Here we present a novel classical model to describe the near-inner horizon geometry of a rotating, accreting black hole. The model assumes spacetime is homogeneous and is sourced by radial streams of a collisionless, null fluid, and it predicts that the standard Poisson-Israel mass inflation phenomenon will be interrupted by a Kasner-like collapse toward a spacelike singularity. Thus, the inner horizon and any additional structure beyond it never gets a chance to form. Such a model is shown to be valid near the inner horizon of astrophysically realistic black holes through comparison to the conformally separable Kerr model, which provides a natural connection of the Kerr metric to a self-similar, accreting spacetime. We then analyze the behavior of null geodesics in our model, connecting them to the Kerr metric in order to answer the practical question of what an infalling observer approaching the inner horizon might see. We find that an infalling zero-energy observer near the inner horizon will initially see streams of matter accelerating at high energies along the radial direction. Then, within a short span of proper time, the geometry will enter a collapse phase as most of the background sky becomes squashed along the edge of the black hole’s shadow. Classically, the observer’s journey will end here at a spacelike singularity, though the final outcome of the collapse will require a future calculation of the quantum back reaction within this framework.

We derive the exact analytical solutions to the symmetron field theory equations in the presence of a one- or two-mirror system in the case of a spontaneously broken phase in vacuum as well as in matter. This complements a similar analysis performed in a previous article, in which the symmetron is in the spontaneously broken phase in vacuum but in the symmetric phase in matter. Here again, the one-dimensional equations of motion are integrated exactly for both systems, and their solutions are expressed in terms of Jacobi elliptic functions. In the case of two parallel mirrors, the equations of motion provide also in this case a discrete set of solutions with an increasing number of nodes and energies. The solutions obtained herein can be applied to $q\mathrm{BOUNCE}$ experiments, to neutron interferometry, and to the calculation of the symmetron-field-induced “Casimir force” in the cannex experiment and allow us to extend the investigation to hitherto unavailable regions in symmetron parameter space.

Violations of Lorentz (and specifically boost) invariance can make gravity renormalizable in the ultraviolet, as initially noted by Hořava, but are increasingly constrained in the infrared. At low energies, Hořava gravity is characterized by three dimensionless couplings, $\alpha$, $\beta$ and $\lambda$, which vanish in the general relativistic limit. Solar system and gravitational wave experiments bound two of these couplings ($\alpha$ and $\beta$) to tiny values, but the third remains relatively unconstrained ($0\le \lambda \lesssim 0.01–0.1$). Moreover, demanding that (slowly moving) black-hole solutions are regular away from the central singularity requires $\alpha$ and $\beta$ to vanish exactly. Although a canonical constraint analysis shows that the class of khronometric theories resulting from these constraints ($\alpha =\beta =0$ and $\lambda \ne 0$) cannot be equivalent to General Relativity, even in vacuum, previous calculations of the dynamics of the solar system, binary pulsars and gravitational-wave generation show perfect agreement with general relativity. Here, we analyze spherical collapse and compute black-hole quasinormal modes, and find again that they behave exactly as in general relativity, as far as observational predictions are concerned. Nevertheless, we find that spherical collapse leads to the formation of a regular universal horizon, i.e., a causal boundary for signals of arbitrary propagation speeds, inside the usual event horizon for matter and tensor gravitons. Our analysis also confirms that the additional scalar degree of freedom present alongside the spin-2 graviton of general relativity remains strongly coupled at low energies, even on curved backgrounds. These puzzling results suggest that any further bounds on Hořava gravity will probably come from cosmology.

We consider the motion of massive and massless particles in a five-dimensional spacetime with a compactified extra-dimensional space where a black hole is localized, i.e., a caged black hole spacetime. We show the existence of circular orbits and reveal their sequences and stability. In the asymptotic region, stable circular orbits always exist, which implies that four-dimensional gravity is more dominant because of the small extra-dimensional space. In the vicinity of a black hole, they do not exist because the effect of compactification is no longer effective. We also clarify the dependence of the sequences of circular orbits on the size of the extra-dimensional space by determining the appearance of the innermost stable circular orbit and the last circular orbit (i.e., the unstable photon circular orbit).

