In this work, we study regular black holes from a global perspective, looking to evade some of the well-known singularity theorems by using their “reverses.” Then, model geometries for the slices of typical spherically symmetric, (locally) static four-dimensional regular black hole solutions are described from both an analytical and a topological point of view. While the finiteness of both the scalar and Kretchmann curvature of the slices around the regular center determines the geometry of the core, the positive answer to the Poincaré conjecture assures that, under two assumptions, its topology is that of a three-sphere. However, in general, the cores are shown to be $S^3$, $H^3$, $\mathbb{R}\times S^1$, or $S^1\times S^2$, depending on whether de Sitter, anti–de Sitter, Nariai, or Bertotti-Robinson geometries are employed to describe the slices at the regular center. Then, a description of the aforementioned slices in terms of Seifert fiber spaces is given in order to show that the Euler characteristic of the bundle can be used to track the transition between the core of the regular black hole and the rest of the slices in most of the cases considered in the literature. After Geroch and Tipler’s theorems are employed to study the consequences of topology change on regular black hole spacetimes, we show that Borde and Vilenkin’s singularity theorem is used to restrict their possible types. We end by noting that Nariai cores can be safely used to construct regular black holes without topology changes.

Scalarization is a very interesting phenomenon allowing a compact object to be endowed with scalar hair while leaving all the predictions in the weak field limit unaltered. In Gauss-Bonnet gravity, the source of the scalar field can be the curvature of the spacetime. It was recently shown that for a particular type of coupling function between the scalar field and the Gauss-Bonnet invariant, spin-induced black hole scalarization is possible. In the present paper, we study this phenomenon by performing a $2+1$ time evolution of the relevant linearized scalar field perturbation equation and examine the region where the Kerr black hole becomes unstable, giving rise to new scalarized rotating black holes. This is a direct numerical approach to study the development of spin-induced scalarization, and it can serve as an independent check of previously published results.

We study nonlinear gravitational perturbations of vacuum Einstein equations, with $\mathrm{\Lambda }<0$ in ($n+2$) dimensions, with $n>2$, generalizing previous studies for $n=2$. We follow the formalism by Ishibashi, Kodama, and Seto to decompose the metric perturbations into tensor, vector, and scalar sectors, and simplify the Einstein equations. We study, for the first time, general tensor perturbations. We render the metric perturbations in each sector asymptotically anti–de Sitter. Finally, we analyze the resonant structure of the perturbed equations at second order for the five-dimensional case, by a partial study of single mode tensor-type perturbations at the linear level. For the cases we studied, resonant terms vanish at second order.

There are several studies proposing phenomenological consequences of a deformation of special and general relativity. Here, we cast novel constraints on the deformation parameter of a metric in the cotangent bundle accounting for a curved momentum space. In an expanding universe, we study three possible observations that could restrict our model, focusing on the deformations of velocity, redshift and luminosity distance, which in the aforementioned framework, depend on the energy of the particles. We find that for an energy dependent velocity there would be no time delay for massless particles since also the observed distance to the source depends on the energy. For the redshift and luminosity distance we see that a scale of the order of some keV could be compatible with our model. This shows that the constraints on the high-energy scale parametrizing the momentum dependent deviation from a Friedmann-Robertson-Walker metric are at the moment weak due to the fact that the precision (rather than energies) needed in the observational constraints are extremely high. However, this is not the case when considering the synchrotron radiation. Indeed, the observation of such emission from the Crab Nebula, for deformations leading to subluminal propagation at high energies, leads to a constraint for the high-energy scale of the order of 1 PeV.

We propose a top down construction for Jackiw-Teitelboim (JT) gravity using compactification of $D=5$ gravity theories in the presence of Abelian [$U(1)$] as well as $SU(2)$ Yang-Mills fields. The background solutions corresponding to the $D=2$ model have been obtained in the perturbative regime of the theory where the corrections have been estimated over the uncharged JT solutions while treating the gauge couplings as the parameters of the expansion. Our analysis reveals the existence of two classes of solutions, namely, (i) the interpolating vacuum solution with ${\mathrm{AdS}}_2$ in the IR and ${\mathrm{Lifshitz}}_2$ in the UV (which serves as the thermal radiation background for our analysis) and (ii) the charged 2D black hole solution exhibiting ${\mathrm{Lifshitz}}_2$ asymptotics. The analysis on thermal stability reveals the onset of a first-order phase transition at $T\sim T_0$ such that for $T<T_0$ the only possible state is the thermal radiation background without any black hole. On the other hand, as the temperature is gradually increased beyond a certain crossover value $T\sim T_2(>T_0)$, an emerging globally stable black-hole-dominated phase has been observed which clearly indicates the onset of a Hawking-Page transition in 2D gravity models.

