We consider an alternative explanation for the deficit of ${\nu }_e$ in Ga solar neutrino calibration experiments and of the ${\overline{\nu }}_e$ in short-baseline reactor experiments by a model where neutrinos can oscillate into sterile Kaluza-Klein modes that can propagate in compactified submicrometer flat extra dimensions. We have analyzed the results of the gallium radioactive source experiments and 19 reactor experiments with baseline shorter than 100 m, and showed that these data can be fit into this scenario. The values of the lightest neutrino mass and of the size of the largest extra dimension that are compatible with these experiments are mostly not excluded by other neutrino oscillation experiments.

We study the effect of a magnetic field on the pairing dynamics in two-flavor color superconducting dense quark matter. The study is performed in the weakly coupled regime of QCD at asymptotically high density, using the framework of the Schwinger-Dyson equation in the improved rainbow approximation. We show that the superconducting gap function develops a directional dependence in momentum space. Quasiparticles with momenta perpendicular to the direction of the magnetic field have the largest gaps, while quasiparticles with momenta parallel to the field have the smallest gaps. We argue that the directional dependence is a consequence of a long-range interaction in QCD. The quantitative measure of the ellipticity of the gap function is determined by a dimensionless ratio, proportional to the square of the magnetic field and inversely proportional to the fourth power of the quark chemical potential. For magnetic fields in stars, $B\lesssim {10}^{18}\mathrm{G}$, the corresponding ratio is estimated to be less than about ${10}^{-2}$, justifying the use of the weak magnetic field limit in all stellar applications.

The mixing of photons with axionlike particles (ALPs) in the large-scale magnetic field $B$ induces a rotation in the polarization angle of a linearly polarized photon beam from active galactic nuclei in radio galaxies as it propagates over cosmological distances. Using available ultraviolet polarization data concerning these sources, we derive a new bound on the product of the photon-ALP coupling $g_{a\gamma }$ times $B$. We find $g_{a\gamma }B\lesssim {10}^{-11}{\mathrm{GeV}}^{-1}\mathrm{nG}$ for ultralight ALPs with $m_a\lesssim {10}^{-15}\mathrm{eV}$. We compare our new bound with the ones present in the literature and we comment about possible improvements with observations of more sources.

We perform a fully relativistic analysis of odd-type linear perturbations around a static and spherically symmetric solution in the most general scalar-tensor theory with second-order field equations in four-dimensional spacetime. It is shown that, as in the case of general relativity, the quadratic action for the perturbations reduces to the one having only a single dynamical variable, from which concise formulas for no-ghost and no-gradient instability conditions are derived. Our result is applicable to all the theories of gravity with an extra scalar degree of freedom. We demonstrate how the generic formulas can be applied to some particular examples such as the Brans-Dicke theory, $f(R)$ models, and Galileon gravity.

We discuss the propagation of gravitational waves in a recently discussed class of theories containing $N\ge 2$ metric tensors and a corresponding number of standard model copies. Using the formalism of gauge-invariant linear perturbation theory we show that all gravitational waves propagate at the speed of light. We then employ the Newman-Penrose formalism to show that two to six polarizations of gravitational waves may exist, depending on the parameters entering the field equations. This corresponds to E(2) representations $N_2$, $N_3$, ${\mathrm{III}}_5$ and ${\mathrm{II}}_6$. We finally apply our general discussion to a recently presented concrete multimetric gravity model and show that it is of class $N_2$, i.e., it allows only two tensor polarizations, as it is the case for general relativity. Our results provide the theoretical background for tests of multimetric gravity theories using the upcoming gravitational wave experiments.

We construct a deformed brane solution generated by a double-kink scalar field and a dilaton scalar field. In this brane scenario the extra dimension is finite, which is due to the introduction of the dilaton field with a special form. The finity of the extra dimension will result in the localization of the zero mode for the vector fields, while the localization of the Kalb-Ramond fields depends on the coupling to the dilaton. For the fermion fields, with different values of the dilaton-fermion coupling constant, there are three types of the effective potential for the fermion Kaluza-Klein modes. Moreover, we investigate the effect of the deformation of the brane on the localization, and find that the number of the resonances will increase with the distances of the two sub-branes.

We construct finite mass, asymptotically flat black hole solutions in $d=4$ Einstein—Yang-Mills theory augmented with higher order curvature terms of the gauge field. They possess non-Abelian hair in addition to Coulomb electric charge, and, below some nonzero critical temperature, they are thermodynamically preferred over the Reissner-Nordström solution. Our results indicate the existence of hairy non-Abelian black holes which are stable under linear, spherically symmetric perturbations.

