Classical and Quantum Gravity

Interstellar is the first Hollywood movie to attempt depicting a black hole as it would actually be seen by somebody nearby. For this, our team at Double Negative Visual Effects, in collaboration with physicist Kip Thorne, developed a code called Double Negative Gravitational Renderer (DNGR) to solve the equations for ray-bundle (light-beam) propagation through the curved spacetime of a spinning (Kerr) black hole, and to render IMAX-quality, rapidly changing images. Our ray-bundle techniques were crucial for achieving IMAX-quality smoothness without flickering; and they differ from physicists’ image-generation techniques (which generally rely on individual light rays rather than ray bundles), and also differ from techniques previously used in the film industry’s CGI community. This paper has four purposes: (i) to describe DNGR for physicists and CGI practitioners, who may find interesting and useful some of our unconventional techniques. (ii) To present the equations we use, when the camera is in arbitrary motion at an arbitrary location near a Kerr black hole, for mapping light sources to camera images via elliptical ray bundles. (iii) To describe new insights, from DNGR, into gravitational lensing when the camera is near the spinning black hole, rather than far away as in almost all prior studies; we focus on the shapes, sizes and influence of caustics and critical curves, the creation and annihilation of stellar images, the pattern of multiple images, and the influence of almost-trapped light rays, and we find similar results to the more familiar case of a camera far from the hole. (iv) To describe how the images of the black hole Gargantua and its accretion disk, in the movie Interstellar, were generated with DNGR—including, especially, the influences of (a) colour changes due to doppler and gravitational frequency shifts, (b) intensity changes due to the frequency shifts, (c) simulated camera lens flare, and (d) decisions that the film makers made about these influences and about the Gargantua’s spin, with the goal of producing images understandable for a mass audience. There are no new astrophysical insights in this accretion-disk section of the paper, but disk novices may find it pedagogically interesting, and movie buffs may find its discussions of Interstellar interesting.

We propose a gravitational wave (GW) detector based on ultrastable optical cavities enabling the detection of GW signals in the mostly unexplored 10ドル^{-5}-1$ Hz frequency band. We illustrate the working principle of the detector and discuss that several classes of GW sources, both of astrophysical and cosmological origin, may be within the detection range of this instrument. Our work suggests that terrestrial GW detection in the milli-Hz frequency range is potentially within reach with current technology.

We study the internal dynamics of a hypothetical spaceship traveling on a close timelike curve in an axially symmetric Universe. We choose the curve so that the generator of evolution in proper time is the angular momentum. Using Wigner’s theorem, we prove that the energy levels internal to the spaceship must undergo spontaneous discretization. The level separation turns out to be finely tuned so that, after completing a roundtrip of the curve, all systems are back to their initial state. This implies, for example, that the memories of an observer inside the spaceship are necessarily erased by the end of the journey. More in general, if there is an increase in entropy, a Poincaré cycle will eventually reverse it by the end of the loop, forcing entropy to decrease back to its initial value. We show that such decrease in entropy is in agreement with the eigenstate thermalization hypothesis. The non-existence of time-travel paradoxes follows as a rigorous corollary of our analysis.

The simplest ΛCDM model provides a good fit to a large span of cosmological data but harbors large areas of phenomenology and ignorance. With the improvement of the number and the accuracy of observations, discrepancies among key cosmological parameters of the model have emerged. The most statistically significant tension is the 4σ to 6σ disagreement between predictions of the Hubble constant, H₀, made by the early time probes in concert with the ‘vanilla’ ΛCDM cosmological model, and a number of late time, model-independent determinations of H₀ from local measurements of distances and redshifts. The high precision and consistency of the data at both ends present strong challenges to the possible solution space and demands a hypothesis with enough rigor to explain multiple observations—whether these invoke new physics, unexpected large-scale structures or multiple, unrelated errors. A thorough review of the problem including a discussion of recent Hubble constant estimates and a summary of the proposed theoretical solutions is presented here. We include more than 1000 references, indicating that the interest in this area has grown considerably just during the last few years. We classify the many proposals to resolve the tension in these categories: early dark energy, late dark energy, dark energy models with 6 degrees of freedom and their extensions, models with extra relativistic degrees of freedom, models with extra interactions, unified cosmologies, modified gravity, inflationary models, modified recombination history, physics of the critical phenomena, and alternative proposals. Some are formally successful, improving the fit to the data in light of their additional degrees of freedom, restoring agreement within 1–2σ between Planck 2018, using the cosmic microwave background power spectra data, baryon acoustic oscillations, Pantheon SN data, and R20, the latest SH0ES Team Riess, et al (2021 Astrophys. J.908 L6) measurement of the Hubble constant (H₀ = 73.2 ± 1.3 km s⁻¹ Mpc⁻¹ at 68% confidence level). However, there are many more unsuccessful models which leave the discrepancy well above the 3σ disagreement level. In many cases, reduced tension comes not simply from a change in the value of H₀ but also due to an increase in its uncertainty due to degeneracy with additional physics, complicating the picture and pointing to the need for additional probes. While no specific proposal makes a strong case for being highly likely or far better than all others, solutions involving early or dynamical dark energy, neutrino interactions, interacting cosmologies, primordial magnetic fields, and modified gravity provide the best options until a better alternative comes along.

The theory of general relativity predicts the existence of closed time-like curves (CTCs), which theoretically would allow an observer to travel back in time and interact with their past self. This raises the question of whether this could create a grandfather paradox, in which the observer interacts in such a way to prevent their own time travel. Previous research has proposed a framework for deterministic, reversible, dynamics compatible with non-trivial time travel, where observers in distinct regions of spacetime can perform arbitrary local operations with no contradiction arising. However, only scenarios with up to three regions have been fully characterised, revealing only one type of process where the observers can verify to both be in the past and future of each other. Here we extend this characterisation to an arbitrary number of regions and find that there exist several inequivalent processes that can only arise due to non-trivial time travel. This supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.

