The null geodesic equation in the Kerr spacetime can be expressed as a set of integral equations involving certain potentials. We classify the roots of these potentials and express the integrals in manifestly real Legendre elliptic form. We then solve the equations using Jacobi elliptic functions, providing the complete set of null geodesics of the Kerr exterior as explicit parametrized curves.

Interpreting horizon-scale observations of astrophysical black holes demands a general understanding of null geodesics in the Kerr spacetime. These may be divided into two classes: “direct” rays that primarily determine the observational appearance of a given source, and highly bent rays that produce a nested sequence of exponentially demagnified images of the main emission: the so-called “photon ring.” We develop heuristics that characterize the direct rays and study the highly bent geodesics analytically. We define three critical parameters $\gamma$, $\delta$, and $\tau$ that respectively control the demagnification, rotation, and time delay of successive images of the source, thereby providing an analytic theory of the photon ring. These observable parameters encode universal effects of general relativity, independent of the details of the emitting matter.

A Damour-Solodukhin wormhole with a metric which is similar to a Schwarzschild black hole seems to be a black hole mimicker since it is difficult to distinguish them by practical astrophysical observations. In this paper, we investigate a center-of-mass energy for the collision of two test particles in the Damour-Solodukhin wormhole spacetime. We show that the center-of-mass energy for the head-on collision of the particles is large if the difference between the metrics of the wormhole and the black hole is small. To deeply understand the high energy particle collision, we generalize the head-on collision to static, spherically symmetric black-hole-like wormholes.

We continue our study in four dimensions to derive noncharged and charged (anti)–de Sitter black hole solutions in conformal teleparallel equivalent of general relativity. The noncharged and charged equations of motion are applied to a spherically symmetric tetrad and the nonlinear differential equations are derived. It is shown that the output solutions of the two cases are identical to those obtained in teleparallel equivalent theory to general relativity. As a result, it is found that in the conformal teleparallel equivalent theory to general relativity, the scalar field cannot influence on the manifold of spherical symmetry, i.e., the scalar field must equal one in order to have a well-known asymptote spacetime.

Though under nonlinear accretion, all black holes in four and higher dimensions obey the weak cosmic censorship conjecture (CCC); however, they generally violate it for linear test particle accretion with the exception of five-dimensional rotating black hole with a single rotation. In dimensions greater than five, there exists no extremal condition for black hole with single rotation and hence it can never be overspun. However, the extremal condition does exist for five-dimensional black hole with two rotations and then it could indeed be overspun under linear accretion. In this paper, we study the case of six-dimensional rotating black hole with two rotations and show that unlike the five-dimensional black hole it cannot be overspun under linear accretion. Though for nonlinear accretion, this result is anyway expected to hold good, yet we have verified it with an explicit calculation. Further, we would like to conjecture that so should be the case in all dimensions greater than six. Thus, the weak CCC may always be obeyed even at linear accretion process for rotating black hole in all dimensions greater than five.

We study the induced primordial gravitational waves (GWs) coming from the effect of scalar perturbation on the tensor perturbation at the second order of cosmological perturbation theory. We use the evolution of the standard model degrees of freedom with respect to temperature in the early Universe to compute the induced gravitational waves background. Our result shows that the spectrum of the induced GWs is affected differently by the standard model degrees of freedom than the GWs coming from the first-order tensor perturbation. This phenomenon is due to the presence of scalar perturbations as a source for tensor perturbations, and it is effective around the quark gluon deconfinement and electroweak transition. In case of considering a scalar spectral index larger than 1 at small scales or a non-Gaussian curvature power spectrum, this effect can be observed by gravitational wave observatories.

We examine the dynamics of a closed cosmology of which the matter source is that of a conformally coupled scalar field with a broken $\mathrm{SO}(1,1)$ symmetry, which corresponds to the $\alpha$ attractors proposed by Linde and Kallosh. Following a field redefinition, such models give rise to “$T$-model” inflationary potentials, the dynamics of which provide both an inflationary phase and a classical bounce. We show that the Universe can undergo bounces far from the regime of quantum gravity (i.e., at energy densities lower than the Planck density). We analyze perturbations on this background with particular attention given to the effects of a double-bouncing scenario (with rapid recollapse between bounces) on the long wavelength modes. We demonstrate that the predictions of such models agree well with observations and might explain the suppression of power in the low multiples of the cosmic microwave background.

