The anomalous orbits of trans-Neptunian objects can be accounted for by the planet 9 hypothesis. One intriguing possibility is that planet 9 could be a dilute axion star captured by the Solar System, with the ratio of the axion star to dark matter being approximately $1/10$. Although dilute axion stars can emit monochromatic signals through two-photon decay, the spontaneous decay signal is too weak to be detected by radio telescopes. However, we find that stimulated decay of the dilute axion star, which explains planet 9, can occur by directing a radio beam with a power of 50 MW into the star. The resulting echo can be detected by terrestrial telescopes such as SKA, FAST, ngLOBO, and LOFAR. Therefore, the dilute axion star can be distinguished from other planet 9 candidates, such as a primordial black hole or a free-floating planet captured by the Solar System.

The South Atlantic Anomaly (SAA) is a region where the Earth’s magnetic field is lower by a factor of 2–3 with respect to the mean field value, resulting in a remarkably higher density of trapped charged particles. Updated surveys of this unique region are of capital importance to track the evolution of the geomagnetic field and improve models of the Earth’s ionosphere. The High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite (CSES-01), operational between the 24th and 25th Solar cycles, is one of the few instruments able to probe the radiation environment of the SAA at high energies. Sensitive to $>1\mathrm{MeV}$ electrons and $>10\mathrm{MeV}$ protons, HEPD-01 counters can reliably measure rates as large as several $\mathrm{kHz}/{\mathrm{cm}}^2$, overcoming SAA saturation issues that often affect spaceborne detectors. By mapping the total particle flux observed within the SAA, we report here on the geographical shift of its center during the 2018–2022 period and deliver HEPD-01’s estimate of the associated integral particle flux, which validates NASA’s AP9-AE9 model. The present assessment of the SAA drift, besides featuring a notable accuracy, is unique in its time coverage, altitude, and energy range. In addition, our flux estimate covers the proton energy range above $\sim 200\mathrm{MeV}$ where the AP9 model relies on extrapolated data. The present results enrich the available observations of the radiation environment within the SAA, which are crucial for the investigation of the temporal evolution of the geomagnetic field, for models of geomagnetically trapped particles, and for the investigation of perturbations generated by space weather events. Knowledge of the SAA radiation environment is also relevant for spacecraft system design and to safeguard human crew health.

This study examines the impact of $CP$-violation on the signal strength ${\mu }^{Z\gamma }$, which was reported as $2.2\pm 0.7$ by the LHC. We obtain constraints on the real and absorptive parts of the $CP$-violating form factor $h_3^{Z\gamma }$ and find that they are less than 1.15 GeV. Additionally, we revisit the leading order Standard Model (SM) contributions to the $H\to Z\gamma$ decay and calculate contributions to $h_3^{Z\gamma }$ from flavor-changing neutral current (FCNC) complex couplings mediated by the $Z$ and $H$ bosons. By using the current bounds on such couplings, we find that the FCNC contribution to $h_3^{Z\gamma }$ with top and charm quarks in the loop is of order ${10}^{-5}\mathrm{GeV}$. While in a model with new quarks that preserves the SM predictions on Higgs decays, the $CP$-violating form factor $h_3^{Z\gamma }$ can be of order ${10}^{-1}\mathrm{GeV}$ and could explain the excess on the signal strength ${\mu }^{Z\gamma }$.

Forward photoproduction of ${\pi }^{+}{\pi }^{-}$ pairs with invariant mass of the order of $m_{\rho }\sim 770\mathrm{MeV}$ is traditionally attributed to Pomeron exchange. Based on a detailed analysis of the CEBAF Large Acceptance Spectrometer photoproduction data collected at photon energies below 4 GeV, it is shown from a study of the angular moments that the dynamics of two-pion photoproduction for $|t|\gtrsim 0.5{\mathrm{GeV}}^2$ cannot be explained by Pomeron exchange alone. This motivates the development of a new theoretical model of two-pion photoproduction which incorporates both two-pion and pion-nucleon resonant contributions. After fitting free parameters, the model provides an excellent description of the low moments of the angular distribution measured at CLAS and enables an assessment of the relative contributions of particular production mechanisms and an interpretation of the various features of the data in terms of these mechanisms.

We establish the anisotropic hydrodynamics (aHydro) equations based on a boost-noninvariant longitudinally expanding system. Good consistency is found in the comparison between the aHydro results with those from the Boltzmann equation under relaxation time approximation. We also obtain a corresponding attractor solution and observe the time delay of convergence when increasing the absolute value of the rapidity. We finally show that the rapidity dependence is solved by introducing the redefined attractor variables to compensate the time delay effect.