The LIGO Scientific Collaboration and the Virgo Collaboration have cataloged eleven confidently detected gravitational-wave events during the first two observing runs of the advanced detector era. All eleven events were consistent with being from well-modeled mergers between compact stellar-mass objects: black holes or neutron stars. The data around the time of each of these events have been made publicly available through the gravitational-wave open science center. The entirety of the gravitational-wave strain data from the first and second observing runs have also now been made publicly available. There is considerable interest among the broad scientific community in understanding the data and methods used in the analyses. In this paper, we provide an overview of the detector noise properties and the data analysis techniques used to detect gravitational-wave signals and infer the source properties. We describe some of the checks that are performed to validate the analyses and results from the observations of gravitational-wave events. We also address concerns that have been raised about various properties of LIGO–Virgo detector noise and the correctness of our analyses as applied to the resulting data.

Teleparallel gravity and its popular generalization gravity can be formulated as fully invariant (under both coordinate transformations and local Lorentz transformations) theories of gravity. Several misconceptions about teleparallel gravity and its generalizations can be found in the literature, especially regarding their local Lorentz invariance. We describe how these misunderstandings may have arisen and attempt to clarify the situation. In particular, the central point of confusion in the literature appears to be related to the inertial spin connection in teleparallel gravity models. While inertial spin connections are commonplace in special relativity, and not something inherent to teleparallel gravity, the role of the inertial spin connection in removing the spurious inertial effects within a given frame of reference is emphasized here. The careful consideration of the inertial spin connection leads to the construction of a fully invariant theory of teleparallel gravity and its generalizations. Indeed, it is the nature of the spin connection that differentiates the relationship between what have been called good tetrads and bad tetrads and clearly shows that, in principle, any tetrad can be utilized. The field equations for the fully invariant formulation of teleparallel gravity and its generalizations are presented and a number of examples using different assumptions on the frame and spin connection are displayed to illustrate the covariant procedure. Various modified teleparallel gravity models are also briefly reviewed.

We present a major update to the Simulating eXtreme Spacetimes (SXSs) Collaboration’s catalog of binary black hole (BBH) simulations. Using highly efficient spectral methods implemented in the Spectral Einstein Code (SpEC), we have nearly doubled the total number of binary configurations from 2018 to 3756. The catalog now more densely covers the parameter space with precessing simulations up to mass ratio q = 8 and dimensionless spins up to $|\vec{\chi}|\unicode{x2A7D}0.8$ with near-zero eccentricity. The catalog also includes some simulations at higher mass ratios with moderate spin and more than 250 eccentric simulations. We have also deprecated and rerun some simulations from our previous catalog (e.g. simulations run with a much older version of SpEC or that had anomalously high errors in the waveform). The median waveform difference (which is similar to the mismatch) between resolutions over the simulations in the catalog is 4ドル\times10^{-4}$. The simulations have a median of 22 orbits, while the longest simulation has 148 orbits. We have corrected each waveform in the catalog to be in the binary’s center-of-mass frame and exhibit gravitational-wave memory. We estimate the total CPU cost of all simulations in the catalog to be 480 000 000 core-hours. We find that using spectral methods for BBH simulations is over 1000 times more efficient than previously published finite-difference simulations. The full catalog is publicly available through the sxs Python package and at https://data.black-holes.org .

As a crucial step towards the completion of the fourth post-Newtonian (4PN) gravitational-wave generation from compact binary systems, we obtain the expressions of the so-called ‘canonical’ multipole moments of the source in terms of the ‘source’ and ‘gauge’ moments. The canonical moments describe the propagation of gravitational waves outside the source’s near zone, while the source and gauge moments encode explicit information about the matter source. Those two descriptions, in terms of two sets of canonical moments or in terms of six sets of source and gauge moments, are isometric. We thus construct the non-linear diffeomorphism between them up to the third post-Minkowskian order, and we exhibit the concrete expression of the canonical mass-type quadrupole moment at the 4PN order. This computation is one of the last missing pieces for the determination of the gravitational-wave phasing of compact binary systems at 4PN order.

The Advanced LIGO gravitational wave detectors are second-generation instruments designed and built for the two LIGO observatories in Hanford, WA and Livingston, LA, USA. The two instruments are identical in design, and are specialized versions of a Michelson interferometer with 4 km long arms. As in Initial LIGO, Fabry–Perot cavities are used in the arms to increase the interaction time with a gravitational wave, and power recycling is used to increase the effective laser power. Signal recycling has been added in Advanced LIGO to improve the frequency response. In the most sensitive frequency region around 100 Hz, the design strain sensitivity is a factor of 10 better than Initial LIGO. In addition, the low frequency end of the sensitivity band is moved from 40 Hz down to 10 Hz. All interferometer components have been replaced with improved technologies to achieve this sensitivity gain. Much better seismic isolation and test mass suspensions are responsible for the gains at lower frequencies. Higher laser power, larger test masses and improved mirror coatings lead to the improved sensitivity at mid and high frequencies. Data collecting runs with these new instruments are planned to begin in mid-2015.

Utilizing the covariant formulation of Penrose’s plane wave limit by Blau et al, we construct for any semi-Riemannian metric g a family of ‘plane wave limits.’ These limits are taken along any geodesic of g, yield simpler metrics of Lorentzian signature, and are isometric invariants. We show that they generalize Penrose’s limit to the semi-Riemannian regime and, in certain cases, encode g’s tensorial geometry and its geodesic deviation. As an application of the latter, we partially extend a well known result by Hawking & Penrose to the semi-Riemannian regime: On any semi-Riemannian manifold, if the Ricci curvature is nonnegative along any complete geodesic without conjugate points that is ‘causally independent’ (in a sense we make precise), then the curvature tensor along that geodesic must vanish in all normal directions. A Morse Index Theorem is also proved for such geodesics.

