Working within the post-Minkowskian approach to general relativity, we prove that the radiation-reaction to the emission of gravitational waves during the large-impact-parameter scattering of two (classical) point masses modifies the conservative scattering angle by an additional contribution of order $G^3$ which involves a high-energy (or massless) logarithmic divergence of opposite sign to the one contained in the third-post-Minkowskian result of Bern et al. [Phys. Rev. Lett. 122, 201603 (2019)]. The high-energy limit of the resulting radiation-reaction-corrected (classical) scattering angle is finite, and is found to agree with the one following from the (quantum) eikonal-phase result of Amati et al. [Nucl. Phys. B347, 550 (1990)].

We continue the study of spherically symmetric black hole solutions in torsion bigravity, a class of Einstein-Cartan-type gravity theories involving, besides a metric, a massive propagating torsion field. In the infinite-range limit, these theories admit asymptotically flat black hole solutions related to the presence of attractive fixed points in the asymptotic radial evolution of the metric and the torsion. We discuss these fixed points and the way they are approached at large radii. Several phenomenological aspects of asymptotically flat torsion-hairy black holes are discussed: (i) location of the light ring and of the shadow; (ii) correction to the redshift of orbiting stars; and (iii) modification of the periastron precession of orbiting stars. By comparing the observable properties of torsion-hairy black holes to existing observational data on supermassive black holes obtained by the Event Horizon Telescope Collaboration and by the GRAVITY Collaboration, we derive constraints on the theory parameters of torsion bigravity. The strongest constraint is found to come from the recent measurement of the periastron precession of the star S2 orbiting the Galactic-center massive black hole [R. Abuter et al., Astron. Astrophys. 636, L5 (2020) and to be a thousand times more stringent than Solar-System gravitational tests.

We revisit the problem of the rotating generalization of the Fisher-Janis-Newman-Winicour solution of the minimal Einstein-scalar theory proving that previously known metrics do not satisfy the equations of motion. The same is shown for a putative rotating solution of the Brans-Dicke theory. We then use various generating techniques to derive correct spinning solutions with the scalar charge, in particular, those endowed with oblate deformations.

We present in this paper a 4-dimensional formulation of the Newton equations for gravitation on a Lorentzian manifold (hence distinct from the Newton-Cartan formalism), inspired from the $1+3$ and $3+1$ formalisms of general relativity. We begin by writing the Newton equations in a general time-parametrized coordinate system. We show that the freedom on the coordinate velocity of this system with respect to a Galilean reference system is similar to the shift freedom in the $3+1$-formalism of general relativity. This allows us to write Newton’s theory as living in a 4-dimensional Lorentzian manifold ${\mathcal{M}}^{\mathrm{N}}$. This manifold can be chosen to be curved depending on the dynamics of the Newtonian fluid. In this paper, we focus on a specific choice for ${\mathcal{M}}^{\mathrm{N}}$ leading to what we call the $1+3$-Newton equations. We show that these equations can be recovered from general relativity with a Newtonian limit performed in the rest frames of the relativistic fluid. The $1+3$ formulation of the Newton equations along with the Newtonian limit we introduce also allow us to define a dictionary between Newton’s theory and general relativity. This dictionary is defined in the rest frames of the dust fluid, i.e., a nonaccelerating observer. A consequence of this is that it is only defined for irrotational fluids. As an example supporting the $1+3$-Newton equations and our dictionary, we show that the parabolic free-fall solution in $1+3$-Newton exactly translates into the Schwarzschild spacetime, and this without any approximations. The dictionary might then be an additional tool to test the validity of Newtonian solutions with respect to general relativity. It however needs to be further tested for nonvacuum, nonstationary, and nonisolated Newtonian solutions, as well as to be adapted for rotational fluids. One of the main applications we consider for the $1+3$ formulation of Newton’s equations is to define new models suited for the study of backreaction and global topology in cosmology.

