Gravitational waves emitted from a generic binary black hole merger carry away linear momentum anisotropically, resulting in a gravitational recoil, or “kick,” of the center of mass. For certain merger configurations the time evolution of the magnitude of the kick velocity has a local maximum followed by a sudden drop. Perturbative studies of this “antikick” in a limited range of black hole spins have found that the antikick decreases for retrograde orbits as a function of negative spin. We analyze this problem using a recently developed code to evolve gravitational perturbations from a point particle in Kerr spacetime driven by an effective-one-body resummed radiation reaction force at linear order in the mass ratio $\nu \ll 1$. Extending previous studies to nearly extremal negative spins, thus complementing current numerical relativity knowledge about the recoil, we find that the well-known decrease of the antikick is overturned and, instead of approaching zero, the antikick increases again to reach $\mathrm{\Delta }v/(c{\nu }^2)=3.37\times {10}^{-3}$ for dimensionless spin $\widehat{a}=-0.9999$. The corresponding final kick velocity is $v_{\text{end}}/(c{\nu }^2)=0.076$. We interpret the antikick result analytically by means of the quality factor $Q$ of the linear momentum flux, that is used to quantify the amount of nonadiabaticity of the emission process. We show that, besides capturing qualitatively the global properties over the whole spin range, $Q$ actually predicts the return of the antikick for $\widehat{a}\to -1$. Since $Q$ is computed only from the, gauge-invariant, flux of linear momentum, the herein presented verification of its reliability advocates its systematic use also in numerical relativity calculations. In addition, we also connect, in a new way, the properties of the flux to the noncircular character of the plunge dynamics, highlighting the central role of subdominant waveform multipoles in shaping the characteristic interference pattern exhibited by the linear momentum flux as $\widehat{a}\to -1$.

Using the reconstruction method, we investigate which $F(R)$ theories, with or without the presence of matter fluids, can produce the matter bounce scenario of holonomy corrected loop quantum cosmology. We focus our study in two limits of the cosmic time, the large cosmic time limit and the small cosmic time limit. For the former, we find that, in the presence of noninteracting and nonrelativistic matter, the $F(R)$ gravity that reproduces the late time limit of the matter bounce solution is actually the Einstein-Hilbert gravity plus a power law term. In the early time limit, since it corresponds to large spacetime curvatures, assuming that the Jordan frame is described by a general metric that, when it is conformally transformed to the Einstein frame, produces an accelerating Friedmann-Robertson-Walker metric, we find explicitly the scalar field dependence on time. After demonstrating that the solution in the Einstein frame is indeed accelerating, we calculate the spectral index derived from the Einstein frame scalar-tensor counterpart theory of the $F(R)$ theory and compare it with the Planck experiment data. In order to implement the resulting picture, we embed the $F(R)$ gravity explicitly in a loop quantum cosmology framework by introducing holonomy corrections to the $F(R)$ gravity. In this way, the resulting inflation picture corresponding to the $F(R)$ gravity can be corrected in order that it coincides to some extent with the current experimental data.

I derive the Komar mass/function for the Schwarszchild–de Sitter black hole inside the de Rham–Gabadadze–Tolley nonlinear theory of massive gravity by taking the usual notion of timelike Killing vector in unitary gauge. The de Rham–Gabadadze–Tolley Komar function depends on the dynamics of the Stückelberg fields through the gauge transformation function. It goes to the standard value obtained in general relativity if the spatial derivative of the gauge function vanishes. In such a case, the (gauge) function corresponds to the usual notion of time as in general relativity.

We show that in the first-order gravity theory coupled to axions the instanton number of the Giddings-Strominger wormhole can be interpreted as the Nieh-Yan topological index. The axion charge of the baby universes is quantized in terms of the Nieh-Yan integers. Tunneling between universes of different Nieh-Yan charges implies a nonperturbative vacuum state. The associated topological vacuum angle can be identified with the Barbero-Immirzi parameter.