Predictions in an eternally inflating multiverse are meaningless unless we specify the probability measure. The scale-factor cutoff is perhaps the simplest and most successful measure which avoids catastrophic problems such as the youngness paradox, runaway problem, and Boltzmann brain problem, but it is not well defined in contracting regions with a negative cosmological constant. In this paper, we propose a new measure with properties similar to the scale-factor cutoff which is well-defined everywhere. The measure is defined by a cutoff in the four volume spanned by infinitesimal comoving neighborhoods in a congruence of timelike geodesics. The probability distributions for the cosmological constant and for the curvature parameter in this measure are similar to those for the scale-factor cutoff and are in a good agreement with observations.

The ultralight scalar fields can arise ubiquitously, for instance, as a result of the spontaneous breaking of an approximate symmetry such as the axion and, more generally, the axionlike particles. In addition to the particle physics motivations, these particles can also play a major role in cosmology by contributing to dark matter abundance and affecting the structure formation at sub-Mpc scales. In this paper, we propose to use the 21 cm forest observations to probe the nature of ultralight dark matter. The 21 cm forest is the system of narrow absorption lines appearing in the spectra of high redshift background sources due to the intervening neutral hydrogen atoms, similar to the Lyman-$\alpha$ forest. Such features are expected to be caused by the dense neutral hydrogen atoms in a small starless collapsed object called a minihalo. The 21 cm forest can probe much smaller scales than the Lyman-$\alpha$ forest, that is, $k\gtrsim 10{\mathrm{Mpc}}^{-1}$. We explore the range of the ultralight dark matter mass $m_u$ and $f_u$, the fraction of ultralight dark matter with respect to the total matter, which can be probed by the 21 cm forest. We find that 21 cm forest can potentially put the dark matter mass lower bound $m_u\gtrsim {10}^{-18}\mathrm{eV}$ for $f_u=1$, which is a 3 orders of magnitude bigger mass scale than those probed by the current Lyman-$\alpha$ forest observations. While the effects of the ultralight particles on the structure formation become smaller when the dominant component of dark matter is composed of the conventional cold dark matter, we find that the 21 cm forest is still powerful enough to probe the subcomponent ultralight dark matter mass up to the order of ${10}^{-19}\mathrm{eV}$. The Fisher matrix analysis shows that $(m_u,f_u)\sim ({10}^{-20}\mathrm{eV},0.3)$ is the most optimal parameter set which the 21 cm forest can probe with the minimal errors for a subcomponent ultralight dark matter scenario.

We advocate an idea that the presence of the daily and annual modulations of the axion flux on the Earth’s surface may dramatically change the strategy of the axion searches. Our computations are based on the so-called axion quark nugget (AQN) dark-matter model which was originally put forward to explain the similarity of the dark and visible cosmological matter densities ${\mathrm{\Omega }}_{\mathrm{dark}}\sim {\mathrm{\Omega }}_{\text{visible}}$. In our framework, the population of galactic axions with mass ${10}^{-6}\mathrm{eV}\lesssim m_a\lesssim {10}^{-3}\mathrm{eV}$ and velocity $⟨v_a⟩\sim {10}^{-3}c$ will be always accompanied by the axions with typical velocities $⟨v_a⟩\sim 0.6c$ emitted by AQNs. We formulate the broadband detection strategy to search for such relativistic axions by studying the daily and annual modulations. We describe several tests which could effectively discriminate a true signal from noise. These AQN-originated axions can be observed as correlated events which could be recorded by synchronized stations in the global network. The correlations can be effectively studied if the detectors are positioned at distances shorter than a few hundred kilometers.

