We show that the simplicity constraints, which define the dynamics of spin foam models, imply, and are implied by, the first law of thermodynamics, when the latter is applied to causal diamonds in the quantum spacetime. This result reveals an intimate connection between the holographic nature of gravity, as reflected by the Bekenstein entropy, and the fact that general relativity and other gravitational theories can be understood as constrained topological field theories. To state and derive this correspondence we describe causal diamonds in the causal structure of spin foam histories and generalize arguments given for the near horizon region of black holes by Frodden, Gosh and Perez [Phys. Rev. D 87, 121503 (2013); Phys. Rev. D 89, 084069 (2014); Phys. Rev. Lett. 107, 241301 (2011); Phys. Rev. Lett.108, 169901(E) (2012).] and Bianchi [arXiv:1204.5122.]. This allows us to apply a recent argument of Jacobson [Phys. Rev. Lett. 116, 201101 (2016).] to show that if a spin foam history has a semiclassical limit described in terms of a smooth metric geometry, that geometry satisfies the Einstein equations. These results suggest also a proposal for a quantum equivalence principle.

We present and assess a Bayesian method to interpret gravitational wave signals from binary black holes. Our method directly compares gravitational wave data to numerical relativity (NR) simulations. In this study, we present a detailed investigation of the systematic and statistical parameter estimation errors of this method. This procedure bypasses approximations used in semianalytical models for compact binary coalescence. In this work, we use the full posterior parameter distribution for only generic nonprecessing binaries, drawing inferences away from the set of NR simulations used, via interpolation of a single scalar quantity (the marginalized log likelihood, $\ln \mathcal{L}$) evaluated by comparing data to nonprecessing binary black hole simulations. We also compare the data to generic simulations, and discuss the effectiveness of this procedure for generic sources. We specifically assess the impact of higher order modes, repeating our interpretation with both $l\le 2$ as well as $l\le 3$ harmonic modes. Using the $l\le 3$ higher modes, we gain more information from the signal and can better constrain the parameters of the gravitational wave signal. We assess and quantify several sources of systematic error that our procedure could introduce, including simulation resolution and duration; most are negligible. We show through examples that our method can recover the parameters for equal mass, zero spin, GW150914-like, and unequal mass, precessing spin sources. Our study of this new parameter estimation method demonstrates that we can quantify and understand the systematic and statistical error. This method allows us to use higher order modes from numerical relativity simulations to better constrain the black hole binary parameters.

Spherically symmetric bosonic stars are one of the few examples of gravitating solitons that are known to form dynamically, via a classical process of (incomplete) gravitational collapse. As stationary solutions of the Einstein–Klein-Gordon or the Einstein–Proca theory, bosonic stars may also become sufficiently compact to develop light rings and hence mimic, in principle, gravitational-wave observational signatures of black holes (BHs). In this paper, we discuss how these horizonless ultracompact objects (UCOs) are actually distinct from BHs, both phenomenologically and dynamically. In the electromagnetic channel, the light ring associated phenomenology reveals remarkable lensing patterns, quite distinct from a standard BH shadow, with an infinite number of Einstein rings accumulating in the vicinity of the light ring, both inside and outside the latter. The strong lensing region, moreover, can be considerably smaller than the shadow of a BH with a comparable mass. Dynamically, we investigate the fate of such UCOs under perturbations, via fully nonlinear numerical simulations and observe that, in all cases, they decay into a Schwarzschild BH within a time scale of $\mathcal{O}(M)$, where $M$ is the mass of the bosonic star. Both these studies reinforce how difficult it is for horizonless UCOs to mimic BH phenomenology and dynamics, in all its aspects.

We propose a scenario that can naturally explain the observed dark matter-baryon ratio in the context of bimetric theory with a chameleon field. We introduce two additional gravitational degrees of freedom, the massive graviton and the chameleon field, corresponding to dark matter and dark energy, respectively. The chameleon field is assumed to be nonminimally coupled to dark matter, i.e., the massive graviton, through the graviton mass terms. We find that the dark matter-baryon ratio is dynamically adjusted to the observed value due to the energy transfer by the chameleon field. As a result, the model can explain the observed dark matter-baryon ratio independently from the initial abundance of them.

