There is no proof that black holes contain singularities when they are generated by real physical bodies. Roger Penrose claimed sixty years ago that trapped surfaces inevitably lead to light rays of finite affine length (FALL's). Penrose and Stephen Hawking then asserted that these must end in actual singularities. When they could not prove this they decreed it to be self evident. It is shown that there are counterexamples through every point in the Kerr metric. These are asymptotic to at least one event horizon and do not end in singularities.

The Einstein equivalence principle in general relativity allows us to interpret accelerating black holes as a black hole immersed into the gravitational field of a larger companion black hole. Indeed it is demonstrated that C-metrics can be obtained as a limit of a binary system where one of the black holes grows indefinitely large, becoming a Rindler horizon. When the bigger black hole, before the limiting process, is of Schwarzschild type we recover usual accelerating black holes belonging to the Plebanski-Demianski class, thus type D. Whether the greater black hole carries some extra features, such as electric charges or rotations, we get generalised accelerating black holes which belongs to a more general class, the type I. In that case the background has a richer structure, reminiscent of the physical features of the inflated companion, with respect to the standard Rindler spacetime.

We show that, given any static spacetime whose spatial slices are asymptotically Euclidean (or, more generally, asymptotically conic) manifolds modeled on the large end of the Schwarzschild exterior, there exist stationary solutions to the Klein--Gordon equation having Schwartz initial data. In fact, there exist infinitely many independent such solutions. The proof is a variational argument based on the long range nature of the effective potential. We give two sets of test functions which serve to verify the hypothesis of the variational argument. One set consists of cutoff versions of the hydrogen bound states and is used to prove the existence of eigenvalues near the hydrogen spectrum.

We study the correlation between a part of the gravitational field at the common dynamical horizon in the strong field regime and the news of the gravitational radiation received from the system in the weak field regime, in the post-merger phase of quasi-circular, non-spinning binary black hole mergers using numerical relativity simulations. We find that, as in the inspiral phase Phys.Rev.Lett.125,121101, the shear of the common dynamical horizon formed late into the inspiral continues to be well correlated with the news of the outgoing gravitational radiation even at early times. We show by fitting that the shear contains certain quasi-normal frequencies and information about the masses and spins of the remnant and the parent black holes, providing evidence to support the horizon correlation conjecture holds for dynamical horizons in binary black hole mergers.

In this work, we consider the corrections to the cosmological models based on the teleparralel equivalent of general relativity and the scalar-torsion gravity implying non-minimal coupling between scalar field and torsion. To determine these corrections, we consider a power-law parameterization of the deviations between teleparralel equivalent of general relativity and the scalar-torsion gravity. The impact of these deviations on cosmological dynamics, scalar field potential and parameters of cosmological perturbations is considered for different inflationary models.

We consider the cosmological models based on the scalar-torsion gravity implying non-minimal coupling between torsion and the scalar field as modification of teleparallel equivalent of general relativity. Based on generalized exact solutions of the equations of cosmological dynamics, the type of scalar-torsion gravity was reconstructed. Also, based on observational restrictions on the values of the tensor-scalar ratio, the type of the coupling function was determined. It was noted that any models of cosmological inflation obtained on the basis of the proposed approach are verified by observational constraints on the parameters of cosmological perturbations.

In the first three observation runs, ground-based gravitational wave (GW) detectors have observed close to 100 compact binary coalescence (CBC) events. The GW detection rates for CBCs are expected to increase with improvements in the sensitivity of the International Gravitational-Wave Observatory Network (IGWN). However, with improved sensitivity, non-Gaussian instrumental transients or ``glitches'' are expected to adversely affect GW searches and characterisation algorithms. The most detrimental effect is due to short-duration glitches, which mimic the morphology of short-duration GW transients, in particular Intermediate-mass black hole (IMBH) binaries. They can be easily misidentified as astrophysical signals by current searches, and if included in astrophysical analyses, glitches mislabelled as IMBH binaries can affect IMBH population studies. In this work, we introduce a new similarity metric that quantifies the consistency of astrophysical parameters across the detector network and helps to distinguish between IMBH binaries and short-duration, loud glitches which mimic such binaries. We develop this method using a simulated set of IMBH binary signals and a collection of noise transients identified during the third observing run of the Advanced LIGO and Advanced Virgo detectors.

