We consider pion condensation in QCD at finite isospin density ${\mu }_I$ and zero temperature using two-flavor chiral perturbation theory ($\chi \mathrm{PT}$). The pressure is calculated to next-to-leading order (NLO) in the low-energy expansion. In the nonrelativistic limit, we recover the classic result by Lee, Huang, and Yang for the energy density of a dilute Bose gas with an $s$-wave scattering length that includes loop corrections from $\chi \mathrm{PT}$. In the chiral limit, higher-order calculations are tractable. We calculate the pressure to next-to-next-to-leading order (NNLO) in the low-energy expansion, which is an expansion in powers of ${\mu }_I^2/(4\pi {)}^2{f}^2$, where $f$ is the (bare) pion decay constant. The spontaneous breakdown of the global internal symmetry $U(1{)}_{I_3}$ gives rise to a massless Goldstone boson or phonon. We discuss the properties of the low-energy effective theory describing this mode. Finally, we compare our results for the pressure and the speed of sound with recent lattice simulations with $2+1$ flavors. The agreement is very good for isospin chemical potentials up to 180–200 MeV, depending on the physical quantity.

We construct noncommutative theories with the Moyal-Weyl product in the double field theory (DFT) framework. We deform the infinitesimal generalized diffeomorphisms and the Leibniz rule in a consistent way. The prescription requires a generalized star metric, which can be thought of as the fundamental double metric, in order to construct the action. Finally we use the generalized scalar field dynamics and the generalized scalar field-perfect fluid correspondence to construct the generalized energy-momentum tensor of a perfect fluid in the noncommutative double geometry. The present formalism paves the way to the study of string cosmologies scenarios including the Moyal-Weyl product in a T-duality invariant way.

We present a fully relativistic model for localized probes in quantum field theory. Furthermore, we show that it is possible to obtain particle detector models from localized quantum field theories that interact with a free quantum field. In particular, a particle detector model is obtained when one traces out over inaccessible degrees of freedom of the localized field. This gives rise to a particle detector model, that is, a quantum degree of freedom that couples to a free field theory in an extended region of spacetime. Moreover, we show that the predictions of traditional particle detector models and fully relativistic localized fields completely coincide to leading order in perturbation theory.

A previous paper [Z. Bern et al., Binary dynamics through the fifth power of spin at $O(G^2)$, Phys. Rev. Lett. 130, 201402 (2023)] identified a puzzle stemming from the amplitudes-based approach to spinning bodies in general relativity: additional Wilson coefficients appear compared to current worldline approaches to conservative dynamics of generic astrophysical objects, including neutron stars. In this paper we clarify the nature of analogous Wilson coefficients in the simpler theory of electrodynamics. We analyze the original field-theory construction, identifying definite-spin states some of which have negative norms, and relating the additional Wilson coefficients in the classical theory to transitions between different quantum spin states. We produce a new version of the theory which also has additional Wilson coefficients, but no negative-norm states. We match, through $\mathcal{O}({\alpha }^2)$ and $\mathcal{O}(S^2)$, the Compton amplitudes of these field theories with those of a modified worldline theory with extra degrees of freedom introduced by releasing the spin supplementary condition. We build an effective two-body Hamiltonian that matches the impulse and spin kick of the modified field theory and of the worldline theory, displaying additional Wilson coefficients compared to standard worldline approaches. The results are then compactly expressed in terms of an eikonal formula. Our key conclusion is that, contrary to standard approaches, while the magnitude of the spin tensor is still conserved, the magnitude of the spin vector can change under conserved Hamiltonian dynamics and this change is governed by the additional Wilson coefficients. For specific values of Wilson coefficients the results are equivalent to those from a definite spin obeying the spin supplementary condition, but for generic values they are physically inequivalent. These results warrant detailed studies of the corresponding issues in general relativity.

We develop a method for accurately calculating vacuum decay rates beyond the thin-wall regime in pure scalar field theory at the one-loop level of the effective action. It accounts for radiative effects resulting from quantum corrections to the classical bounce, including gradient effects stemming from the inhomogeneity of the bounce background. To achieve this, it is necessary to compute not only the functional determinant of the fluctuation operator in the background of the classical bounce but also its functional derivative evaluated at the classical bounce. The former is efficiently calculated using the Gel’fand-Yaglom method. We illustrate how the latter can also be calculated with the same method, combined with a computation of various Green’s functions.