By determining the relation between topological M-theory and the Chern-Simons actions for a gauge field constructed from the Lie algebra of either $SL(2,\mathbb{R})\times SL(2,\mathbb{R})$ or $SL(2,\mathbb{C})\times SL(2,\mathbb{C})$, depending on the sign of the space-time curvature, we show that the standard and exotic actions of three-dimensional gravity can be recovered from topological M-theory. With this result, we provide a concrete realization of a conjecture by Dijkgraaf et al. stating that the partition function of topological M-theory is equivalent to the partition function of a black hole in a related theory. We do this for the standard and exotic BTZ black holes in three-dimensional gravity.

We study the circular orbits of charged particles around a weakly charged Schwarzschild black hole immersed in a weak, axisymmetric magnetic field. We start by reviewing the circular orbits of neutral particles and charged particles around only weakly charged and only weakly magnetized black holes. The case of a weakly magnetized and charged black hole is investigated then. In particular, we study the effect of the electromagnetic forces on the charged particle innermost stable circular orbits. We show that negative energy stable circular orbits are possible and that two bands of charged particles circular orbits, separated by a gap of no stable circular orbits can exist. The astrophysical aspects of our findings are discussed too.

Gravitational wave (GW) detections have considerably enriched our understanding of the universe. To date, all GW events from individual sources have been found by interferometer-type detectors. In this paper, we study a GW detection technique based on astrometric solutions from photometric surveys and demonstrate that it offers a highly flexible frequency range that can uniquely complement existing detection methods. From repeated point-source astrometric measurements, periodic GW-induced deflections can be extracted and wave parameters inferred. We emphasize that this method can be applied widely to any photometric surveys relying on relative astrometric measurements, in addition to surveys designed to measure absolute astrometry, such as Gaia. We illustrate how high-cadence observations of the galactic bulge, such as offered by the Roman Space Telescope’s Exoplanet MicroLensing (EML) survey, have the potential to be a potent GW probe with a complementary frequency range to Gaia, pulsar timing arrays, and the Laser Interferometer Space Antenna. We calculate that the Roman EML survey is sensitive to GWs with frequencies ranging from $7.7\times {10}^{-8}\mathrm{Hz}$ to $5.6\times {10}^{-4}\mathrm{Hz}$, which opens up a unique GW observing window for supermassive black hole binaries and their waveform evolution. While the detection threshold assuming the currently expected performance proves too high for detecting individual GWs in light of the expected supermassive black hole binary population distribution, we show that binaries with chirp mass ${\mathcal{M}}_c>{10}^{8.3}{M}_{\odot }$ out to 100 Mpc can be detected if the telescope is able to achieve an astrometric accuracy of 0.11 mas. To confidently detect binaries with ${\mathcal{M}}_c>{10}^7{M}_{\odot }$ out to 50 Mpc, a factor of 100 sensitivity improvement is required. We propose several improvement strategies, including recovering the mean astrometric deflection and increasing astrometric accuracy, number of observed stars, field-of-view size, and observational cadence. We also discuss how other existing and planned photometric surveys could contribute to detecting GWs via astrometry.