Identifying the presence of a gravitational wave transient buried in nonstationary, non-Gaussian noise, which can often contain spurious noise transients (glitches), is a very challenging task. For a given dataset, transient gravitational wave searches produce a corresponding list of triggers that indicate the possible presence of a gravitational wave signal. These triggers are often the result of glitches mimicking gravitational wave signal characteristics. To distinguish glitches from genuine gravitational wave signals, search algorithms estimate a range of trigger attributes, with thresholds applied to these trigger properties to separate signal from noise. Here, we present the use of Gaussian mixture models, a supervised machine learning approach, as a means of modeling the multidimensional trigger attribute space. We demonstrate this approach by applying it to triggers from the coherent Waveburst search for generic bursts in LIGO O1 data. By building Gaussian mixture models for the signal and background noise attribute spaces, we show that we can significantly improve the sensitivity of the coherent Waveburst search and strongly suppress the impact of glitches and background noise, without the use of multiple search bins as employed by the original O1 search. We show that the detection probability is enhanced by a factor of 10, leading enhanced statistical significance for gravitational wave signals such as GW150914.

We perform the first fully general relativistic, magnetohydrodynamic simulations of dynamically stable hypermassive neutron stars with and without ergoregions to assess the impact of ergoregions on launching magnetically driven outflows. The hypermassive neutron stars are modeled by a compressible and causal equation of state and are initially endowed with a dipolar magnetic field extending from the stellar interior into its exterior. We find that, after a few Alfvén times, magnetic field lines in the ergostar (star that contains ergoregions) and the normal star, have been tightly wound in both cases into a helical funnel within which matter begins to flow outward. The maximum Lorentz factor in the outflow is ${\mathrm{\Gamma }}_L\sim 2.5$, while the force-free parameter holds at $B^2/8\pi {\rho }_0\lesssim 10$. These values are incompatible with highly relativistic, magnetically driven outflows (jets) and short $\gamma$-ray bursts. We compare these results with those of a spinning black hole surrounded by a magnetized, massless accretion disk that launches a bona fide jet. Our simulations suggest that the Blandford-Znajek mechanism for launching relativistic jets only operates when a black hole is present, though the Poynting luminosity in all cases is comparable. Therefore, one cannot distinguish a magnetized, accreting black hole from a magnetized hypermassive neutron star in the so-called mass-gap based solely on the value of the observed Poynting luminosity. These results complement our previous studies of supramassive remnants and suggest that it would be challenging for either normal neutron stars or ergostars in a hypermassive state to be the progenitors of short $\gamma$-ray bursts.

We investigate domain wall and other defect solutions in the weak-field limit of Eddington-inspired Born-Infeld gravity as a function of $\kappa$, the only additional parameter of the theory with respect to general relativity. We determine, both analytically and numerically, the internal structure of domain walls, quantifying its dependency on $\kappa$ as well as the impact of such dependency on the value of the tension measured by an outside observer. We find that the pressure in the direction perpendicular to the domain wall can be, in contrast to the weak-field limit of general relativity, significantly greater or smaller than zero, depending, respectively, on whether $\kappa$ is positive or negative. We further show that the generalized von Laue condition, which states that the average value of the perpendicular pressure is approximately equal to zero in the weak-field limit of general relativity, does not generally hold in Eddington-inspired Born-Infeld gravity not only for domain walls, but also in the case of cosmic strings and spherically symmetric particles. We argue that a violation of the generalized von Laue condition should, in general, be expected in any theory of gravity whenever geometry plays a significant role in determining the defect structure.

Rapid parameter estimation of gravitational waves from binary neutron star coalescence, in particular accurate sky localization in minutes after the initial detection stage, is crucial for the success of multimessenger observations. One of the techniques to speed up the parameter estimation, which has been applied for the production analysis of the LIGO-Virgo collaboration, is reduced order quadrature (ROQ). While it speeds up parameter estimation significantly, the time required is still on the order of hours. Focusing on the fact that the parameter-estimation follow-up can be tuned with the information available at the detection stage, we improve the ROQ technique and develop a new technique, which we designate focused reduced order quadrature (FROQ). We find that FROQ speeds up the parameter estimation by a factor of $\mathcal{O}({10}^3)$ to $\mathcal{O}({10}^4)$ and enables providing accurate source properties such as the location of a source in several tens of minutes after detection.