One striking feature of binary black hole (BBH) mergers observed in the first decade of gravitational-wave astronomy is an excess of events with component masses around 35ドル,円\mathrm{M}_{\odot}$. Multiple formation channels have been proposed to explain this excess. To distinguish among these channels, it is essential to examine their predicted population-level distributions across additional parameters. In this work, we focus on BBH mergers near the 35ドル,円\mathrm{M}_{\odot}$ peak and infer the population distributions of primary mass (m₁), mass ratio (q), effective spin ($,円\chi_{\mathrm{eff}}$), and redshift (z). We observe a gradual increase in the merger rate with m₁, rising by a factor of 3 from 20ドル,円\mathrm{M}_{\odot}$ to a peak around 34ドル,円\mathrm{M}_{\odot}$, followed by a sharp, order-of-magnitude decline by 50ドル,円\mathrm{M}_{\odot}$. This population also shows a weak preference for equal-mass mergers and has a $,円\chi_{\mathrm{eff}}$ distribution skewed toward positive values, with a median of zero excluded at approximately 90% confidence. We find no significant $q-,円\chi_{\mathrm{eff}}$ correlation in the 35ドル ,円\mathrm{M}_{\odot}$ peak population, suggesting that lower-mass systems ($m_1 \lt 20,円\mathrm{M}_{\odot}$) likely drive the $q-,円\chi_{\mathrm{eff}}$ anti-correlation observed in the full BBH merger catalog. The redshift evolution of the merger rate is consistent with the cosmic star formation rate. We compare our findings with predictions from a wide range of formation channels. We find that common variants of the pair-instability supernova scenario, as well as hierarchical mergers in absence of sufficient gas-accretion, are incompatible with the observed features of the 35ドル,円\mathrm{M}_{\odot}$ population. Ultimately, none of the formation channels we consider can explain all or even most of the features observed in this population. The ‘mid-thirties’ of black hole mergers are in crisis.

We combine the well-known Beltrami–Klein model of non-Euclidean geometry on a two-dimensional disk, where the geodesics are the chords of the disk, with the two—dimensional de Sitter space. The geometry of the de Sitter space is defined on the complement of the Beltrami–Klein disk in the plane, with the de Sitter metric being the unique Lorentzian Einstein metric whose light cones form cones tangent to the disk in this complement. This leads to a Beltrami–de Sitter model on the plane $\mathbb{R}^2$, which is endowed with the Riemannian Beltrami metric on the disk and the Lorentzian de Sitter metric outside the disk in $\mathbb{R}^2$. We explore the relevance of this model for Penrose’s conformal cyclic cosmology, first in the two-dimensional setting and subsequently in higher dimensions, including the physically significant case of four dimensions. In this context, we define a Radon-like transform between the de Sitter and Beltrami spaces, facilitating the purely geometric transformation of physical fields from the Lorentzian de Sitter space to the Riemannian Beltrami space. In the two –, and three-dimensional cases, we also uncover a hidden G₂ symmetry associated with the de Sitter spaces in these dimensions, which is related to a certain vector distribution naturally defined by the geometry of the model. We suggest the potential for discovering similar hidden symmetries in the n-dimensional Beltrami–de Sitter model.

We propose a minimal, fully thermal mechanism that resolves the long-standing tension between achieving the observed dark-matter relic abundance and explaining the astrophysical signatures of self-interactions. The framework introduces two mediators: a light scalar φ (MeV scale) that yields the required, velocity-dependent self-interactions, and a heavy scalar resonance $\Phi_h$ (TeV scale) with mass $m_{\Phi_h}\!\approx\!2m_\chi$ that opens an s-channel resonant annihilation during freeze-out. This clearly decouples early-Universe annihilation from late-time halo dynamics. A detailed numerical analysis identified a narrow predictive island of viability. A representative benchmark with $m_\chi\! = \!600$ GeV, $m_\phi\! = \!15$ MeV, and $m_{\Phi_h}\!\simeq\!1.2$ TeV reproduces the relic density and yields $\sigma_T/m_\chi\sim 0.1$–1ドル~\mathrm{cm}^2\!,円\mathrm{g}^{-1}$ at dwarf-galaxy velocities while satisfying cluster bounds. The model makes sharp, testable predictions: a narrow $t\bar t$ resonance near 1.2 TeV within HL-LHC reach, and a spin-independent direct-detection signal $\sigma_\textrm{SI}\!\sim\!7\times10^{-48},円\mathrm{cm}^2$ within next-generation sensitivity. As an optional UV completion, we show that walking $\mathrm{SU}(3)_H$ gauge theory with $N_f = 10$ naturally realizes the near-threshold relation $m_{\Phi_h}\!\approx\!2m_\chi$ and can furnish an effective anomalous dimension $\gamma\!\approx\!0.5$ which underlies a density-responsive dark-energy sector, suggesting a unified origin for the dark sector.

Gravitational optical properties are here investigated under the hypothesis of spherically-symmetric spacetimes behaving as media. To do so, we first consider two different definitions of the refractive index, n, of a spacetime medium and show how to pass from one definition to another by means of a coordinate transformation. Accordingly, the corresponding physical role of n is discussed by virtue of the Misner–Sharp mass and the redshift definition. Afterwards, we discuss the inclusion of the electromagnetic fields and the equivalence with nonlinear effects induced by geometry. Accordingly, the infrared and ultraviolet gravity regimes are thus discussed, obtaining bounds from the Solar System, neutron stars and white dwarfs, respectively. To do so, we also investigate the Snell’s law and propose how to possibly distinguish regular solutions from black holes. As a consequence of our recipe, we speculate on the existence of gravitational metamaterials with negative refraction and explore the corresponding physical implications, remarking that n < 0 may lead to invisible optical properties, as light is bent in the opposite direction compared to what occurs in ordinary cases. Further, we conjecture that those spacetimes that exhibit negative refraction can have particle-like behavior, contributing to dark matter and propose three toy models, highlighting possible advantages and limitations of their use. Finally, we suggest that such particle-like configurations can be ‘dressed’ by interaction, giving rise to geometric quasiparticles. We thus construct modifications of the quantum propagator as due to nonminimal couplings between curvature and external matter-like fields, finding the corresponding effective mass through a boson mixing mechanism.