We propose a new test of strong-field general relativity (GR) based on the universal interferometric signature of the black hole photon ring. The photon ring is a narrow ring-shaped feature, predicted by GR but not yet observed, that appears on images of sources near a black hole. It is caused by extreme bending of light within a few Schwarzschild radii of the event horizon and provides a direct probe of the unstable bound photon orbits of the Kerr geometry. We show that the precise shape of the observable photon ring is remarkably insensitive to the astronomical source profile and can therefore be used as a stringent test of GR. We forecast that a tailored space-based interferometry experiment targeting M87* could test the Kerr nature of the source to the sub-subpercent level.

Motivated by the prospect of measuring a black hole photon ring, in previous work we explored the interferometric signature produced by a bright, narrow curve in the sky. Interferometric observations of such a curve measure its “projected position function” $\mathit{r}·\widehat{\mathit{n}}$, where $\mathit{r}$ parametrizes the curve and $\widehat{\mathit{n}}$ denotes its unit normal vector. In this paper, we show by explicit construction that a curve can be fully reconstructed from its projected position, completing the argument that space interferometry can in principle determine the detailed photon ring shape. In practice, near-term observations may be limited to the visibility amplitude alone, which contains incomplete shape information: for convex curves, the amplitude only encodes the set of projected diameters (or “widths”) of the shape. We explore the freedom in reconstructing a convex curve from its widths, giving insight into the shape information probed by technically plausible future astronomical measurements. Finally, we consider the Kerr “critical curve” in this framework and present some new results on its shape. We analytically show that the critical curve is an ellipse at small spin or inclination, while at extremal spin it becomes the convex hull of a Cartesian oval. We find a simple oval shape, the “phoval,” which reproduces the critical curve with high fidelity over the whole parameter range.

We apply the full theory of Loop Quantum Gravity (LQG) to cosmology and present a top-down derivation of gauge-invariant cosmological perturbation theory from quantum gravity. The derivation employs the reduced phase space formulation of LQG and the new discrete path integral formulation defined in M. Han and H. Liu, Phys. Rev. D 101, 046003 (2020). We demonstrate that in the semiclassical approximation and continuum limit, the result coincides with the existing formulation of gauge-invariant cosmological perturbation theory in, e.g., K. Giesel et al., Classical Quantum Gravity 27, 055006 (2010). Time evolution of cosmological perturbations is computed numerically from the new cosmological perturbation theory of LQG, and various power spectrums are studied for scalar mode and tensor mode perturbations. Comparing these power spectrums with predictions from the classical theory demonstrate corrections in the ultralong wavelength regime. These corrections are results from the lattice discretization in LQG. In addition, tensor mode perturbations at late time demonstrate the emergence of spin-2 gravitons as low energy excitations from LQG. The graviton has a modified dispersion relation and reduces to the standard graviton in the long wavelength limit.

We develop new high-order results for the post-Newtonian (PN) expansions of the energy and angular momentum fluxes at infinity for eccentric-orbit extreme-mass-ratio inspirals on a Schwarzschild background. The series are derived through direct expansion of the Mano-Suzuki-Takasugi solutions within the Regge-Wheeler-Zerilli formalism for first-order black hole perturbation theory (BHPT). By utilizing factorization and a few computational simplifications, we are able to compute the fluxes to 19PN, with each PN term calculated as a power series in (Darwin) eccentricity to $e^{10}$. This compares favorably with the numeric fitting approach used in previous work. We also compute PN terms to $e^{20}$ through 10PN. Then, we analyze the convergence properties of the composite energy flux expansion by checking against numeric data for several orbits, both for the full flux and also for the individual 220 mode, with various resummation schemes tried for each. The match between the high-order series and numerical calculations is generally strong, maintaining relative error better than ${10}^{-5}$ except when $p$ (the semilatus rectum) is small and $e$ is large. However, the full-flux expansion demonstrates superior fidelity (particularly at high $e$), as it is able to incorporate additional information from PN theory. For the orbit $(p=10,e=1/2)$, the full flux achieves a best fractional error near ${10}^{-5}$, while the 220 mode exhibits error worse than 1%. The full series can be accessed electronically on the black hole perturbation toolkit and are also given as Supplemental Material. Finally, we describe a procedure for transforming these expansions to the harmonic gauge of PN theory by analyzing Schwarzschild geodesic motion in harmonic coordinates. This will facilitate future comparisons between BHPT and PN theory.

