We give a thorough description of the shape of rotating axisymmetric stable black-hole (apparent) horizons applicable in dynamical or stationary regimes. It is found that rotation manifests in the widening of their central regions (rotational thickening), limits their global shapes to the extent that stable holes of a given area $A$ and angular momentum $J\ne 0$ form a precompact family (rotational stabilization) and enforces their whole geometry to be close to the extreme-Kerr horizon geometry at almost maximal rotational speed (enforced shaping). The results, which are based on the stability inequality, depend only on $A$ and $J$. In particular they are entirely independent of the surrounding geometry of the space-time and of the presence of matter satisfying the strong energy condition. A complete set of relations between $A$, $J$, the length $L$ of the meridians and the length $R$ of the greatest axisymmetric circle, is given. We also provide concrete estimations for the distance between the geometry of horizons and that of the extreme Kerr, in terms only of $A$ and $J$. Besides its own interest, the work has applications to the Hoop conjecture as formulated by Gibbons in terms of the Birkhoff invariant, to the Bekenstein-Hod entropy bounds and to the study of the compactness of classes of stationary black-hole space-times.

Recently, alternatives to the Bañados, Silk, and West effect were proposed which are (i) due to the existence of naked singularities instead of the horizon and (ii) require neither horizon nor naked singularity. We reveal the main features of such alternatives in a model-independent way. The metric should be close to that of the extremal black hole, but the horizon should not form. Then, one can gain the unbound energy $E_{\mathrm{c}.\mathrm{m}.}$ in the center-of-mass frame due to head-on collisions of particles near the would-be horizon. The energy measured at infinity can also be unbound. If self-gravitating shells collide, instead of particles, the underlying reason leading to unbound $E_{\mathrm{c}.\mathrm{m}.}$ is the same.

We propose a geometric interpretation for the Stokes phenomenon in de Sitter spacetime that particles are produced in even dimensions but not in odd dimensions. The scattering amplitude square for a quantum field between the in-vacuum and the transported one along a closed path in the complex-time plane gives the particle-production rate that explains not only the Boltzmann factor from the simple pole at infinity, corresponding to the cosmological horizon, but also the sinusoidal behavior from simple poles at the north and south poles of the Euclidean geometry. The Stokes phenomenon is a consequence of interference among four independent closed paths in the complex plane.

The white dwarf luminosity function, which provides information about the cooling of such stars, has been measured with high precision in the past few years. Simulations that include well known Standard Model physics give a good fit to the data. This leaves little room for new physics and makes these astrophysical objects a good laboratory for testing models beyond the Standard Model. It has already been suggested that white dwarfs might provide some evidence for the existence of axions. In this work we study the constraints that the white dwarf luminosity function puts on physics beyond the Standard Model involving new light particles (fermions or bosons) that can be pair-produced in a white dwarf and then escape to contribute to its cooling. We show, in particular, that we can severely constrain the parameter space of models with dark forces and light hidden sectors (lighter than a few tens of keV). The bounds we find are often more competitive than those from current lab searches and those expected from most future searches.

We combine recent measurements of cosmic microwave background anisotropies, supernovae luminosity distances, and baryonic acoustic oscillations to derive constraints on the dark energy equation of state $w$ in the redshift range $0<z<2$ using a principal components approach. We find no significant deviations from the expectations of a cosmological constant. However, combining the data sets, we find a slight indication for $w<-1$ at low redshift, thus highlighting how these data sets prefer a nonconstant $w$. Nevertheless, the cosmological constant is still in agreement with these observations, while we find that some classes of alternative models may be in tension with the inferred $w(z)$ behavior.

Core-collapse supernovae emit on the order of $3\times {10}^{53}\mathrm{ergs}$ in high-energy neutrinos over a time of order 10 seconds, and so decrease their mass by about $0.2M_{\odot }$. If the explosion is nearly spherically symmetric, there will be little gravitational-wave emission. Nevertheless, the sudden decrease of mass of the progenitor may cause a change in the gravitational time delay of signals from a nearby pulsar. We calculate the change in arrival times as successive pulses pass through the neutrino shell at different times, and find that the effect may be detectable in ideal circumstances.

We examine the effect of the cosmological constant $\Lambda$ on the angle of deflection of null geodesics in the equatorial plane of the Kerr–de Sitter spacetime. This is done by employing a procedure used recently by Rindler and Ishak to obtain the bending angle of light in the Schwarzschild–de Sitter geometry. We show that this approach yields a contribution from the cosmological constant in the expression for the bending angle.