In this work, we study the impact of the environment around a black hole in detail. We introduce non-vanishing radial pressure in a manner analogous to compact stars. We examine both isotropic and anisotropic fluid configurations with and without radial pressure respectively. Our focus extends beyond just dark matter density to the vital role of the energy condition and sound speed in the spacetime of a black hole immersed in matter. In cases of anisotropic pressure with vanishing radial pressure, all profiles violate the dominant energy condition near the BH, and the tangential sound speed exceeds light speed for all dark matter profiles. In our second approach, without assuming vanishing radial pressure, we observe similar violations and superluminal sound speeds. To rectify this, we introduce a hard cutoff for the sound speed, ensuring it remains subluminal. As a consequence, the energy condition is also satisfied. However, this results in increased density and pressure near the BH. This raises questions about the sound speed and its impact on the density structure, as well as questions about the validity of the model itself. With the matter distribution, we also compute the metric for different configurations. It reveals sensitivity to the profile structure. The metric components point towards the horizon structure.

It is commonly believed that black holes are the smallest self-gravitating objects of the same mass in the Universe. Here, we demonstrate, in a subclass of higher-order pure gravities known as quasi-topological gravity, that by modifying general relativity (GR) to reduce the strength of gravity in strong-field regimes while keeping GR unchanged in weak-field regimes, it is possible for stars to collapse to radii less than $2M$ while still maintaining equilibrium between gravity and pressure gradients, leading to physically-reasonable neutron stars smaller in size than a black hole of the same mass. We present concrete solutions for such objects and discuss some of their observational consequences. These objects may furnish new avenues for understanding the nature of gravity in strong-field regimes and leave imprints on gravitational wave echoes from compact binary mergers. An observation of these imprints may constitute evidence for new physics beyond GR when effects of gravity in strong-field regimes are concerned.

We show that the spherically symmetric Einstein-scalar-field equations with potential for small wave-like decaying initial data at null infinity have unique global solutions when potential is dominated by four powers of scalar fields essentially.

We look for a classical double copy structure between gravity and electrodynamics by connecting the descriptions of the scattering of two point masses, and of two point charges, in terms of perturbative (post-Minkowskian or post-Lorentzian) expansions. We do so by recasting available analytical information within the effective-one-body formalism using Kerr-Schild gauges in both cases. Working at the third perturbative level, we find that the usual linear relation (holding in the probe limit) between the adimensionalized electric potential, $\tilde{\phi}= \frac{G M}{e_1 e_2} \phi^{\rm el}$, and the Schwarzschildlike gravitational one, $\Phi^{\rm grav}$, is deformed, in the comparable-mass, comparable-charge, case, into a {\it nonlinear relation} which becomes {\it universal in the high energy limit}: $\Phi^{\rm grav}= 2\tilde{\phi}-5\tilde{\phi}^2 + 18\tilde{\phi}^3$.

Although ordinary laboratory thermodynamic systems are known to be homogeneous systems, black holes are different and cannot be considered within this class. Using the formalism of geometrothermodynamics, we show that black holes should be considered as quasi-homogeneous systems. As a consequence, we argue that coupling constants in generalized gravity theories should be considered as thermodynamic variables, giving raise to extended versions of black hole thermodynamics.

