Singularities are common features of general relativity black holes. However, within general relativity, one can construct black holes that present no singularities. These regular black hole solutions can be achieved by, for instance, relaxing one of the energy conditions on the stress-energy tensor sourcing the black hole. Some regular black hole solutions were found in the context of nonlinear electrodynamics, the Bardeen black hole being the first one proposed. In this paper, we consider a planar massless scalar wave scattered by a Bardeen black hole. We compare the scattering cross section computed using a partial-wave description with the classical geodesic scattering of a stream of null geodesics, as well as with the semiclassical glory approximation. We obtain that, for some values of the corresponding black hole charge, the scattering cross section of a Bardeen black hole has a similar interference pattern to a Reissner-Nordström black hole.

We investigate the thick brane model in Palatini $f(\mathcal{R})$ gravity. The brane is generated by a real scalar field with a scalar potential. We solve the system analytically and obtain a series of thick brane solutions for the $f(\mathcal{R})=\mathcal{R}+\alpha {\mathcal{R}}^2$-brane model. It is shown that tensor perturbations of the metric are stable for $df(\mathcal{R})/d\mathcal{R}>0$. For nonconstant curvature solutions, the graviton zero mode can be localized on the brane, which indicates that the four-dimensional gravity can be recovered on the brane. The mass spectrum of graviton Kaluza-Klein modes and their corrections to the Newtonian potential are also discussed.

Dark energy is often assumed to be composed of a single scalar field. The background cosmic expansion is not sufficient to determine whether this is true or not. We study multifield scalar-tensor models with a general dark matter source and write the observable modified gravity parameters (effective gravitational constant and anisotropic stress) in the form of a ratio of polynomials in the Fourier wave number $k$ of order $2N$, where $N$ is the number of scalar fields. By comparing these observables to real data it is in principle possible to determine the number of dark energy scalar fields coupled to gravity. We also show that there are no realistic nontrivial cases in which the order of the polynomials is reduced.

We construct a massive spin-2 theory in $2+1$ dimensions that is immune to the bulk-boundary unitarity conflict in anti-de Sitter space and hence amenable to holography. The theory is an extension of topologically massive gravity (TMG), just like the recently found minimal massive gravity (MMG), but it has two massive helicity modes instead of a single one. The theory admits all the solutions of TMG with a redefined topological parameter. We calculate the Shapiro time delay and show that flat-space (local) causality is not violated. We show that there is an interesting relation between the theory we present here (which we call ${\mathrm{MMG}}_2$), MMG, and the earlier new massive gravity (NMG): namely, field equations of these theories are nontrivially related. We study the bulk excitations and boundary charges of the conformal field theory that could be dual to gravity. We also find the chiral gravity limit for which one of the massive modes becomes massless. The virtue of the model is that one does not have to go to the chiral limit to achieve unitarity in the bulk and on the boundary, and the log-terms that appear in the chiral limit and cause instability do not exist in the generic theory.

We construct approximate solutions of the hybrid quantum Gowdy cosmology with three-torus topology, linear polarization, and local rotational symmetry, in the presence of a massless scalar field. More specifically, we determine some families of states for which the complicated inhomogeneous and anisotropic Hamiltonian constraint operator of the Gowdy model is approximated by a much simpler one. Our quantum states follow the dynamics governed by this simpler constraint, while being at the same time also approximate solutions of the full Gowdy model. This is so thanks to the quantum correlations that the considered states present between the isotropic and anisotropic sectors of the model. Remarkably, this simpler constraint can be regarded as that of a flat Friedmann-Robertson-Walker universe filled with different kinds of perfect fluids and geometrically corrected by homogeneous and isotropic curvaturelike terms. Therefore, our quantum states, which are intrinsically inhomogeneous, admit approximate homogeneous and isotropic effective descriptions similar to those considered in modified theories of gravity.