A novel approach for extracting gauge-invariant information about spin-orbit coupling in gravitationally interacting binary systems is introduced. This approach is based on the “scattering holonomy”, i.e. the integration (from the infinite past to the infinite future) of the differential spin evolution along the two worldlines of a binary system in hyperboliclike motion. We apply this approach to the computation, at the first post-Minkowskian approximation (i.e. first order in $G$ and all orders in $v/c$), of the values of the two gyrogravitomagnetic ratios describing spin-orbit coupling in the effective one-body formalism. These gyrogravitomagnetic ratios are found to tend to zero in the ultrarelativistic limit.

To derive a power spectrum for energy density inhomogeneities in a closed universe, we study a spatially-closed inflation-modified hot big bang model whose evolutionary history is divided into three epochs: an early slowly-rolling scalar field inflation epoch and the usual radiation and nonrelativistic matter epochs. (For our purposes it is not necessary to consider a final dark energy dominated epoch.) We derive general solutions of the relativistic linear perturbation equations in each epoch. The constants of integration in the inflation epoch solutions are determined from de Sitter invariant quantum-mechanical initial conditions in the Lorentzian section of the inflating closed de Sitter space derived from Hawking’s prescription that the quantum state of the universe only include field configurations that are regular on the Euclidean (de Sitter) sphere section. The constants of integration in the radiation and matter epoch solutions are determined from joining conditions derived by requiring that the linear perturbation equations remain nonsingular at the transitions between epochs. The matter epoch power spectrum of gauge-invariant energy density inhomogeneities is not a power law, and depends on spatial wave number in the way expected for a generalization to the closed model of the standard flat-space scale-invariant power spectrum. The power spectrum we derive appears to differ from a number of other closed inflation model power spectra derived assuming different (presumably non de Sitter invariant) initial conditions.

In the large $N_c$ limit both heavy and light baryons are described by the universal mean field, which allows us to relate the properties of heavy baryons to light ones. With the only input from the decays of light octet baryons (due to the universality of the chiral mean field), excellent description of strong decays of both charm and bottom sextets is obtained. The parameter-free prediction for the widths of exotic antidecapentaplet ($\overline{\mathbf{15}}$) baryons is also made. The exotic heavy baryons should be anomalously narrow despite the large phase space available. In particular, the widths of ${\mathrm{\Omega }}_c(3050)$ and ${\mathrm{\Omega }}_c(3119)$, interpreted as members of $\overline{\mathbf{15}}$-plet, are very small: 0.48 MeV and 1.12 MeV respectivly. This result is in very good agreement with the measurements of the LHCb Collaboration and provides natural and parameter-free explanation of the LHCb observation that ${\mathrm{\Omega }}_c(3050)$ and ${\mathrm{\Omega }}_c(3119)$ have anomalously small widths among five recently observed states.

We study the spectrum of the large $N$ quantum field theory of bosonic rank-3 tensors, the quartic interactions of which are such that the perturbative expansion is dominated by the melonic diagrams. We use the Schwinger-Dyson equations to determine the scaling dimensions of the bilinear operators of arbitrary spin. Using the fact that the theory is renormalizable in $d=4$, we compare some of these results with the $4-\epsilon$ expansion, finding perfect agreement. This helps elucidate why the dimension of operator ${\varphi }^{abc}{\varphi }^{abc}$ is complex for $d<4$: the large $N$ fixed point in $d=4-\epsilon$ has complex values of the couplings for some of the $O(N{)}^3$ invariant operators. We show that a similar phenomenon holds in the $O(N{)}^2$ symmetric theory of a matrix field ${\varphi }^{ab}$, where the double-trace operator has a complex coupling in $4-\epsilon$ dimensions. We also study the spectra of bosonic theories of rank-$q-1$ tensors with ${\varphi }^q$ interactions. In dimensions $d>1.93$, there is a critical value of $q$, above which we have not found any complex scaling dimensions. The critical value is a decreasing function of $d$, and it becomes 6 in $d\approx 2.97$. This raises a possibility that the large $N$ theory of rank-5 tensors with sextic potential has an IR fixed point which is free of perturbative instabilities for $2.97<d<3$. This theory may be studied using renormalized perturbation theory in $d=3-\epsilon$.