Past studies have empirically demonstrated a surprising agreement between gravitational waveforms computed using adiabatic-driven-inspiral point-particle black hole perturbation theory (ppBHPT) and numerical relativity (NR) following a straightforward calibration step, sometimes referred to as $\alpha$-$\beta$ scaling. Specifically focusing on the quadrupole mode, this calibration technique necessitates only two time-independent parameters to scale the overall amplitude and time coordinate. In this article, part of a special issue, we investigate this scaling for non-spinning binaries at the equal mass limit. Even without calibration, NR and ppBHPT waveforms exhibit an unexpected degree of similarity after accounting for different mass scale definitions. Post-calibration, good agreement between ppBHPT and NR waveforms extends nearly up to the point of the merger. We also assess the breakdown of the time-independent assumption of the scaling parameters, shedding light on current limitations and suggesting potential generalizations for the $\alpha$-$\beta$ scaling technique.

In general relativity, the asymptotically flat space-time of a charged, spherically symmetric (non-rotating) body is described by the Reissner-Nordstr\"om metric. This metric corresponds to a naked singularity when the absolute value of charge, $Q$, exceeds the mass, $M$. For all Reissner-Nordstr\"om naked singularities, there exists a zero gravity sphere where a test particle can remain at rest. Outside that sphere gravity is attractive, inside it gravity is repulsive. For values of $Q/M>\sqrt{9/8}$ the angular frequency of circular test-particle orbits has a maximum at radius $r=(4/3)\,Q^2/M$. We construct polytropic tori with uniform values of specific angular momentum in the naked singularity regime of the Reissner-Nordstr\"om metric, $(Q/M>1)$.

Impulsive gravitational waves are theoretical models of short but violent bursts of gravitational radiation. They are commonly described by two distinct spacetime metrics, one of local Lipschitz regularity, the other one even distributional. These two metrics are thought to be `physically equivalent' since they can be formally related by a `discontinuous coordinate transformation'. In this paper we provide a mathematical analysis of this issue for the entire class of nonexpanding impulsive gravitational waves propagating in a background spacetime of constant curvature. We devise a natural geometric regularisation procedure to show that the notorious change of variables arises as the distributional limit of a family of smooth coordinate transformations. In other words, we establish that both spacetimes arise as distributional limits of a smooth sandwich wave taken in different coordinate systems which are diffeomorphically related.

Effective models of gravitational collapse in loop quantum gravity for the Lema\^itre-Tolman-Bondi spacetime predict that collapsing matter reaches a maximum finite density, bounces, and then expands outwards. We show that in the marginally trapped case, shell-crossing singularities commonly occur for inhomogeneous initial profiles of the dust energy density; this is the case in particular for all profiles that are continuous and of compact support, including configurations arbitrarily close to the Oppenheimer-Snyder model. When a shell-crossing singularity occurs, it is necessary to seek weak solutions to the dynamics; we argue that weak solutions typically contain shock waves.

We calculate the gravitational-electromagnetic phase for a charged particle in the Kerr-Newman spacetime. The result is applied to an interference experiment, in which the phase differences and the fringe shifts are derived. We find that both the charge of the particle and the charge of the black hole contribute to the gravitational phase difference, for which we give some qualitative explanations. Finally, we extend the results to the case of dyonic particles in the spacetime of a dyonic Kerr-Newman black hole.

We examine various exact solutions in "Cotton Gravity" (CG), a new gravity theory that provides an extension of General Relativity (GR) based on the Cotton tensor. Using an alternative formulation of the field equations in terms of a Codazzi tensor, we obtain various non-trivial CG exact solutions that generalize known GR solutions: FLRW cosmologies, Lemaitre-Tolman-Bondi (LTB) and Szekeres dust solutions, as well as static perfect fluid spheres and solutions with a shear-free 4 velocity. We show that CG modifies the spatial curvature of the nonstatic GR solutions. Demanding a well posed initial value formulation keeps the same dynamics of FLRW models of GR, but with the cosmological constant interpreted as constant spatial curvature. In other solutions the modification of spatial curvature allows for self-consistent significant changes in the dynamics, an time and spece dependent evolution from decelerated to accelerated expansion driven by negative spatial curvature and without necessarily assuming a dark energy source or imposing a cosmological constant. The $\Lambda$CDM model naturally emerges as the unique FLRW dust model of CG with constant negative spatial curvature. Static fluid spheres in the weak field regime of CG allow for modeling the flattening of rotation velocities in galactic systems without assuming dark matter. The methods we have presented can be improved to be able to obtain more general solutions that will facilitate the application of CG to current open problems in gravitational systems in general.