Quantum polyhedra constructed from angular momentum operators are the building blocks of space in its quantum description as advocated by loop quantum gravity. Here we extend previous results on the semiclassical properties of quantum polyhedra. Regarding tetrahedra, we compare the results from a canonical quantization of the classical system with a recent wave-function-based approach to the large-volume sector of the quantum system. Both methods agree in the leading order of the resulting effective operator (given by an harmonic oscillator), while minor differences occur in higher corrections. Perturbative inclusion of such corrections improves the approximation to the eigenstates. Moreover, the comparison of both methods leads also to a full wave function description of the eigenstates of the (square of the) volume operator at negative eigenvalues of large modulus. For the case of general quantum polyhedra described by discrete angular momentum quantum numbers we formulate a set of quantum operators fulfilling in the semiclassical regime the standard commutation relations between momentum and position. Differently from previous formulations, the position variable here is chosen to have dimension of (Planck) length squared which facilitates the identification of quantum corrections. Finally, we provide expressions for the pentahedral volume in terms of Kapovich-Millson variables.

The angular momentum of the Kerr singularity should not be larger than a threshold value so that it is enclosed by an event horizon: The Kerr singularity with the angular momentum exceeding the threshold value is naked. This fact suggests that if the cosmic censorship exists in our Universe, an over-spinning body without releasing its angular momentum cannot collapse to spacetime singularities. A simple kinematical estimate of two particles approaching each other supports this expectation and suggests the existence of a minimum size of an over-spinning body. But this does not imply that the geometry near the naked singularity cannot appear. By analyzing initial data, i.e., a snapshot of a spinning body, we see that an over-spinning body may produce a geometry close to the Kerr naked singularity around itself at least as a transient configuration.

We consider a broad class of asymptotically flat, maximal initial data sets satisfying the vacuum constraint equations, admitting two commuting rotational symmetries. We construct a mass functional for “$t-{\varphi }^i$”-symmetric data which evaluates to the Arnowitt-Deser-Misner mass. We then show that $\mathbb{R}\times \mathrm{U}(1{)}^2$-invariant solutions of the vacuum Einstein equations are critical points of this functional amongst this class of data. We demonstrate the positivity of this functional for a class of rod structures which include the Myers-Perry initial data. The construction is a natural extension of Dain’s mass functional to $D=5$, although several new features arise.

We study the linear stability of a nongravitating, steady-state, spherically symmetric ghost condensate accreting onto a Schwarzschild black hole using two methods. The first one is based on the conservation of the energy-momentum tensor of the perturbations (of which the propagation is determined by the effective metric) and involves determination of the sign of the time derivative of the energy of the perturbations. The second method employs the positivity of the effective potential. Both methods yield the result that the system is stable, but the second one is less practical, since it involves lengthier calculations and requires the explicit form of the background solution for the scalar field.

Different spacetime regions separated by horizons are not related to each other. We know that this statement holds for classical spacetimes. In this paper we carry out a canonical quantization of a Kantowski-Sachs minisuperspace model whose classical solutions exhibit both an event horizon and a cosmological horizon in order to check whether the above statement also holds from the quantum gravitational point of view. Our analysis shows that in fact this is not the case: Quantum gravitational states with support in spacetime configurations that exclusively describe either the region between horizons or outside them are not consistent in the sense that there exist unitary operators describing a natural notion of evolution that connect them. In other words, unitarity is only preserved in this quantization when dealing with the whole spacetime and not in each region separately.

We provide strong evidence for universality of the inflationary field range: given an accurate measurement of $(n_s,r)$, one can infer $\mathrm{\Delta }\varphi$ in a model-independent way in the sub-Planckian regime for a range of universality classes of inflationary models. Both the tensor-to-scalar ratio as well as the spectral tilt are essential for the field range. Given the Planck constraints on $n_s$, the Lyth bound is strengthened by two orders of magnitude: whereas the original bound gives a sub-Planckian field range for $r\lesssim 2\times {10}^{-3}$, we find that $n_s=0.96$ brings this down to $r\lesssim 2\times {10}^{-5}$.