We present a complete numerical treatment of inflationary dynamics under the influence of stochastic corrections from sub-Hubble modes. We discuss how to exactly model the stochastic noise terms arising from the sub-Hubble quantum modes that give rise to the coarse-grained inflaton dynamics in the form of stochastic differential equations. The stochastic differential equations are solved event by event on a discrete time grid. We then compute the power spectrum of curvature perturbations that can be compared with the power spectrum computed in the traditional fashion using the Mukhanov-Sasaki equation by canonically quantizing the inflaton fluctuations. Our numerical procedure helps us to easily extend the formalism to ultra slow-roll inflation and study the possibility of primordial black hole formation.

The reconstruction of a $k$-essence inflationary universe, considering the unification between the swampland criteria and the attractor given by the scalar spectral index $n_S(N)$ together with the slow-roll parameter $\epsilon (N)$ in terms of the number of $e$-folds $N$ is studied. In the context of a coupling of the form $L(\varphi )X$ in the $k$-essence model, we find the effective potential $V$ and the coupling parameter $L$ in terms of the scalar spectral index and the slow-roll parameter under a general formalism. To apply the unification in our model, we consider some examples in order to rebuild the effective potential $V(\varphi )$ and the coupling parameter $L(\varphi )$ as a function of the inflaton field $\varphi$. Here, we find that the reconstruction gives rise to an exponential potential and also to natural and hyperbolic inflation, respectively. Thus, in this article we show that it is possible to unify the theoretical foundations from the swampland criteria and the observational parameters corroborated by observations, in the reconstruction of an inflationary universe.

Studies of intrinsic alignment effects mostly focus on the correlations between the shapes of galaxies with each other or with the underlying density field of the large scale structure of the Universe. Lately, the correlation between the shapes of galaxies and the large-scale velocity field has been proposed as an additional probe of the large scale structure. We use a Fisher forecast to make a prediction for the detectability of this velocity-shape correlation with a combination of redshifts and shapes from the $4\mathrm{MOST}+\mathrm{LSST}$ surveys, and radial velocity reconstruction from the Simons Observatory. The signal-to-noise ratio for the velocity-shape (dipole) correlation is 23, relative to 44 for the galaxy density-shape (monopole) correlation and for a maximum wave number of $0.2\text{Mp}{\mathrm{c}}^{-1}$. Increasing the signal-to-noise ratio for higher values of the maximum wave numbers (respectively, 56 and 69, for a maximum wave number of $1\text{Mp}{\mathrm{c}}^{-1}$) indicate potential gains in the nonlinear regime. Encouraged by these predictions, we discuss two possible applications for the velocity-shape correlation. Measuring the velocity-shape correlation could improve the mitigation of selection effects induced by intrinsic alignments on galaxy clustering. We also find that velocity-shape measurements could potentially aid in determining the scale dependence of intrinsic alignments when multiple shape measurements of the same galaxies are provided.

Deep neural networks (DNNs) are powerful algorithms that have been proven capable of extracting non-Gaussian information from weak lensing (WL) datasets. Understanding which features in the data determine the output of these nested, nonlinear algorithms is an important but challenging task. We analyze a DNN that has been found in previous work to accurately recover cosmological parameters in simulated maps of the WL convergence ($\kappa$). We derive constraints on the cosmological parameter pair $({\mathrm{\Omega }}_m,{\sigma }_8)$ from a combination of three commonly used WL statistics (power spectrum, lensing peaks, and Minkowski functionals), using ray-traced simulated $\kappa$ maps. We show that the network can improve the inferred parameter constraints relative to this combination by 20% even in the presence of realistic levels of shape noise. We apply a series of well-established saliency methods to interpret the DNN and find that the most relevant pixels are those with extreme $\kappa$ values. For noiseless maps, regions with negative $\kappa$ account for 69%–89%. of the attribution of the DNN output, defined as the square of the saliency in input space. In the presence of shape noise, the attribution concentrates in high-convergence regions, with 36%–68% of the attribution in regions with $\kappa >3{\sigma }_{\kappa }$.