Solutions of the field equations in $f(R)$ theory of gravity are found for a spherically symmetric and static spacetime in the non-linear electrodynamic theory of Born-Infeld (BI). It is found that the models allowed under these conditions must have the parametric form $f^{\prime }(R){|}_r=m+nr$, where $m$ and $n$ are constants, whose values and signs have a strong impact on the solutions, as well as on the form and range of the function $f(R)$. When $n=0$, $f(R)=mR+m_0$ and Einstein-BI solution is found. When $m\ne 0$ and $n\ne 0$, the theory $f(R)$ is asymptotically equivalent to General Relativity (GR), so that the solutions of Schwarzschild and $f(R)$-Reissner-Nordstrm can be written in some limits. Similarly, if $n>0$ and $r\gg 1$, the form of $f(R)$ can be approximated by an expansion in series and as a particular case, when $R_S=-\frac{m^2}{3n}$, can be found explicitly $f(R)=mR+2n\sqrt{R}+m_0$. Finally, the solutions, scalar curvature and parametric form of the function $f(r)$ in the nonlinear regime ($m=0$) of the $f(R)$ theory are found, and some models are plotted for specific values of $m$ and $n$.

An interesting class of scalar-tensor models, denoted by kinetic gravity braiding (KGB), has recently been proposed. These models contain interactions of the second derivatives of the scalar field that do not lead to additional degrees of freedom and exhibit peculiar features, such as an essential mixing of the scalar $\varphi$ and tensor kinetic $X$ terms. In this work, we consider the possibility that wormhole geometries are sustained by the KGB theory. More specifically, we present the full gravitational field equations in a static and spherically symmetric traversable wormhole background, and outline the general constraints at the wormhole throat, imposed by the flaring-out conditions. Furthermore, we present a plethora of analytical and numerical wormhole solutions by considering particular choices of the KGB factors. The analysis explicitly demonstrates that the KGB theory exhibits a rich structure of wormhole geometries, ranging from asymptotically flat solutions to asymptotically anti–de Sitter spacetimes.

We revisit the mechanism of primordial magnetogenesis during inflation by taking into account the dynamics of the stochastic noises of the electromagnetic perturbations. We obtain the associated Langevin and Fokker-Planck equations for the electromagnetic fields and solve them analytically. It is shown that while the backreactions of the electric field energy density may spoil inflation too early, there are regions of parameter space where the usual decaying behavior of the magnetic fields are replaced by a mean-reverting process of stochastic dynamics. As a result the magnetic fields settle down into an equilibrium state with the amplitude significantly larger than what is obtained in the absence of the stochastic noises. We show that magnetic fields with present time amplitude $\sim {10}^{-13}$ Gauss and correlation length Mpc can be generated while the backreactions of the electric field perturbations are under control.

We study the Bose condensation of scalar dark matter in the presence of both gravitational and self-interactions. Axions and other scalar dark matter in gravitationally bound miniclusters or dark matter halos are expected to condense into Bose-Einstein condensates called Bose stars. This process has been shown to occur through attractive self-interactions of the axionlike particles or through the field’s self gravitation. We show that in the high-occupancy regime of scalar dark matter, the Boltzmann collision integral does not describe either gravitational or self-interactions, and derive kinetic equations valid for these interactions. We use this formalism to compute relaxation times for the Bose-Einstein condensation, and find that condensation into Bose stars could occur within the lifetime of the Universe. The self-interactions reduce the condensation time only when they are very strong.

The energy radiated in supernova neutrinos is a fundamental quantity that is closely related to the gravitational binding energy of a neutron star. Recently the tidal deformability of neutron stars was constrained by gravitational wave observations. By considering several equations of state, we find a strong correlation between the tidal deformability and neutron star binding energy. We use this correlation to sharpen predictions of the binding energy of neutron stars and the total neutrino energy in supernovae. We find a minimum binding energy for a neutron star formed in a supernova of $\sim 1.5\times {10}^{53}\mathrm{ergs}$. Should the neutrino energy in a supernova be significantly below this value, it would strongly suggest new unobserved particles are carrying away some of the supernova energy. Alternatively, if the neutrino energy is observed above $\sim 6\times {10}^{53}\mathrm{ergs}$, it would strongly imply the formation of a (perhaps surprisingly) massive neutron star.