In this paper, we revisit the infrared (IR) divergences in de Sitter (dS) space using the wavefunction method, and explicitly explore how the resummation of higher-order loops leads to the stochastic formalism. In light of recent developments of the cosmological bootstrap, we track the behaviour of these nontrivial IR effects from perturbation theory to the non-perturbative regime. Specifically, we first examine the perturbative computation of wavefunction coefficients, and show that there is a clear distinction between classical components from tree-level diagrams and quantum ones from loop processes. Cosmological correlators at loop level receive contributions from tree-level wavefunction coefficients, which we dub classical loops. This distinction significantly simplifies the analysis of loop-level IR divergences, as we find the leading contributions always come from these classical loops. Then we compare with correlators from the perturbative stochastic computation, and find the results there are essentially the ones from classical loops, while quantum loops are only present as subleading corrections. This demonstrates that the leading IR effects are contained in the semi-classical wavefunction which is a resummation of all the tree-level diagrams. With this insight, we go beyond perturbation theory and present a new derivation of the stochastic formalism using the saddle-point approximation. We show that the Fokker-Planck equation follows as a consequence of two effects: the drift from the Schr\"odinger equation that describes the bulk time evolution, and the diffusion from the Polchinski's equation which corresponds to the exact renormalization group flow of the coarse-grained theory on the boundary.

It is shown that in the presence of a nonvanishing cosmological constant, Strominger's infinite-dimensional $\mathrm{w_{1+\infty}}$ algebra of soft graviton symmetries is modified in a simple way. The deformed algebra contains a subalgebra generating $ SO(1,4)$ or $SO(2,3)$ symmetry groups of $\text{dS}_4$ or $\text{AdS}_4$, depending on the sign of the cosmological constant. The transformation properties of soft gauge symmetry currents under the deformed $\mathrm{w_{1+\infty}}$ are also discussed.

We prove there is a unique vacuum solution in split-signature spacetimes with Kleinian SO(2,1) spherical symmetry. We extend our analysis to accommodate a positive or negative cosmological constant and we prove the Kleinian spherically symmetric solutions to Einstein's equation are locally isomorphic to the split-signature analogues of Schwarzschild-(Anti)-de Sitter or Nariai spacetimes. Our analysis provides a Kleinian extension of Birkhoff's theorem to metrics with split-signature. Axisymmetric vacuum solutions are also considered, including (2,2) signature formulations of the Kerr and Taub-NUT metrics.

The Aether-Scalar-Tensor (AeST) theory is an extension of General Relativity (GR) which can support Modified Newtonian Dynamics (MOND) behaviour in its static weak-field limit, and cosmological evolution resembling $\Lambda$CDM. We consider static spherically symmetric weak-field solutions in this theory and show that the resulting equations can be reduced to a single equation for the gravitational potential. The reduced equation has apparent isolated singularities when the derivative of the potential passes through zero and we show how these are removed by evolving, instead, the canonical momentum of the corresponding Hamiltonian system that we find. We construct solutions in three cases: (i) vacuum outside a bounded spherical object, (ii) within an extended prescribed source, and (iii) isothermal gas in hydrostatic equilibrium, serving as a simplified model for galaxy clusters. We show that the oscillatory regime that follows the Newtonian and MOND regimes, obtained in previous works in the vacuum case, also persists for isothermal spheres, and we show that the gas density profiles in AeST may become more compressed than their Newtonian or MOND counterparts. We construct the Radial Acceleration Relation (RAR) in AeST for isothermal spheres and find that it can display a peak, an enhancement with respect to the MOND RAR, at an acceleration range determined by the value of the AeST weak-field mass parameter, the mass of the system and the boundary value of the gravitational potential. For lower accelerations, the AeST RAR drops below the MOND expectation, as if there is a negative mass density. Similar observational features of the galaxy cluster RAR have been reported. This illustrates the potential of AeST to address the shortcomings of MOND in galaxy clusters, but a full quantitative comparison with observations will require going beyond the isothermal case.