The structure of the electromagnetic vertex function of spin-$3/2$ particles is analyzed in a general way, for the diagonal and off-diagonal couplings, of charged as well as neutral particles including the case of self-conjugate (Majorana) particles. The restrictions imposed by common principles such as electromagnetic gauge invariance and Hermiticity are studied, and the implications due to the discrete space-time symmetries or the Majorana condition are deduced when they are applicable. In some cases certain features of the vertex function are analogous to the known ones for the spin-$1/2$ or spin-1 particles, but we find and discuss other particular properties which are related to the spin-$3/2$ Rarita–Schwinger representation. For example, in the diagonal Majorana case, the vertex function can contain a term of the form ${\gamma }_{\mu }{\gamma }_5$, which resembles the axial charge radius term for Majorana neutrinos, plus another one that resembles the vertex function for self-conjugate spin-1 particles, but with the particularity that the two terms may not appear independently of each other, but instead with a specific relative coefficient. In essence this is due to the requirement that the vertex function does not mix the genuine spin-$3/2$ degrees of freedom with the spurious spin-$1/2$ components of the Rarita–Schwinger representation. The analogous results for the other cases (off-diagonal and charged couplings) are discussed as well.

The mass and current coupling constant of the $D_{s2}^{*}(2573)$ charmed-strange meson is calculated in the framework of the two-point QCD sum rule approach. Although the quantum numbers of this meson are not exactly known, its width and decay modes are consistent with $I(J^P)=0(2^{+})$, which we consider to write the interpolating current used in our calculations. Replacing the light strange quark with the up or down quark, we also compare the results with those of the $D_2^{*}$ charmed tensor meson and estimate the order of SU(3) flavor symmetry violation.

Some extensions of the standard model predict the existence of particles having two units of leptonic charge, known as bileptons. One such model is based on the $SU(3{)}_c\times SU(3{)}_L\times U(1{)}_X$ symmetry group (3-3-1 model, for short). Our search uses the minimal version of the 3-3-1 model, which has exotic charges for the quarks and new gauge bosons. This model predicts the existence of bileptons as vector particles having one ($V^{\pm }$) and two ($Y^{\pm \pm }$) units of electric charge. We study the signatures for the production of four leptons by considering the contribution of a pair of bileptons in $pp$ collisions for three energy and luminosity regimes at the Large Hadron Collider (LHC). We present invariant mass and transverse momentum distributions, the total cross section, and we determine the expected number of events for each bilepton type. Finally, we analyze the LHC potential for discovering single and double charged vector bileptons at 95% C.L. We conclude that the LHC collider can show a clear signature for the existence of bileptons as a signal of new physics.

In this paper we study the phenomenological implications of the one-zero textures of low energy neutrino mass matrices in the presence of a sterile neutrino. We consider the $3+1$ scheme and use the results from a global fit for short baseline neutrino oscillation data, which provides the bounds on the three additional mixing angles. We find that the mass matrix elements $m_{\alpha \beta }$ ($\alpha$, $\beta =e$, $\mu$, $\tau$) involving only the active states can assume vanishing values in the allowed parameter space for all of the mass spectrum. Among the mass matrix elements connecting the active and sterile states, $m_{es}$ and $m_{\mu s}$ can become small only for the quasidegenerate neutrinos. The element $m_{\tau s}$, on the other hand, can vanish even for lower values of masses since the 3–4 mixing angle only has an upper bound from current data. The mass matrix element ($m_{ss}$) involving only the sterile state stays $\sim \mathcal{O}(1)\mathrm{eV}$ in the whole parameter region. We study the possible correlations between the sterile mixing angles and the Majorana phases to give a zero element in the mass matrix.

We perform a systematic study of neutrino mass matrices having a vanishing cofactor and an equality between two cofactors of the mass matrix. Such texture structures of the effective neutrino mass matrix arise from the type-I seesaw mechanism when the Dirac neutrino mass matrix is diagonal with two equal elements and the right-handed Majorana neutrino mass matrix has hybrid textures with one equality of matrix elements and one zero matrix element. For three right-handed neutrinos there are sixty possible hybrid textures out of which only six are excluded by the present experimental data. We show that such textures can be derived using discrete symmetries. The predictions of experimentally allowed textures are examined for unknown parameters such as the effective Majorana mass of the electron neutrino and the Dirac-type $CP$-violating phase.

We carry out a detailed study of the branching fractions and lepton-pair invariant-mass spectrum of ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }{\ell }^{+}{\ell }^{-}$ decays ($\ell =e$, $\mu$). In addition to the model-independent (QED) contributions, we include the structure-dependent (SD) terms, which encode information on the hadronization of QCD currents. The form factors describing the SD contributions are evaluated by supplementing Chiral Perturbation Theory with the inclusion of the lightest multiplet of spin-1 resonances as active degrees of freedom. The Lagrangian couplings have been determined by demanding the known QCD short-distance behavior to the relevant Green functions and associated form factors in the limit where the number of colors goes to infinity. As a result, we predict $\mathrm{BR}({\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }{e}^{+}{e}^{-})=({1.7}_{-0.3}^{+1.1})\times {10}^{-5}$ and $\mathrm{BR}({\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }{\mu }^{+}{\mu }^{-})\in [0.03,1.0]\times {10}^{-5}$. According to this, the first decay could be measured in the near future, which is not granted for the second one.