TianQin is a future space-based gravitational wave (GW) observatory targeting the frequency window of 10⁻⁴–1 Hz. A large variety of GW sources are expected in this frequency band, including the merger of massive black hole binaries, the inspiral of extreme/intermediate mass ratio systems, stellar-mass black hole binaries, Galactic compact binaries, and so on. TianQin will consist of three Earth orbiting satellites on nearly identical orbits with orbital radii of about 10⁵ km. The satellites will form a normal triangle constellation whose plane is nearly perpendicular to the ecliptic plane. The TianQin project has been progressing smoothly following the ‘0123’ technology roadmap. In step ‘0’, the TianQin laser ranging station has been constructed and it has successfully ranged to all the five retro-reflectors on the Moon. In step ‘1’, the drag-free control technology has been tested and demonstrated using the TianQin-1 satellite. In step ‘2’, the inter-satellite laser interferometry technology will be tested using the pair of TianQin-2 satellites. The TianQin-2 mission has been officially approved and the satellites will be launched around 2026. In step ‘3’, i.e. the TianQin-3 mission, three identical satellites will be launched around 2035 to form the space-based GW detector, TianQin, and to start GW detection in space.

If a set of massive objects collide in space and the fragments disperse, possibly forming black holes, then this process will emit gravitational waves. Computing the detailed gravitational wave-form associated with this process is a complicated problem, not only due to the non-linearity of gravity but also due to the fact that during the collision and subsequent fragmentation the objects could undergo complicated non-gravitational interactions. Nevertheless the classical soft graviton theorem determines the power law fall-off of the wave-form at late and early times, including logarithmic corrections, in terms of only the momenta of the incoming and outgoing objects without any reference to what transpired during the collision. In this short review I shall explain the results and very briefly outline the derivation of these results.

Differences in the values of the Hubble constant obtained from the local Universe and the early Universe have resulted in a significant tension. This tension signifies that our understanding of cosmology (physical processes and/or cosmological data) is incomplete. Some of the suggested solutions include physics of the early Universe. In this paper we aim to investigate common features of various early-Universe solutions to the Hubble constant tension. The physics of the early Universe affects the size of the sound horizon which is probed with the Cosmic Microwave Background data. Within the standard model, the size of the horizon (within limits of current measurements) is affected by processes that could occur between (approximately) 1 d after the Big Bang and the last scattering instant. We focus on simple extensions incorporating Early Dark Energy (EDE) and show how such a model affects the inferred values of the Hubble constant. We compare this model to ΛCDM models using MCMC analysis, likelihoods over the parameter space and Bayesian evidence. The MCMC analysis shows that EDE leads to a decrease in the size of the sound horizon that is consistent with $H_0 = 73.56$ km s⁻¹ Mpc⁻¹ but we also show that MCMC analysis favours increasing redshift and proportion of EDE. The Bayesian evidence favours our EDE model for very narrow, finely-tuned parameter space. The ΛCDM model used for comparison has good evidence across a wide parameter space. We interpret this as an indication that more sophisticated models are required. We conclude that if the Hubble tension were to be related to the physics of the early Universe, EDE could be used as a window to explore conditions of the early Universe and extend our understanding of that era.

Supermassive binary black holes (SMBBHs) are natural products of the hierarchical mergers of galaxies with central black holes in the Λ cold dark matter cosmogony. We briefly introduce the formation and evolution processes of SMBBHs and population synthesis modeling of SMBBHs across cosmic time. Both the semi-analytical analysis and numerical simulations suggest that close SMBBHs are abundant in the Universe, with rich electromagnetic signatures and enormous gravitational wave radiation. However, observational evidence for their existence is still uncertain. We summarize the current status of the electromagnetic searches and observations of these binaries, focusing on their morphological signatures, continuum spectra, line properties, and periodic variations modulated by their orbital motions. We review pulsar timing array observations of nanohertz gravitational waves from these SMBBHs, including from gravitational wave signals from individual SMBBH sources and the stochastic background from the whole population of SMBBHs. Finally, we discuss the prospects of multimessenger studies for SMBBHs.

Group field theory is a background-independent approach to quantum gravity whose starting point is the definition of a quantum field theory on an auxiliary group manifold (not interpreted as spacetime, but rather as the finite-dimensional configuration space of a single ‘atom’ of geometry). Group field theory models can be seen as an extension of matrix and tensor models by additional data, and are traditionally defined through a functional integral whose perturbative expansion generates a sum over discrete geometries. More recently, some efforts have been directed towards formulations of group field theory based on a Hilbert space and operators, in particular in applications to cosmology. This is an attempt to review some of these formulations and their main ideas, to disentangle these constructions as much as possible from applications and phenomenology, and to put them into a wider context of quantum gravity research.

In the context of the general effort to model black hole dynamics, and in particular their return-to-equilibrium through quasi-normal modes, it is crucial to understand how much test-field perturbations deviate from physical perturbations in modified gravity scenarios. On the one hand, physical perturbations follow the modified Einstein equations of the considered extension of general relativity. The complexity of those equations can quickly escalate with extra fields and non-linear couplings. On the other hand, test-field perturbations, with negligible back-reaction on the space-time geometry, describe the propagation of both matter fields and spin s=2 gravitational waves on the black hole geometry. They are not subject to the intricacies of the modified Einstein equations, and only probe the background spacetime metric. If their physics were to not deviate significantly from physical perturbations, they would be especially useful to investigate predictions from quantum gravity scenarios which lack explicit detailed Einstein equations.&#xD;Here we focus on a specific modified gravity solution -- Babichev-Charmousis-Leh\'ebel (BCL) black holes in scalar-tensor theories -- for which physical perturbations and related QNM frequencies have already been studied and computed numerically. We compute the test-field QNM frequencies and compare the two QNM spectra. This provides a concrete example of the significant differences arising between test-fields and physical perturbations, and flags unphysical deviations related to the test-field framework.