Simulations of binary collisions involving compact objects require initial data that satisfy the constraint equations of general relativity. For binary boson star simulations it is common practice to use a superposition of two isolated star solutions to construct an approximate solution to the constraint equations. Such superposed data is simple to set up compared to solving these equations explicitly, but also introduces extra constraint violations in the time evolution. In this work we investigate how physical observables depend on the quality of initial data in the case of head-on boson star collisions. In particular we compare results obtained from data prepared using four different methods: the standard method to superpose isolated stars, a heuristic improvement to this superposition technique and two versions of this data where excess constraint violations were removed through a conformal thin-sandwich solver. We find that differences in the time evolutions are dominated by differences in the way the two superposition methods differ, whereas additionally constraint solving the superposed data has smaller impact. The numerical experiments are conducted using the pseudospectral code bamps. Our work demonstrates that bamps is a code suited for generating high accuracy numerical waveforms for boson star collisions due to the exponential convergence in the polynomial resolution of the numerical approximation.

With recent advancements in observing supermassive black holes with the Event Horizon Telescope, there has been persistent exploration into what the images can reveal about fundamental physics, including space-time geometries and astrophysical emission sources. Inspired by Penrose’s aberration formula for a rigid sphere, which clarified that increased speed does not flatten the appearance of the sphere, we extend the studies to the behavior of the images of accretion emissions. This paper examines the impact of aberration effects on the images of a thin accretion disk around Kerr–de Sitter black holes for finite distant observers, specifically focusing on the primary, secondary, and $n=2$ images. We employ the analytical ray-tracing scenario and extend the astrometric approach to investigate the images in the presence of aberration. This study is nontrivial because we do not assume a specific form of the aberration formula; instead, all aberration effects emerge from a coordinate-independent and tetrad-independent framework referred to as the astrometric approach. Our study finds that the shapes of the lower-order images get highly distorted for finite observers in motion, and the shapes and sizes of primary images are more sensitive to aberration than those of the $n=2$ images. This finding suggests that the primary images could theoretically be distinguished from the shadow based on their distinctive variations.

Spacetime perturbations due to scalar, vector, and tensor fields on a fixed background geometry can be described in the framework of Teukolsky’s equation. In this work, wave scattering is treated analytically, using the Green’s function method and solutions to the separated radial and angular differential equations in combination with a partial wave technique for a scalar and monochromatic perturbation. The results are applied to analytically describe wave-optical imaging via Kirchhoff-Fresnel diffraction, leading to, e.g., the formation of observable black hole shadows. A comparison to the ray-optical description is given, providing new insights into wave-optical effects and properties. On a Kerr–de Sitter spacetime, the cosmological constant changes the singularity structure of the Teukolsky equation and allows for an analytical, exact solution via a transformation into Heun’s differential equation, which is the most general, second-order differential equation with four regular singularities. The scattering of waves originating from a point source involves a solution in terms of the so-called Heun’s function $Hf$. It is used to find angular solutions that form a complete set of orthonormal functions similar to the spherical harmonics. Our approach allows to solve the scattering problem while taking into account the complex interplay of Heun’s functions around local singularities.

In this work we obtain a static spherically symmetric charged black hole solution in the framework of minimal Horndeski gravity with additional Maxwell and Yang-Mills fields. The obtained solution is examined; in particular, its asymptotics are studied. Thermodynamics of the black hole is investigated, namely, we use an effective surface gravity to derive black hole temperature. To obtain the first law of black hole thermodynamics, the Wald method is applied. We also use the extended thermodynamics approach, namely, it allows us to derive the Smarr relation, Gibbs free energy, and the thermal equation of state. The study of thermal values in the extended space shows rich phase behavior, in particular, the domain where the first-order phase transition takes place and the critical point with the second-order phase transition. We also study thermal behavior near the critical point, obtain critical exponents, and analyze the Ehrenfest equations at the critical point. Finally, we calculate the Prigogine-Defay ratio confirming the conclusion about the second-order phase transition at the critical point.

The most general charged and rotating black hole in Kaluza-Klein theory is known to be described by the Rasheed-Larsen solution. When the under-rotating extremal limit of this solution is taken, it falls into a general class of solutions of Kaluza-Klein theory found by Clément, and is specified by two harmonic functions on a three-dimensional flat base space. We use this fact to generalize the single extremal black hole solution to one describing an arbitrary superposition of such black holes. These black holes carry nonzero electric and magnetic charges, which we set to be equal for simplicity, and are in general rotating with parallel or antiparallel spin vectors. It is checked that the space-time outside the black holes is free of pathologies such as naked singularities and closed time-like curves.

