Fluctuations play a key role in the study of QCD phases. Lattice QCD is a valuable tool to calculate them, but going to high orders is challenging. Up to the fourth order, continuum results have been available since 2015. We present the first continuum results for sixth-order baryon fluctuations for temperatures between $T=130$ and 200 MeV and eighth order at $T=145\mathrm{MeV}$ in a fixed volume. Comparison with earlier studies with imaginary chemical potential suggests that the volume effect is under control for $T<145\mathrm{MeV}$. Our results are in sharp contrast with well-known results in the literature obtained at finite lattice spacing.

We present a new vacuum of the bosonic higher-spin gauge theory in $d+1$ dimensions, which has leftover symmetry of the Poincaré algebra in $d$ dimensions. Its structure is very simple: the space-time geometry is that of anti–de Sitter space, while the only nonzero field is a scalar. The scalar extends along the Poincaré radial coordinate $\mathrm{z}$ and is shown to be linearly exact for an arbitrary mixture of its two $\mathrm{\Delta }=2$ and $\mathrm{\Delta }=d-2$ conformal branches. The obtained vacuum breaks the global higher-spin symmetry, leading to a broken phase that lives in the Minkowski space-time.

We construct higher-derivative gravity theories in three dimensions that admit holographic $c$-theorems and exhibit a unique maximally symmetric vacuum, at arbitrary order $n$ in the curvature. We show that these theories exhibit special properties, the most salient ones being the decoupling of ghost modes around anti–de Sitter (AdS) space, the enhancement of symmetries at linearized level, and the existence of a one-parameter generalization of the Bañados-Teitelboim-Zanelli (BTZ) black hole that, while being asymptotically AdS, is not of constant curvature but rather exhibits a curvature singularity. For such black holes, we provide a holographic derivation of their thermodynamics. This gives a microscopic picture of black hole thermodynamics for nonsupersymmetric solutions, of nonconstant curvature in higher-derivative theories of arbitrary order in the curvature.

We investigate the implications of decoherence induced by quantum spacetime properties on neutrino oscillation phenomena. We develop a general formalism where the evolution of neutrinos is governed by a Lindblad-type equation and we compute the oscillation damping factor for various models that have been proposed in the literature. Furthermore, we discuss the sensitivity to these effects of different types of neutrino oscillation experiments, encompassing astrophysical, atmospheric, solar, and reactor neutrino experiments. By using neutrino oscillation data from long-baseline reactors and atmospheric neutrino observations, we establish stringent constraints on the energy scale governing the strength of the decoherence induced by stochastic metric fluctuations, amounting to, respectively, $E_{QG}\ge 2.6\times {10}^{34}\mathrm{GeV}$ and $E_{QG}\ge 2.5\times {10}^{55}\mathrm{GeV}$.

We present a general construction of a geometric notion of circuit complexity for Gaussian states (both bosonic and fermionic) in terms of Riemannian geometry. We lay out general conditions that a Riemannian metric function on the space of Gaussian states should satisfy in order for it to yield a physically reasonable measure of complexity. This general formalism can naturally accommodate modifications to complexity geometries that arise from cost functions that depend nontrivially on the instantaneous state and on the direction on circuit space at each point. We explore these modifications and, as a particular case, we show how to account for time-reversal symmetry breaking in measures of complexity, which is often natural from an experimental (and thermodynamical) perspective, but is absent in commonly studied complexity measures. This establishes a first step towards building a quantitative, geometric notion of complexity that faithfully mimics what is experienced as “easy” or “hard” to implement in a lab from a physically motivated point of view.

We study numerically the existence in a false vacuum of magnetic monopoles that are “thin walled,” i.e., which correspond to a spherical region of radius $R$ that is essentially trivial surrounded by a wall of thickness $\mathrm{\Delta }\ll R$, hence the name thin wall, and finally an exterior region that essentially corresponds to a pure Abelian magnetic monopole. Such monopoles were dubbed false monopoles and can occur in non-Abelian gauge theories where the symmetry-broken vacuum is actually the false vacuum. This idea was first proposed in Kumar et al. [Phys. Rev. D 82, 025022 (2010)]; however, the proof of the existence of thin-wall, false monopoles given there was incorrect. Here, we fill this lacuna and demonstrate numerically, for an appropriately modified potential, the existence of thin-wall false monopoles. The decay via quantum tunneling of the false monopoles could be of importance to cosmological scenarios that entertain epochs in which the Universe is trapped in a symmetry-broken false vacuum.

