Showing papers in "Classical and Quantum Gravity in 2021"
TL;DR: In this paper, the authors present a thorough review of recent Hubble constant estimates and a summary of the proposed theoretical solutions, including early or dynamical dark energy, neutrino interactions, interacting cosmologies, primordial magnetic fields, and modified gravity.
Abstract: The $\Lambda$CDM model provides a good fit to a large span of cosmological data but harbors areas of phenomenology. 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 $4-6\sigma$ disagreement between predictions of the Hubble constant $H_0$ by early time probes with $\Lambda$CDM model, and a number of late time, model-independent determinations of $H_0$ 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 demand a hypothesis with enough rigor to explain multiple observations--whether these invoke new physics, unexpected large-scale structures or multiple, unrelated errors. We present a thorough review of the problem, including a discussion of recent Hubble constant estimates and a summary of the proposed theoretical solutions. Some of the models presented are formally successful, improving the fit to the data in light of their additional degrees of freedom, restoring agreement within $1-2\sigma$ between {\it Planck} 2018, using CMB power spectra data, BAO, Pantheon SN data, and R20, the latest SH0ES Team measurement of the Hubble constant ($H_0 = 73.2 \pm 1.3{\rm\,km\,s^{-1}\,Mpc^{-1}}$ at 68\% confidence level). Reduced tension might not simply come from a change in $H_0$ but also from an increase in its uncertainty due to degeneracy with additional physics, 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.[Abridged]
603 citations
TL;DR: The characterization of the Advanced LIGO detectors in the second and third observing runs has increased the sensitivity of the instruments, allowing for a higher number of detectable gravitational-wave signals, and provided confirmation of all observed gravitational wave events as discussed by the authors.
Abstract: The characterization of the Advanced LIGO detectors in the second and third observing runs has increased the sensitivity of the instruments, allowing for a higher number of detectable gravitational-wave signals, and provided confirmation of all observed gravitational-wave events. In this work, we present the methods used to characterize the LIGO detectors and curate the publicly available datasets, including the LIGO strain data and data quality products. We describe the essential role of these datasets in LIGO–Virgo Collaboration analyses of gravitational-waves from both transient and persistent sources and include details on the provenance of these datasets in order to support analyses of LIGO data by the broader community. Finally, we explain anticipated changes in the role of detector characterization and current efforts to prepare for the high rate of gravitational-wave alerts and events in future observing runs.
103 citations
TL;DR: In this article, the authors derived effective equations with loop quantum gravity corrections for the Lemaitre-Tolman-Bondi family of space-times, and used these to study quantum gravity effects in the Oppenheimer-Snyder collapse model.
Abstract: We derive effective equations with loop quantum gravity corrections for the Lemaitre–Tolman–Bondi family of space-times, and use these to study quantum gravity effects in the Oppenheimer–Snyder collapse model. For this model, after the formation of a black hole with an apparent horizon, quantum gravity effects become important in the space-time region where the energy density and space-time curvature scalars become comparable to the Planck scale. These quantum gravity effects first stop the collapse of the dust matter field when its energy density reaches the Planck scale, and then cause the dust field to begin slowly expanding. Due to this continued expansion, the matter field will eventually extend beyond the apparent horizon, at which point the horizon disappears and there is no longer a black hole. There are no singularities anywhere in this space-time. In addition, in the limit that edge effects are neglected, we show that the dynamics for the interior of the star of uniform energy density follow the loop quantum cosmology effective Friedman equation for the spatially flat Friedman–Lemaitre–Robertson–Walker space-time. Finally, we estimate the lifetime of the black hole, as measured by a distant observer, to be ∼(GM)2/l Pl.
51 citations
TL;DR: The architecture and functioning of the pipeline, including transient noise mitigation strategies, parameter space for the searched signals, detection of candidates and evaluation of a false alarm rate associated to them, are described.