In this work, we examine the propagation of gravitational waves in cosmological and astrophysical spacetimes in the context of Einstein-Gauss-Bonnet gravity, in view of the GW170817 event. The perspective we approach the problem with is to obtain a theory which can produce a gravitational wave speed that is equal to that of light in the vacuum, or at least the speed can be compatible with the constraints imposed by the GW170817 event. As we show, in the context of Einstein-Gauss-Bonnet gravity, the propagation speed of gravity waves in cosmological spacetimes can be compatible with the GW170817 event, and we reconstruct some viable models. However, the propagation of gravity waves in spherically symmetric spacetimes violates the GW170817 constraints, thus it is impossible for the gravitational wave that propagates in a spherically symmetric spacetime to have a propagating speed which is equal to that of light in the vacuum. The same conclusion applies to the Einstein-Gauss-Bonnet theory with two scalars. We discuss the possible implications of our results on spherically symmetric spacetimes.

Massive Brans-Dicke (BD) theory is among the simplest general relativity extensions. It is commonly found as the weak-field limit of other gravitational theories. Here we do a detailed post-Newtonian analysis of massive BD theories. We start by expanding the massive BD field equations following the Will-Nordtvedt parametrized post-Newtonian (PPN) formalism, without point-particle approximations. A single potential that is not present in the standard PPN formalism is found. This new potential hinders immediate PPN conclusions. To proceed, we do a complete first-order post-Newtonian analysis and explicitly derive all the conserved quantities. After demanding that there exists a Newtonian limit by requiring the BD mass to be sufficiently large, we find, as expected, that $\gamma =1$; but there is no effective $\beta$ parameter that can have the same physical role of the standard $\beta$ in PPN formalism. All the others standard PPN parameters can be extended to the massive BD case without issues and are shown to have the same values of general relativity. At last, we consider numerical relations on the periastron advance and the BD mass in two different physical contexts, the orbit of Mercury about the Sun and the orbit of the star S2 about the expected supermassive black hole in the Milky Way.

We compare four loop quantum gravity inspired black hole metrics near the Planck scale. Spin 0, $1/2$, 1, and 2 field perturbations on these backgrounds are studied. The axial-gravitational quasinormal modes are calculated and compared. The time evolution of the ringdown is examined. We also calculate the quasinormal modes in the eikonal limit and compare with the predictions from circular null geodesics.

Boson stars arise as solutions of a massive complex scalar field coupled to gravity. A variety of scalar potentials, giving rise to different types of boson stars, have been studied in the literature. Here we study instead the effect of promoting the kinetic term of the scalar field to a nonlinear sigma model—an extension that is naturally motivated by UV completions of gravity like string theory. We consider the O(3) and $\mathrm{SL}(2,\mathbb{R})$ sigma models with minimally interacting potentials and obtain their boson star solutions. We study the maximum mass and compactness of the solutions as a function of the curvature of the sigma model and compare the results to the prototypical case of mini boson stars, which are recovered in the case of vanishing curvature. The effect of the curvature turns out to be dramatic. While O(3) stars are massive and can reach a size as compact as $R\sim 3.3GM$, $\mathrm{SL}(2,\mathbb{R})$ stars are much more diffuse and only astrophysically viable if the bosons are ultralight. These results show that the scalar kinetic term is at least as important as the potential in determining the properties of boson stars.

We investigate the time evolution of spherically symmetric boson stars in Palatini $f(\mathcal{R})$ gravity through numerical relativity computations. Employing a novel approach that establishes a correspondence between modified gravity with scalar matter and general relativity with modified scalar matter, we are able to use the techniques of numerical relativity to simulate these systems. Specifically, we focus on the quadratic theory $f(\mathcal{R})=\mathcal{R}+\xi {\mathcal{R}}^2$ and compare the obtained solutions with those in general relativity, exploring both positive and negative values of the coupling parameter $\xi$. Our findings reveal that boson stars in Palatini $f(\mathcal{R})$ gravity exhibit both stable and unstable evolutions. The latter give rise to three distinct scenarios: migration toward a stable configuration, complete dispersion, and gravitational collapse leading to the formation of a baby universe structure.

In this paper, we investigate the thermodynamic properties of a regular black hole model which exhibits the most significant subleading corrections to the Schwarzchild asymptotic behavior, in the context of general relativity, using the Euclidean path integral approach. We review the derivation of the Lagrangian for the matter fields which act as a source for this geometry, explicitly derive the proper thermodynamic quantities introduced in the first law of black hole mechanics, and show that they satisfy the Smarr formula. This analysis naturally leads to the emergence of an effective temperature that is distinct from the one associated with surface gravity. Furthermore, we study the phase structure in anti–de Sitter, Minkowski, and de Sitter spacetimes in the canonical ensemble, considering this effective temperature as the appropriate choice. We show that in this case the regularization of the singularity prevents the Hawking-Page transition and also leads to a deviation from the “universal” mean-field theory critical ratio. We conjecture that the way a singularity is rendered smooth plays a pivotal role to the degree of this deviation. Finally, we provide remarks on constraints imposed on the minimal length scale by observational data and the viability of regular black holes.