An Unruh-DeWitt detector falling radially into a four-dimensional Schwarzschild black hole, coupled linearly to a massless scalar field that has been prepared in the Hartle-Hawking or Unruh state, has recently been shown to exhibit a local extremum in its transition probability near the black hole horizon [K. K. Ng et al., New J. Phys. 24, 103018 (2022)]. We show that a similar phenomenon is present in the transition rate of an Unruh-DeWitt detector falling radially into a spinless Bañados-Teitelboim-Zanelli black hole, with the scalar field prepared in the Hartle-Hawking state. We give extensive numerical results as a function of the detector’s energy gap, the black hole’s mass, and the detector’s drop-off radius. Our results suggest that the effect is robust, motivating a search for a similar effect in other black hole spacetimes, and calling for an explanation of the physical origin of the effect.

As gravitational wave detectors improve in sensitivity, signal-to-noise ratios of compact binary coalescences will dramatically increase, reaching values in the hundreds and potentially thousands. Such strong signals offer both exciting scientific opportunities and pose formidable challenges to the template waveforms used for interpretation. Current waveform models are informed by calibrating or fitting to numerical relativity waveforms and such strong signals may unveil computational errors in generating these waveforms. In this paper, we isolate a single source of computational error, that of the finite grid resolution, and investigate its impact on parameter estimation for aLIGO and Cosmic Explorer. We demonstrate that increasing the inclination angle or decreasing the mass ratio $q$ ($q\le 1$) raises the resolution required for unbiased parameter estimation. We quantify the error associated with the highest-resolution waveform utilized in our study using an extrapolation procedure on the median of recovered posteriors and confirm the accuracy of current waveforms for the synthetic sources. We introduce a measure to predict the necessary numerical resolution for unbiased parameter estimation and use it to predict that current waveforms are suitable for equal and moderately unequal mass binaries for both detectors. However, current waveforms fail to meet accuracy requirements for high signal-to-noise ratio signals from highly unequal mass ratio binaries $(q\lesssim 1/6)$, for all inclinations in Cosmic Explorer, and for high inclinations in future updates to LIGO. Given that the resolution requirement becomes more stringent with more unequal mass ratios, current waveforms may lack the necessary accuracy, even at median signal-to-noise ratios for future detectors.

A small quantum system within the gravitational field of a massive body will be entangled with the quantum degrees of freedom of the latter. Hence, the massive body acts as an environment, and it induces nonunitary dynamics, noise, and decoherence to the quantum system. It is impossible to shield systems on Earth from this gravity-mediated decoherence, which could severely affect all experiments with macroscopic quantum systems. We undertake a first-principles analysis of this effect, by deriving the corresponding open system dynamics. We find that near-future quantum experiments are not affected, but there is a strong decoherence effect at the human scale. The decoherence time for a superposition of two localized states of a human with a one meter separation is of the order of one second.

The so-called black-bounce mechanism of singularity suppression, proposed by Simpson and Visser, consists of replacing the spherical radius $r$ in the metric tensor with $\sqrt{r^2+a^2}$, $a=\text{const}>0$. This removes a singularity at $r=0$ and its neighborhood from space-time and there emerges a regular minimum of the spherical radius that can be a wormhole throat or a regular bounce (if located inside a black hole). Instead, it is proposed here to make $r=0$ a regular center by proper (Bardeen type) replacements in the metric, preserving its form at large $r$. Such replacements are applied to a class of metrics satisfying the condition $R_t^t=R_r^r$ for their Ricci tensor, in particular, to the Schwarzschild, Reissner-Nordström, and Einstein-Born-Infeld solutions. A simpler version of nonlinear electrodynamics (NED) is considered, for which a black hole solution is similar to the Einstein-Born-Infeld one but is simpler expressed analytically. All new regular metrics can be presented as solutions to NED-Einstein equations with radial magnetic fields.

We investigate the well-posedness of the characteristic initial-boundary value problem for the Einstein equations in Bondi-like coordinates (including Bondi, double-null and affine). We propose a definition of strong hyperbolicity of a system of partial differential equations of any order, and show that the Einstein equations in Bondi-like coordinates in their second-order form used in numerical relativity do not meet it, in agreement with results of Giannakopoulos et al. for specific first-order reductions. In the principal part, frozen coefficient approximation that one uses to examine hyperbolicity, we explicitly construct the general solution to identify the solutions that obstruct strong hyperbolicity. Independently, we present a first-order symmetric hyperbolic formulation of the Einstein equations in Bondi gauge, linearized about Schwarzschild, thus completing work by Frittelli. This establishes an energy norm ($L^2$ in the metric perturbations and selected first and second derivatives), in which the initial-boundary value problem, with initial data on an outgoing null cone and boundary data on a timelike cylinder or an ingoing null cone, is well posed, thus verifying a conjecture by Giannakopoulos et al. Unfortunately, our method does not extend to the pure initial-value problem on a null cone with regular vertex.