Operator product expansions (OPEs) in quantum field theory (QFT) provide an asymptotic relation between products of local fields defined at points $x_1, \dots, x_n$ and local fields at point $y$ in the limit $x_1, \dots, x_n \to y$. They thereby capture in a precise way the singular behavior of products of quantum fields at a point as well as their ``finite trends.'' In this article, we shall review the fundamental properties of OPEs and their role in the formulation of interacting QFT in curved spacetime, the ``flow relations'' in coupling parameters satisfied by the OPE coefficients, the role of OPEs in conformal field theories, and the manner in which general theorems -- specifically, the PCT theorem -- can be formulated using OPEs in a curved spacetime setting.

The Kibble-Zurek mechanism (KZM) describes the non-equilibrium dynamics and topological defect formation in systems undergoing second-order phase transitions. KZM has found applications in fields such as cosmology and condensed matter physics. However, it is generally not suitable for describing first-order phase transitions. It has been demonstrated that transitions in systems like superconductors or charged superfluids, typically classified as second-order, can exhibit weakly first-order characteristics when the influence of fluctuations is taken into account. Moreover, the order of the phase transition (i.e., the extent to which it becomes first rather than second order) can be tuned. We explore quench-induced formation of topological defects in such tunable phase transitions and propose that their density can be predicted by combining KZM with nucleation theory.

This study explores the impact of cosmic curvature on structure formation through general relativistic first-order perturbation theory, focusing on scalar fluctuations and excluding anisotropic stress sources. We analyze continuity and Euler equations, incorporating cosmic curvature into Einstein equations. Emphasizing late-time dynamics, we investigate matter density contrast evolution in the presence of cosmic curvature and dark energy perturbations, with a specific focus on sub-Hubble scales. Solving the evolution equation, we conduct data analysis using cosmic chronometers, baryon acoustic oscillations, type Ia supernova observations, and $f\sigma_8$ data. While constraints on certain parameters remain consistent, excluding cosmic curvature tightens constraints on $\Omega_{\rm m0}$ and $\sigma_{\rm 80}$ in $\Lambda$CDM and wCDM models. Intriguingly, the non-phantom behavior of dark energy proves more favorable in both wCDM and CPL models across diverse data combinations.

A protocol of quantum dense coding with gravitational cat states is proposed. We explore the effects of temperature and system parameters on the dense coding capacity and provide an efficient strategy to preserve the quantum advantage of dense coding for these states. Our results might open new opportunities for secure communication and possibly insights into the fundamental nature of gravity in the context of quantum information processing.

Recent observations from several pulsar timing array (PTA) collaborations have unveiled compelling evidence for a stochastic signal in the nanohertz band. This signal aligns remarkably with a gravitational wave (GW) background, potentially originating from the first-order color charge confinement phase transition. Distinct quantum chromodynamics (QCD) matters, such as quarks or gluons, and diverse phase transition processes thereof can yield disparate GW energy density spectra. In this letter, employing the Bayesian analysis on the NANOGrav 15-year data set, we explore the compatibility with the observed PTA signal of the GW from phase transitions of various QCD matter scenarios in the framework of the holographic QCD. We find that the PTA signal can be effectively explained by the GW from the confinement-deconfinement phase transition of pure quark systems in a hard wall model of the holographic QCD where the bubble dynamics, one important source of the GWs, is of the Jouguet detonations. Notably, our analysis decisively rules out the plausibility of the pure gluon QCD-matter scenario and the non-runaway bubble dynamics model for the phase transition in explaining the observed PTA signal.