We explore the use of field solvers as approximations of classical Vlasov-Poisson systems. This correspondence is investigated in both electrostatic and gravitational contexts. We demonstrate the ability of field solvers to be excellent approximations of problems with cold initial condition into the nonlinear regime. We also investigate extensions of the Schrödinger-Poisson system that employ multiple stacked cold streams, and the von Neumann–Poisson equation as methods that can successfully reproduce the classical evolution of warm initial conditions. We then discuss how appropriate simulation parameters need to be chosen to avoid interference terms, aliasing, and wave behavior in the field solver solutions. We present a series of criteria clarifying how parameters need to be chosen in order to effectively approximate classical solutions.

There are many simplifications in current relativistic reflection models and this introduces systematic uncertainties in the measurement of the properties of the source. In this paper, we study the impact of the radiation crossing the equatorial plane between the black hole and the inner edge of the accretion disk; that is, the radiation produced by the other side of the disk or circling the black hole one or more times (higher order disk images). For slow-rotating black holes with a larger plunging region, the effect is not very sensitive to the exact inclination angle of the disk. For fast-rotating black holes with a smaller plunging region, the effect is very weak for low inclination angles and becomes more important as the angle increases. We simulate some observations without and with higher order disk images and fit the data with the reflection model relxill_nk to check its capability of recovering the correct input parameters. Our results suggest that the effect of higher order disk images can be safely ignored for observations with present and near future X-ray missions, even for tests of the Kerr hypothesis.

Recent theoretical work indicates that the neutrino radiation in core-collapse supernovae may be susceptible to flavor instabilities that set in far behind the shock, grow extremely rapidly, and have the potential to profoundly affect supernova dynamics and composition. Here we analyze the nonlinear collective oscillations that are prefigured by these instabilities. We demonstrate that a zero crossing in $n_{{\nu }_e}-n_{{\overline{\nu }}_e}$ as a function of propagation angle is not sufficient to generate instability. Our analysis accounts for this fact and allows us to formulate complementary criteria. Using fornax simulation data, we show that fast collective oscillations qualitatively depend on how forward peaked the neutrino angular distributions are.

The next Galactic core-collapse supernova (SN) is a highly anticipated observational target for neutrino telescopes. However, even prior to collapse, massive dying stars shine copiously in “pre-supernova” (pre-SN) neutrinos, which can potentially act as efficient SN warning alarms and provide novel information about the very last stages of stellar evolution. We explore the sensitivity to pre-SN neutrinos of large-scale direct dark matter detection experiments, which, unlike dedicated neutrino telescopes, take full advantage of coherent neutrino-nucleus scattering. We find that argon-based detectors with target masses of $\mathcal{O}(100)$ tons (i.e., comparable in size to the proposed ARGO experiment) operating at sub-keV thresholds can detect $\mathcal{O}(10–100)$ pre-SN neutrinos coming from a source at a characteristic distance of $\sim 200\mathrm{pc}$, such as Betelgeuse ($\alpha$ Orionis). Large-scale xenon-based experiments with similarly low thresholds could also be sensitive to pre-SN neutrinos. For a Betelgeuse-type source, large-scale dark matter experiments could provide a SN warning siren $\sim 10$ hours prior to the explosion. We also comment on the complementarity of large-scale direct dark matter detection experiments and neutrino telescopes in the understanding of core-collapse SN.

We investigate how the coherent spreading of the center-of-mass wave function of a particle, such as an atom, molecule or ion, affects the particle’s interaction with fields such as the electromagnetic field or a phonon field, in view also of possible applications to emerging quantum technologies. To this end, we develop a suitably generalized Unruh-DeWitt model for the interaction between a delocalizing first-quantized particle and a second-quantized field. We study how the coherent spreading of the center-of-mass wave function of the particle affects emission and absorption rates and we find, in particular, that in the case of a supersonic coherent spreading in a medium, there should occur Cherenkov-like emissions, along with the excitation of the particle.

We investigate the impact parameter dependence of the $\cos 4\varphi$ azimuthal asymmetry for electromagnetic lepton pair production in heavy ion collisions. The asymmetry induced by linearly polarized coherent photons exhibits strong impact parameter dependence.