Cold dark matter (DM) models for structure formation predict that DM subhalos are present in the Galaxy. In the standard paradigm of DM as weakly interacting massive particle, subhalos are expected to shine in gamma rays and to provide a signal detectable with current instruments, notably with the Large Area Telescope (LAT) aboard the Fermi satellite. This is the main motivation behind searches for DM signals towards dwarf spheroidal galaxies and unidentified Fermi-LAT sources. A significant angular extension detected from unassociated sources located at relatively high latitudes is considered a “smoking gun” signature for identifying DM subhalos. In the present work, we systematically explore, by means of state-of-the-art models of cold DM halos in the Galaxy, the detectability of extended subhalos with Fermi-LAT. We simulate a DM signal exploring different assumptions of subhalos distribution in the Galaxy and DM profile, and reconstruct its flux through a realistic Fermi-LAT analysis pipeline. In the most optimistic case, we show that a detection of extended DM subhalos can be made for annihilation cross sections higher than $3\times {10}^{-26}{\mathrm{cm}}^3/\mathrm{s}$ (for a 100 GeV DM mass), still compatible with existing gamma-ray constraints, and that, in this case, the preference for extension of the source (vs a pointlike hypothesis) is significant. For fainter signals, instead, halos not only do not show significant extension, but they are not even detectable significantly as pointlike sources.

The Novikov-Thorne model is the standard framework for the description of geometrically thin and optically thick accretion disks around black holes and is widely used to study the electromagnetic spectra of accreting black holes. One of the assumptions of the model is that the particles of the gas move on nearly geodesic circular orbits on the equatorial plane. In this work, we propose to test the Keplerian velocity of the particles in the accretion disk using x-ray reflection spectroscopy. We present a modified version of relxill in which we introduce a phenomenological parameter, $\alpha$, to quantify possible deviations from Keplerian motion. We use our model to fit a Suzaku observation of the black hole binary GRS $1915+105$. We find that the estimate of $\alpha$ is correlated to that of the inclination angle of the disk, $i$, and that we could test the Keplerian disk hypothesis in the presence of a robust and independent measurement of $i$.

We argue that the stable (color-singlet) supermassive gravitinos proposed in our previous work can serve as seeds for giant primordial black holes. These seeds are hypothesized to start out as tightly bound states of fractionally charged gravitinos in the radiation-dominated era, whose formation is supported by the universally attractive combination of gravitational and electric forces between the gravitinos and antigravitinos (reflecting their “almost BPS-like” nature). When lumps of such bound states coalesce and undergo gravitational collapse, the resulting mini black holes can escape Hawking evaporation if the radiation temperature exceeds the Hawking temperature. Subsequently the black holes evolve according to an exact solution of Einstein’s equations, to emerge as macroscopic black holes in the transition to the matter-dominated era, with masses on the order of the solar mass or larger. The presence of these seeds at such an early time provides ample time for further accretion of matter and radiation, and would imply the existence of black holes of almost any size in the Universe, up to the observed maximum.

We consider a version of the Gross-Neveu model in $1+1$ dimensions with discrete chiral and continuous flavor symmetry (isospin). In $2+1$ dimensions, this model is known as the chiral Heisenberg-Gross-Neveu model. Spontaneous symmetry breaking and the emergence of two massless and one massive scalar bosons are shown. A duality to the Nambu–Jona-Lasinio model with isospin is exhibited, provided that the isovector pseudoscalar mean field is constrained to a plane in isospin space. This enables us to find the phase diagram as a function of temperature, chemical potential, and isospin chemical potential as well as twisted kinks. A bare mass term acts quite differently when added to this model as compared to other chiral variants of the Gross-Neveu model.

Thorough modeling of the physics involved in liquid argon calorimetry is essential for accurately predicting the performance of DUNE and optimizing its design and analysis pipeline. At the fundamental level, it is essential to quantify the detector response to individual hadrons—protons, charged pions, and neutrons—at different injection energies. We report such a simulation, analyzed under different assumptions about event reconstruction, such as particle identification and neutron detection. The role of event containment is also quantified. The results of this simulation can help inform the ProtoDUNE test-beam data analysis, while also providing a framework for assessing the impact of various cross section uncertainties.

We employ the functional renormalization group approach formulated on the Schwinger-Keldysh contour to calculate real-time correlation functions in scalar field theories. We provide a detailed description of the formalism, discuss suitable truncation schemes for real-time calculations, as well as the numerical procedure to self-consistently solve the flow equations for the spectral function. Subsequently, we discuss the relations to other perturbative and nonperturbative approaches to calculate spectral functions and present a detailed comparison and benchmark in $d=0+1$ dimensions.