By using the Gauss-Bonnet theorem, the bending angle of light in a static, spherically symmetric and asymptotically flat spacetime has been recently discussed, especially by taking account of the finite distance from a lens object to a light source and a receiver [Ishihara, Suzuki, Ono, Asada, Phys. Rev. D 95, 044017 (2017)]. We discuss a possible extension of the method of calculating the bending angle of light to stationary, axisymmetric and asymptotically flat spacetimes. For this purpose, we consider the light rays on the equatorial plane in the axisymmetric spacetime. We introduce a spatial metric to define the bending angle of light in the finite-distance situation. We show that the proposed bending angle of light is coordinate-invariant by using the Gauss-Bonnet theorem. The nonvanishing geodesic curvature of the photon orbit with the spatial metric is caused in gravitomagnetism, even though the light ray in the four-dimensional spacetime follows the null geodesic. Finally, we consider Kerr spacetime as an example in order to examine how the bending angle of light is computed by the present method. The finite-distance correction to the gravitomagnetic deflection angle due to the Sun’s spin is around a pico-arcsecond level. The finite-distance corrections for Sgr ${\mathrm{A}}^{*}$ also are estimated to be very small. Therefore, the gravitomagnetic finite-distance corrections for these objects are unlikely to be observed with present technology.

The minimal theory of quasidilaton massive gravity allows for a stable self-accelerating de Sitter solution in a wide range of parameters. On the other hand, in order for the theory to be compatible with local gravity tests, the fifth force due to the quasidilaton scalar needs to be screened at local scales. The present paper thus extends the theory by inclusion of a cubic Horndeski term in a way that (i) respects the quasidilaton global symmetry, that (ii) maintains the physical degrees of freedom in the theory being 3, that (iii) can accommodate the Vainshtein screening mechanism, and that (iv) still allows for a stable self-accelerating de Sitter solution. After adding the Horndeski term (and a $k$-essence type nonlinear kinetic term as well) to the precursor action, we switch to the Hamiltonian language and find a complete set of independent constraints. We then construct the minimal theory with 3 physical degrees of freedom by carefully adding a pair of constraints to the total Hamiltonian of the precursor theory. Switching back to the Lagrangian language, we study cosmological solutions and their stability in the minimal theory. In particular, we show that a self-accelerating de Sitter solution is stable for a wide range of parameters. Furthermore, as in the minimal theory of massive gravity, the propagation speed of the massive gravitational waves in the high momentum limit precisely agrees with the speed of light.

We study Dirac quasinormal modes of Schwarzschild–anti–de Sitter (Schwarzschild-AdS) black holes, following the generic principle for allowed boundary conditions proposed in [M. Wang, C. Herdeiro, and M. O. P. Sampaio, Phys. Rev. D 92, 124006 (2015).]. After deriving the equations of motion for Dirac fields on the aforementioned background, we impose vanishing energy flux boundary conditions to solve these equations. We find a set of two Robin boundary conditions are allowed. These two boundary conditions are used to calculate Dirac normal modes on empty AdS and quasinormal modes on Schwarzschild-AdS black holes. In the former case, we recover the known normal modes of empty AdS; in the latter case, the two sets of Robin boundary conditions lead to two different branches of quasinormal modes. The impact on these modes of the black hole size, the angular momentum quantum number and the overtone number are discussed. Our results show that vanishing energy flux boundary conditions are a robust principle, applicable not only to bosonic fields but also to fermionic fields.

