Extremal black holes have vanishing Hawking temperatures. In this paper, we argue that for asymptotically anti–de Sitter (AdS) black holes, at extremality, a particular class of correlators in the dual conformal field theory can exhibit exponential, maximally chaotic growth with a nonvanishing temperature. Our approach, at extremality, is twofold. First, we geometrically investigate the modes that are responsible for chaos. Second, we study the dynamics of a probe string to capture chaos in world sheet correlators. For rotating Banados-Teitelboim-Zanelli at extremality, the corresponding Lyapunov exponent is determined by the left-moving temperature. In higher dimensional AdS-Kerr geometries, on the other hand, the corresponding Lyapunov exponent becomes a nontrivial function of the Frolov-Thorne temperatures.

The extendibility of spacetime and the existence of weak solutions to the Einstein field equations beyond Cauchy horizons is a crucial ingredient to examine the limits of general relativity. Strong cosmic censorship serves as a firewall for gravitation by demanding the inextendibility of spacetime beyond the Cauchy horizon. For asymptotically flat spacetimes, the predominance of the blueshift instability and the subsequent formation of a mass-inflation singularity at the Cauchy horizon have, so far, substantiated the conjecture. Accelerating black holes, described by the $C$ metric, are exact solutions of the field equations without a cosmological constant, which possess an acceleration horizon with similar causal properties to the cosmological horizon of de Sitter spacetime. Here, by considering linear scalar field perturbations, we provide numerical evidence for the stability of the Cauchy horizon of charged accelerating black holes. In particular, we show that the stability of Cauchy horizons in accelerating charged black holes is connected to quasinormal modes, discuss the regularity requirement for which weak solutions to the field equations exist at the Cauchy horizon, and show that strong cosmic censorship may be violated near extremality.

Exact formulas relating the radii of the spherical photon orbits to the black hole’s rotation parameter and the effective inclination angle of the orbit have been known only for equatorial and polar orbits up to now. Here we provide exact formulas for nonequatorial orbits that lie between these extreme limits. For a given rotation parameter of the black hole, there is a critical inclination angle below which there are four null photon orbits two of which are in the exterior region. At the critical angle, the radii of these orbits are explicitly found. We also provide approximate but very accurate formulas for any orbit. To arrive at these analytical solutions, we carefully study the sextic polynomial in the radius that arises for null spherical geodesics.

A unified description of inflation, dark energy, and dark matter is presented within a two-scalar-field cosmological model. Inflation, assumed to be of the warm type, is driven by one of the scalar fields, which, shortly after the end of the inflationary period, decouples from radiation and begins to oscillate rapidly around the minimum of its potential, thus behaving like cold dark matter; the second scalar field emerges, at recent times, as the dominant component of the Universe, giving rise to a second era of accelerated expansion. For certain values of the parameters of the model, the cosmological solutions arising in this triple unification of inflation, dark energy, and dark matter are viable, reproducing the main features of the evolution of the Universe.

The presence of complex hierarchical gravitational structures is one of the main features of the observed universe. Here, structure formation is studied both for the standard ($\mathrm{\Lambda }\mathrm{CDM}$) cosmological model and for the Dirac-Milne universe, a matter-antimatter symmetric universe that was recently proposed as an alternative “coasting” cosmological scenario. One-dimensional numerical simulations reveal the analogies and differences between the two models. Although structure formation is faster in the Dirac-Milne universe, both models predict that it ends shortly before the present epoch, at cosmological redshift $z\approx 3$ for the Dirac-Milne cosmology, and at $z\approx 0.5$ for the $\mathrm{\Lambda }\mathrm{CDM}$ universe. The present results suggest that the matter power spectrum observed by the Sloan Digital Sky Survey might be entirely due to the nonlinear evolution of matter and antimatter domains of relatively small initial dimensions, of the order of a few tens of parsecs comoving at cosmological redshift $z=1080$.