We consider an inflationary scenario in the holographic braneworld with a cosmological fluid occupying the $3+1$ dimensional brane located at the holographic boundary of an asymptotic ${\mathrm{ADS}}_5$ bulk. The contribution of the boundary conformal field can be represented as a modification of Einstein’s equations on the boundary. Using these effective Einstein equations we calculate the cosmological perturbations and derive the corresponding power spectra assuming a general $k$-essence type of inflaton. We find that the braneworld scenario affects the scalar power spectrum only in the speed of sound dependence on the slow-roll parameters whereas there is no change in the tensor power spectrum. This implies that the changes in the spectral indices appear at the second order in the slow-roll parameter expansion.

We extend the formalism of stochastic inflation to the setup of nonattractor inflation with a sound speed $c_s$. We obtain the Langevin equations for the superhorizon perturbations and calculate the stochastic corrections to curvature perturbation power spectrum. It is shown that the fractional stochastic corrections in mean number of e-folds and power spectrum are at the order of power spectrum. We also calculate the boundary crossing and the first hitting probabilities in a hypothetical dS space with two boundaries in field space. Furthermore, the stochastic corrections in power spectrum in a setup akin to eternal inflation with large diffusion term are calculated.

There is a variety of cosmological models for dark matter and dark energy in which a possible interaction is considered between these two significant components of the Universe. We focus on five suggested models of interacting dark matter and dark energy and derive the modified virial theorem for them by developing a previous approach. It provides an opportunity to study the evolution of this modified virial theorem with time and interacting constants for different interacting models. Then, we use this obtained virial condition to investigate the modified mass-temperature relation in galaxy clusters via three various methods. It reveals that the effect of interaction between dark matter and dark energy merely appears in the normalization factor of $M\propto T^{\frac{3}{2}}$. This relation also leads to a new constraint on the constants of interacting models, which only depends on the concentration parameter and density profile of the cluster. Then, we use five observational datasets to check some proposed figures for the constants of interaction which have resulted from other observational constraints. Finally, by fitting the observational results to the modified mass-temperature relation, we obtain values for interacting constants of three models and four specific cases of the two remaining models. In agreement with many other observational outcomes, we find that, according to observational data for masses and temperatures of the galaxy clusters, energy transfer occurs from dark matter to dark energy in the seven investigated models.

The emergence of an increasingly strong tension between the Hubble rate inferred from early- and late-time observations has reinvigorated interest in nonstandard scenarios, with the aim of reconciling these measurements. One such model involves interactions between dark matter and dark energy. Here we consider a specific form of the coupling between these two fluids proportional to the dark energy density, which has been studied extensively in the literature and claimed to substantially alleviate the Hubble tension. We complement the work already discussed in several previous analyses and show that, once all relevant cosmological probes are included simultaneously, the value of the Hubble parameter in this model is $H_0=69.82_{-0.76}^{+0.63}\mathrm{km}/(\mathrm{s}\mathrm{Mpc})$, which reduces the Hubble tension to $2.5\sigma$. Furthermore, we also perform a statistical model comparison, finding a $\mathrm{\Delta }{\chi }^2$ of $-2.15$ (corresponding to a significance of $1.5\sigma$) with the inclusion of one additional free parameter, showing no clear preference for this model with respect to $\mathrm{\Lambda }\mathrm{CDM}$, which is further confirmed with an analysis of the Bayes ratio.