We show that there exists a critical point for the coupling constants in Einsteinian cubic gravity in which the linearized equations on the maximally symmetric vacuum vanish identically. We construct an exact isotropic bounce universe in the critical theory in four dimensions. The comoving time runs from minus infinity to plus infinity, yielding a smooth universe bouncing between two de Sitter vacua. In five dimensions, we adopt a numerical approach to construct a bounce solution, in which a singularity occurs before the bounce takes place. We then construct exact anisotropic bounces that connect two isotropic de Sitter spacetimes with flat spatial sections. We further construct exact anti-de Sitter black holes in the critical theory in four and five dimensions and obtain an exact anti-de Sitter worm brane in four dimensions.

We study primordial perturbations from hyperinflation, proposed recently and based on a hyperbolic field-space. In the previous work, it was shown that the field-space angular momentum supported by the negative curvature modifies the background dynamics and enhances fluctuations of the scalar fields qualitatively, assuming that the inflationary background is almost de Sitter. In this work, we confirm and extend the analysis based on the standard approach of cosmological perturbation in multifield inflation. At the background level, to quantify the deviation from de Sitter, we introduce the slow-varying parameters and show that steep potentials, which usually cannot drive inflation, can drive inflation. At the linear perturbation level, we obtain the power spectrum of primordial curvature perturbation and express the spectral tilt and running in terms of the slow-varying parameters. We show that hyperinflation with power-law type potentials has already been excluded by the recent Planck observations, while exponential-type potential with the exponent of order unity can be made consistent with observations as far as the power spectrum is concerned. We also argue that, in the context of a simple $D$-brane inflation, the hyperinflation requires exponentially large hyperbolic extra dimensions but that masses of Kaluza-Klein gravitons can be kept relatively heavy.

We examine the Hamiltonian dynamics of bouncing Bianchi IX cosmologies with three scale factors in Hořava-Lifshitz (HL) gravity. We assume a positive cosmological constant plus noninteracting dust and radiation as the matter content of the models. In this framework the modified field equations contain additional terms which turn the dynamics nonsingular. The six-dimensional phase space presents (i) two critical points in a finite region of the phase space, (ii) one asymptotic de Sitter attractor at infinity and (iii) a two-dimensional invariant plane containing the critical points; together they organize the dynamics of the phase space. We identify four distinct parameter domains $A$, $B$, $C$ and $D$ for which the pair of critical points engenders distinct features in the dynamics, connected to the presence of centers of multiplicity 2 and saddles of multiplicity 2. In the domain $A$ the dynamics consists basically of periodic bouncing orbits, or oscillatory orbits with a finite number of bounces before escaping to the de Sitter attractor. The center with multiplicity 2 engenders in its neighborhood the topology of stable and unstable cylinders $R\times S^3$ of orbits, where $R$ is a saddle direction and $S^3$ is the center manifold of unstable periodic orbits. We show that the stable and unstable cylinders coalesce, realizing a smooth homoclinic connection to the center manifold, a rare event of regular/nonchaotic dynamics in bouncing Bianchi IX cosmologies. The presence of a saddle of multiplicity 2 in the domain $B$ engenders a high instability in the dynamics so that the cylinders emerging from the center manifold about $P_2$ towards the bounce have four distinct attractors: the center manifold itself, the de Sitter attractor at infinity and two further momentum-dominated attractors with infinite anisotropy. In the domain $C$ we examine the features of invariant manifolds of orbits about a saddle of multiplicity 2 $P_2$. The presence of the saddle of multiplicity 2 engenders bifurcations of the invariant manifold as the energy $E_0$ of the system increases relative to the energy $E_{c{r}_2}$ of $P_2$: (i) for $E_0<E_{c{r}_2}$ the invariant manifold has the topology $S^3$; (ii) for $E_0=E_{c{r}_2}$ two points of $S^3$ pinch into the point $P_2$, so that the invariant manifold contains infinitely many orbits homoclinic to $P_2$; (iii) for $E_0>E_{c{r}_2}$ the center manifold bifurcates into a 3-torus; (iv) for $E_0$ sufficiently large the 3-torus bifurcates into three $S^3$, an invariant manifold multiply connected. Such structures were not yet observed in the literature. The domain $D$ is not examined as most of its features are present already in the previous domains.