We studied the ionization of hydrogen by the scattering of the neutrino magnetic moment, relativistic muon, and a weakly interacting massive particle with a QED-like interaction. Analytic results were obtained and compared with several approximation schemes often used in atomic physics. As current searches for the neutrino magnetic moment and dark matter have lowered the detector threshold down to the sub-keV regime, we tried to deduce from this simple case study the influence of the atomic structure on the cross sections and the applicabilities of various approximations. The general features being found will be useful for cases where practical detector atoms are considered.

If black holes are not to be dreaded sinks of information but rather fully described by unitary evolution, they must scramble in-falling data and eventually leak it through Hawking radiation. Sekino and Susskind have conjectured that black holes are fast scramblers; they generate entanglement at a remarkably efficient rate, with the characteristic time scaling logarithmically with the entropy. In this work, we focus on Matrix theory—M-theory in the light-cone frame—and directly probe the conjecture. We develop a concrete test bed for quantum gravity using the fermionic variables of Matrix theory and show that the problem becomes that of chains of qubits with an intricate network of interactions. We demonstrate that the black hole system evolves much like a Brownian quantum circuit, with strong indications that it is indeed a fast scrambler. We also analyze the Berenstein-Maldacena-Nastase model and reach the same tentative conclusion.

Massive (hypermassive and supramassive) neutron stars are likely to be often formed after the merger of binary neutron stars. We explore the evolution process of the remnant massive neutron stars and gravitational waves emitted by them, based on numerical-relativity simulations for binary neutron star mergers employing a variety of equations of state and choosing a plausible range of the neutron star mass of binaries. We show that the lifetime of remnant hypermassive neutron stars depends strongly on the total binary mass and also on the equations of state. Gravitational waves emitted by the remnant massive neutron stars universally have a quasiperiodic nature of an approximately constant frequency although the frequency varies with time. We also show that the frequency and time-variation feature of gravitational waves depend strongly on the equations of state. We derive a fitting formula for the quasiperiodic gravitational waveforms, which may be used for the data analysis of a gravitational-wave signal.

We study the geodesic motion of test particles in the space-time of noncompact boson stars. These objects are made of a self-interacting scalar field and—depending on the scalar field’s mass—can be as dense as neutron stars or even black holes. In contrast to the former these objects do not contain a well-defined surface, while in contrast to the latter the space-time of boson stars is globally regular and can—however—only be given numerically. Hence, the geodesic equation also has to be studied numerically. We discuss the possible orbits for massive and massless test particles and classify them according to the particle’s energy and angular momenta. The space-time of a boson star approaches the Schwarzschild space-time asymptotically; however, it deviates strongly from it close to the center of the star. As a consequence, we find additional bound orbits of massive test particles close to the center of the star that are not present in the Schwarzschild case. Our results can be used to make predictions about extreme-mass-ratio inspirals, and we hence compare our results to recent observational data of the stars orbiting Sagittarius $A^{*}$—the radio source at the center of our own Galaxy.

We reconsider the cosmological model discussed by Sung-Won Kim [Phys. Rev. D 53, 6889 (1996)] in the context of Friedmann-Robertson-Walker cosmologies with a traversable wormhole, where it is assumed that the matter content is divided into two parts: the cosmic part that depends on time only and the wormhole part that depends on space only. The cosmic part obeys the barotropic equation of state $p_c=\gamma {\rho }_c$. The complete analysis requires further care and reveals more interesting results than what was previously shown by the author. They can be readily applied to the evolution of a large class of cosmological models which are more general than Friedmann-Robertson-Walker models.

I show that the high energy neutrino flux predicted to arise from active galactic nuclei cores can explain the PeV neutrinos detected by IceCube without conflicting with the constraints from the observed extragalactic cosmic-ray and $\gamma$-ray backgrounds.

We study dynamics of probe D-branes in ($d+2$)-dimensional background with general semilocality. The background is characterized by a parameter $\eta$ and is conformal to ${\mathrm{AdS}}_2\times {\mathbb{R}}^d$. We discuss thermodynamics of the probe D-branes and find that the entropy density is vanishing in the extremal limit, which indicates that the background may correspond to the true ground state of the system. We also clarify the conditions under which the specific heat matches to the behavior of a Fermi liquid or a Bose liquid. We calculate the current-current and density-density retarded Green’s functions, from which we can obtain the ac conductivity and the zero sound mode. The ac conductivity scales as ${\omega }^{-1}$ when $d/\eta <2$ and ${\omega }^{-2\eta /d}$ when $d/\eta >2$, while it contains a logarithmic term in $\omega$ when $d/\eta =2$. We also observe that there is no quasiparticle excitation when $d/\eta \ge 2$ and classify the conditions under which the quasiparticle description is valid when $d/\eta <2$.