Abstract: We describe the MBTA search for gravitational waves signals from coalescences of compact objects in the LIGO-Virgo data, at the time of the third observing run (2019-2020), both for low-latency detections and for offline analysis. Details are given on the architecture and functioning of the pipeline, including transient noise mitigation strategies, parameter space for the searched signals, detection of candidates and evaluation of a false alarm rate associated to them. The performance of the low-latency search is demonstrated based on the LIGO-Virgo third observing run, during which MBTA has contributed to 42 alerts, submitting candidates with a median latency of 36 seconds. The performance of the offline search is illustrated on a subset of data collected during the second LIGO-Virgo observation run in 2017, and are quantified based on injections of simulated signal events on the same data.
45 citations
TL;DR: In this article, the authors present extensive studies of moving mirrors in conformal field theories by employing both field theoretic as well as holographic methods, and calculate the time evolution of entanglement entropy in various moving mirror models.
Abstract: Moving mirrors have been known as tractable setups modeling Hawking radiation from black holes. In this paper, motivated by recent developments regarding the black hole information problem, we present extensive studies of moving mirrors in conformal field theories by employing both field theoretic as well as holographic methods. Reviewing first the usual field theoretic formulation of moving mirrors, we construct their gravity dual by resorting to the AdS/BCFT construction. Based on our holographic formulation, we then calculate the time evolution of entanglement entropy in various moving mirror models. In doing so, we mainly focus on three different setups: escaping mirror, which models constant Hawking radiation emanating from an eternal black hole; kink mirror, which models an evaporating black hole formed from collapse; and the double escaping mirror, which models two constantly radiating eternal black holes. In particular, by computing the holographic entanglement entropy, we show that the kink mirror gives rise to an ideal Page curve. We also find that an interesting phase transition arises in the case of the double escaping mirror. Furthermore, we argue and provide evidence for an interpretation of moving mirrors in terms of two dimensional Liouville gravity. We also discuss the connection between quantum energy conditions and the time evolution of holographic entanglement entropy in moving mirror models.
39 citations
TL;DR: This work clarifies some of the aspects of minimal length models, with particular reference to the representation of the position operator, in relation to the commutation relation between position and momentum.
Abstract: Quantum mechanical models with a minimal length are often described by modifying the commutation relation between position and momentum. Although this represents a small complication when described in momentum space, at least formally, the (quasi-)position representation acquires numerous issues, source of misunderstandings. In this work, we review these issues, clarifying some of the aspects of minimal length models, with particular reference to the representation of the position operator.
38 citations
TL;DR: In this paper, the entanglement entropy between the states inside the cosmological horizon and states outside, as a function of time, is computed using the quantum Ryu-Takayanagi or island formula.
Abstract: Observers in de Sitter space can only access the space up to their cosmological horizon. Assuming thermal equilibrium, we use the quantum Ryu-Takayanagi or island formula to compute the entanglement entropy between the states inside the cosmological horizon and states outside, as a function of time. We obtain a Page curve that is bound at a value corresponding to the Gibbons-Hawking entropy.
At this transition an 'island' forms, which is in a significantly different location as compared to when considering black hole horizons and even moves back in time. These differences turn out to be essential for non-violation of the no-cloning theorem in combination with entanglement wedge reconstruction. This consideration furthermore introduces the need for a scrambling time, the entropy dependence of which turns out to coincide with what is expected for black holes.
The model we employ has pure three-dimensional de Sitter space as a solution. We dimensionally reduce to two dimensions in order to take into account semi-classical effects. Nevertheless, we expect the aforementioned qualitative features of the island to persist in higher dimensions.
37 citations
TL;DR: In this article, the authors characterize the scattering noise for LIGO's third observing run O3 from April, 2019 to March, 2020, and investigate the multiple origins of one of them as well as its mitigation.