Single-field inflaton models in the kinetic dominance period admit formal solutions given by generalized asymptotic expansions called psi series. We present a method for computing psi series for the Hubble parameter as a function of the inflaton field in the Hamilton-Jacobi formulation of inflaton models. Similar psi series for the scale factor, the conformal time, and the Hubble radius are also derived. They are applied to determine the value of the inflaton field when the inflation period starts and to estimate the contribution of the kinetic dominance period to calculate the duration of inflation. These psi series are also used to obtain explicit two-term truncated psi series near the singularity for the potentials of the Mukhanov-Sasaki equation for curvature and tensor perturbations. The method is illustrated with wide families of inflaton models determined by potential functions combining polynomial and exponential functions, as well as with generalized Starobinsky models.

Cosmological phase transitions in the early Universe may produce relics in the form of a network of cosmic defects. Independently of the order of a phase transition, topology of the defects, and their global or gauge nature, the defects are expected to emit gravitational waves (GWs) as the network energy-momentum tensor adapts itself to maintaining scaling. We show that the evolution of any defect network (and for that matter any scaling source) emits a GW background with spectrum ${\mathrm{\Omega }}_{\mathrm{GW}}\propto f^3$ for $f\ll f_0$, ${\mathrm{\Omega }}_{\mathrm{GW}}\propto 1/f^2$ for $f_0\lesssim f\lesssim f_{\mathrm{eq}}$, and ${\mathrm{\Omega }}_{\mathrm{GW}}\propto \mathrm{const}$ (i.e., exactly scale invariant) for $f\gg f_{\mathrm{eq}}$, where $f_0$ and $f_{\mathrm{eq}}$ denote respectively the frequencies corresponding to the present and matter-radiation equality horizons. This background represents an irreducible emission of GWs from any scaling network of cosmic defects, with its amplitude characterized only by the symmetry-breaking scale and the nature of the defects. Using classical lattice simulations we calculate the GW signal emitted by defects created after the breaking of a global symmetry $O(N)\to O(N-1)$. We obtain the GW spectrum for $N$ between 2 and 20 with two different techniques: integrating over unequal-time correlators of the energy-momentum tensor, updating our previous work on smaller lattices, and for the first time, comparing the result with the real-time evolution of the tensor perturbations sourced by the same defects. Our results validate the equivalence of the two techniques. Using cosmic microwave background upper bounds on the defects’ energy scale, we discuss the difficulty of detecting this GW background in the case of global defects.

We revisit models of natural inflation and show that the single-field effective theory described by the potential $V(a)\sim \cos \frac{a}{f}$ breaks down as the inflaton $a$ makes large-field excursions, even for values of $f$ smaller than the Planck scale. To remedy the problem, we modify the potential in order to account for the heavy degrees of freedom (hadrons) that become intertwined with the light inflaton as the latter rolls down its potential. By embedding the low-energy degrees of freedom into an ultraviolet-complete gauge theory, we argue that the intertwining between the two scales can be explained as the result of a generalized mixed ’t Hooft anomaly between the discrete chiral symmetry and background fractional fluxes in the baryon number, color, and flavor directions. Further, we study the multifield inflation and show that it entertains rich dynamics. Inflating near the hilltop excites the hadrons and spoils the slow-roll parameters, in contradistinction with the expectations in the single-field inflation. Nevertheless, we identify a safe zone where inflation can proceed successfully. We determine the conditions under which the Universe inflates by at least 60 $e$-foldings and inflation leads to a power spectrum and tensor to scalar ratio that are consistent with the cosmic microwave background data.

The diffusive motion of charged particles in synthetic magnetic turbulence with different properties is investigated by using numerical simulations with unprecedented dynamical range, which allow us to ensure that both the inertial range and the long wavelength part of the turbulent spectrum are properly described. This is of particular importance in evaluating previous suggestions that parallel and perpendicular diffusion coefficients differ in their energy dependence, an assertion at odds with the many claims of universality of the $D_{\perp }$ and $D_{\parallel }$ as functions of particle energy. Cases with and without an ordered magnetic field are discussed. Results of the numerical simulations are compared with available theoretical models, for slab, slab/2D and isotropic turbulence. We find widespread evidence that universality is broken, and that the ratio $D_{\perp }/D_{\parallel }$ is not independent of energy. The implications of this finding for the physics of cosmic ray transport are discussed in depth.