Future observations with next-generation large-area radio telescopes are expected to discover radio pulsars (PSRs) closely orbiting around Sagittarius~A* (Sgr~A*), the supermassive black hole (SMBH) dwelling at our Galactic Center (GC). Such a system can provide a unique laboratory for testing General Relativity (GR), as well as the astrophysics around the GC. In this paper, we provide a numerical timing model for PSR-SMBH systems based on the post-Newtonian (PN) equation of motion, and use it to explore the prospects of measuring the black hole (BH) properties with pulsar timing. We further consider the perturbation caused by the dark matter (DM) distribution around Sgr~A*, and the possibility of constraining DM models with PSR-SMBH systems. Assuming a 5-year observation of a normal pulsar in an eccentric ($e=0.8$) orbit with an orbital period $P_b = 0.5\,$yr, we find that -- with weekly recorded times of arrival (TOAs) and a timing precision of 1 ms -- the power-law index of DM density distribution near the GC can be constrained to about 20%. Such a measurement is comparable to those measurements at the Galactic length scale but can reveal small-scale properties of the DM.

In this work we make use of the generalized zeta function technique to investigate the vacuum energy, temperature corrections and heat kernel coefficients associated with a scalar field under a quasiperiodic condition in a $(D+1)$-dimensional conical spacetime. In this scenario we find that the renormalized vacuum energy, as well as the temperature corrections, are both zero. The nonzero heat kernel coefficients are the ones related to the usual Euclidean divergence, and also to the nontrivial aspects of the quaisperiodically identified conical spacetime topology. An interesting result that arises in this configuration is that for some values of the quasiperiodic parameter, the heat kernel coefficient associated with the nontrivial topology vanishes. In addition, we also consider the scalar field in a $(D+1)$-dimensional spacetime formed by the combination of a conical and screw dislocation topological defects. In this case, we obtain a nonzero renormalized vacuum energy density and its corresponding temperature corrections. Again, the nonzero heat kernel coefficients found are the ones related to the Euclidean and nontrivial topology divergences. For $D=3$ we explicitly show, in the massless scalar field case, the limits of low and high temperatures for the free energy. In the latter, we show that the free energy presents a classical contribution.

In the present article, we review the classical covariant formulation of Yang-Mills theory and general relativity in the presence of spacetime boundaries, focusing mainly on the derivation of the presymplectic forms and their properties. We further revisit the introduction of the edge modes and the conditions which justify them, in the context where only field-independent gauge transformations are considered. We particularly show that the presence of edge modes is not justified by gauge invariance of the presymplectic form, but rather by the condition that the presymplectic form is degenerate on the initial field space, which allows to relate this presymplectic form to the symplectic form on the gauge reduced field space via pullback.

We investigate the Casimir effect of a rough membrane within the framework of the Horava-Lifshitz theory in 2+1 dimensions. Quantum fluctuations are induced by an anisotropic scalar field subject to Dirichlet boundary conditions. We implement a coordinate transformation to render the membrane completely flat, treating the remaining terms associated with roughness as a potential. The spectrum is obtained through perturbation theory and regularized using the $\zeta$--function method. We present an explicit example of a membrane with periodic border. Additionally, we consider the effect of temperature. Our findings reveal that the Casimir energy and force depend on roughness, the anisotropic scaling factor and temperature.