Abstract: Noise due to scattered light has been a frequent disturbance in the Advanced LIGO gravitational wave detectors, hindering the detection of gravitational waves. The non stationary scatter noise caused by low frequency motion can be recognized as arches in the time-frequency plane of the gravitational wave channel. In this paper, we characterize the scattering noise for LIGO's third observing run O3 from April, 2019 to March, 2020. We find at least two different populations of scattering noise and we investigate the multiple origins of one of them as well as its mitigation. We find that relative motion between two specific surfaces is strongly correlated with the presence of scattered light and we implement a technique to reduce this motion. We also present an algorithm using a witness channel to identify the times this noise can be present in the detector.
36 citations
TL;DR: In this paper, it was shown that a non-minimal kinetic coupling correction term in the Lagrangian can produce primordial massless gravitons, or equivalently primordial gravitational waves with speed $c_T^2=1$ in natural units, thus complying with the GW170817 event.
Abstract: After the recent GW170817 event of the two neutron stars merging, many string corrected cosmological theories confronted the non-viability peril. This was due to the fact that most of these theories produce massive gravitons primordially. Among these theories were the ones containing a non-minimal kinetic coupling correction term in the Lagrangian, which belong to a subclass of Horndeski theories. In this work we demonstrate how these theories may be revived and we show how these theories can produce primordial gravitational waves with speed $c_T^2=1$ in natural units, thus complying with the GW170817 event. As we show, if the gravitational action of an Einstein-Gauss-Bonnet theory also contains a kinetic coupling of the form $\sim \xi(\phi) G^{\mu
u}\partial_\mu\phi\partial_
u\phi$, the condition of having primordial massless gravitons, or equivalently primordial gravitational waves with speed $c_T^2=1$ in natural units, results to certain conditions on the scalar field dependent coupling function of the Gauss-Bonnet term, which is also the non-minimal coupling of the kinetic coupling. We extensively study the phenomenological implications of such a theory focusing on the inflationary era, by only assuming slow-roll dynamics for the scalar field. Accordingly, we briefly study the case that the scalar field evolves in a constant-roll way. By using some illustrative examples, we demonstrate that the viability of the theoretical framework at hand may easily be achieved. Also, theories containing terms of the form $\sim \xi(\phi)\Box\phi g^{\mu
u}\partial_\mu\phi\partial_
u\phi$ and $\sim \xi(\phi)\left(g^{\mu
u}\partial_\mu\phi\partial_
u\phi\right)^2$ also lead to the same gravitational wave speed as the theory we shall study in this paper, so this covers a larger class of Horndeski theories.
33 citations
TL;DR: In this article, a detailed analysis on the dynamics of a Chiral-like cosmological model where the scalar fields can have negative kinetic terms was performed, and the asymptotic dynamics for the gravitational field equations for four different models in a spatially flat Friedmann--Lema\^itre--Robertson--Walker background space.
Abstract: We perform a detailed analysis on the dynamics of a Chiral-like cosmological model where the scalar fields can have negative kinetic terms In particular, we study the asymptotic dynamics for the gravitational field equations for four different models in a spatially flat Friedmann--Lema\^itre--Robertson--Walker background space When one of the scalar fields is phantom, we calculated that the cosmological fluid can evolves such that the parameter for the equation of state crosses twice the phantom divide line without the appearance of ghosts Moreover, the cosmological viability of these four models is discussed
32 citations
TL;DR: In this article, a simplified Einstein-Gauss-Bonnet inflationary theoretical framework was proposed, which is compatible with the GW170817 observational constraints on the gravitational wave speed.
Abstract: We provide a refined and much more simplified Einstein-Gauss-Bonnet inflationary theoretical framework, which is compatible with the GW170817 observational constraints on the gravitational wave speed. As in previous works, the constraint that the gravitational wave speed is $c_T^2=1$ in natural units, results to a constraint differential equation that relates the coupling function of the scalar field to the Gauss-Bonnet invariant $\xi(\phi)$ and the scalar potential $V(\phi)$. Adopting the slow-roll conditions for the scalar field and the Hubble rate, and in contrast to previous works, by further assuming that $\kappa \frac{\xi '}{\xi''}\ll 1$, which is motivated by slow-roll arguments, we succeed in providing much more simpler expressions for the slow-roll indices and for the tensor and scalar spectral indices and for the tensor-to-scalar ratio. We exemplify our refined theoretical framework by using an illustrative example with a simple power-law scalar coupling function $\xi(\phi)\sim \phi^{
u}$ and as we demonstrate the resulting inflationary phenomenology is compatible with the latest Planck data. Moreover, this particular model produces a blue-tilted tensor spectral index, so we discuss in brief the perspective of describing the NANOGrav result with this model as is indicated in the recent literature.