The thermodynamics of the Universe is restudied by requiring its compatibility with the holographic-style gravitational equations that govern the dynamics of both the cosmological apparent horizon and the entire Universe, and possible solutions are proposed to the existent confusions regarding the apparent-horizon temperature and the cosmic entropy evolution. We start from the generic Lambda cold dark matter cosmology of general relativity to establish a framework for the gravitational thermodynamics. The Cai–Kim Clausius equation $\delta Q=T_{\mathrm{A}}d{S}_{\mathrm{A}}=-d{E}_{\mathrm{A}}=-A_{\mathrm{A}}{\mathit{\psi }}_t$ for the isochoric process of an instantaneous apparent horizon indicates that the Universe and its horizon entropies encode the positive-heat-out thermodynamic sign convention, which encourages us to adjust the traditional positive-heat-in Gibbs equation into the positive-heat-out version $d{E}_m=-T_{m}d{S}_m-P_{m}dV$. It turns out that the standard and the generalized second laws (GSLs) of nondecreasing entropies are always respected by the event-horizon system as long as the expanding Universe is dominated by nonexotic matter $-1\le w_m\le 1$, while for the apparent-horizon simple open system, the two second laws hold if $-1\le w_m<-1/3$; also, the artificial local equilibrium assumption is abandoned in the GSL. All constraints regarding entropy evolution are expressed by the equation-of-state parameter, which shows that from a thermodynamic perspective the phantom dark energy is less favored than the cosmological constant and the quintessence. Finally, the whole framework is extended from general relativity and Lambda cold dark matter to modified gravities with field equations $R_{\mu \nu }-R{g}_{\mu \nu }/2=8\pi G_{\text{eff}}{T}_{\mu \nu }^{(\text{eff})}$. Furthermore, this paper argues that the Cai–Kim temperature is more suitable than Hayward, both temperatures are independent of the inner or outer trappedness of the apparent horizon, and the Bekenstein–Hawking and Wald entropies cannot unconditionally apply to the event and particle horizons.

The reionization of helium describes the transition from its singly ionized state to a doubly ionized state in the intergalactic medium. This process is important for the thermal evolution of the intergalactic medium and influences the mean free path of photons with energies above 54.4 eV. While it is well known that helium reionization is mostly driven by the contribution of energetic quasars at $z<6$, we study here how helium reionization proceeds if there is an additional contribution due to the annihilation of dark matter. We explore the effects of different dark matter profiles for the dark matter clumping factor, which can significantly enhance the annihilation rate at late times. We find that the presence of dark matter annihilation enhances the ${\mathrm{He}}^{++}$ abundance at early stages where it would be zero within the standard model, and it can further increase during structure formation, reflecting the increase of the dark matter clumping factor. The latter is, however, degenerate with the buildup of the quasar contribution, and we therefore expect no significant changes at late times. We expect that future studies of the ${\mathrm{He}}^{+}$ Lyman $\alpha$ forest may help to assess whether the evolution is consistent with the contribution from quasars alone, or if an additional component may be required.

In the absence of gravity, one can prove that tunneling instantons exhibit exactly one negative mode in their spectrum of fluctuations. It is precisely the existence of this tunneling negative mode that warrants an interpretation of these solutions as mediating the decay of a metastable vacuum. In the presence of gravity the situation is much more subtle, not least because of diffeomorphism invariance. New complications arise here: in particular, the kinetic term of the fluctuations can change sign somewhere along the instanton. We show that in this case the mode functions remain nonsingular, and the tunneling negative mode continues to exist. Moreover, the eigenvalues vary continuously when the potential is varied such that the kinetic term of the fluctuations switches sign. However, the negative kinetic term implies the additional existence of an infinite tower of negative modes, whose significance and interpretation remain elusive.

We consider a higher-order term in the $\delta N$ expansion for the cosmic microwave background power asymmetry generated by a superhorizon isocurvature field fluctuation. The term can generate the asymmetry without requiring a large value of $f_{\mathrm{NL}}$. Instead it produces a nonzero value of $g_{\mathrm{NL}}$. A combination of constraints leads to an allowed region in $f_{\mathrm{NL}}-g_{\mathrm{NL}}$ space. To produce the asymmetry with this term without a large value of $f_{\mathrm{NL}}$ we find that the isocurvature field needs to contribute less than the inflation towards the power spectrum of the curvature perturbation.