The peeling behavior of the Weyl tensor near null infinity is determined for an asymptotically flat higher dimensional spacetime. The result is qualitatively different from the peeling property in 4 dimensions. To leading order, the Weyl tensor is type $N$. The first subleading term is type II. The next term is algebraically general in 6 or more dimensions, but in 5 dimensions another type $N$ term appears before the algebraically general term. The Bondi energy flux is written in terms of “Newman-Penrose” Weyl components.

Viable corrections to the matter sector of Poisson’s equation may result in qualitatively different astrophysical phenomenology, for example, the gravitational collapse and the properties of compact objects can change drastically. We discuss a class of modified nonrelativistic theories and focus on a relativistic completion, Eddington-inspired Born-Infeld gravity. This recently proposed theory is equivalent to General Relativity in vacuum, but its nontrivial coupling to matter prevents singularities in early cosmology and in the nonrelativistic collapse of noninteracting particles. We extend our previous analysis, discussing further developments. We present a full numerical study of spherically symmetric nonrelativistic gravitational collapse of dust. For any positive coupling, the final state of the collapse is a regular pressureless star rather than a singularity. We also argue that there is no Chandrasekhar limit for the mass of a nonrelativistic white dwarf in this theory. Finally, we extend our previous results in the fully relativistic theory by constructing static and slowly rotating compact stars governed by nuclear-physics inspired equations of state. In the relativistic theory, there exists an upper bound on the mass of compact objects, suggesting that black holes can still be formed in the relativistic collapse.

We generalize the domain structure for stationary black hole space-times to include asymptotically de Sitter and anti-de Sitter space-times. Given a set of commuting Killing vector fields of a space-time the domain structure lives on the submanifold of the orbit space on which at least one of the Killing vector fields has zero norm. In general the domain structure provides topological and geometrical invariants of black hole space-times that in specific cases have proven to be a crucial part of a full characterization leading to uniqueness theorems. In four and five dimensions the domain structure generalizes the rod structure. We examine in detail the domain structure for four-, five-, six- and seven-dimensional black hole space-times including a very general class of spherically symmetric and static black hole space-times as well as the exact solutions for Kerr-(anti)-de Sitter black holes. While for asymptotically anti-de Sitter space-times the domain structures resemble that of asymptotically flat space-times, the domain structures of asymptotically de Sitter space-times are shown to be compact. Finally, we find under certain assumptions that the horizon topologies for four- and five-dimensional black holes are restricted by our domain structure analysis.

We give a detailed calculation for the Hamiltonian of single Galileon field theory, keeping track of all the surface terms. We calculate the energy of static, spherically symmetric configuration of the single Galileon field at cubic order coupled to a point source and show that the 2-branches of the solution possess energy of equal magnitude and opposite sign, the sign of which is determined by the coefficient of the kinetic term ${\alpha }_2$. Moreover, the energy is regularized in the short distance (ultraviolet) regime by the dominant cubic term even though the source is divergent at the origin. We argue that the origin of the negativity is due to the ghostlike modes in the corresponding branch in the presence of the point source. This seems to be a nonlinear manifestation of the ghost instability.

The possibility that the apparent anomalous acceleration of the Pioneer 10 and 11 spacecrafts may be due, at least in part, to a chameleon field effect is examined. A small spacecraft, with no thin shell, can have a more pronounced anomalous acceleration than a large compact body, such as a planet, having a thin shell. The chameleon effect seems to present a natural way to explain the differences seen in deviations from pure Newtonian gravity for a spacecraft and for a planet, and it appears to be compatible with the basic features of the Pioneer anomaly, including the appearance of a jerk term. However, estimates of the size of the chameleon effect indicate that its contribution to the anomalous acceleration is negligible. We conclude that any inverse square component in the anomalous acceleration is more likely caused by an unmodeled reaction force from solar radiation pressure rather than a chameleon field effect.

We present a novel approach to modified theories of gravity which consists of adding to the Einstein-Hilbert Lagrangian an $f(\mathcal{R})$ term constructed à la Palatini. Using the respective dynamically equivalent scalar-tensor representation, we show that the theory can pass the Solar System observational constraints even if the scalar field is very light. This implies the existence of a long-range scalar field, which is able to modify the cosmological and galactic dynamics but leaves the Solar System unaffected. We also verify the absence of instabilities in perturbations and provide explicit models which are consistent with local tests and lead to the late-time cosmic acceleration.