TL;DR: In this paper, the role of such observables in black hole polymer models is discussed in detail, and a new model based on polymerisation of new variables is proposed, which allows to overcome previous restrictions on initial conditions.
Abstract: In the past years, black holes and the fate of their singularity have been heavily studied within loop quantum gravity Effective spacetime descriptions incorporating quantum geometry corrections are provided by the so-called polymer models Despite the technical differences, the main common feature shared by these models is that the classical singularity is resolved by a black-to-white hole transition In a recent paper (Bodendorfer et al 2019 Class Quantum Grav 36 195015), we discussed the existence of two Dirac observables in the effective quantum theory respectively corresponding to the black and white hole mass Physical requirements about the onset of quantum effects then fix the relation between these observables after the bounce, which in turn corresponds to a restriction on the admissible initial conditions for the model In the present paper, we discuss in detail the role of such observables in black hole polymer models First, we revisit previous models and analyse the existence of the Dirac observables there Observables for the horizons or the masses are explicitly constructed In the classical theory, only one Dirac observable has physical relevance In the quantum theory, we find a relation between the existence of two physically relevant observables and the scaling behaviour of the polymerisation scales under fiducial cell rescaling We present then a new model based on polymerisation of new variables which allows to overcome previous restrictions on initial conditions Quantum effects cause a bound of a unique Kretschmann curvature scale, independently of the relation between the two masses
TL;DR: In this paper, the observed shadow size of M87$^*$ can be used to unambiguously and robustly constrain the black hole geometry in the vicinity of the circular photon orbit.
Abstract: Following up on a recent analysis by Psaltis et al. [Phys. Rev. Lett. 125, 141104 (2020)], we show that the observed shadow size of M87$^*$ can be used to unambiguously and robustly constrain the black hole geometry in the vicinity of the circular photon orbit. Constraints on the post-Newtonian weak-field expansion of the black hole's metric are instead more subtle to obtain and interpret, as they rely on combining the shadow-size measurement with suitable theoretical priors. We provide examples showing that post-Newtonian constraints resulting from shadow-size measurements should be handled with extreme care. We also discuss the similarities and complementarity between the EHT shadow measurements and black-hole gravitational quasi-normal modes.
TL;DR: In this paper, the authors compare Kerr black holes' photon ring structure with those produced by three rotating regular spacetimes, viz Bardeen, Hayward, and nonsingular.
Abstract: The Kerr black holes possess a photon region with prograde and retrograde orbits radii, respectively, and , and thereby always cast a closed photon ring or a shadow silhouette for a ⩽ M. For a > M, it is a no-horizon spacetime (naked singularity) wherein prograde orbits spiral into the central singularity, and retrograde orbits produce an arc-like shadow with a dark spot at the center. We compare Kerr black holes’ photon ring structure with those produced by three rotating regular spacetimes, viz Bardeen, Hayward, and nonsingular. These are non-Kerr black hole metrics with an additional deviation parameter of g related to the nonlinear electrodynamics charge. It turns out that for a given a, there exists a critical value of g, g E such that Δ = 0 has no zeros for g > g E, one double zero at r = r E for g = g E, respectively, corresponding to a no-horizon regular spacetime and extremal black hole with degenerate horizon. We demonstrate that, unlike the Kerr naked singularity, no-horizon regular spacetimes can possess closed photon ring when g E < g ⩽ g c, e.g. for a = 0.10M, Bardeen (g E = 0.763 332M < g ⩽ g c = 0.816 792M), Hayward (g E = 1.052 97M < g ⩽ g c = 1.164 846M) and nonsingular (g E = 1.2020M < g ⩽ g c = 1.222 461M) no-horizon spacetimes have closed photon ring. These results confirm that the mere existence of a closed photon ring does not prove that the compact object is necessarily a black hole. The ring circularity deviation observable ΔC for the three no-horizon rotating spacetimes satisfy ΔC ⩽ 0.10 as per the M87* black hole shadow observations. We have also appended the case of Kerr–Newman no-horizon spacetimes (naked singularities) with similar features.