Recent developments have exposed close connections between quantum information and holography. In this paper, we explore the geometrical interpretations of the recently introduced $Q$-correlation and $R$-correlation, $E_Q$ and $E_R$. We find that $E_Q$ admits a natural geometric interpretation via the surface-state correspondence: it is a minimal mutual information between a surface region $A$ and a cross section of $A$’s entanglement wedge with $B$. We note a strict trade-off between this minimal mutual information and the symmetric side-channel-assisted distillable entanglement from the environment $E$ to $A$, $I^{ss}(E⟩A)$. We also show that the $R$-correlation, $E_R$, coincides holographically with the entanglement wedge cross section. This further elucidates the intricate relationship between entanglement, correlations, and geometry in holographic field theories.

Weyl conformal geometry may play a role in early cosmology where effective theory at short distances becomes conformal. Weyl conformal geometry also has a built-in geometric Stueckelberg mechanism: it is broken spontaneously to Riemannian geometry after a particular Weyl gauge transformation (of gauge fixing) while the Stueckelberg mechanism rearranges the degrees of freedom, conserving their number ($n_{df}$). The Weyl gauge field (${\omega }_{\mu }$) of local scale transformations acquires a mass after absorbing a compensator (dilaton), decouples, and Weyl connection becomes Riemannian. Mass generation has thus a dynamic origin, corresponding to a transition from Weyl to Riemannian geometry. In applications, we show that a gauge fixing symmetry transformation of the original Weyl’s quadratic gravity action immediately gives the Einstein-Proca action for the Weyl gauge field and a positive cosmological constant, plus matter action (if present). As a result, the Planck scale is an emergent scale, where Weyl gauge symmetry is spontaneously broken and Einstein action is a broken phase of Weyl action. This is in contrast to local scale invariant models (no gauging) where a negative kinetic term (ghost dilaton) remains present and $n_{df}$ is not conserved when this symmetry is broken. The mass of ${\omega }_{\mu }$, setting the nonmetricity scale, can be much smaller than $M_{\text{Planck}}$, for ultraweak values of the coupling ($q$), with implications for phenomenology. If matter is present, a positive contribution to the Planck scale from a scalar field (${\varphi }_1$) VEV(vacuum expectation value) induces a negative ${(\mathrm{mass})}^2$ term for ${\varphi }_1$ and spontaneous breaking of the symmetry under which it is charged. These results are immediate when using Weyl-covariant (invariant) scalar (tensor) curvatures, respectively, instead of their Riemannian form. Briefly, Weyl gauge symmetry is physically relevant and its role in high scale physics should be reconsidered.

We present a systematic construction of the most general first order Lagrangian describing an arbitrary number of interacting Maxwell and Proca fields on Minkowski spacetime. To this aim, we first formalize the notion of a Proca field, in analogy to the well-known Maxwell field. Our definition allows for a nonlinear realization of the Proca mass, in the form of derivative self-interactions. Consequently, we consider so-called generalized Proca/vector Galileons. We explicitly demonstrate the ghost-freedom of this complete Maxwell-Proca theory by obtaining its constraint algebra. We find that, when multiple Proca fields are present, their interactions must fulfill nontrivial differential relations in order to ensure the propagation of the correct number of degrees of freedom. These relations had so far been overlooked, which means previous multi-Proca proposals generically contain ghosts. This is a companion paper to the paper by Diez et al. [arXiv:1905.06967]. It puts on a solid footing the theory there introduced.

We present the most general ghostfree classical Lagrangian containing first-order derivatives and describing interacting real Abelian spin-one fields on Minkowski spacetime. We study both massive Proca and massless Maxwell fields and allow for a nonlinear realization of mass, in the form of derivative self-interactions. Within this context, our construction notoriously extends the existing literature, which is limited to the case of a single Proca field and to multiple interacting Proca fields in the presence of a global rotational symmetry. In the limit of a single Proca field, we reproduce the known healthy interaction terms. We provide the necessary and sufficient conditions to ensure ghost-freedom in any multifield setup. We observe that, in general, the said conditions are not satisfied by the rotationally symmetric multi-Proca interactions suggested so far, which implies that they propagate ghosts. Our theory admits a plethora of applications in a wide range of subjects. For illustrative purposes, we provide concrete proposals in holographic condensed matter and black hole physics.