We study the loop quantum cosmology of a flat Friedmann-Lemaître-Robertson-Walker space-time with a Maxwell field. We show that many of the qualitative properties derived for the case of a massless scalar field also hold for a Maxwell field. In particular, the big-bang singularity is replaced by a quantum bounce, and the operator corresponding to the matter energy density is bounded above by the same critical energy density. We also numerically study the evolution of wave functions that are sharply peaked in the low energy regime, and derive effective equations which very closely approximate the full quantum dynamics of sharply peaked states at all times, including the near-bounce epoch. In the process, the analytical and numerical methods originally used to study the dynamics in loop quantum cosmology for the case of a massless scalar field are substantially improved to handle the difficulties (that generically arise for matter content other than a massless scalar field) related to the presence of a Maxwell field.

Tentative evidence of a 3.5 keV x-ray line has been found in the stacked spectra of galaxy clusters, individual clusters, the Andromeda galaxy and the Galactic center, leading to speculation that it could be due to decays of metastable dark matter such as sterile neutrinos. However, searches for the line in other systems such as dwarf satellites of the Milky Way have given negative or ambiguous results. We reanalyze both the positive and negative searches from the point of view that the line is due to inelastic scattering of dark matter to an excited state that subsequently decays—the mechanism of excited dark matter (XDM). Unlike the metastable dark matter scenario, XDM gives a stronger signal in systems with higher velocity dispersions, such as galaxy clusters. We show that the predictions of XDM can be consistent with null searches from dwarf satellites, while the signal from the closest individual galaxies can be detectable having a flux consistent with that from clusters. We discuss the impact of our new fits to the data for two specific realizations of XDM.

We report on the possibility of using dark matter particle’s mass and its interaction cross section as a smoking gun signal of the existence of a big bounce at the early stage in the evolution of our currently observed universe. A model independent study of dark matter production in the pre-bounce contraction and the post-bounce expansion epochs of the bounce universe reveals a new venue for achieving the observed relic abundance of our present universe, in which a significantly smaller amount of dark matter with a smaller cross section—as compared to the prediction of standard cosmology—is produced and the information about the bounce universe evolution is preserved by the out-of-thermal-equilibrium process. Once the value of dark matter mass and interaction cross section are obtained by direct detection in laboratories, this alternative route becomes a signature prediction of the bounce universe scenario.

Motivated by the idea that quantum gravity corrections usually suppress the power of the scalar primordial spectrum (E-mode) more than the power of the tensor primordial spectrum (B-mode), in this paper we construct a concrete gravitational theory in five-dimensions for which $V(\varphi )\propto {\varphi }^n$-type inflation ($n\ge 1$) generates an appropriate tensor-to-scalar ratio that may be compatible with the BICEP2 and Planck data together. The true nature of gravity is five-dimensional and described by the action $S=\int d^{5}x\sqrt{|g|}{M}^3(-6\lambda M^2+R+\alpha M^{-2}{\mathcal{R}}^2)$ where $M$ is the five-dimensional Planck mass and ${\mathcal{R}}^2=R^2-4R_{ab}{R}^{ab}+R_{abcd}{R}^{abcd}$ is the Gauss-Bonnet (GB) term. The five-dimensional “bulk” spacetime is anti-de Sitter ($\lambda <0$) for which inflation ends naturally. The effects of ${\mathcal{R}}^2$ term on the magnitudes of scalar and tensor fluctuations and spectral indices are shown to be important at the energy scale of inflation. For GB-assisted $m^2{\varphi }^2$-inflation, inflationary constraints from BICEP2 and Planck, such as, $n_s\simeq 0.9603(\pm 0.0073)$, $r=0.16(+0.06-0.05)$ and $V_{*}^{1/4}\gtrsim 1.5\times 10^{16}\mathrm{GeV}$ are all satisfied for $(-\lambda \alpha )\simeq (3-300)\times 10^{-5}$.

