We undertake a first-principles analysis of the thermodynamics of a small body near a black hole horizon. In particular, we study the paradigmatic system of a quantum ideal gas in a small box hovering over the Schwarzschild horizon. We describe the gas in terms of free quantum fields, bosonic and fermionic, massive and massless. We identify thermodynamic properties through the microcanonical distribution. We first analyze the more general case of a box in Rindler spacetime, and then specialize to the black hole case. The physics depends strongly on the distance of the box from the horizon, which we treat as a macroscopic thermodynamic variable. We find that the effective dimension of the system transitions from three-dimensional to two-dimensional as we approach the horizon, that Bekenstein’s bound fails when the box is adiabatically lowered towards the black hole, and that the pressure is highly anisotropic. The pressure difference between the upper and lower wall leads to an effective force that must be added to the gravitational acceleration. We also show that the approximation of quantum fields propagating on a fixed background for matter breaks down when the system is brought to microscopic distances from the horizon, in which case backreaction effects must be included.

We examine the semiclassical backreaction of a conformally coupled scalar field on an overspinning (Bañados-Teitelboim-Zanelli) BTZ geometry. This extends the work done on a similar problem for ($2+1$)-anti–de Sitter geometries of the BTZ family with $|M|>|J|$. The overspinning classical solutions correspond to $|M|<|J|$ and possess a naked singularity at $r=0$. Using the renormalized quantum stress-energy tensor for a conformally coupled scalar field on such a spacetime, we obtain the semiclassical Einstein equations, which we attempt to solve perturbatively. We show that the stress-energy tensor is nonrenormalizable in this approach, and consequently the perturbative solution to the semiclassical equations in the overspinning case does not exist. This could be an indication of the fact that the naked singularity at the center of an overspinning geometry is of a more severe nature than the conical singularity found in the same family of BTZ geometries.

We consider the low-energy effective action of string theory at order ${\alpha }^{\prime }$, including $R^2$ corrections to the Einstein-Hilbert gravitational action and nontrivial dilaton coupling. By means of a convenient field redefinition, we manage to express the theory in a frame that enables us to solve its field equations analytically and perturbatively in ${\alpha }^{\prime }$ for a static spherically symmetric ansatz in an arbitrary number of dimensions. The set of solutions we obtain is compatible with asymptotically flat geometries exhibiting a regular event horizon at which the dilaton is well behaved. For the four-dimensional case, we also derive the stationary black hole configuration at first order in ${\alpha }^{\prime }$ and in the slowly rotating approximation. This yields string theory modifications to the Kerr geometry, including terms of the form $a$, $a^2$, ${\alpha }^{\prime }$, and $a{\alpha }^{\prime }$. In addition, we obtain the first ${\alpha }^{\prime }$ correction to the $C$ metrics, which accommodates accelerating black holes. We work in the string frame and discuss the connection to the Einstein frame, for which rotating black holes have already been obtained in the literature.

Near-Kerr black hole initial datasets are constructed by applying either the parabolic-hyperbolic or the algebraic-hyperbolic form of the constraints. In both cases, strongly and weakly asymptotically flat initial datasets with desirable falloff rates are produced by controlling only the monopole part of one of the freely specifiable variables. The viability of the applied method is verified by numerically integrating the evolutionary forms of the constraint equations in the case of various near-Kerr configurations.

We use an asteroseismology method to calculate the frequencies of gravitational waves (GWs) in a long-term core-collapse supernova simulation, with a mass of 9.6 solar mass. The simulation, which includes neutrino radiation transport in general relativity is performed from core-collapse, bounce, explosion and cooling of protoneutron stars (PNSs) up to 20 s after the bounce self-consistently. Based on the hydrodynamics background, we calculate eigenmodes of the PNS oscillation through a perturbation analysis on fluid and metric. We classify the modes by the number of nodes and find that there are several eigenmodes. In the early phase before 1 s, there are low-frequency $g$-modes around 0.5 kHz, midfrequency $f$-modes around 1 kHz, and high-frequency $p$-modes above them. Beyond 1 second, the $g$-modes drop too low in frequency and the $p$-modes become too high to be detected by ground-based interferometers. However, the $f$-mode persists at 1 kHz. We present a novel fitting formula for the ramp-up mode, comprising a mixture of $g$-mode and $f$-mode, using postbounce time as a fitting parameter. Our approach yields improved results for the long-term simulation compared to prior quadratic formulas. We also fit frequencies using combinations of gravitational mass, $M$, and radius, $R$, of the PNS. We test three types of fitting variables: compactness $M/R$, surface gravity $M/R^2$, and average density $\sqrt{M/R^3}$. We present results of the time evolution of each mode and the fitting for three different ranges, from 0.2 s to 1 s, 4 s, and 20 s for each formula. We then compare the deviation of the formulas from the eigenmodes to determine which fitting formula is the best. In conclusion, any combination of $M$ and $R$ fits the eigenmodes well to a similar degree. Comparing three variables in detail, the fitting with compactness is slightly the best among them. We also find that the fitting using less than 1 s of simulation data cannot be extrapolated to the long-term frequency prediction.