TL;DR: In this article, the authors use Dirac's method for the quantization of constrained systems in order to quantize a spatially flat Friedmann-Lemaitre-Robertson-Walker spacetime in the context of cosmology.
Abstract: We use Dirac's method for the quantization of constrained systems in order to quantize a spatially flat Friedmann-Lemaitre-Robertson-Walker spacetime in the context of $f(Q)$ cosmology. When the coincident gauge is considered, the resulting minisuperspace system possesses second class constraints. This distinguishes the quantization process from the typical Wheeler-DeWitt quantization, which is applied for cosmological models where only first class constraints are present (e.g. for models in General Relativity or in $f(R)$ gravity). We introduce the Dirac brackets, find appropriate canonical coordinates and then apply the canonical quantization procedure. We perform this method both in vacuum and in the presence of matter: a minimally coupled scalar field and a perfect fluid with a linear equation of state. We demonstrate that the matter content changes significantly the quantization procedure, with the perfect fluid even requiring to put in use the theory of fractional Quantum Mechanics in which the power of the momentum in the Hamiltonian is associated with the fractal dimension of a Levy flight. The results of this analysis can be applied in $f(T)$ teleparallel cosmology, since $f(Q)$ and $f(T)$ theories have the same degrees of freedom and same dynamical constraints in cosmological studies.
TL;DR: In this article, the authors introduce and generalize the horizon, range, response response, and reach distances of a GW detector, and provide answers to the question: "How far can GW detectors hear?"
Abstract: We present quantities which characterize the sensitivity of gravitational-wave observatories to sources at cosmological distances. In particular, we introduce and generalize the horizon, range, response, and reach distances. These quantities incorporate a number of important effects, including cosmologically well-defined distances and volumes, cosmological redshift, cosmological time dilation, and rate density evolution. In addition, these quantities incorporate unique aspects of gravitational wave detectors, such as the variable sky sensitivity of the detectors and the scaling of the sensitivity with inverse distance. An online calculator (this https URL) and python notebook (this https URL) to determine GW distances are available. We provide answers to the question: "How far can gravitational-wave detectors hear?"
TL;DR: In this article, the authors explore the construction and stability of asymptotically anti-de Sitter Euclidean wormholes in a variety of models and show that the structure of such wormholes resembles the Hawking-Page phase transition for AdS-Schwarzschild black holes.
Abstract: We explore the construction and stability of asymptotically anti-de Sitter Euclidean wormholes in a variety of models. In simple ad hoc low-energy models, it is not hard to construct two-boundary Euclidean wormholes that dominate over disconnected solutions and which are stable (lacking negative modes) in the usual sense of Euclidean quantum gravity. Indeed, the structure of such solutions turns out to strongly resemble that of the Hawking-Page phase transition for AdS-Schwarzschild black holes, in that for boundary sources above some threshold we find both a `large' and a `small' branch of wormhole solutions with the latter being stable and dominating over the disconnected solution for large enough sources. We are also able to construct two-boundary Euclidean wormholes in a variety of string compactifications that dominate over the disconnected solutions we find and that are stable with respect to field-theoretic perturbations. However, as in classic examples investigated by Maldacena and Maoz, the wormholes in these UV-complete settings always suffer from brane-nucleation instabilities (even when sources that one might hope would stabilize such instabilities are tuned to large values). This indicates the existence of additional disconnected solutions with lower action. We discuss the significance of such results for the factorization problem of AdS/CFT.