In the standard model neutrinos are assumed to have streamed across the Universe since they last scattered when the standard-model plasma temperature was $\sim \mathrm{MeV}$. The shear stress of free-streaming neutrinos imprints itself gravitationally on the cosmic microwave background (CMB) and makes the CMB a sensitive probe of neutrino scattering. Yet, the presence of nonstandard physics in the neutrino sector may alter this standard chronology and delay neutrino free streaming until a much later epoch. We use observations of the CMB to constrain the strength of neutrino self interactions $G_{\text{eff}}$ and put limits on new physics in the neutrino sector from the early Universe. Within the context of conventional $\mathrm{\Lambda }\mathrm{CDM}$ parameters cosmological data are compatible with $G_{\text{eff}}\lesssim 1/(56\mathrm{MeV}{)}^2$ and neutrino free streaming might be delayed until their temperature has cooled to as low as $\sim 25\mathrm{eV}$. Intriguingly, we also find an alternative cosmology compatible with cosmological data in which neutrinos scatter off each other until $z\sim 10^4$ with a preferred interaction strength in a narrow region around $G_{\text{eff}}\simeq 1/(10\mathrm{MeV}{)}^2\simeq 8.6\times 10^8{G}_{\mathrm{F}}$, where $G_{\mathrm{F}}$ is the Fermi constant. This distinct self-interacting neutrino cosmology is characterized by somewhat lower values of both the scalar spectral index and the amplitude of primordial fluctuations. While we phrase our discussion here in terms of a specific scenario, our constraints on the neutrino visibility function are very general.

Using publicly available Monte Carlo codes as well as compilation of published data on $pp$ interactions for proton kinetic energy below 2 GeV, we parametrize the energy spectra and production rates of $\gamma$-rays by simple but quite accurate ($\le 20\%$) analytical expressions in a broad range from the kinematic threshold to PeV energies.

We have proposed a new ghost-free model with interactions of massive spin-two particles in Phys. Rev. D 90, 043006 (2014). Although the model is ghost-free on the Minkowski spacetime, it is not obvious whether or not this desirable property is preserved on curved spacetime. In fact, Buchbinder et al. already pointed out that the Fierz-Pauli theory is not ghost-free on curved spacetime without nonminimal coupling terms. In this paper, we construct a new theory of massive spin-two particles with nonminimal coupling on curved spacetime and show that the model can be ghost-free. Furthermore, we propose new nonminimal coupling terms.

In the pure-gravity sector of the minimal standard-model extension, nine Lorentz-violating coefficients of a vacuum-condensed tensor field describe dominant observable deviations from general relativity, out of which eight were already severely constrained by precision experiments with lunar laser ranging, atom interferometry, and pulsars. However, the time-time component of the tensor field, ${\overline{s}}^{\mathrm{TT}}$, dose not enter into these experiments, and was only very recently constrained by Gravity Probe B. Here we propose a novel idea of using the Lorentz boost between different frames to mix different components of the tensor field, and thereby obtain a stringent limit of ${\overline{s}}^{\mathrm{TT}}$ from binary pulsars. We perform various tests with the state-of-the-art white dwarf optical spectroscopy and pulsar radio timing observations, in order to get new robust limits of ${\overline{s}}^{\mathrm{TT}}$. With the isotropic cosmic microwave background as a preferred frame, we get $|{\overline{s}}^{\mathrm{TT}}|<1.6\times {10}^{-5}$ (95% C.L.), and without assuming the existence of a preferred frame, we get $|{\overline{s}}^{\mathrm{TT}}|<2.8\times {10}^{-4}$ (95% C.L.). These two limits are respectively about 500 times and 30 times better than the current best limit.

We present a study of three-mode parametric instability in large-scale gravitational-wave detectors. Previous work used a linearized model to study the onset of instability. This paper presents a nonlinear study of this phenomenon, which shows that the initial stage of an exponential rise of the amplitudes of a higher-order optical mode and the mechanical internal mode of the mirror is followed by a saturation phase, in which all three participating modes reach a new equilibrium state with constant oscillation amplitudes. Results suggest that stable operation of interferometers may be possible in the presence of such instabilities, thereby simplifying the task of suppression.

We investigate the interference pattern in the spectrum of nondipole bremsstrahlung on two amorphous foils. It contains a suppression at lowest photon energies, an enhancement in the neighboring region, and subsequent evanescent oscillations. The net effect of suppressions and enhancements proves to be zero. The location and the origin of the spectral features is explained. Predictions of the rigorous Molière averaging are compared with those of the Gaussian averaging with Coulomb contributions to the rms multiple scattering angle. Comparison with experimental data and with previous theoretical predictions is included.