Theoretical investigations have provided proof-of-principle calculations suggesting measurements of stellar or pulsar orbits near the Galactic Center could strongly constrain the properties of the Galactic Center black hole, local matter, and even the theory of gravity itself. As in previous studies, we use a Markov chain Monte Carlo to quantify what properties of the Galactic Center environment measurements can constrain. In this work, however, we also develop an analytic model (Fisher matrix) to understand what parameters are well-constrained and why. Using both tools, we conclude that existing astrometric measurements cannot constrain the spin of the Galactic Center black hole. Extrapolating to the precision and cadence of future experiments, we anticipate that the black hole spin can be measured with the known star S2. Our calculations show that we can measure the dimensionless black hole spin to a precision of $\sim 0.1$ with weekly measurements of the orbit of S2 for 40 years using the GRAVITY telescope’s best resolution at the Galactic Center, i.e., an angular resolution of $10\mu \mathrm{arcsec}$ and a radial velocity resolution of $500\mathrm{m}/\mathrm{s}$. An analytic expression is derived for the measurement uncertainty of the black hole spin using the Fisher matrix in terms of observation strategy, star’s orbital parameters, and instrument resolution. From it we conclude that highly eccentric orbits can provide better constraints on the spin, and that an orbit with a higher eccentricity is more favorable even when the orbital period is longer. We also apply it to S62, S4711, and S4714 to show whether they can constrain the black hole spin sooner than S2. If in addition future measurements include the discovery of a new, tighter stellar orbit, then future data could conceivably enable tests of strong field gravity, by directly measuring the black hole quadrupole moment. Our simulations show that with a stellar orbit similar to that of S2 but at one-fifth the distance to the Galactic Center and GRAVITY’s resolution limits on the Galactic Center, we can start to test the no-hair theorem with 20 years of weekly orbital measurements.

It is now known that the shadow is not only the property of a black hole, it can also be cast by other compact objects like naked singularities. However, there exist some novel features of the shadow of the naked singularities which are elaborately discussed in some recent articles. In the earlier literature, it is also shown that a naked singularity may admit negative precession of bound timelike orbits which cannot be seen in Schwarzschild and Kerr black hole spacetimes. This distinguishable behavior of timelike bound orbit in the presence of the naked singularity along with the novel features of the shadow may be useful to distinguish between a black hole and a naked singularity observationally. However, in this paper, it is shown that deformed Kerr spacetime can allow negative precession of bound timelike orbits, when the central singularity of that spacetime is naked. We also show that negative precession and shadow both can exist simultaneously in deformed Kerr naked singularity spacetime. Therefore, any observational evidence of negative precession of bound orbits, along with the central shadow may indicate the presence of a deformed Kerr naked singularity.

In this work we provide a framework that connects the corotating and counterrotating $f$-mode frequencies of rotating neutron stars with their stellar structure. The accurate computation of these modes for realistic equations of state has been presented recently and they are here used as input for a Bayesian analysis of the inverse problem. This allows to quantitatively reconstruct basic neutron star parameters, such as the mass, radius, rotation rate or universal scaling parameters. We find that future observations of both $f$-mode frequencies, in combination with a Bayesian analysis, would provide a promising direction to solve the inverse stellar problem. We provide two complementary approaches, one that is equation of state dependent and one that only uses universal scaling relations. We discuss advantages and disadvantages of each approach, such as possible bias and robustness. The focus is on astrophysically motivated scenarios in which informed prior information on the neutron star mass or rotation rate can be provided and study how they impact the results.

In the present paper, we construct spontaneously scalarized rotating black hole solutions in dynamical Chern-Simons (dCS) gravity by following the scalar field evolution in the decoupling limit. For the range of parameters where the Kerr black hole becomes unstable within dCS gravity, the scalar field grows exponentially until it reaches an equilibrium configuration that is independent of the initial perturbation. Interestingly, the ${\mathbb{Z}}_2$ symmetry of the scalar field is broken, and a strong maximum around only one of the rotational axes can be observed. The black hole scalar charge is calculated for two coupling functions, suggesting that the main observations would remain qualitatively correct even if one considers coupling functions/coupling parameters producing large deviations from the Kerr solution beyond the decoupling limit approximation.