Ultralight dark matter (ULDM) is usually taken to be a single scalar field. Here we explore the possibility that ULDM consists of $N$ light scalar fields with only gravitational interactions. This configuration is more consistent with the underlying particle physics motivations for these scenarios than a single ultralight field. ULDM halos have a characteristic granular structure that increases stellar velocity dispersion and can be used as observational constraints on ULDM models. In multifield simulations, we find that inside a halo the amplitude of the total density fluctuations decreases as $1/\sqrt{N}$ and that the fields do not become significantly correlated over cosmological timescales. Smoother halos heat stellar orbits less efficiently, reducing the velocity dispersion relative to the single field case and thus weakening the observational constraints on the field mass. Analytically, we show that for $N$ equal-mass fields with mass $m$ the ULDM contribution to the stellar velocity dispersion scales as $1/(N{m}^3)$. Lighter fields heat the most efficiently and if the smallest mass $m_L$ is significantly below the other field masses the dispersion scales as $1/(N^2{m}_L^3)$.

The presence of an extra radio background besides the cosmic microwave background has important implications for the observation of the 21-cm signal during the cosmic dark ages, cosmic dawn, and the epoch of reionization. The strong absorption trough found in the 21-cm global spectrum measured by the EDGES experiment, which has a much greater depth than the standard model prediction, has drawn great interest to this scenario, but more generally it is still of great interest to consider such a cosmic radio background in the early Universe. To be effective in affecting the 21-cm signal at early time, such a radio background must be produced by sources that can emit strong radio signals but a modest amount of x rays so that the gas is not heated up too early. We investigate the scenario where such a radio background is produced by the primordial black holes (PBHs). For PBHs with a single mass, we find that if the PBHs’ abundance $\log (f_{\mathrm{PBH}})$ (ratio of total PBH mass density to total matter density) and mass satisfy the relation $\log (f_{\mathrm{PBH}})\sim -1.8\log (M_{•}/M_{\odot })-3.5$ for $1M_{\odot }\lesssim M_{•}\lesssim 300M_{\odot }$, and have jet emission, they can generate a cosmic radio background required for reproducing the 21-cm absorption signal seen by the EDGES. The accretion rate can be boosted if the PBHs are surrounded by dark matter halos, which permits a lower $f_{\mathrm{PBH}}$ value to satisfy the EDGES observation. In the latter scenario, since the accretion rate can evolve rapidly during the cosmic dawn, the frequency (redshift), and depth of the absorption trough can determine the mass and abundance of the PBHs simultaneously. For absorption trough redshift $\sim 17$ and depth $\sim -500\mathrm{mK}$, it corresponds to $M_{•}\sim 1.05M_{\odot }$ and $f_{\mathrm{PBH}}\sim 1.5\times {10}^{-4}$.

Nonannihilating dark matter particles, owing to their interactions with ordinary baryonic matter, can efficiently accumulate inside celestial objects. For heavy mass, they gravitate toward the core of the celestial objects, thermalize in a small core region, and eventually form tiny black holes via core collapse, resulting destruction of the host objects. We demonstrate that the existence of a variety of celestial objects provides stringent constraints on strongly interacting heavy dark matter, a blind spot for the terrestrial dark matter detectors as well as for the cosmological probes. Celestial objects with larger sizes and lower core temperatures, such as Jupiter, are the most optimal detectors to probe the strongly interacting heavy nonannihilating dark matter.

It is challenging to build a model that can correctly and unifiedly account for the deconfinement phase transition and thermodynamics of the hot $SU(N)$ pure Yang-Mills (PYM) system, for any $N$. In this article, we slightly generalize the massive PYM model to the situation with a quasigluon mass $M_g(T)$ varying with temperature, inspired by the quasigluon model. In such a framework, we can acquire an effective potential for the temporal gauge field background by perturbative calculation, rather than adding by hand. The resulting potential works well to describe the behavior of the hot PYM system for all $N$, via the single parameter $M_g(T)$. Moreover, under the assumption of unified eigenvalue distribution, the $M_g(T)$ fitted by machine learning is found to follow $N$-universality.