We investigate the consequences of the hybrid quantization approach for primordial perturbations in loop quantum cosmology, obtaining predictions for the cosmic microwave background and comparing them with data collected by the Planck mission. In this work, we complete previous studies about the scalar perturbations and incorporate tensor modes. We compute their power spectrum for a variety of vacuum states. We then analyze the tensor-to-scalar ratio and the consistency relation between this quantity and the spectral index of the tensor power spectrum. We also compute the temperature-temperature, electric-electric, temperature-electric, and magnetic-magnetic correlation functions. Finally, we discuss the effects of the quantum geometry in these correlation functions and confront them with observations.

We describe a general mathematical framework for ${\chi }^2$ discriminators in the context of the compact binary coalescence (CBC) search. We show that with any ${\chi }^2$ is associated a vector bundle over the signal manifold, that is, the manifold traced out by the signal waveforms in the function space of data segments. The ${\chi }^2$ is then defined as the square of the $L_2$ norm of the data vector projected onto a finite-dimensional subspace (the fibre) of the Hilbert space of data trains and orthogonal to the signal waveform. Any such fibre leads to a ${\chi }^2$ discriminator, and the full vector bundle comprising the subspaces and the base manifold constitute the ${\chi }^2$ discriminator. We show that the ${\chi }^2$ discriminators used so far in the CBC searches correspond to different fibre structures constituting different vector bundles on the same base manifold, namely, the parameter space. Several benefits accrue from this general formulation. It most importantly shows that there are a plethora of ${\chi }^2$’s available and further gives useful insights into the vetoing procedure. It indicates procedures to formulate new ${\chi }^2$’s that could be more effective in discriminating against commonly occurring glitches in the data. It also shows that no ${\chi }^2$ with a reasonable number of degrees of freedom is foolproof. It could also shed light on understanding why the traditional ${\chi }^2$ works so well. We show how to construct a generic ${\chi }^2$ given an arbitrary set of vectors in the function space of data segments. These vectors could be chosen such that glitches have maximum projection on them. Further, for glitches that can be modeled, we are able to quantify the efficiency of a given ${\chi }^2$ discriminator by a probability. Second, we propose a family of ambiguity ${\chi }^2$ discriminators that is an alternative to the traditional one [B. Allen, Phys. Rev. D 71, 062001 (2005), B. Allen et al., Phys. Rev. D 85, 122006 (2012).]. Any such ambiguity ${\chi }^2$ makes use of the filtered output of the template bank, thus adding negligible cost to the overall search. It is termed so because it makes significant use of the ambiguity function. We first describe the formulation with the help of the Newtonian waveform, apply the ambiguity ${\chi }^2$ to the spinless TaylorF2 waveforms, and test it on simulated data. We show that the ambiguity ${\chi }^2$ essentially gives a clean separation between glitches and signals. We indicate how the ambiguity ${\chi }^2$ can be generalized to detector networks for coherent observations. The effects of mismatch between signal and templates on a ${\chi }^2$ discriminator using general arguments and the geometrical framework are also investigated.

A theoretical analysis of possible influence of neutrino magnetic moments on the propagation of ultrahigh-energy cosmic neutrinos in the interstellar space is carried out under the assumption of two-neutrino mixing. The exact solution of the effective equation for neutrino evolution in the presence of a magnetic field and matter is obtained, which accounts for four neutrino species corresponding to two different flavor states with positive and negative helicities. Using most stringent astrophysical bounds on the putative neutrino magnetic moment, probabilities of neutrino flavor and spin oscillations are calculated on the basis of the obtained exact solution. Specific patterns of spin-flavor oscillations are determined for neutrino-energy values characteristic of, respectively, the cosmogenic neutrinos, the Greisen-Zatsepin-Kuz’min (GZK) cutoff, and well above the cutoff.