We study the conformal window of asymptotically free gauge theories containing $N_f$ flavors of fermion matter transforming to the vector and two-index representations of $SO(N),SU(N)$ and $Sp(2N)$ gauge groups. For $SO(N)$ we also consider the spinorial representation. We determine the critical number of flavors $N_f^{\mathrm{cr}}$, corresponding to the lower end of the conformal window, by using the conjectured critical condition on the anomalous dimension of the fermion bilinear at an infrared fixed point, ${\gamma }_{\overline{\psi }\psi ,\mathrm{IR}}=1$ or equivalently ${\gamma }_{\overline{\psi }\psi ,\mathrm{IR}}(2-{\gamma }_{\overline{\psi }\psi ,\mathrm{IR}})=1$. To compute ${\gamma }_{\overline{\psi }\psi ,\mathrm{IR}}$ we employ the (scheme-independent) Banks-Zaks conformal expansion up to the 4th order in ${\mathrm{\Delta }}_{N_f}=N_f^{\mathrm{AF}}-N_f$ with $N_f^{\mathrm{AF}}$ corresponding to the onset of the loss of asymptotic freedom, where we show that the latter critical condition provides a better performance along with this conformal expansion. To quantify the uncertainties in our analysis, which potentially originate from nonperturbative effects, we propose two distinct approaches by assuming the large order behavior of the conformal expansion separately, either convergent or divergent asymptotic. In the former case, we take the difference in the Padé approximants to the two definitions of the critical condition, whereas in the latter case the truncation error associated with the singularity in the Borel plane is taken into account. Our results are further compared to other analytical methods as well as lattice results available in the literature. In particular, we find that $SU(2)$ with six and $SU(3)$ with ten fundamental flavors are likely on the lower edge of the conformal window, which are consistent with the recent lattice results. We also predict that $Sp(4)$ theories with fundamental and antisymmetric fermions have the critical numbers of flavors, approximately ten and five, respectively.

We consider the nonoscillatory explanations of the low-energy excess of events detected by MiniBooNE. We present a systematic search for phenomenological scenarios based on new physics which can produce the excess. We define scenarios as series of transitions and processes which connect interactions of accelerated protons in target with single-shower events in the MiniBooNE detector. The key elements of the scenarios are production and decay of new light $\mathcal{O}(\mathrm{keV}-100\mathrm{MeV})$ particles (fermions or/and bosons). We find about 20 scenarios with the minimal possible number of new particles and interaction points. In practice, they are all reduced to a few generic scenarios, and in this way we develop the effective theory of the MiniBooNE excess. We consider tests of the scenarios with near or close detectors in neutrino experiments T2K ND280, $\mathrm{NO}\nu \mathrm{A}$, and $\mathrm{MINER}\nu \mathrm{A}$ as well as in NOMAD and PS191. The scenarios immediately connect the MiniBooNE excess and expected numbers of new physics events in these detectors. We compute the expected numbers of events as functions of the lifetimes and masses of new particles and confront them with the corresponding experimental bounds. We indicate scenarios that are excluded or strongly disfavored by one or several experiments. Given our general approach, this work can also be regarded as the effective theory of new physics at accelerator-based neutrino experiments, being relevant for future projects such as DUNE.

Light axionlike particles occur in many theories of beyond-Standard-Model physics, and may make up some or all of the Universe’s dark matter. One of the ways they can couple to the Standard Model is through the electromagnetic $F_{\mu \nu }{\tilde{F}}^{\mu \nu }$ portal, and there is a broad experimental program, covering many decades in mass range, aiming to search for axion dark matter via this coupling. In this paper, we derive limits on the absorbed power, and coupling sensitivity, for a broad class of such searches. We find that standard techniques, such as resonant cavities and dielectric haloscopes, can achieve $\mathcal{O}(1)$-optimal axion-mass-averaged signal powers, for given volume and magnetic field. For low-mass (frequency $\ll \mathrm{GHz}$) axions, experiments using static background magnetic fields generally have suppressed sensitivity; we discuss the physics of this limitation, and propose experimental methods to avoid it, such as microwave up-conversion experiments. We also comment on the detection of other forms of dark matter, including dark photons, as well as the detection of relativistic hidden-sector particles.

Very recently, the LHCb Collaboration reported a fully charmed tetraquark state $X(6900)$ in the invariant mass spectrum of $J/\psi$ pairs. If one $J/\psi$ meson is replaced with a fully charmed baryon, we obtain a fully charmed pentaquark candidate. In this work, we perform a systematic study on the mass spectra of the $S$-wave fully heavy pentaquark $QQQQ\overline{Q}$ in the framework of the chromomagnetic interaction model. Based on our results in two different schemes, we further investigate the decay behaviors for them. We hope that our study will be helpful in searching for such types of exotic pentaquark states in experiments in the future.