We explore the newly discovered “hangup-kick” effect, which greatly amplifies the recoil for configurations with partial spin-/orbital angular momentum alignment, by studying a set of 48 new simulations of equal-mass, spinning black-hole binaries. We propose a phenomenological model for the recoil that takes this new effect into account and then use this model, in conjunction with statistical distributions for the spin magnitude and orientations, based on accretion simulations, to find the probabilities for observing recoils of several thousand $\mathrm{km}{\mathrm{s}}^{-1}$. In addition, we provide initial parameters, eccentricities, radiated linear and angular momentum, precession rates and remnant mass, spin, and recoils for all 48 configurations. Our results indicate that surveys exploring peculiar (redshifted or blueshifted) differential line-of-sight velocities should observe at least one case above $2000\mathrm{km}{\mathrm{s}}^{-1}$ out of 4000 merged galaxies. On the other hand, the probability that a remnant black hole recoils in any direction at a velocity exceeding the $\sim 2000\mathrm{km}{\mathrm{s}}^{-1}$ escape velocity of large elliptical galaxies is 0.03%. Probabilities of recoils exceeding the escape velocity quickly rise to 5% for galaxies with escape velocities of $1000\mathrm{km}{\mathrm{s}}^{-1}$ and nearly 20% for galaxies with escape velocities of $500\mathrm{km}{\mathrm{s}}^{-1}$. In addition the direction of these large recoils is strongly peaked toward the angular momentum axis, with very low probabilities of recoils exceeding $350\mathrm{km}{\mathrm{s}}^{-1}$ for angles larger than 45° with respect to the orbital angular momentum axis.

A recent experimental claim of the detection of analogue Hawking radiation in an optical system [Phys. Rev. Lett. 105, 203901 (2010)] has led to some controversy [Phys. Rev. Lett. 107, 149401 (2011)]. While this experiment strongly suggests some form of particle creation from the quantum vacuum (and hence it is per se very interesting), it is also true that it seems difficult to completely explain all features of the observations by adopting the perspective of a Hawking-like mechanism for the radiation. For instance, the observed photons are emitted parallel to the optical horizon, and the relevant optical horizon is itself defined in an unusual manner by combining group and phase velocities. This raises the question: Is this really Hawking radiation, or some other form of quantum vacuum radiation? Naive estimates of the amount of quantum vacuum radiation generated due to the rapidly changing refractive index—sometimes called the dynamical Casimir effect—are not encouraging. However we feel that naive estimates could be misleading depending on the quantitative magnitude of two specific physical effects: “pulse steepening” and “pulse cresting”. Plausible bounds on the maximum size of these two effects results in estimates much closer to the experimental observations, and we argue that the dynamical Casimir effect is now worth additional investigation.

We consider the evolution of an initially FLRW universe after the formation of a long, straight, cosmic string with arbitrary tension and mass per unit length. The birth of the string sources scalar and tensor-type perturbations in the background metric and both density and velocity perturbations in the background fluid, which compensate for the string mass and maintain energy conservation. The former generate the deficit angle within the light cone of the string and a gravitational shock front at the cosmological horizon, whereas the latter are confined within the sound cone. We study the properties of the metric within each region of the resulting spacetime and give the explicit coordinate transformations which demonstrate non-violation of causality. This paper generalizes the work of previous studies for the Nambu-Goto string.

We employ the formalism of the effective field theory of inflation to study the effects of a sudden change in the speed of sound of the inflationary perturbations. Such an event generates a feature with high frequency oscillations both in the two- and in the three-point functions of the curvature fluctuations. We study, at first order in the magnitude of the change of the speed of sound, the dependence of the power spectrum and of the bispectrum on the duration of the change. In the limit of a very short duration, the oscillations in the power spectrum persist up to very large momenta and the amplitude of the feature in the bispectrum diverges while its location moves to increasing momenta.

We calculate the limits on the fraction of viable dark matter minihalos in the early universe to host Population III.1 stars, surviving today as dark matter spikes in our Milky Way halo. Motivated by potential hints of light dark matter from the DAMA and CoGeNT direct dark matter searches, we consider thermal relic WIMP dark matter with masses of 5, 10, and 20 GeV, and annihilation to ${\mu }^{+}{\mu }^{-}$, ${\tau }^{+}{\tau }^{-}$, and $q\overline{q}$. From this brief study we conclude that, if dark matter is light, either the typical black hole size is $\lesssim 100M_{\odot }$ (i.e. there is no significant Dark Star phase), and/or dark matter annihilates primarily to ${\mu }^{+}{\mu }^{-}$ or other final states that result in low gamma-ray luminosity, and/or that an extremely small fraction of minihalos in the early universe that seem suitable to host the formation of the first stars actually did.