We investigate the magnetized QCD matter and chiral phase transition in a ($2+1$)-flavor Nambu–Jona-Lasinio (NJL) model at finite temperature and chemical potential by comparing the contributions from the tensor spin polarization (TSP) and anomalous magnetic moment (AMM) of quarks. For light $u$ and $d$ quarks, when TSP and AMM are not considered, the magnetized system is characterized by magnetic catalysis. The introduction of TSP will further enhance the magnetic catalytic characteristics. On the other hand, when AMM is introduced, the phase-transition temperature decreases with the magnetic field, which is the feature of inverse magnetic catalysis. The phase diagram of $u$ and $d$ quarks will change from the crossover phase transition to the first order phase transition with the increase of magnetic field and chemical potential when AMM is induced. The phase diagram will not change from the crossover phase transition to the first-order phase transition when TSP is induced. For the phase diagram of strange $s$ quark, whether TSP or AMM is induced, the phase diagram will keep a crossover phase transition with the increase of magnetic field and chemical potential.

In this work, we study the annihilation of a pair of ‘t Hooft-Polyakov monopoles due to confinement by a string. We analyze the regime in which the scales of monopoles and strings are comparable. We compute the spectrum of the emitted gravitational waves and find it to agree with the previously calculated pointlike case for wavelengths longer than the system width and before the collision. However, we observe that in a head-on collision, monopoles are never recreated. Correspondingly, not even once the string oscillates. Instead, the system decays into waves of Higgs and gauge fields. We explain this phenomenon by the loss of coherence in the annihilation process. Due to this, the entropy suppression makes the recreation of a monopole pair highly improbable. We argue that in a similar regime, analogous behavior is expected for the heavy quarks connected by a QCD string. There too, instead of restretching a long string after the first collapse, the system hadronizes and decays in a high multiplicity of mesons and glueballs. We discuss the implications of our results.

A large set of models beyond the Standard Model of particle physics suggest that the top quark plays a special role in fundamental interactions. At the same time some of these models predict that a particle responsible for dark matter is present in the physical spectrum of the theory, and in particular cases dark matter may be specially linked to the top sector. Such a new physics configuration occurs, for instance, in composite frameworks that additionally feature vectorlike quarks as mediators of the interactions of the dark matter with the Standard Model. We investigate the interplay of top-philic dark matter and the presence of several vectorlike top partners. Such a nonminimal setup with several vectorlike fermions is typical of composite scenarios, and we make use of an effective model to study to which extent such nonminimality modifies the collider and cosmological phenomenology of the minimal model, the latter only including one vectorlike mediator of the dark matter interactions with the Standard Model.

We investigate how the charge distributions of both the unpolarized and transversely polarized $\mathrm{\Delta }$ baryon change as the longitudinal momentum ($P_z$) of the $\mathrm{\Delta }$ baryon increases from $P_z=0$ to $P_z=\infty$ in a Wigner phase-space perspective. When the $\mathrm{\Delta }$ baryon is longitudinally polarized, its two-dimensional charge distribution is kept to be spherically symmetric with $P_z$ varied, whereas when the $\mathrm{\Delta }$ baryon is transversely polarized along the $x$-axis, the quadrupole contribution emerges at the rest frame ($P_z=0$). When $P_z$ grows, the electric dipole and octupole moments are induced. The induced dipole moment dominates over other higher multipole contributions and governs the deformation of the charge distribution of the $\mathrm{\Delta }$ baryon.

We present a measurement of the dressed cross sections for $e^{+}{e}^{-}\to \varphi {\eta }^{\prime }$ at different center-of-mass energies between 3.508 and 4.600 GeV based on $15.1{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ annihilation data collected with the BESIII detector operating at the BEPCII collider. In addition, a search for the decay $Y(4230)\to \varphi {\eta }^{\prime }$ is performed. No clear signal is observed and the corresponding upper limit is provided.

We investigate the effects of quantum backreaction on the Schwarzschild geometry in the semiclassical approximation. The renormalized stress-energy tensor (RSET) of a scalar field is modeled via an order reduction of the analytical approximation derived by Anderson, Hiscock, and Samuel (AHS). As the resulting AHS semiclassical Einstein equations are of fourth-derivative order in the metric, we follow a reduction of order prescription to shrink the space of solutions. Motivated by this prescription, we develop a method that allows us to obtain a novel analytic approximation for the RSET that exhibits all the desired properties for a well-posed RSET: conservation, regularity, and correct estimation of vacuum-state contributions. We derive a set of semiclassical equations sourced by the order-reduced AHS-RSET in the Boulware state. We classify the self-consistent solutions to this set of field equations, discuss their main features and address how well they resemble the solutions of the higher-order semiclassical theory. Finally, we establish a comparison with previous results in the literature obtained through the Polyakov approximation for minimally coupled scalar fields.