We examine the extent to which primordial black holes (PBHs) can constitute the observed dark matter while also giving rise to the measured matter-antimatter asymmetry and account for the observed baryon abundance through asymmetric Hawking radiation generated by a derivative coupling of curvature to the baryon-lepton current. We consider both broad and monochromatic mass spectra for this purpose. For the monochromatic spectrum we find that the correct dark matter and baryon energy densities are recovered for peak masses of the spectrum of $M_{\mathrm{pk}}\ge {10}^{12}\mathrm{kg}$ whereas for the broad case the observed energy densities can be reproduced regardless of peak mass. Adopting some simplifications for the early-time expansion history as a first approximation, we also find that the measured baryon asymmetry can be recovered within an order of magnitude. We argue furthermore that the correct value of the baryon-lepton yield can in principle be retrieved for scenarios where a significant amount of the radiation is produced by PBH decay during or after reheating, as is expected when the decaying PBHs also cause reheating, or when an early matter-dominated phase is considered. We conclude from this first analysis that the model merits further investigation.

A nuclear reactor is a powerful tool to study neutrinos and light dark sector particles. Some reactor experiments have already proven to be extremely useful in these searches. Considering the great interest in the power of the Intensity Frontier to search for new light particles, it would be desirable to explore the possibility of exploiting the existing reactor power sources for particle physics research. We suggest a new reactor experiment searching for the dark sector. The dark photon can be produced in a reactor core and decay into a photon and an axion in the presence of the dark axion portal through an axion-photon-dark photon vertex. We investigate the potential to search for this new vertex with a monophoton signature and present the expected sensitivities at some of the existing reactor neutrino experiment detectors.

We derive the solutions of gravitational waves in the future (F) expanding and past (P) shrinking Kasner spacetimes, as well as in the left (L) and right (R) Rindler wedges in the Regge-Wheeler gauge. The solutions for all metric components are obtained in an analytic form in each region. We identify the master variables, which are equivalent to massless scalar fields, to describe the gravitational degrees of freedom for the odd-parity and even-parity modes under the transformation in the two-dimensional plane-symmetric space. Then, the master variables are quantized, and we develop the quantum field theory of the gravitational waves in the F, P, L, and R regions. We demonstrate that the mode functions of the quantized gravitational waves in the left and right Rindler wedges are obtained by an analytic continuation of the left-moving and right-moving wave modes in Kasner spacetime. On the basis of these analyses, we discuss the Unruh effect of the quantized gravitational waves for an observer in a uniformly accelerated motion in Minkowski spacetime in an explicit manner for the first time.

Indirect dark matter (DM) detection typically involves the observation of standard model (SM) particles emerging from DM annihilation/decay inside regions of high dark matter concentration. We consider an annihilation scenario in which this reaction has to be initiated by one of the DMs involved being boosted while the other is an ambient nonrelativistic particle. This “trigger” DM must be created, for example, in a previous annihilation or decay of a heavier component of DM. Remarkably, boosted DM annihilating into gamma rays at a specific point in a galaxy could actually have traveled from its source at another point in the same galaxy or even from another galaxy. Such a “nonlocal” behavior leads to a nontrivial dependence of the resulting photon signal on the galactic halo parameters, such as DM density and core size, encoded in the so-called “astrophysical” $J$-factor. These nonlocal $J$-factors are strikingly different than the usual scenario. A distinctive aspect of this model is that the signal from dwarf galaxies relative to the Milky Way tends to be suppressed from the typical value to various degrees depending on their characteristics. This feature can thus potentially alleviate the mild tension between the DM annihilation explanation of the observed excess of approximately GeV photons from the Milky Way’s Galactic Center vs the apparent nonobservation of the corresponding signal from dwarf galaxies.