We present exact solutions describing a fake Schwarzschild black hole that cannot be distinguished from the Schwarzschild black hole by observations. They are constructed by attaching a spherically symmetric dynamical interior solution with a matter field to the Schwarzschild exterior solution at the event horizon without a lightlike thin shell. The dynamical region inside a Killing horizon of a static spherically symmetric perfect-fluid solution obeying an equation of state p = χρ for χ ∈ [−1/3, 0) can be the interior of a fake Schwarzschild black hole. The matter field inside such a black hole is an anisotropic fluid that violates at least the weak energy condition and can be interpreted as a spacelike (tachyonic) perfect fluid. While the author constructed the first model of fake Schwarzschild black holes using Semiz’s solution for χ = −1/5, we present another one using Whittaker’s solution for χ = −1/3 in this paper. We also present a model of fake Kerr black holes whose interior is filled with a different matter field violating only the dominant energy condition near the event horizon. Whether a fake Schwarzschild or Kerr black hole can be realized under the dominant energy condition is an open question.&#xD;

We present results for the numerical evaluation of scalar quasinormal modes in Taub-NUT-AdS4 spacetimes. To achieve this we consider angular modes that correspond to non-unitary highest weight SU(2) representations since global regularity is not consistent with the presence of complex quasinormal modes. We show that for any non-zero value of the NUT charge n there exists a region in the complex plane that contains only stable quasinormal modes. The radius of this region increases with the horizon distance and decreases with n towards a constant value for infinite n. We also find analytic and numerical evidence for the existence of a critical NUT charge ncr beyond which the lowest lying stable quasinormal modes become overdamped. Our results draw an intricate picture for the holographic fluid of Taub-NUT-AdS4.&#xD;

We investigate the quasinormal modes of static and spherically symmetric black holes in vacuum within the framework of $f(\mathbb{Q}) = \mathbb{Q} + \alpha \mathbb{Q}^2$ gravity, and compare them with those in $f(\mathbb{T}) = \mathbb{T} + \alpha \mathbb{T}^2$ gravity. Based on the Symmetric Teleparallel Equivalent of General Relativity, we notice that the gravitational effects arise from non-metricity (the covariant derivative of metrics) in $f(\mathbb{Q})$ gravity rather than curvature in $f(R)$ or torsion in $f(\mathbb{T})$.&#xD;Using the finite difference method and the sixth-order WKB method, we compute the quasinormal modes of massless scalar field and electromagnetic field perturbations. Tables of quasinormal frequencies for various parameter configurations are provided based on the sixth-order WKB method. Our findings reveal the differences in the quasinormal modes of black holes in $f(\mathbb{Q})$ gravity &#xD;compared to those in $f(R)$ and $f(\mathbb{T})$ gravity. This variation demonstrates the impact of different parameter values, &#xD;offering insights into the characteristics of $f(\mathbb{Q})$ gravity. These results provide the theoretical groundwork for assessing alternative gravities' viability through gravitational wave data, and aid probably in picking out the alternative gravity theory that best aligns with the empirical reality.

In this work, we revisit/reinterpret/extend the model-independent analysis method (which we now call spread -luminosity distance fitting, spread-LDF) from our previous work. We apply it to the updated supernova type Ia catalogue, Pantheon+ and recent GRB compilations. The procedure allows us, using only FLRW assumption, to construct good approximations for expansion history of the universe, re-confirming its acceleration to be a robust feature. When we also assume General Relativity ("GR"), we can demonstrate, without any matter/energy model in mind, the need for (possibly nonconstant) dark energy ("GDE"). We find hints for positive pressure of GDE at z > 1 with implications on either the complexity of dark energy, or the validity of one of the cosmological principle, interpretation of SN Ia data, or GR.

One striking feature of binary black hole (BBH) mergers observed in the first decade of gravitational-wave astronomy is an excess of events with component masses around 35ドル,円\mathrm{M}_{\odot}$. Multiple formation channels have been proposed to explain this excess. To distinguish among these channels, it is essential to examine their predicted population-level distributions across additional parameters. In this work, we focus on BBH mergers near the 35ドル,円\mathrm{M}_{\odot}$ peak and infer the population distributions of primary mass (m₁), mass ratio (q), effective spin ($,円\chi_{\mathrm{eff}}$), and redshift (z). We observe a gradual increase in the merger rate with m₁, rising by a factor of 3 from 20ドル,円\mathrm{M}_{\odot}$ to a peak around 34ドル,円\mathrm{M}_{\odot}$, followed by a sharp, order-of-magnitude decline by 50ドル,円\mathrm{M}_{\odot}$. This population also shows a weak preference for equal-mass mergers and has a $,円\chi_{\mathrm{eff}}$ distribution skewed toward positive values, with a median of zero excluded at approximately 90% confidence. We find no significant $q-,円\chi_{\mathrm{eff}}$ correlation in the 35ドル ,円\mathrm{M}_{\odot}$ peak population, suggesting that lower-mass systems ($m_1 \lt 20,円\mathrm{M}_{\odot}$) likely drive the $q-,円\chi_{\mathrm{eff}}$ anti-correlation observed in the full BBH merger catalog. The redshift evolution of the merger rate is consistent with the cosmic star formation rate. We compare our findings with predictions from a wide range of formation channels. We find that common variants of the pair-instability supernova scenario, as well as hierarchical mergers in absence of sufficient gas-accretion, are incompatible with the observed features of the 35ドル,円\mathrm{M}_{\odot}$ population. Ultimately, none of the formation channels we consider can explain all or even most of the features observed in this population. The ‘mid-thirties’ of black hole mergers are in crisis.

We combine the well-known Beltrami–Klein model of non-Euclidean geometry on a two-dimensional disk, where the geodesics are the chords of the disk, with the two—dimensional de Sitter space. The geometry of the de Sitter space is defined on the complement of the Beltrami–Klein disk in the plane, with the de Sitter metric being the unique Lorentzian Einstein metric whose light cones form cones tangent to the disk in this complement. This leads to a Beltrami–de Sitter model on the plane $\mathbb{R}^2$, which is endowed with the Riemannian Beltrami metric on the disk and the Lorentzian de Sitter metric outside the disk in $\mathbb{R}^2$. We explore the relevance of this model for Penrose’s conformal cyclic cosmology, first in the two-dimensional setting and subsequently in higher dimensions, including the physically significant case of four dimensions. In this context, we define a Radon-like transform between the de Sitter and Beltrami spaces, facilitating the purely geometric transformation of physical fields from the Lorentzian de Sitter space to the Riemannian Beltrami space. In the two –, and three-dimensional cases, we also uncover a hidden G₂ symmetry associated with the de Sitter spaces in these dimensions, which is related to a certain vector distribution naturally defined by the geometry of the model. We suggest the potential for discovering similar hidden symmetries in the n-dimensional Beltrami–de Sitter model.