We present a pseudoclassical mechanics model which exhibits gauge symmetry and time-reparametrization invariance. As such, first- and second-class constraints restrict the phase space, and the Hamiltonian weakly vanishes. We show that the Dirac conjecture does not hold—the secondary first-class constraint is not a symmetry generator—and only the gauge fixing condition associated with the primary first-class constraint is needed to remove the gauge ambiguities. The gauge fixed theory is equivalent to the Fermi harmonic oscillator extended by a boundary term. We quantize in the deformation quantization and in the Schrödinger representation approaches and observe that the boundary term prepares the system in the state of positive energy.

We report on numerical simulations of critical phenomena near the threshold of black-hole formation in the collapse of axisymmetric gravitational waves in vacuum. We discuss several new features of our numerical treatment, and then compare results obtained from families of quadrupolar and hexadecapolar initial data. Specifically, we construct (nonlinear) initial data from quadrupolar and hexadecapolar, time-symmetric wavelike solutions to the linearized Einstein equations (often referred to as Teukolsky waves), and evolve these using a shock-avoiding slicing condition. While our degree of fine-tuning to the onset of black-hole formation is rather modest, we identify several features of the threshold solutions formed for the two families. Both threshold solutions appear to display an at least approximate discrete self-similarity with an accumulation event at the center, and the characteristics of the threshold solution for the quadrupolar data are consistent with those found previously by other authors. The hexadecapolar threshold solution appears to be distinct from the quadrupolar one, providing further support to the notion that there is no universal critical solution for the collapse of vacuum gravitational waves.

The dynamics of a binary system moving in the background of a black hole is affected by tidal forces. In this work, for the Kerr black hole, we derive the electric and magnetic tidal moments at quadrupole order, where the latter are computed for the first time in full generality. We make use of these moments in the scenario of a hierarchical triple system made of a Kerr black hole and an extreme-mass ratio binary system consisting of a Schwarzschild black hole and a test particle. We study how the secular dynamics of the test particle in the binary system is distorted by the presence of tidal forces from a much larger Kerr black hole. Our treatment includes strong gravitational effects beyond the post-Newtonian approximation both for the binary system and for the tidal forces since the binary system is allowed to be close to the event horizon of the Kerr black hole. We compute the shifts in the physical quantities for the secular dynamics of the test particle and show that they are gauge invariant. In particular, we apply our formalism to the innermost stable circular orbit for the test particle and to the case of the photon sphere. Our results are relevant for the astrophysical situation in which the binary system is in the vicinity of a supermassive black hole.

Cosmological observations precisely measure primordial variations in the density of the Universe at megaparsec and larger scales, but much smaller scales remain poorly constrained. However, sufficiently large initial perturbations at small scales can lead to an abundance of ultradense dark matter minihalos that form during the radiation epoch and survive into the late-time Universe. Because of their early formation, these objects can be compact enough to produce detectable microlensing signatures. We investigate whether the EROS, OGLE, and HSC surveys can probe these halos by fully accounting for finite source size and extended lens effects. We find that current data may already constrain the amplitudes of primordial curvature perturbations in a new region of parameter space, but this conclusion is strongly sensitive to yet undetermined details about the internal structures of these ultradense halos. Under optimistic assumptions, current and future HSC data would constrain a power spectrum that features an enhancement at scales $k\sim {10}^7/\mathrm{Mpc}$, and an amplitude as low as ${\mathcal{P}}_{\zeta }\simeq {10}^{-4}$ may be accessible. This is a particularly interesting regime because it connects to primordial black hole formation in a portion of the LIGO/Virgo/Kagra mass range and the production of scalar-induced gravitational waves in the nanohertz frequency range reachable by pulsar timing arrays. These prospects motivate further study of the ultradense halo formation scenario to clarify their internal structures.

Cold interstellar gas systems have been used to constrain dark matter (DM) models by the condition that the heating rate from DM must be lower than the astrophysical cooling rate of the gas. Following the methodology of Wadekar and Farrar [1], we use the interstellar medium of a gas-rich dwarf galaxy, Leo T, and a Milky Way-environment gas cloud, G33.4-8.0 to constrain DM. Leo T is a particularly strong system as its gas can have the lowest cooling rate among all the objects in the late Universe (owing to the low volume density and metallicity of the gas). Milky Way clouds, in some cases, provide complementary limits as the DM-gas relative velocity in them is much larger than that in Leo T. We derive constraints on the following scenarios in which DM can heat the gas: (i) interaction of axions with hydrogen atoms or free electrons in the gas, (ii) deceleration of relic magnetically charged DM in gas plasma, (iii) dynamical friction from compact DM, (iv) hard sphere scattering of composite DM with gas. Our limits are complementary to DM direct detection searches. Detection of more gas-rich low-mass dwarfs like Leo T from upcoming 21 cm and optical surveys can improve our bounds.