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TL;DR: In this article, the authors study stable circular orbits in spherically symmetric AdS black holes in various dimensions and their limiting innermost stable circular orbit (ISCOs) and provide analytic expressions for their size, angular velocity and angular momentum.
Abstract: We study stable circular orbits in spherically symmetric AdS black holes in various dimensions and their limiting innermost stable circular orbits (ISCOs). We provide analytic expressions for their size, angular velocity and angular momentum in a large black hole mass regime. The dual interpretation is in terms of meta-stable states not thermalising in typical thermal scales and whose existence is due to non-perturbative effects on the spatial curvature. Our calculations reproduce the binding energy known in the literature, but also include a binding energy in the radial fluctuations corresponding to near circular trajectories. We also describe how particles are placed on these orbits from integrated operators on the boundary: they tunnel inside in a way that can be computed from both complex geodesics in the black hole background and from the WKB approximation of the wave equation. We explain how these two computations are related.
TL;DR: In this article, the authors describe the methods used and how they have led to the mitigation of noise sources, the role that environmental monitoring has played in the validation of gravitational wave events, and plans for future observing runs.
Abstract: The sensitivity of the Advanced LIGO detectors to gravitational waves can be affected by environmental disturbances external to the detectors themselves. Since the transition from the former initial LIGO phase, many improvements have been made to the equipment and techniques used to investigate these environmental effects. These methods have aided in tracking down and mitigating noise sources throughout the first three observing runs of the advanced detector era, keeping the ambient contribution of environmental noise below the background noise levels of the detectors. In this paper we describe the methods used and how they have led to the mitigation of noise sources, the role that environmental monitoring has played in the validation of gravitational wave events, and plans for future observing runs.
TL;DR: In this paper, the photon calibrators (PCal) were used to calibrate the interferometer, using radiation pressure of a modulated auxiliary laser beam impinging on the Advanced Virgo end mirrors.
Abstract: As the sensitivities of LIGO, Virgo and KAGRA detectors improve, calibration of the interferometers output is becoming more and more important and may impact scientific results. For the observing run O3, Virgo used for the first time photon calibrators (PCal) to calibrate the interferometer, using radiation pressure of a modulated auxiliary laser beam impinging on the Advanced Virgo end mirrors. Those optical devices, also used in LIGO, are now the calibration reference for the global gravitational wave detectors network. The intercalibration of LIGO and Virgo PCals, based on the same absolute reference called the Gold Standard, has allowed to remove a systematic bias of 3.92% that would have been present in Virgo calibration using the PCal. The uncertainty budget on the PCal-induced displacement of the end mirrors (NE and WE) of Advanced Virgo has been estimated to be 1.36% for O3a and 1.40% on NE PCal (resp. 1.74% on WE PCal) for O3b. This uncertainty is the limiting one for the global calibration of Advanced Virgo. It is expected to be reduced below 1% for the next observing runs.
TL;DR: In this paper, the authors proposed that several anomalies observed at large angular scales in the CMB have a common origin in a cosmic bounce that took place before the inflationary era.
Abstract: We propose that several of the anomalies that have been observed at large angular scales in the CMB have a common origin in a cosmic bounce that took place before the inflationary era. The bounce introduces a new physical scale in the problem, which breaks the almost scale invariance of inflation. As a result, the state of scalar perturbations at the onset of inflation is no longer the Bunch-Davies vacuum, but it rather contains excitations and non-Gaussianity, which are larger for infrared modes. We argue that the combined effect of these excitations and the correlations between CMB modes and longer wavelength perturbations, can account for the observed power suppression, for the dipolar asymmetry, and it can also produce a preference for odd-parity correlations. The model can also alleviate the tension in the lensing amplitude $A_L$. We adopt a phenomenological viewpoint by characterizing the model with a few free parameters, rather than restricting to specific bouncing theories. We identify the minimum set of ingredients needed for our ideas to hold, and point out examples of theories in the literature where these conditions are met.