We propose a minimal, fully thermal mechanism that resolves the long-standing tension between achieving the observed dark-matter relic abundance and explaining the astrophysical signatures of self-interactions. The framework introduces two mediators: a light scalar φ (MeV scale) that yields the required, velocity-dependent self-interactions, and a heavy scalar resonance $\Phi_h$ (TeV scale) with mass $m_{\Phi_h}\!\approx\!2m_\chi$ that opens an s-channel resonant annihilation during freeze-out. This clearly decouples early-Universe annihilation from late-time halo dynamics. A detailed numerical analysis identified a narrow predictive island of viability. A representative benchmark with $m_\chi\! = \!600$ GeV, $m_\phi\! = \!15$ MeV, and $m_{\Phi_h}\!\simeq\!1.2$ TeV reproduces the relic density and yields $\sigma_T/m_\chi\sim 0.1$–1ドル~\mathrm{cm}^2\!,円\mathrm{g}^{-1}$ at dwarf-galaxy velocities while satisfying cluster bounds. The model makes sharp, testable predictions: a narrow $t\bar t$ resonance near 1.2 TeV within HL-LHC reach, and a spin-independent direct-detection signal $\sigma_\textrm{SI}\!\sim\!7\times10^{-48},円\mathrm{cm}^2$ within next-generation sensitivity. As an optional UV completion, we show that walking $\mathrm{SU}(3)_H$ gauge theory with $N_f = 10$ naturally realizes the near-threshold relation $m_{\Phi_h}\!\approx\!2m_\chi$ and can furnish an effective anomalous dimension $\gamma\!\approx\!0.5$ which underlies a density-responsive dark-energy sector, suggesting a unified origin for the dark sector.

Gravitational optical properties are here investigated under the hypothesis of spherically-symmetric spacetimes behaving as media. To do so, we first consider two different definitions of the refractive index, n, of a spacetime medium and show how to pass from one definition to another by means of a coordinate transformation. Accordingly, the corresponding physical role of n is discussed by virtue of the Misner–Sharp mass and the redshift definition. Afterwards, we discuss the inclusion of the electromagnetic fields and the equivalence with nonlinear effects induced by geometry. Accordingly, the infrared and ultraviolet gravity regimes are thus discussed, obtaining bounds from the Solar System, neutron stars and white dwarfs, respectively. To do so, we also investigate the Snell’s law and propose how to possibly distinguish regular solutions from black holes. As a consequence of our recipe, we speculate on the existence of gravitational metamaterials with negative refraction and explore the corresponding physical implications, remarking that n < 0 may lead to invisible optical properties, as light is bent in the opposite direction compared to what occurs in ordinary cases. Further, we conjecture that those spacetimes that exhibit negative refraction can have particle-like behavior, contributing to dark matter and propose three toy models, highlighting possible advantages and limitations of their use. Finally, we suggest that such particle-like configurations can be ‘dressed’ by interaction, giving rise to geometric quasiparticles. We thus construct modifications of the quantum propagator as due to nonminimal couplings between curvature and external matter-like fields, finding the corresponding effective mass through a boson mixing mechanism.

We investigate the gravitational Poissonian spontaneous localization (GPSL) model, a hybrid classical-quantum model in which classical Newtonian gravity emerges from stochastic collapses of the mass density operator, and consistently couples to quantum matter. Unlike models based on continuous weak measurement schemes, we show that GPSL ensures vacuum stability; this, together with its applicability to identical particles and fields, makes it a promising candidate for a relativistic generalization. We analyze the model’s general properties, and compare its predictions with those based on continuous weak measurement schemes. Notably, here the gravitational feedback enters entirely through the non-Hermitian jump operators, without modifying the unitary part of the dynamics. We show that this leads to a short-range gravitational back-reaction and permits decoherence rates below those of any model based on continuous weak measurement schemes. We provide explicit examples, including the dynamics of a single particle and a rigid sphere, to illustrate the distinctive phenomenology of the model. Finally, we discuss the experimental testability of GPSL, highlighting both interferometric and non-interferometric strategies to constrain its parameters and distinguish it from competing models.

The field of gravitational wave (GW) detection is progressing rapidly, with several next-generation observatories on the horizon, including LISA. GW data is challenging to analyze due to highly variable signals shaped by source properties and the presence of complex noise. These factors emphasize the need for robust, advanced analysis tools. In this context, we have initiated the development of a low-latency GW detection pipeline based on quantum neural networks (QNNs). Previously, we demonstrated that QNNs can recognize GWs simulated using post-Newtonian approximations in the Newtonian limit. We then extended this work using data from the LISA Consortium, training QNNs to distinguish between noisy GW signals and pure noise. Currently, we are evaluating performance on the Sangria LISA Data Challenge dataset and comparing it against classical methods. Our results show that QNNs can reliably distinguish GW signals embedded in noise, achieving classification accuracies above 98%. Notably, our QNN identified 5 out of 6 mergers in the Sangria blind dataset. The missed event corresponds to the lowest signal-to-noise ratio (SNR) source, indicating that model sensitivity improvements are needed for weak signals. This can potentially be addressed using additional mock training datasets, and by testing different QNN architectures and ansatzes. Compared with a recurrent neural network baseline, the QNN achieves comparable accuracy on higher-SNR events while using orders of magnitude fewer trainable parameters. These results demonstrate the feasibility of QNNs for GW detection and motivate further investigation of quantum-enhanced data analysis techniques for LISA.

We present results for the numerical evaluation of scalar quasinormal modes in Taub-NUT-AdS4 spacetimes. To achieve this we consider angular modes that correspond to non-unitary highest weight SU(2) representations since global regularity is not consistent with the presence of complex quasinormal modes. We show that for any non-zero value of the NUT charge n there exists a region in the complex plane that contains only stable quasinormal modes. The radius of this region increases with the horizon distance and decreases with n towards a constant value for infinite n. We also find analytic and numerical evidence for the existence of a critical NUT charge ncr beyond which the lowest lying stable quasinormal modes become overdamped. Our results draw an intricate picture for the holographic fluid of Taub-NUT-AdS4.&#xD;

Landau's criterion for superfluidity is a special case of a broader principle: A moving fluid cannot be stopped by frictional forces if its state of motion is a local minimum of the grand potential. We employ this general thermodynamic criterion to derive a set of inequalities that any superfluid mixture (with an arbitrary number of order parameters) must satisfy for a certain state of motion to be long-lived and unimpeded by friction. These macroscopic constraints complement Landau's original criterion, in that they hold at all temperatures, and remain valid even for gapless superfluids. They are only necessary conditions for the existence of a frictionless hydrodynamic motion, since they presuppose the validity of a fluid description, but they provide sufficient conditions for stability against stochastic hydrodynamic fluctuations. We first formulate our analysis within General Relativity (with neutron star applications in mind), and then we take the Newtonian limit.