We study multifield inflation models that contain a nontrivial field-space metric and a nonminimal coupling between the gravity and inflaton sectors. In such models it is known that even in the absence of explicit interaction terms the inflaton sector can decay into matter as a result of its nonminimal coupling to gravity, thereby reheating the Universe gravitationally. Using the Bogoliubov approach we evaluate the gravitational decay rates of the inflaton fields into both scalars and fermions, and analyze the reheating dynamics. We also discuss how the interpretation of the reheating dynamics differs in the so-called Jordan and Einstein frames, highlighting that the calculation of the Bogoliubov coefficients is independent of the frame in which one starts.

We introduce a highly parallelizable architecture for estimating parameters of compact binary coalescence using gravitational-wave data and waveform models. Using a spherical harmonic mode decomposition, the waveform is expressed as a sum over modes that depend on the intrinsic parameters (e.g., masses) with coefficients that depend on the observer dependent extrinsic parameters (e.g., distance, sky position). The data is then prefiltered against those modes, at fixed intrinsic parameters, enabling efficiently evaluation of the likelihood for generic source positions and orientations, independent of waveform length or generation time. We efficiently parallelize our intrinsic space calculation by integrating over all extrinsic parameters using a Monte Carlo integration strategy. Since the waveform generation and prefiltering happens only once, the cost of integration dominates the procedure. Also, we operate hierarchically, using information from existing gravitational-wave searches to identify the regions of parameter space to emphasize in our sampling. As proof of concept and verification of the result, we have implemented this algorithm using standard time-domain waveforms, processing each event in less than one hour on recent computing hardware. For most events we evaluate the marginalized likelihood (evidence) with statistical errors of $\lesssim 5\%$, and even smaller in many cases. With a bounded runtime independent of the waveform model starting frequency, a nearly unchanged strategy could estimate neutron star (NS)-NS parameters in the 2018 advanced LIGO era. Our algorithm is usable with any noise curve and existing time-domain model at any mass, including some waveforms which are computationally costly to evolve.

Radiatively inefficient accretion flows (RIAFs) in low-luminosity active galactic nuclei (LLAGNs) have been suggested as cosmic-ray and neutrino sources that may largely contribute to the observed diffuse neutrino intensity. We show that this scenario naturally predicts hadronic multi-TeV gamma-ray excesses around Galactic centers. The protons accelerated in the RIAF in Sagittarius ${\mathrm{A}}^{*}$ (Sgr ${\mathrm{A}}^{*}$) escape and interact with dense molecular gas surrounding Sgr ${\mathrm{A}}^{*}$, which is known as the central molecular zone (CMZ), and produce gamma rays as well as neutrinos. Based on a theoretical model that is compatible with the IceCube data, we calculate gamma-ray spectra of the CMZ and find that the gamma rays with $\gtrsim 1\mathrm{TeV}$ may have already been detected with the High Energy Stereoscopic System if Sgr ${\mathrm{A}}^{*}$ was more active in the past than it is today, as indicated by various observations. Our model predicts that neutrinos should come from the CMZ with a spectrum similar to the gamma-ray spectrum. We also show that such a gamma-ray excess is expected for some nearby galaxies hosting LLAGNs.

The LHCb Collaboration has recently reported on some anomalies in $b\to s$ transitions. In addition to discrepancies with the Standard Model (SM) predictions in some angular observables and branching ratios, an intriguing hint for lepton universality violation was found. Here we propose a simple model that extends the SM with a dark sector charged under an additional $U(1)$ gauge symmetry. The spontaneous breaking of this symmetry gives rise to a massive $Z^{\prime }$ boson, which communicates the SM particles with a valid dark matter candidate, while solving the $b\to s$ anomalies with contributions to the relevant observables.