Observations by HAWC and Milagro have detected bright and spatially extended TeV $\gamma$-ray sources surrounding the Geminga and Monogem pulsars. We argue that these observations, along with a substantial population of other extended TeV sources coincident with pulsar wind nebulae, constitute a new morphological class of spatially extended TeV halos. We show that HAWCs wide field of view unlocks an expansive parameter space of TeV halos not observable by atmospheric Cherenkov telescopes. Under the assumption that Geminga and Monogem are typical middle-aged pulsars, we show that ten-year HAWC observations should eventually observe ${37}_{-13}^{+17}$ middle-aged TeV halos that correspond to pulsars whose radio emission is not beamed towards Earth. Depending on the extrapolation of the TeV halo efficiency to young pulsars, HAWC could detect more than 100 TeV halos from misaligned pulsars. These pulsars have historically been difficult to detect with existing multiwavelength observations. TeV halos will constitute a significant fraction of all HAWC sources, allowing follow-up observations to efficiently find pulsar wind nebulae and thermal pulsar emission. The observation and subsequent multi-wavelength follow-up of TeV halos will have significant implications for our understanding of pulsar beam geometries, the evolution of pulsar wind nebulae, the diffusion of cosmic rays near energetic pulsars, and the contribution of pulsars to the cosmic-ray positron excess.

A hidden Markov model (HMM) scheme for tracking continuous-wave gravitational radiation from neutron stars in low-mass x-ray binaries (LMXBs) with wandering spin is extended by introducing a frequency-domain matched filter, called the $\mathcal{J}$-statistic, which sums the signal power in orbital sidebands coherently. The $\mathcal{J}$-statistic is similar but not identical to the binary-modulated $\mathcal{F}$-statistic computed by demodulation or resampling. By injecting synthetic LMXB signals into Gaussian noise characteristic of the Advanced Laser Interferometer Gravitational-wave Observatory (Advanced LIGO), it is shown that the $\mathcal{J}$-statistic HMM tracker detects signals with characteristic wave strain $h_0\ge 2\times {10}^{-26}$ in 370 d of data from two interferometers, divided into 37 coherent blocks of equal length. When applied to data from Stage I of the Scorpius X-1 Mock Data Challenge organized by the LIGO Scientific Collaboration, the tracker detects all 50 closed injections ($h_0\ge 6.84\times {10}^{-26}$), recovering the frequency with a root-mean-square accuracy of $\le 1.95\times {10}^{-5}\mathrm{Hz}$. Of the 50 injections, 43 (with $h_0\ge 1.09\times {10}^{-25}$) are detected in a single, coherent 10 d block of data. The tracker employs an efficient, recursive HMM solver based on the Viterbi algorithm, which requires $\sim {10}^5$ CPU-hours for a typical broadband (0.5 kHz) LMXB search.

Particle scattering and vacuum instability in a constant inhomogeneous electric field of particular peak configuration that consists of two (exponentially increasing and exponentially decreasing) independent parts are studied. It presents a new kind of external field where exact solutions of the Dirac and Klein-Gordon equations can be found. We obtain and analyze in- and out-solutions of the Dirac and Klein-Gordon equations in this configuration. By their help we calculate probabilities of particle scattering and characteristics of the vacuum instability. In particular, we consider in details three configurations: a smooth peak, a sharp peak, and a strongly asymmetric peak configuration. We find asymptotic expressions for total mean numbers of created particles and for vacuum-to-vacuum transition probability. We discuss a new regularization of the Klein step by the sharp peak and compare this regularization with another one given by the Sauter potential.