Two inhomogeneous single-fluid models for the Universe, which are able to naturally solve the $H_0$ tension problem, are discussed. The analysis is based on a Bayesian Machine Learning approach that uses a generative process. The method here adopted allows for constraint of the free parameters of each model by making use of the model itself only. The observable is taken to be the Hubble parameter, obtained from the generative process. Using the full advantages of our method, the models are constrained for two redshift ranges. Namely, first this is done with mock $H(z)$ data over $z\in [0,2.5]$, thus covering known $H(z)$ observational data, which are most helpful to validate the fit results. Then, aiming to extend to redshift ranges to be covered by the most recent ongoing and future planned missions, the models are constrained for the range $z\in [0,5]$, too. Full validation of the results for this extended redshift range will have to wait for some years, for when higher-redshift $H(z)$ data become available. This makes our models fully falsifiable. In addition, our second model is able to explain the BOSS reported value for $H(z)$ at $z=2.34$.

We propose that the nearly massless dark photons produced from the annihilation of keV dark fermions in the Galaxy can induce the excess of electron recoil events recently observed in the XENON1T experiment. The minimal model for this is the extension of a $U(1{)}_X$ gauge symmetry, under which the dark photon couples to both dark and visible matter currents. We find that the best-fit parameters of the dark sector are compatible with the most stringent constraints from stellar cooling. We also show that in the freeze-out scenario, the dark fermions can explain the anomaly while contributing $\gtrsim 1\%$ of the DM relic density.

The cumulative emission of axionlike particles (ALPs) from all past core-collapse supernovae (SNe) would lead to a diffuse flux with energies $\mathcal{O}(50)\mathrm{MeV}$. We use this to constrain ALPs featuring couplings to photons and to nucleons. ALPs coupled only to photons are produced in the SN core via the Primakoff process and then converted into gamma rays in the Galactic magnetic field. We set a bound on $g_{a\gamma }\lesssim 5\times {10}^{-10}{\mathrm{GeV}}^{-1}$ for $m_a\lesssim {10}^{-11}\mathrm{eV}$, using recent measurements of the diffuse gamma-ray flux observed by the Fermi-LAT telescope. However, if ALPs couple also with nucleons, their production rate in SN can be considerably enhanced due to the ALPs nucleon-nucleon bremsstrahlung process. Assuming the largest ALP-nucleon coupling phenomenologically allowed, bounds on the diffuse gamma-ray flux lead to a much stronger $g_{a\gamma }\lesssim 6\times {10}^{-13}{\mathrm{GeV}}^{-1}$ for the same mass range. If ALPs are heavier than $\sim \mathrm{keV}$, the decay into photons becomes significant, leading again to a diffuse gamma-ray flux. In the case of only photon coupling, we find, e.g., $g_{a\gamma }\lesssim 5\times {10}^{-11}{\mathrm{GeV}}^{-1}$ for $m_a\sim 5\mathrm{keV}$. Allowing for a (maximal) coupling to nucleons, the limit improves to the level of $g_{a\gamma }\lesssim {10}^{-19}{\mathrm{GeV}}^{-1}$ for $m_a\sim 20\mathrm{MeV}$, which represents the strongest constraint to date.

The contribution to the cosmic opacity of ultra-high-energy (UHE) ($E>{10}^{18}\mathrm{eV}$) $\gamma$ rays has been computed so far for the $\gamma \gamma \to e^{+}{e}^{-}$ and $\gamma \gamma \to e^{+}{e}^{-}{e}^{+}{e}^{-}$ processes alone. We go a step further by systematically evaluating the additional opacity brought about by other leptons and hadrons. We find that the new dominant channels are those leading to the production of ${\mu }^{\pm }$ and hadrons (mainly ${\pi }^{\pm }$, ${\pi }^0$, $K^{\pm }$, $K^0$, $\eta$). Our result can be expressed in terms of the probability distribution for the interaction lengths. For a few interaction lengths such a probability gets reduced by up to 20%–30% with respect to current estimates.