Precise direct cosmic-ray (CR) measurements provide an important probe to study the energetic particle sources in our Galaxy, and the interstellar environment through which these particles propagate. Uncertainties on hadronic models, ion-nucleon cross sections in particular, are currently the limiting factor toward obtaining more accurate CR ion flux measurements with calorimetric space-based experiments. We present an energy-dependent measurement of the inelastic cross section of protons and helium-4 nuclei (alpha particles) on a ${\mathrm{Bi}}_4$${\text{Ge}}_3{\mathrm{O}}_{12}$ target, using 88 months of data collected by the DAMPE space mission. The kinetic energy range per nucleon of the measurement points ranges from 18 GeV to 9 TeV for protons, and from $5\mathrm{GeV}/\mathrm{n}$ to $3\mathrm{TeV}/\mathrm{n}$ for helium-4 nuclei. Our results lead to a significant improvement of the CR flux normalization. In the case of helium-4, these results correspond to the first cross section measurements on a heavy target material at energies above $10\mathrm{GeV}/\mathrm{n}$.

Computation of spin-resummed observables in post-Minkowskian dynamics typically involve evaluation of Feynman integrals deformed by an exponential factor, where the exponent is a linear sum of the momenta being integrated. Such integrals can be viewed as tensor integral generating functions, which provide alternative approaches to tensor reduction of Feynman integrals. We develop a systematic method to evaluate tensor integral generating functions using conventional multiloop integration techniques. The spin-resummed aligned-spin eikonal at second post-Minkowskian order is considered as a phenomenologically relevant example where evaluation of tensor integral generating functions is necessary.

Herein we exhibit exact, analytic, fully explicit, photon escape cones and the related silhouettes (“shadows”) at arbitrary distance from an extremal ($|a|=m$) Kerr black hole. In contrast to the situation for nonextremal ($0<|a|<m$) Kerr, where one typically has to resort to numerical, and/or graphics based, and/or perturbative investigations, the situation for extremal Kerr is amenable to direct analytic and fully explicit investigation. Interest in these results is twofold: In view of the technical complexity of Kerr spacetime, any fully explicit analytic result (even if only for the special case of extremality) is a significant aid to physical insight. Furthermore a significant fraction of observationally accessible galactic black holes are high spin, relatively close to extremal, objects ($|a|\gtrsim 0.85m$) so the extremal case is a good analytically tractable zeroth order approximation for such objects.

Placing a black hole in a cavity provides a natural framework for exploring gravitational scales, thermodynamic instabilities, and effective gravity theories. In this paper, we examine the evolution of entanglement entropy and entanglement islands in a two-sided extension of the Schwarzschild black hole in a cavity. By introducing a reflecting boundary in the eternal black hole exteriors, we regulate the infrared modes of Hawking radiation, finding that the entanglement entropy eventually saturates at a constant value. This value can be lower than the black hole’s thermodynamic entropy, thus avoiding the Page formulation of the information paradox. Regarding entanglement islands, we identify a universal effect induced by the boundary, which we term the “blinking island”—where the entanglement island temporarily disappears, resulting in a short-time information paradox.

We show that there exists a natural analog of the Yang-Mills equations using the Frölicher-Nijenhuis bracket between vector-valued differential forms. The gauge field is a rank-2 tensor, and when one constrains it to be symmetric, then the system exhibits fractonic behaviors. In the linearized limit, the constrained equations of motion reduce to those of the covariant fracton model [Phys. Rev. D 106, 125008 (2022)., Phys. Lett. B 833, 137304 (2022)., and Notes from the bulk, Ph.D. thesis, Università di Genova, 2024.].

Chern-Simons theory with certain gauge groups is known to be equivalent to a first-order formulation of three-dimensional Einstein gravity with a cosmological constant, where both are purely topological. Here, we extend this correspondence to theories with dynamical degrees of freedom. As an example, we show that three-dimensional Yang-Mills theory with gauge group $\mathrm{SL}(2,\mathbb{R})$ is equivalent to the first-order formulation of three-dimensional Einstein gravity with no cosmological constant coupled to a background stress-energy tensor density (which breaks the diffeomorphism symmetry). The local degree of freedom of three-dimensional Yang-Mills theory corresponds to degenerate “gravitational waves” in which the metric is degenerate and the spin connection is no longer completely determined by the metric. Turning on a cosmological constant produces the third-way (for $\mathrm{\Lambda }<0$) or the imaginary third-way (for $\mathrm{\Lambda }>0$) gauge theories with a background stress-energy tensor density.