TL;DR: In this article, a generalization of the Heisenberg uncertainty principle is presented, which introduces the existence of a maximal observable momentum and at the same time does not entail a minimal indeterminacy in position.
Abstract: We present a novel generalization of the Heisenberg uncertainty principle which introduces the existence of a maximal observable momentum and at the same time does not entail a minimal indeterminacy in position. The above result is an exact generalized uncertainty principle (GUP), valid at all energy scales. For small values of the deformation parameter $\beta$, our ansatz is consistent with the usual expression for GUP borrowed from string theory, doubly special relativity and other quantum gravity candidates that provide $\beta$ with a negative sign. As a preliminary analysis, we study the implications of this new model on some quantum mechanical applications and on the black hole thermodynamics.
TL;DR: In this article, effective spin foam models were adopted to define a path integral over discrete Lorentzian quantum geometric configurations, which include metric and torsion degrees of freedom, which are parametrized by the Barbero-Immirzi parameter.
Abstract: Making the Lorentzian path integral for quantum gravity well-defined and computable has been a long standing challenge. In this work we adopt the recently proposed effective spin foam models to the Lorentzian case. This defines a path integral over discrete Lorentzian quantum geometric configurations, which include metric and torsion degrees of freedom. The torsion degrees of freedom arise due to an anomaly, which is parametrized by the Barbero--Immirzi parameter. Requiring a semi-classical regime constrains this parameter, but the precise bound has to be determined by probing the dynamics. The effective models provide the computationally most efficient spin foam models yet, which allows us to perform first tests for determining the semi-classical regime. This includes explorations specific to the Lorentzian case, e.g investigating quantum geometries with null lengths and null areas as well as geometries that describe a change of spatial topology.
TL;DR: In this article, an equivalent reformulation of the equations at the basis of extended irreversible thermodynamics is provided for the case of bulk viscosity, which is, to date, one of the most successful theories for relativistic dissipation.
Abstract: The approach of extended irreversible thermodynamics consists of promoting the dissipative fluxes to non-equilibrium thermodynamic variables. In a relativistic context, this naturally leads to the formulation of the theory of Israel and Stewart (1979), which is, to date, one of the most successful theories for relativistic dissipation. Although the generality of the principle makes it applicable to any dissipative fluid, a connection of the Israel-Stewart theory with microphysics has been established, through kinetic theory, only for the case of ideal quantum gases. By performing a convenient change of variables, we provide, for the case of bulk viscosity, an equivalent reformulation of the equations at the basis of extended irreversible thermodynamics. This approach maps any thermodynamic process which contributes to the bulk viscosity into a set of chemical reactions, whose reaction coordinates are abstract parameters describing the displacement from local thermodynamic equilibrium of the fluid element. We apply our new formalism to the case of the relativistic fluids, showing that the Israel-Stewart model for bulk viscosity is just the second-order expansion of a minimal model belonging to a larger class of non-perturbative theories for bulk viscosity which include the nuclear-reaction-mediated bulk viscosity of neutron star matter as a particular case. Furthermore, we show with concrete examples that our formalism provides new ways of computing the bulk viscosity directly and defines a simple prescription for constructing the Israel-Stewart model for a generic bulk-viscous fluid.
TL;DR: In this paper, the authors present a thorough observational investigation of the heuristic quantised ringdown model presented in [FOIT-KLEBAN (2019), which is based on the Bekenstein-Mukhanov conjecture, stating that the area of a black hole horizon is an integer multiple of the Planck area.