The realization that first- and second-generation Yukawa couplings can be probed by decays of the Higgs boson to a meson in association with a photon has renewed interest in such rare exclusive decays. We present here a detailed study of the rare $Z$-boson processes $Z\to J/\psi +\gamma$, $Z\to \mathrm{\Upsilon }+\gamma$, and $Z\to \varphi +\gamma$ that can serve as benchmarks for the analogous Higgs-boson decays. We include both direct-production and fragmentation contributions to these decays and consider the leading QCD corrections and the relativistic corrections to the $J/\psi$ and $\mathrm{\Upsilon }$ processes. We present numerical predictions for the branching ratios that include a careful accounting of the theoretical uncertainties.

We study decays of $D^0$, $D^{+}$, and $D_s^{+}$ mesons into two pseudoscalar mesons by expressing the decay amplitudes in terms of topological amplitudes. Including consistently $SU(3{)}_F$ breaking to linear order, we show how the topological-amplitude decomposition can be mapped onto the standard expansion using reduced amplitudes characterized by $SU(3)$ representations. The tree and annihilation amplitudes can be calculated in factorization up to corrections which are quadratic in the color-counting parameter $1/N_c$. We find new sum rules connecting $D^{+}\to K_S{K}^{+}$, $D_s^{+}\to K_S{\pi }^{+}$, and $D^{+}\to K^{+}{\pi }^0$, which test the quality of the $1/N_c$ expansion. Subsequently, we determine the topological amplitudes in a global fit to the data, taking the statistical correlations among the various measurements into account. We carry out likelihood ratio tests in order to quantify the role of specific topological contributions. While the $SU(3{)}_F$ limit is excluded with a significance of more than 5 standard deviations, a good fit (with $\mathrm{\Delta }{\chi }^2<1$) can be obtained with less than 28% of $SU(3{)}_F$ breaking in the decay amplitudes. The magnitude of the penguin amplitude $P_{\text{break}}$, which probes the Glashow-Iliopoulos-Maiani mechanism, is consistent with zero; the hypothesis $P_{\text{break}}=0$ is rejected with a significance of just $0.7\sigma$. We obtain the Standard-Model correlation between $\mathcal{B}(D^0\to K_L{\pi }^0)$ and $\mathcal{B}(D^0\to K_S{\pi }^0)$, which probes doubly Cabibbo-suppressed amplitudes, and find that $\mathcal{B}(D^0\to K_L{\pi }^0)<\mathcal{B}(D^0\to K_S{\pi }^0)$ holds with a significance of more than $4\sigma$. We finally predict $\mathcal{B}(D_s^{+}\to K_L{K}^{+})=0.012_{-0.002}^{+0.006}$ at $3\sigma$ C.L.

The use of charged pileup tracks in a jet to predict the neutral pileup component in that same jet could potentially lead to improved pileup removal techniques, provided there is a strong local correlation between charged and neutral pileup. In a Monte Carlo simulation we find that the correlation is however moderate, a feature that we attribute to characteristics of the underlying nonperturbative dynamics. Consequently, pure “neutral-proportional-to-charge” (NpC) pileup mitigation approaches do not outperform existing, area-based pileup removal methods. This finding contrasts with the arguments made in favor of a new method, “jet cleansing,” in part based on the NpC approach. We identify the critical differences between the performances of linear cleansing and trimmed NpC as being due to the former’s rejection of subjets that have no charged tracks from the leading vertex, a procedure that we name “zeroing.” Zeroing, an extreme version of the “charged-track trimming” proposed by ATLAS, can be combined with a range of pileup mitigation methods, and appears to have both benefits and drawbacks. We show how the latter can be straightforwardly alleviated. We also discuss the limited potential for improvement that can be obtained by linear combinations of the NpC and area-subtraction methods.