We present the first numerical simulations in null coordinates of the collapse of nonspherical regular initial data to a black hole. We restrict to twist-free axisymmetry, and reinvestigate the critical collapse of a nonspherical massless scalar field. We find that the Choptuik solution governing scalar field critical collapse in spherical symmetry persists when fine-tuning moderately nonspherical initial data to the threshold of black hole formation. The nonsphericity evolves as an almost-linear perturbation until the end of the self-similar phase, and becomes dominant only in the final collapse to a black hole. We compare with numerical results of Choptuik et al., Baumgarte, and Marouda et al., and conclude that they have been able to evolve somewhat more nonspherical solutions. Future work with larger deviations from spherical symmetry, and in particular vacuum collapse, will require a different choice of radial coordinate that allows the null generators to reconverge locally.

We present a code for numerical simulations of the collapse of regular initial data to a black hole in null coordinates. We restrict to twist-free axisymmetry with scalar field matter. Our coordinates are $(u,x,\theta ,\phi )$, where the retarded time $u$ labels outgoing null cones emerging from a regular central worldline, the angles $(\theta ,\phi )$ label the null generators of each null cone, and the radial coordinate $x$ labels points along these generators. We focus on a class of generalized Bondi radial coordinates $x$ with the twin properties that $x=0$ is the central worldline and that the numerical domain ($u\ge 0,0\le x\le x_{\max }$) is a subset of the domain of dependence of the initial data on ($u=0,0\le x\le x_{\max }$). In critical collapse, an appropriate choice of these coordinates can be made to zoom in on the accumulation point of scale echoes of the critical solution, without the need for explicit mesh refinement. We introduce a novel numerical scheme that in effect reduces the angular resolution at small radius, such that the time step $\mathrm{\Delta }u$ for an explicit numerical scheme is limited by the radial resolution $\mathrm{\Delta }x$, rather than $\mathrm{\Delta }x(\mathrm{\Delta }\theta {)}^2$. We present convergence tests in the weak-field regime, where we have exact solutions to the linearized scalar and gravitational-wave equations.

We formulate cosmological perturbation theory around the spatially curved FLRW background in the context of metric-affine gauge theory of gravity which includes torsion and nonmetricity. Performing scalar-vector-tensor decomposition of the spatial perturbations, we find that the theory displays a rich perturbation spectrum with helicities 0, 1, 2, and 3, on top of the usual scalar, vector, and tensor metric perturbations arising from Riemannian geometry. Accordingly, the theory provides a diverse phenomenology, e.g. the helicity-2 modes of the torsion and/or nonmetricity tensors source helicity-2 metric tensor perturbation at the linear level leading to the production of gravitational waves. As an immediate application, we study linear perturbation of the nonmetricity helicity-3 modes for a general parity-preserving action of metric-affine gravity which includes quadratic terms in curvature, torsion, and nonmetricity. We then find the conditions to avoid possible instabilities in the helicity-3 modes of the spin-3 field.

We investigate the parametrized black hole quasinormal ringdown formalism, which is a robust framework used to analyze quasinormal modes in systems that closely resemble general relativity, paying particular attention to the higher overtones. We find that larger deviations from the general relativity case typically appear in the quasinormal frequencies for the higher overtones. This growing tendency for higher overtones can be understood using an analytical method, and its relations to previous works are briefly discussed. Our findings indicate that we can impose a strong constraint on gravity theories by considering the overtones of quasinormal modes.

We consider higher-order derivative gauge field corrections that arise in the fundamental context of dimensional reduction of string theory and Lovelock-inspired gravities and obtain an exact and asymptotically flat black hole solution, in the presence of nontrivial dilaton configurations. Specifically, by considering the gravitational theory of Euler-Heisenberg nonlinear electrodynamics coupled to a dilaton field with specific coupling functions, we perform an extensive analysis of the characteristics of the black hole, including its geodesics for massive particles, the energy conditions, thermodynamical and stability analysis. The inclusion of a dilaton scalar potential in the action can also give rise to asymptotically (anti) de Sitter spacetimes and an effective cosmological constant. Moreover, we find that the black hole can be thermodynamically favored when compared to the Gibbons-Maeda-Garfinkle-Horowitz-Strominger black hole for those parameters of the model that lead to a larger black hole horizon for the same mass. Finally, it is observed that the energy conditions of the obtained black hole are indeed satisfied, further validating the robustness of the solution within the theoretical framework, but also implying that this self-gravitating dilaton-nonlinear-electrodynamics system constitutes another explicit example of bypassing modern versions of the no-hair theorem without any violation of the energy conditions.