There has been much recent interest in the necessity of an observer degree of freedom in the description of local algebras in semiclassical gravity. In this work, we describe an example where the observer can be constructed intrinsically from the quantum fields. This construction involves the slow-roll inflation example recently analyzed by Chen and Penington, in which the gauge-invariant gravitational algebra arises from marginalizing over modular flow in a de Sitter static patch. We relate this procedure to the Connes–Takesaki theory of the flow of weights for type III von Neumann algebras, and further show that the resulting gravitational algebra can naturally be presented as a crossed product. This leads to a decomposition of the gravitational algebra into quantum field and observer degrees of freedom, with different choices of observer being related to changes in a quantum reference frame for the algebra. We also connect this example to other constructions of type II algebras in semiclassical gravity, and argue they all share the feature of being the result of gauging modular flow. The arguments in this work involve various properties of automorphism groups of hyperfinite factors, and so in an appendix we review the structure of these groups, which may be of independent interest for further investigations into von Neumann algebras in quantum gravity.

A ray model has been developed to characterize how a new, redundant fiber optic harness design affects the UV light propagation in the Laser Interferometer Space Antenna (LISA) charge management system. LISA will be the first gravitational wave observatory in space, detecting gravitational waves in the 0.1 mHz to 1 Hz range. The endpoint of each LISA interferometer arm is a free-falling test mass, which accumulates charge from cosmic rays over time and leads to force noise in gravitational wave measurements. LISA will utilize photoelectric discharge via UV light to control the test mass charge, which was successfully demonstrated on LISA Pathfinder. The delivery of UV light on LISA differs from LISA Pathfinder's design by using UV LEDs, rather than Hg lamps, and a novel, redundant multicore fiber optic harness (FOH), which has four different configurations. The FOH light output is injected into an optical feedthrough, which injects the light into the test mass housing. In MATLAB, a Gaussian model of the FOH light output was developed for each configuration and used to generate a ray distribution of each FOH light output. This ray model was propagated through the geometry of the optical feedthrough in COMSOL Multiphysics to simulate the final ray distribution that will be used in future simulations. The simulated power and beam divergence of the ray model is consistent with experimental values for each FOH configuration to 8%. The ray model accurately characterizes the difference in each FOH configuration such that the effects of the redundant LISA FOH design can be further investigated with a photoelectric charge management simulation of the LISA UV discharge process.

Advanced Virgo is the project to upgrade the Virgo interferometric detector of gravitational waves, with the aim of increasing the number of observable galaxies (and thus the detection rate) by three orders of magnitude. The project is now in an advanced construction phase and the assembly and integration will be completed by the end of 2015. Advanced Virgo will be part of a network, alongside the two Advanced LIGO detectors in the US and GEO HF in Germany, with the goal of contributing to the early detection of gravitational waves and to opening a new window of observation on the universe. In this paper we describe the main features of the Advanced Virgo detector and outline the status of the construction.

The Advanced LIGO gravitational wave detectors are second-generation instruments designed and built for the two LIGO observatories in Hanford, WA and Livingston, LA, USA. The two instruments are identical in design, and are specialized versions of a Michelson interferometer with 4 km long arms. As in Initial LIGO, Fabry–Perot cavities are used in the arms to increase the interaction time with a gravitational wave, and power recycling is used to increase the effective laser power. Signal recycling has been added in Advanced LIGO to improve the frequency response. In the most sensitive frequency region around 100 Hz, the design strain sensitivity is a factor of 10 better than Initial LIGO. In addition, the low frequency end of the sensitivity band is moved from 40 Hz down to 10 Hz. All interferometer components have been replaced with improved technologies to achieve this sensitivity gain. Much better seismic isolation and test mass suspensions are responsible for the gains at lower frequencies. Higher laser power, larger test masses and improved mirror coatings lead to the improved sensitivity at mid and high frequencies. Data collecting runs with these new instruments are planned to begin in mid-2015.

TianQin is a proposal for a space-borne detector of gravitational waves in the millihertz frequencies. The experiment relies on a constellation of three drag-free spacecraft orbiting the Earth. Inter-spacecraft laser interferometry is used to monitor the distances between the test masses. The experiment is designed to be capable of detecting a signal with high confidence from a single source of gravitational waves within a few months of observing time. We describe the preliminary mission concept for TianQin, including the candidate source and experimental designs. We present estimates for the major constituents of the experiment’s error budget and discuss the project’s overall feasibility. Given the current level of technological readiness, we expect TianQin to be flown in the second half of the next decade.

Advanced gravitational wave interferometers, currently under realization, will soon permit the detection of gravitational waves from astronomical sources. To open the era of precision gravitational wave astronomy, a further substantial improvement in sensitivity is required. The future space-based Laser Interferometer Space Antenna and the third-generation ground-based observatory Einstein Telescope (ET) promise to achieve the required sensitivity improvements in frequency ranges. The vastly improved sensitivity of the third generation of gravitational wave observatories could permit detailed measurements of the sources' physical parameters and could complement, in a multi-messenger approach, the observation of signals emitted by cosmological sources obtained through other kinds of telescopes. This paper describes the progress of the ET project which is currently in its design study phase.