A detection of the stochastic gravitational-wave background (SGWB) from unresolved compact binary coalescences could be made by Advanced LIGO and Advanced Virgo at their design sensitivities. However, it is possible for magnetic noise that is correlated between spatially separated ground-based detectors to mimic a SGWB signal. In this paper we propose a new method for detecting correlated magnetic noise and separating it from a true SGWB signal. A commonly discussed method for addressing correlated magnetic noise is coherent subtraction in the raw data using Wiener filtering. The method proposed here uses a parametrized model of the magnetometer-to-strain coupling functions, along with measurements from local magnetometers, to estimate the contribution of correlated noise to the traditional SGWB detection statistic. We then use Bayesian model selection to distinguish between models that include correlated magnetic noise and those with a SGWB. Realistic simulations are used to show that this method prevents a false SGWB detection due to correlated magnetic noise. We also demonstrate that it can be used for a detection of a SGWB in the presence of strong correlated magnetic noise, albeit with reduced significance compared to the case with no correlated noise. Finally, we discuss the advantages of using a global three-detector network for both identifying and characterizing correlated magnetic noise.

The standard model of cosmology is underpinned by the assumption of the statistical isotropy of the Universe. Observations of the cosmic microwave background, galaxy distributions, and supernovae, among other media, support the assumption of isotropy at scales $\gtrsim 100\mathrm{Mpc}$. The recent detections of gravitational waves from merging stellar-mass binary black holes provide a new probe of anisotropy; complementary and independent of all other probes of the matter distribution in the Universe. We present an analysis using a spherical harmonic model to determine the level of anisotropy in the first LIGO/Virgo transient catalog. We find that the ten binary black hole mergers within the first transient catalog are consistent with an isotropic distribution. We carry out a study of simulated events to assess the prospects for future probes of anisotropy. Within a single year of operation, third-generation gravitational-wave observatories will probe anisotropies with an angular scale of $\sim 36°$ at the level of $\lesssim 0.1\%$.

Gravitational waves have been directly detected and astronomical observations indicate that our Universe has a positive cosmological constant $\mathrm{\Lambda }$. Nevertheless, a theoretical gauge-invariant notion of gravitational waves arriving at infinity (escaping from the space-time) in the presence of a positive $\mathrm{\Lambda }$ has been elusive. We find the answer to this long-standing gravitational puzzle and present a geometric, gauge-invariant radiation condition at infinity.

Modifications to electrodynamics from physics beyond the Standard Model can be tested to a high accuracy. Here we use two setups to place bounds on hidden photons, an Abelian boson kinetically mixed with the photon. The first setup involves atomic force microscope measurements, originally designed to study the Casimir effect at sub-$\mu \mathrm{m}$ distances. The second setup consists of two concentric metal shells with the outer one exposed to a high voltage. By measuring the potential difference between the shells it is possible to test Coulomb’s law. The limits obtained here cover regions already excluded, in particular by astrophysical observations, but provide a more direct, laboratory-based confirmation of these bounds.

Lattice QCD calculations of form factors for rare Standard Model processes such as $B\to K{\ell }^{+}{\ell }^{-}$ use tensor currents that require renormalization. These renormalization factors, $Z_T$, have typically been calculated within perturbation theory and the estimated uncertainties from missing higher order terms are significant. Here we study tensor current renormalization using lattice implementations of momentum-subtraction schemes. Such schemes are potentially more accurate but have systematic errors from nonperturbative artifacts. To determine and remove these condensate contributions we calculate the ground-state charmonium tensor decay constant, $f_{J/\psi }^T$, which is also of interest in beyond the Standard Model studies. We obtain $f_{J/\psi }^T(\overline{\mathrm{MS}},2\mathrm{GeV})=0.3927(27)\mathrm{GeV}$, with ratio to the vector decay constant of 0.9569(52), significantly below 1. We also give $Z_T$ factors, converted to the $\overline{\mathrm{MS}}$ scheme, corrected for condensate contamination. This contamination reaches 1.5% at a renormalization scale of 2 GeV (in the preferred Regularisation Invariant Symmetric Momentum subtraction scheme) and so must be removed for accurate results.