We study hairy black hole solutions in Einstein(-Maxwell)-scalar-Gauss-Bonnet theory. The complex scalar coupling function includes quadratic and quartic terms, so the gravitational action has a U(1) symmetry. We argued that when the effective mass of the scalar field is at the critical value, the black holes without hairs transform into hairy black holes in a symmetry-broken vacuum via spontaneous symmetry breaking. These hairy black holes are stable under scalar perturbations, and the Goldstone bosons are trivial. Moreover, we found that the spontaneous symmetry breaking associated with local U(1) is unlikely to occur in this theory.

We present DE-VAE, a variational autoencoder (VAE) architecture to search for a compressed representation of dynamical dark energy (DE) models in observational studies of the cosmic large-scale structure. DE-VAE is trained on matter power spectra boosts generated at wave numbers $k\in (0.01–2.5)h/\mathrm{Mpc}$ and at four redshift values $z\in (0.1,0.48,0.78,1.5)$ for the most typical dynamical DE parametrization with two extra parameters describing an evolving DE equation of state. The boosts are compressed to a lower-dimensional representation, which is concatenated with standard cold dark matter (CDM) parameters and then mapped back to reconstructed boosts; both the compression and the reconstruction components are parametrized as neural networks. Remarkably, we find that a single latent parameter is sufficient to predict 95% (99%) of DE power spectra generated over a broad range of cosmological parameters within $1\sigma$ ($2\sigma$) of a Gaussian error which includes cosmic variance, shot noise, and systematic effects for a stage IV-like survey. This single parameter shows a high mutual information with the two DE parameters, and these three variables can be linked together with an explicit equation through symbolic regression. Considering a model with two latent variables only marginally improves the accuracy of the predictions, and adding a third latent variable has no significant impact on the model’s performance. We discuss how the DE-VAE architecture can be extended from a proof of concept to a general framework to be employed in the search for a common lower-dimensional parametrization of a wide range of beyond-$\mathrm{\Lambda }\mathrm{CDM}$ models and for different cosmological datasets. Such a framework could then both inform the development of cosmological surveys by targeting optimal probes, and provide theoretical insight into the common phenomenological aspects of beyond-$\mathrm{\Lambda }\mathrm{CDM}$ models.

We consider the potential of future microwave spectrometers akin to the Primordial Inflation Explorer (PIXIE) in light of the sky-averaged global signal expected from the total intensity of extragalactic carbon monoxide (CO) and ionized carbon ([C ii]) line emission. We start from models originally developed for forecasts of line-intensity mapping (LIM) observations targeting the same line emission at specific redshifts, extrapolating them across all of cosmic time. We then calculate Fisher forecasts for uncertainties on parameters describing relic spectral deviations, the CO/[C  ii] global signal, and a range of other Galactic and extragalactic foregrounds considered in previous work. We find that the measurement of the CO/[C  ii] global signal with a future CMB spectrometer presents an exciting opportunity to constrain the evolution of metallicity and molecular gas in galaxies across cosmic time. Using PIXIE and its enhanced version, SuperPIXIE, microwave spectrometers would have the fundamental sensitivity to constrain the redshift evolution of the average kinetic temperature and cosmic molecular gas density at levels of 10% and 1%, respectively. Taking a spectral distortion-centric perspective, when combined with other foregrounds, sky-averaged CO/[C ii] emission can mimic $\mu$- and, to a lesser extent, $y$-type distortions. Under fiducial parameters, marginalizing over the CO/[C ii] model parameters increases the error on $\mu$ by $\simeq 86\%$ and the error on $y$ by $\simeq 10\%$. Incorporating information from planned CO LIM surveys can recover some of this loss in precision. Future work should deploy a more general treatment of the microwave sky to quantify in more detail the potential synergies between PIXIE-like and CO LIM experiments, which complement each other strongly in breadth versus depth, and ways to optimize both spectrometer and LIM surveys to improve foreground cleaning and maximize the science return for each.

The bubble expansion velocity is an important parameter in the prediction of gravitational waves from first-order phase transitions. This parameter is difficult to compute, especially in phase transitions in strongly coupled theories. In this work, we present a method to estimate the wall velocity for phase transitions with a large enthalpy jump, valid for weakly and strongly coupled theories. We find that detonations are disfavored in this limit, but wall velocities are not necessarily small. We also investigate the effect of two other features in the equation of state: nonconformal sound speeds and a limited range of temperatures for which the phases exist. We find that the former can increase the wall velocity for a given nucleation temperature, and the latter can restrict the wall velocities to small values. To test our approach, we use holographic phase transitions, which typically display these three features. We find excellent agreement with numerically obtained values of the wall velocity. We also demonstrate that the implications for gravitational waves can be significant.