In this study we investigate the growth index ${\gamma }_L$, which characterizes the growth of linear matter perturbations, while analysing different cosmological datasets. We compare the approaches implemented by two different patches of the cosmological solver camb: mgcamb and camb_gammaprime_growth. In our analysis we uncover a deviation of the growth index from its expected $\mathrm{\Lambda }\mathrm{CDM}$ value of 0.55 when utilizing the Planck dataset, both in the mgcamb case and in the camb_gammaprime_growth case, but in opposite directions. This deviation is accompanied by a change in the direction of correlations with derived cosmological parameters. However, the incorporation of cosmic microwave background lensing data helps reconcile ${\gamma }_L$ with its $\mathrm{\Lambda }$-cold dark matter value in both cases. Conversely, the alternative ground-based telescopes Atacama Cosmology Telescope and South Pole Telescope consistently yield growth index values in agreement with ${\gamma }_L=0.55$. We conclude that the presence of the ${\mathrm{A}}_{\mathrm{lens}}$ problem in the Planck dataset contributes to the observed deviations, underscoring the importance of additional datasets in resolving these discrepancies.

We evaluate the Lorentzian gravitational path integral in the presence of nonvanishing torsion with the application of the Picard-Lefschetz theory for minisuperspaces corresponding to a number of phenomenological bouncing cosmological models as well as for the inflationary paradigm. It turns out that the semiclassical wave function derived from the saddle points of the path-integral formalism coincides with the solutions of the Wheeler-DeWitt equation. Intriguingly, our analysis showed that the relative probability, derived using these semiclassical wave functions favors universes with smaller values of torsion. Moreover, we find that in the inflationary case, nonzero values of a certain parity-violating component of the torsion enhance the power in the large-physical length scales, which can have important observational implications. On the other hand, in the case of bouncing models, the power spectrum is characterized by an initial region of growth, an intermediate oscillatory region, and then again a final region of growth. The shape of the power spectrum in the initial and intermediate regions is sensitive to the abundance of the bounce-enabling matter and torsion, along with the initial wave function of the universe, while the final size modifies the behavior of the power spectrum in the smaller length scales.

Motivated by the recent development in quantum cosmology, we revisit the anisotropic Kantowski-Sachs model in the light of a Lorentzian path-integral formalism. Studies so far have considered the Euclidean method where the choice of the lapse-integration contour is constrained by certain physical considerations rather than mathematical justification. In this paper, we have studied the Hartle-Hawking no-boundary proposal along with the use of Picard-Lefschetz theory in performing the lapse integration. In an isotropic limit, we show our results agree with the studies made in Friedmann-Lemaître-Robertson-Walker cosmology. We also observe that in the large-scale structure the no-boundary proposal tends towards a conical singularity at the beginning of time. We have also performed a massless scalar perturbation analysis with no backreaction. This reveals that if there were any perturbation present at the beginning of the Universe then that would flare up at the final boundary.

Recent upper bounds from the Hydrogen Epoch of Reionization Array on the cosmological 21-cm power spectrum at redshifts $z\approx 8$, 10, have been used to constrain $L_{\mathrm{X}<2\mathrm{keV}}/\mathrm{SFR}$, the soft-band x-ray luminosity measured per unit star formation rate (SFR), strongly disfavoring values lower than $\approx {10}^{39.5}\mathrm{erg}{\mathrm{s}}^{-1}{M}_{\odot }^{-1}\mathrm{yr}$. This conclusion is derived from seminumerical models of the 21-cm signal, specifically focusing on contributions from atomic cooling galaxies that host population II stars. In this work, we first reproduce the bounds on $L_{\mathrm{X}<2\mathrm{keV}}/\mathrm{SFR}$ and other parameters using a pipeline that combines machine learning emulators for the power spectra and the intergalactic medium characteristics, together with a standard Markov chain Monte Carlo parameter fit. We then use this approach when including molecular cooling galaxies that host population III stars in the cosmic dawn 21-cm signal, and show that lower values of $L_{\mathrm{X}<2\mathrm{keV}}/\mathrm{SFR}$ are hence no longer strongly disfavored. The revised Hydrogen Epoch of Reionization Array bound does not require high-redshift x-ray sources to be significantly more luminous than high-mass x-ray binaries observed at low redshift.