Worldline actions for various twistor particles in Anti de Sitter (AdS) spacetimes are constructed from the coadjoint orbits of $Sp(4,\mathbb{R})$, $SU(2,2)$, and $O^{*}(8)$ as constrained Hamiltonian systems. The constraints are associated with the coadjoint orbits of the dual groups, respectively identified as $O(p,M-p)$, $U(p,M-p)$, and $Sp(p,M-p)$. These actions are presented in a universal form by making use of the twistor variables taking values in $\mathbb{R}$, $\mathbb{C}$, and $\mathbb{H}$, respectively. We find that the massless, the massive, and the conformal particles (namely the singletons) of any spins appear for the compact dual groups with $p=0$ reproducing many of the results in earlier literature, whereas more exotic particle species, such as various tachyons, continuous spin particles, and bitemporal anti de Sitter (BdS) particles appear for the noncompact dual groups.

We consider the thermal behavior of a large number of matrix degrees of freedom in the planar limit. We work in $0+1$ dimensions, with $D$ matrices, and use $1/D$ as an expansion parameter. This can be thought of as a noncommutative large-$D$ vector model, with two independent quartic couplings for the two different orderings of the matrices. We compute a thermal two-point correlator to $\mathcal{O}(1/D)$ and find that the degeneracy present at large $D$ is lifted, with energy levels split by an amount $\sim 1/\sqrt{D}$. This implies a timescale for thermal dissipation $\sim \sqrt{D}$. At high temperatures dissipation is predominantly due to one of the two quartic couplings.

The future detection of quasinormal modes (QNMs) from black hole ringdown will allow for consistency and independent tests of general relativity (GR) in the strong-field regime. In this paper, we perform a ringdown Fisher forecast when including the dominant quadratic QNM (QQNM) expected in nearly equal-mass quasicircular binary black holes (BBHs) observed by next-generation ground-based detectors, Einstein Telescope (ET) and Cosmic Explorer (CE). We consider a ringdown model with a total of four modes: three linear QNMs labeled by $(\ell mn)=(220),(330),(440)$ and one QQNM coming from the self-interaction of the dominant linear (220) QNM. We perform a forecast in two scenarios, when the QQNM parameters are considered to be (a) independent of the linear QNMs; (b) dependent on the (220) QNM parameters, according to GR. In Scenario (a), we find the QQNM to generally be measured with better precision than the (440) mode but worse than the (330) mode. As shown in the past, high-spin BBHs tend to have higher relative QQNMs. Even in such cases, we only expect to confidently detect and resolve these four independent QNMs for nearby events, below redshift $z\sim 0.5$ in ET and CE for intermediate-mass BHs. In Scenario (b), we find the QQNM to be extremely useful for improving the precision on the (440) parameters, with negligible improvements on the (220) parameters. In this case, the (440) parameters are expected to be measured even better than those of the (330) QNM. As a result, we expect to confidently detect and resolve the three independent linear QNMs for events even at high redshifts, up to $z\sim 35$ in ET and CE for intermediate-mass BHs. Therefore, because of the inclusion of the QQNM, virtually all second-generation BBH events will provide excellent consistency tests of GR.

Astrophysical environments are ubiquitous in the Universe; from accretion disks around black holes to galactic dark matter halos, distributions of astrophysical material veil the vast majority of Cosmos. Including environmental effects in strong-field gravity and astrophysics is, therefore, a rather tantalizing task in the quest for novel gravitational-wave phenomena. Here, we examine how environments affect the high-energy process of superradiance. In particular, we study the amplification of charged scalar waves under the expense of the electrostatic energy contained in a charged black hole that is embedded in an observationally motivated, and qualitatively generic dark matter halo. We find that the superradiant amplification of massless charged scalar fields scattering off environmentally enriched charged black holes can be equally efficient to those occurring in vacuum charged black holes. This occurs due to the fact that the sole interplay between the scalar wave and the black hole is the electromagnetic interaction. The addition of mass on the charged scalar waves leads to a rapid suppression of superradiant amplification. This transpires to a great extent due to the “friction” that the mass introduces to the black hole potential. Nevertheless, for sufficiently large scalar masses, the amplification factors can be also subdominantly affected by the compactness of the halo. This occurs because the gravitational interaction between the dense halo and the wave’s mass grows, thus further suppressing the superradiant amplification.

