }$, both of which are odd under an imposed $Z_2$ symmetry. The neutral component of $\mathrm{\Phi }$, i.e. the lightest of the $Z_2$-odd particles, is the dark matter candidate. Due to a Yukawa coupling involving $\mathrm{\Phi }$, $N$ and the standard model leptons, the lepton asymmetry is converted into the dark matter asymmetry so that a nonvanishing $B-L$ asymmetry can arise from ($B-L$)-conserving GUT baryogenesis, leading to a nonzero baryon asymmetry after the sphalerons decouple. On the other hand, $\mathrm{\Phi }$ can also generate neutrino masses radiatively. In other words, the existence of $\mathrm{\Phi }$ as the dark matter candidate resuscitates GUT baryogenesis and realizes neutrino masses.

In models where the Higgs is realized as a pseudo-Nambu-Goldstone boson (pNGB) of some global symmetry breaking, there are often remaining pNGBs of some $U(1)$ groups (called “pseudoaxions”) which could lead to smoking-gun signatures of such scenarios and provide important clues on the electroweak symmetry breaking mechanism. As a concrete example, we investigate the phenomenology of the pseudoaxion in the anomaly-free simplest little Higgs (SLH) model. After clarifying a subtle issue related to the effect of symmetric vector-scalar-scalar vertices [e.g., $Z_{\mu }(H{\partial }^{\mu }\eta +\eta {\partial }^{\mu }H)$], we show that for a natural region in the parameter space, the SLH pseudoaxion is top-philic, decaying almost exclusively to a pair of top quarks. The direct and indirect (i.e., via heavy particle decay) production of such a pseudoaxion at the 14 TeV (HL-)LHC turn out to suffer from either large backgrounds or small rates, making its detection quite challenging. A $pp$ collider with higher energy and luminosity, such as the 27 TeV HE-LHC, or even the 100 TeV FCC-hh or SppC, is therefore motivated to capture the trace of such a pNGB.

We calculate the leptonic decay constants of heavy-light pseudoscalar mesons with charm and bottom quarks in lattice quantum chromodynamics on four-flavor QCD gauge-field configurations with dynamical $u$, $d$, $s$, and $c$ quarks. We analyze over twenty isospin-symmetric ensembles with six lattice spacings down to $a\approx 0.03\mathrm{fm}$ and several values of the light-quark mass down to the physical value $\frac{1}{2}(m_u+m_d)$. We employ the highly-improved staggered-quark (HISQ) action for the sea and valence quarks; on the finest lattice spacings, discretization errors are sufficiently small that we can calculate the $B$-meson decay constants with the HISQ action for the first time directly at the physical $b$-quark mass. We obtain the most precise determinations to-date of the $D$- and $B$-meson decay constants and their ratios, $f_{D^{+}}=212.7(0.6)\mathrm{MeV}$, $f_{D_s}=249.9(0.4)\mathrm{MeV}$, $f_{D_s}/f_{D^{+}}=1.1749(16)$, $f_{B^{+}}=189.4(1.4)\mathrm{MeV}$, $f_{B_s}=230.7(1.3)\mathrm{MeV}$, $f_{B_s}/f_{B^{+}}=1.2180(47)$, where the errors include statistical and all systematic uncertainties. Our results for the $B$-meson decay constants are three times more precise than the previous best lattice-QCD calculations, and bring the QCD errors in the standard model predictions for the rare leptonic decays $\overline{\mathcal{B}}(B_s\to {\mu }^{+}{\mu }^{-})=3.64(11)\times {10}^{-9}$, $\overline{\mathcal{B}}(B^0\to {\mu }^{+}{\mu }^{-})=1.00(3)\times {10}^{-10}$, and $\overline{\mathcal{B}}(B^0\to {\mu }^{+}{\mu }^{-})/\overline{\mathcal{B}}(B_s\to {\mu }^{+}{\mu }^{-})=0.0273(9)$ to well below other sources of uncertainty. As a byproduct of our analysis, we also update our previously published results for the light-quark-mass ratios and the scale-setting quantities $f_{p4s}$, $M_{p4s}$, and $R_{p4s}$. We obtain the most precise lattice-QCD determination to date of the ratio $f_{K^{+}}/f_{{\pi }^{+}}=1.1950({}_{-23}^{+16})\mathrm{MeV}$.

We calculate the connected stress-tensor correlation functions that are dual to a certain class of graviton scattering amplitudes in an asymptotically anti-de Sitter, black brane spacetime. This is a continuation of a previous study in which one-particle-irreducible amplitudes were calculated for arbitrarily higher-derivative gravity in a particular kinetic regime of high energies and large scattering angles. The utility of this regime is twofold: It is particularly well suited for translating scattering amplitudes into the language of the gauge theory, and it emphasizes the contributions from higher-derivative corrections (which would otherwise be perturbatively suppressed). Using the new results, we show how it could be possible to identify, experimentally, the gravitational dual to a strongly coupled fluid such as the quark-gluon plasma.

We study universal spatial features of certain nonequilibrium steady states corresponding to flows of strongly correlated fluids over obstacles. This allows us to predict universal spatial features of far-from-equilibrium systems, which in certain interesting cases depend cleanly on the hydrodynamic transport coefficients of the underlying theory, such as $\eta /s$, the shear viscosity to entropy density ratio. In this work we give a purely field-theoretical definition of the spatial collective modes identified earlier and proceed to demonstrate their usefulness in a set of examples, drawing on hydrodynamic theory as well as holographic duality. We extend our earlier treatment by adding a finite chemical potential, which introduces a qualitatively new feature, namely damped oscillatory behavior in space. We find interesting transitions between oscillatory and damped regimes and we consider critical exponents associated with these. We explain in detail the numerical method and add a host of new examples, including fully analytical ones. Such a treatment is possible in the large-dimension limit of the bulk theory, as well as in three dimensions, where we also exhibit a fully analytic nonlinear example that beautifully illustrates the original proposal of spatial universality. This allows us to explicitly demonstrate how an infinite tower of discrete modes condenses into a branch cut in the zero-temperature limit, converting exponential decay into a power-law tail.

The paper discusses backreaction effects in cosmology, (á la Buchert et al.), induced by noncommutative geometry effects in fluid. We have used generalizations of an action formulation of a noncommutative fluid model proposed earlier by us. We show that the noncommutative effects, depending on their nature, can play either roles of (kinematical) dark matter or cosmological constant.