We present a new method, based on Gaussian process regression, for reconstructing the continuous $x$-dependence of parton distribution functions (PDFs) from quasi-PDFs computed using lattice QCD. We examine the origin of the unphysical oscillations seen in current lattice calculations of quasi-PDFs and develop a nonparametric fitting approach to take the required Fourier transform. The method is tested on one ensemble of maximally twisted mass fermions with two light quarks. We find that with our approach oscillations of the quasi-PDF are drastically reduced. However, the final effect on the light-cone PDFs is small. This finding suggests that the deviation seen between current lattice QCD results and phenomenological determinations cannot be attributed solely on the Fourier transform.

We discuss the problem of possible boundary terms at poles of the drift in the complex Langevin method, which spoil correctness of the method. For the simplest, however paradigmatic cases we can find complete answers. Lessons for more generic cases as well as open mathematical problems are discussed.

This paper investigates the electromagnetic production of lepton pairs with low total transverse momentum in relativistic heavy-ion collisions. We estimate the initial photons’ transverse momentum contributions by employing models where the average transverse momentum squared of the incoming photon can be calculated in the equivalent photon approximation. We further derive an all order QED resummation for the soft photon radiation, which gives an excellent description of the ATLAS data in ultraperipheral collisions at the LHC. For peripheral and central collisions, additional $p_T$-broadening effects from multiple interactions with the medium and the magnetic field contributions from the quark-gluon plasma are also discussed.

The physical masses of the lowest scalar mesons strongly disagree with the calculated $q\overline{q}$ pole values. It is the purpose of this paper to explain theoretically the unusual spectrum of scalar mesons for both the ground and excited states, using the known spectra of the corresponding $q\overline{q}$ states and their connection to the meson resonances. The well-known Cornell coupled-channel mechanism is exploited for this connection together with the quark-chiral Lagrangian without fitting parameters. In addition to the scalars previously obtained using this method, $f_0(500)$, $f_0(980)$, $a_0(980)$, we predict all ground and first excited scalar states, $f_0(500)$, $f_0(980)$, $a_0(980)$, $a_0(1450)$, $K_0^{*}(700)$, $K_0^{*}(1430)$, $f_0(1370)$, $f_0(1710)$, which are in reasonably good agreement with experimental data.

We consider the transverse single-spin asymmetry (SSA) for $J/\psi$ production in $p^{\uparrow }p\to J/\psi +X$ within a phenomenological transverse momentum dependent scheme in nonrelativistic QCD. Extending a previous study [U. D’Alesio et al., Eur. Phys. J. C 79, 1029 (2019)], we employ here the color-gauge invariant generalized parton model (CGI-GPM), in which spin and intrinsic transverse momentum effects are taken into account, together with leading-order initial- and final-state interactions (ISIs and FSIs). We find that, even when the heavy-quark pair is produced in a color-octet state, ISIs and FSIs lead to a nonvanishing SSA, allowing us, in principle, to test the process dependence of the gluon Sivers function (GSF). We show that of the two independent contributions, due to the so-called $f$- and $d$-type GSFs, appearing in the CGI-GPM, the $d$-type one turns out to be dynamically suppressed. Therefore, as already found adopting the Color-Singlet Model approach for the $J/\psi$ formation [U. D’Alesio et al., Phys. Rev. D 96, 036011 (2017)], only the $f$-type GSF could play a role in phenomenology. A comparison with the corresponding results obtained in the generalized parton model, without the inclusion of ISIs and FSIs, is also carried out.

A naive application of the heavy quark expansion (HQE) yields theory estimates for the decay rate of neutral $D$ mesons that are 4 orders of magnitude below the experimental determination. It is well known that this huge suppression results from severe Glashow-Iliopoulos-Maiani cancellations. We find that this mismatch can be solved by individually choosing the renormalization scale of the different internal quark contributions. For $b$ and $c$ hadron lifetimes, as well as for the decay rate difference of neutral $B$ mesons, the effect of our scale setting procedure lies within the previously quoted theory uncertainties, while we get enlarged theory uncertainties for the semileptonic $CP$ asymmetries in the $B$ system.