The VuQ model involves the addition of a vector isosinglet up-type quark to the standard model. In this model the full Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix is $4\times 3$. Using present flavor-physics data, we perform a fit to this full CKM matrix, looking for signals of new physics (NP). We find that the VuQ model is very strongly constrained. There are no hints of NP in the CKM matrix, and any VuQ contributions to loop-level flavor-changing $b\to s$, $b\to d$ and $s\to d$ transitions are very small. There can be significant enhancements of the branching ratios of the flavor-changing decays $t\to uZ$ and $t\to cZ$, but these are still below present detection levels.

Measurements of diffractive dissociation cross sections in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ are presented in kinematic regions defined by the masses $M_X$ and $M_Y$ of the two final-state hadronic systems separated by the largest rapidity gap in the event. Differential cross sections are measured as a function of ${\xi }_X=M_X^2/s$ in the region $-5.5<{\log }_{10}{\xi }_X<-2.5$, for ${\log }_{10}{M}_Y<0.5$, dominated by single dissociation (SD), and $0.5<{\log }_{10}{M}_Y<1.1$, dominated by double dissociation (DD), where $M_X$ and $M_Y$ are given in GeV. The inclusive $pp$ cross section is also measured as a function of the width of the central pseudorapidity gap $\mathrm{\Delta }\eta$ for $\mathrm{\Delta }\eta >3$, ${\log }_{10}{M}_X>1.1$, and ${\log }_{10}{M}_Y>1.1$, a region dominated by DD. The cross sections integrated over these regions are found to be, respectively, $2.99\pm 0.02{(\text{stat})}_{-0.29}^{+0.32}(\text{syst})\mathrm{mb}$, $1.18\pm 0.02(\text{stat})\pm 0.13(\text{syst})\mathrm{mb}$, and $0.58\pm 0.01{(\text{stat})}_{-0.11}^{+0.13}(\text{syst})\mathrm{mb}$, and are used to extract extrapolated total SD and DD cross sections. In addition, the inclusive differential cross section, $d\sigma /d\mathrm{\Delta }{\eta }^{\mathrm{F}}$, for events with a pseudorapidity gap adjacent to the edge of the detector, is measured over $\mathrm{\Delta }{\eta }^{\mathrm{F}}=8.4$ units of pseudorapidity. The results are compared to those of other experiments and to theoretical predictions and found compatible with slowly rising diffractive cross sections as a function of center-of-mass energy.

We have performed a calculation in the parton-model approximation of a nonstandard decay process in isotropic modified Maxwell theory coupled to standard Dirac theory, with a single Lorentz-violating parameter $\kappa$ in the photonic sector. Previous calculations of this process (vacuum Cherenkov radiation by a proton for the theory with $\kappa >0$) were performed for pointlike particles, and an upper bound on $\kappa$ at the $10^{-19}$ level was obtained from experimental data on ultra-high-energy cosmic rays. The parton-model results change the decay rate by about an order of magnitude but give the same upper bound on $\kappa$ because the threshold energy is unchanged. The previous point-particle calculation of photon decay into an electron-positron pair for the theory with $\kappa <0$ remains valid, and experimental results on cosmic gamma rays provide a lower bound on $\kappa$ at the $-10^{-15}$ level.

The complete quasiparticle spectrum of a magnetized electromagnetic plasma is systematically explored at zero and nonzero temperatures. To this purpose, the general structure of the one-loop corrected propagator of magnetized fermions is determined, and the dispersion relations arising from the pole of this propagator are numerically solved. It turns out that in the lowest Landau level, where only one spin direction is allowed, the spectrum consists of one positively (negatively) charged fermionic mode with positive (negative) spin. In contrast, in higher Landau levels, as an indirect consequence of the double spin degeneracy of fermions, the spectrum consists of two massless collective modes with left and right chiralities. The mechanism through which these new collective excitations are created in a uniform magnetic field is similar to the production mechanism of dynamical holes (plasminos) at finite temperature and zero magnetic fields. Whereas cold magnetized plasminos appear for moderate magnetic fields and for all positive momenta of propagating fermions, hot magnetized plasminos appear only in the limit of weak magnetic fields and soft momenta.