Gravitational waves (GWs) provide a unique opportunity to test general relativity (GR) in the highly dynamical, strong-field regime. So far, the majority of the tests of GR with GW signals have been carried out following parametrized, theory-independent approaches. An alternative avenue consists in developing inspiral-merger-ringdown (IMR) waveform models in specific beyond-GR theories of gravity, by combining analytical and numerical-relativity results. In this work, we provide the first example of a full IMR waveform model in a beyond-GR theory, focusing on Einstein-scalar-Gauss-Bonnet (ESGB) gravity. This theory has attracted particular attention due to its rich phenomenology for binary black-hole (BH) mergers, thanks to the presence of nontrivial scalar fields. Starting from the state-of-the-art, effective-one-body (EOB) multipolar waveform model for spin-precessing binary BHs SEOBNRv5PHM, we include theory-specific corrections to the EOB Hamiltonian, the metric and scalar energy fluxes, the GW modes, the quasinormal-mode (QNM) spectrum and the mass and spin of the remnant BH. We also propose a way to marginalize over the uncertainty in the merger morphology with additional nuisance parameters. Interestingly, we observe that changes in the frequency of the ringdown waveform due to the final mass and spin corrections are significantly larger than those due to ESGB corrections to the QNM spectrum. By performing Bayesian parameter estimation for the GW events GW190412, GW190814, and GW230529_181500, we place constraints on the fundamental coupling of the theory ($\sqrt{{\alpha }_{\mathrm{GB}}}\lesssim 0.31\mathrm{km}$ at 90% confidence). The bound could be improved by one order of magnitude by observing a single “golden” binary system with next-generation ground-based GW detectors. Our model can be used to improve constraints on modifications of GR with upcoming GW observations, and to provide forecasts for future GW detectors on the ground and in space.

In this paper, we show a local generalized second law (the generalized entropy is nondecreasing) in crossed product constructions for maximally extended static and Kerr black holes using modular theory. The new ingredient is the use of results from a recent paper discussing the entropy of the algebra of operators in subregions of arbitrary spacetimes. These results rely on an assumption which we show is true in our setting. However, we do assume as in that paper, that the gravitational constraints are implemented on each partial Cauchy slice. We employ a slight generalization of the construction, by including an observer degree of freedom even for wedge shaped regions with an asymptotic boundary. In the last part of the paper, we look at modular Hamiltonians of deformed half-spaces in a class of static spacetimes, including the Schwarzschild spacetime. These are computed using path integrals, and we primarily compute them to investigate whether these nonlocal modular Hamiltonians can be made local by subtracting off pieces from the algebra and its commutant, as has been surmised in the literature. Along the way, the averaged null energy condition (ANEC) also follows in this class of spacetimes.

The properties of metric perturbations are determined in the context of an expanding Universe governed by a modified theory of gravity with a nonminimal coupling between curvature and matter. We analyze the dynamics of the 6 components of a general helicity decomposition of the metric and stress-energy perturbations, consisting of scalar, vector and tensor sectors. The tensor polarizations are shown to still propagate luminally, in agreement with recent data from gravitational interferometry experiments, while their magnitude decays with an additional factor sourced by the nonminimal coupling. We show that the production of these modes is associated with a modified quadrupole formula at leading order. The vector perturbations still exhibit no radiative behavior, although their temporal evolution is found to be modified, with spatial dependence remaining unaffected. We establish that the scalar perturbations can no longer be treated as identical. We investigate the scalar sector by writing the modified model as an equivalent two-field scalar-tensor theory and find the same scalar degrees of freedom as in previous literature. The different sectors are paired with the corresponding polarization modes, which can be observationally measured by their effects on the relative motion of test particles, thus providing the possibility of testing the modified theory and constraining its parameters.

We investigate the thermodynamics of regular black hole configurations via quantum analogs of entropy and energy—namely, the entanglement entropy and entanglement energy—near the event horizon of Bardeen and Hayward black holes. Following standard approaches, we introduce a quantum scalar field propagating in such black hole spacetimes and discretize the field degrees of freedom on a lattice of spherical shells. We observe that, at leading order, the entanglement entropy associated with the scalar field is proportional to Bekenstein-Hawking entropy, while the corresponding entanglement energy scales proportionally to Komar energy. We then compute the heat capacity in both scenarios, discussing the black hole stability conditions and the possible appearance of second-order phase transitions. Finally, we extend our analysis to the black hole core, showing that in this sector entanglement energy serves as a valuable tool towards discriminating between singular and regular solutions.

Within the framework of extended black hole thermodynamics, where the cosmological constant acts as the thermodynamic pressure and its conjugate as the thermodynamic volume, we analyze the phase structure and thermodynamic geometry of the three-dimensional quantum Bañados-Teitelboim-Zanelli (qBTZ) black hole. Our results uncover two-phase transitions in the $T\text{−}S$ plane across all pressures except at a critical value. Numerical analysis reveals continuous critical phenomena along the coexistence curve, with critical exponents of 2 and 3 for the heat capacity at constant pressure and the new thermodynamic geometry (NTG) curvature, respectively. Importantly, these values notably deviate from the well-known critical exponents observed in mean-field van der Waals (vdW) fluids, where the NTG curvature and heat capacity demonstrate discontinuous criticality. To our knowledge, our investigation is the first exploration of critical behavior in black holes incorporating consistent semiclassical backreaction.