The simplest ΛCDM model provides a good fit to a large span of cosmological data but harbors large areas of phenomenology and ignorance. With the improvement of the number and the accuracy of observations, discrepancies among key cosmological parameters of the model have emerged. The most statistically significant tension is the 4σ to 6σ disagreement between predictions of the Hubble constant, H₀, made by the early time probes in concert with the ‘vanilla’ ΛCDM cosmological model, and a number of late time, model-independent determinations of H₀ from local measurements of distances and redshifts. The high precision and consistency of the data at both ends present strong challenges to the possible solution space and demands a hypothesis with enough rigor to explain multiple observations—whether these invoke new physics, unexpected large-scale structures or multiple, unrelated errors. A thorough review of the problem including a discussion of recent Hubble constant estimates and a summary of the proposed theoretical solutions is presented here. We include more than 1000 references, indicating that the interest in this area has grown considerably just during the last few years. We classify the many proposals to resolve the tension in these categories: early dark energy, late dark energy, dark energy models with 6 degrees of freedom and their extensions, models with extra relativistic degrees of freedom, models with extra interactions, unified cosmologies, modified gravity, inflationary models, modified recombination history, physics of the critical phenomena, and alternative proposals. Some are formally successful, improving the fit to the data in light of their additional degrees of freedom, restoring agreement within 1–2σ between Planck 2018, using the cosmic microwave background power spectra data, baryon acoustic oscillations, Pantheon SN data, and R20, the latest SH0ES Team Riess, et al (2021 Astrophys. J.908 L6) measurement of the Hubble constant (H₀ = 73.2 ± 1.3 km s⁻¹ Mpc⁻¹ at 68% confidence level). However, there are many more unsuccessful models which leave the discrepancy well above the 3σ disagreement level. In many cases, reduced tension comes not simply from a change in the value of H₀ but also due to an increase in its uncertainty due to degeneracy with additional physics, complicating the picture and pointing to the need for additional probes. While no specific proposal makes a strong case for being highly likely or far better than all others, solutions involving early or dynamical dark energy, neutrino interactions, interacting cosmologies, primordial magnetic fields, and modified gravity provide the best options until a better alternative comes along.

Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strong-field regime. We consider a wide range of well-motivated deviations from classical general relativity (GR) BH solutions, and constrain them using the Event Horizon Telescope (EHT) observations of Sagittarius A$^*$ (Sgr A$^*$), connecting the size of the bright ring of emission to that of the underlying BH shadow and exploiting high-precision measurements of Sgr A$^*$’s mass-to-distance ratio. The scenarios we consider, and whose fundamental parameters we constrain, include various regular BHs, string-inspired space-times, violations of the no-hair theorem driven by additional fields, alternative theories of gravity, novel fundamental physics frameworks, and BH mimickers including well-motivated wormhole and naked singularity space-times. We demonstrate that the EHT image of Sgr A$^*$ places particularly stringent constraints on models predicting a shadow size larger than that of a Schwarzschild BH of a given mass, with the resulting limits in some cases surpassing cosmological ones. Our results are among the first tests of fundamental physics from the shadow of Sgr A$^*$ and, while the latter appears to be in excellent agreement with the predictions of GR, we have shown that a number of well-motivated alternative scenarios, including BH mimickers, are far from being ruled out at present.

Quasinormal modes are eigenmodes of dissipative systems. Perturbations of classical gravitational backgrounds involving black holes or branes naturally lead to quasinormal modes. The analysis and classification of the quasinormal spectra require solving non-Hermitian eigenvalue problems for the associated linear differential equations. Within the recently developed gauge-gravity duality, these modes serve as an important tool for determining the near-equilibrium properties of strongly coupled quantum field theories, in particular their transport coefficients, such as viscosity, conductivity and diffusion constants. In astrophysics, the detection of quasinormal modes in gravitational wave experiments would allow precise measurements of the mass and spin of black holes as well as new tests of general relativity. This review is meant as an introduction to the subject, with a focus on the recent developments in the field.

Gravitational waves (GWs) have a great potential to probe cosmology. We review early universe sources that can lead to cosmological backgrounds of GWs. We begin by presenting proper definitions of GWs in flat space-time and in a cosmological setting (section 2). Following, we discuss the reasons why early universe GW backgrounds are of a stochastic nature, and describe the general properties of a stochastic background (section 3). We recap current observational constraints on stochastic backgrounds, and discuss the basic characteristics of present and future GW detectors, including advanced LIGO, advanced Virgo, the Einstein telescope, KAGRA, and LISA (section 4). We then review in detail early universe GW generation mechanisms, as well as the properties of the GW backgrounds they give rise to. We classify the backgrounds in five categories: GWs from quantum vacuum fluctuations during standard slow-roll inflation (section 5), GWs from processes that operate within extensions of the standard inflationary paradigm (section 6), GWs from post-inflationary preheating and related non-perturbative phenomena (section 7), GWs from first order phase transitions related or not to the electroweak symmetry breaking (section 8), and GWs from general topological defects, and from cosmic strings in particular (section 9). The phenomenology of these early universe processes is extremely rich, and some of the GW backgrounds they generate can be within the reach of near-future GW detectors. A future detection of any of these backgrounds will provide crucial information on the underlying high energy theory describing the early universe, probing energy scales well beyond the reach of particle accelerators.

Advanced gravitational wave detectors, currently under construction, are expected to directly observe gravitational wave signals of astrophysical origin. The Einstein Telescope (ET), a third-generation gravitational wave detector, has been proposed in order to fully open up the emerging field of gravitational wave astronomy. In this paper we describe sensitivity models for ET and investigate potential limits imposed by fundamental noise sources. A special focus is set on evaluating the frequency band below 10 Hz where a complex mixture of seismic, gravity gradient, suspension thermal and radiation pressure noise dominates. We develop the most accurate sensitivity model, referred to as ET-D, for a third-generation detector so far, including the most relevant fundamental noise contributions.

Using the concept of cracking we explore the influence that density fluctuations and local anisotropy have on the stability of local and non-local anisotropic matter configurations in general relativity. This concept, conceived to describe the behavior of a fluid distribution just after its departure from equilibrium, provides an alternative approach to consider the stability of self-gravitating compact objects. We show that potentially unstable regions within a configuration can be identified as a function of the difference of propagations of sound along tangential and radial directions. In fact, it is found that these regions could occur when, at a particular point within the distribution, the tangential speed of sound is greater than the radial one.