Extreme scattering events (ESEs) are observed as dramatic ($>50\%$) drops in flux density that occur over an extended period of weeks to months. Discrete plasma lensing structures are theorized to scatter the radio waves produced by distant sources such as pulsars, causing the signature decrease in flux density and characteristic caustic spikes in ESE light curves. While plasma lens models in the extant literature have reproduced key features of ESE light curves, they have all faced the problem of being highly over-dense and over-pressured relative to the surrounding interstellar medium (ISM) by orders of magnitude. We model ESEs by numerically ray-tracing through analytic, volumetric plasma lens models by solving the eikonal equation. Delaunay triangulation connecting the rays approximates the wavefront, generating a mapping from the observer plane to the source plane to account for multiple-imaging. This eikonal method of ray-tracing is tested against known analytic solutions and is then applied to a three-dimensional Gaussian-distributed electron volume density lens, and a filament model inspired by Grafton et al. (2023). We find convergence of our numerical results with established analytic solutions validating our numerical method, and reproduce ESE-like light curves. Our numerical ray-tracing method lends itself well to exploring the lensing effects of volumetric turbulence as well as sheet-like lenses, which is currently in progress.

We conduct a detailed analysis of quasinormal $f-$mode frequencies in neutron stars (NS), within the linearized General Relativistic formalism. From Bayesian inference, we derived approximately 9000 nuclear Equations of State (EOS) subject to various constraints including nuclear saturation properties, the pure neutron matter EOS constraint obtained within $\chi$EFT, and pQCD at densities relevant to NS cores. The composition and oscillatory dynamics of NS are then investigated using this set. The EOS are transformed into a spectral representation, aiding in the efficient computation of NS properties. The median frequency values of the $f-$mode for NS with masses ranging from 1.4$M_\odot$ to 2.0$M_{\odot}$ lie between 1.80 and 2.20 kHz for our entire EOS set. Our findings do not reveal a strong correlation between $f-$mode frequencies and individual nuclear saturation properties of the EOS. This suggests the need for more complex methods to unravel multiple-parameter relationships. We noticed a strong relationship between the radii and $f-$mode frequencies for different NS masses. Using this correlation along with NICER observations of PSR J0740+6620 and PSR 0030+0451, we obtained constraints that have minimal overlap in the radius domain and differ in the frequency domain from our entire nucleonic EOS set. This indicates that there may be a need to consider additional exotic particles or maybe a deconfined quark phase in the EOS relevant to the NS core. We argue that future observations of the radius or $f-$mode frequency for more than one NS mass, particularly at the extremes, are likely to settle the issue by either ruling out only nucleonic EOS or providing definitive evidence in its favour.

The nature of dark matter is a problem with too many potential solutions. We investigate whether a consistent embedding into quantum gravity can decimate the number of solutions to the dark-matter problem. Concretely, we focus on a hidden sector composed of a gauge field and a charged scalar, with gauge group U(1)$_{\textmd{D}}$ or SU(2)$_\textmd{D}$. The gauge field is the dark-matter candidate, if the gauge symmetry is broken spontaneously. Phenomenological constraints on the couplings in this model arise from requiring that the correct dark matter relic density is produced via thermal freeze-out and that recent bounds from direct-detection experiments are respected. We find that the consistent embedding into asymptotically safe quantum gravity gives rise to additional constraints on the couplings at the Planck scale, from which we calculate corresponding constraints at low energy scales. We discover that phenomenological constraints cannot be satisfied simultaneously with theoretical constraints from asymptotically safe quantum gravity, ruling out these dark-matter models.

The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems with the next-generation Event Horizon Telescope (ngEHT), which will greatly enhance the capabilities of the existing EHT array. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that the ngEHT will enable.

In this paper, we explore correlators of a series of theories in anti-de Sitter space: we present comprehensive results for interactions involving scalars, gluons, and gravitons in multiple dimensions. One aspect of our investigation is the establishment of an intriguing connection between the kinematic factors of these theories; indeed, such a connection directly relates these theories among themselves and with other theories of higher spin fields. Besides providing several explicit results throughout the paper, we also highlight the interconnections and relationships between these different theories, providing valuable insights into their similarities and distinctions.