Abstract: We present a thorough observational investigation of the heuristic quantised ringdown model presented in [FOIT-KLEBAN (2019)]. This model is based on the Bekenstein-Mukhanov conjecture, stating that the area of a black hole horizon is an integer multiple of the Planck area~$l_P^2$ multiplied by a phenomenological constant, α, which can be viewed as an additional black hole intrinsic parameter. Our approach is based on a time-domain analysis of the gravitational wave signals produced by the ringdown phase of binary black hole mergers detected by the LIGO and Virgo collaboration. Employing a full Bayesian formalism and taking into account the complete correlation structure among the black hole parameters, we show that the value of α cannot be constrained using only GW150914, in contrast to what was suggested in [FOIT-KLEBAN (2019)]. We proceed to repeat the same analysis on the new gravitational wave events detected by the LIGO and Virgo Collaboration up to 1 October 2019, obtaining a combined-event measure equal to $α = 15.6^{+20.5}_{-13.3}$ and a combined log odds ratio of $0.1 \pm 0.6$, implying that current data are not informative enough to favour or discard this model against general relativity. We then show that using a population of $\mathcal{O}(20)$ GW150914-like simulated events -- detected by the current infrastructure of ground-based detectors at their design sensitivity -- it is possible to confidently falsify the quantised model or prove its validity, in which case probing α at the few % level. Finally we classify the stealth biases that may show up in a population study.
TL;DR: In this paper, a class of soliton solutions that are capable of superluminal motion and sourced by purely positive energy densities is presented, and the solitons are also shown to be capable of being sourced from the stress energy of a conducting plasma and classical electromagnetic fields.
Abstract: Solitons in space--time capable of transporting time-like observers at superluminal speeds have long been tied to violations of the weak, strong, and dominant energy conditions of general relativity. The negative-energy sources required for these solitons must be created through energy-intensive uncertainty principle processes as no such classical source is known in particle physics. This paper overcomes this barrier by constructing a class of soliton solutions that are capable of superluminal motion and sourced by purely positive energy densities. The solitons are also shown to be capable of being sourced from the stress-energy of a conducting plasma and classical electromagnetic fields. This is the first example of hyper-fast solitons resulting from known and familiar sources, reopening the discussion of superluminal mechanisms rooted in conventional physics.
TL;DR: Gwbench as mentioned in this paper implements the well-established Fisher information formalism as a fast and straightforward tool for the purpose of gravitational-wave benchmarking, i.e. the estimation of signal-to-noise ratios and measurement errors of gravitational waves observed by a network of detectors.
Abstract: We present a new Python package, gwbench, implementing the well-established Fisher information formalism as a fast and straightforward tool for the purpose of gravitational-wave benchmarking, i.e. the estimation of signal-to-noise ratios and measurement errors of gravitational waves observed by a network of detectors. Such an infrastructure is necessary due to the high computational cost of Bayesian parameter estimation methods which renders them less effective for the scientific assessment of gravitational waveforms, detectors, and networks of detectors, especially when determining their effects on large populations of gravitational-wave sources spread throughout the universe. gwbench further gives quick access to detector locations and sensitivities, while including the effects of Earth's rotation on the latter, as well as waveform models and their derivatives, while giving access to the host of waveforms available in the LSC Algorithm Library. With the provided functionality, gwbench is relevant for a wide variety of applications in gravitational-wave astronomy such as waveform modeling, detector development, cosmology, and tests of general relativity.
TL;DR: In this paper, the existence of a wandering null geodesic in general black hole spacetimes mainly in terms of Weyl focusing is examined. But the authors focus on the photon sphere.
Abstract: Causal concept for the general black hole shadow is investigated, instead of the photon sphere. We define several `wandering null geodesics' as complete null geodesics accompanied by repetitive conjugate points, which would correspond to null geodesics on the photon sphere in Schwarzschild spacetime. We also define a `wandering set', that is, a set of totally wandering null geodesics as a counterpart of the photon sphere, and moreover, a truncated wandering null geodesic to symbolically discuss its formation. Then we examine the existence of a wandering null geodesic in general black hole spacetimes mainly in terms of Weyl focusing. We will see the essence of the black hole shadow is not the stationary cycling of the photon orbits which is the concept only available in a stationary spacetime, but their accumulation. A wandering null geodesic implies that this accumulation will be occur somewhere in an asymptotically flat spacetime.
TL;DR: In this article, the accuracy of laser power sensor calibration was improved by using transfer standards to on-line power sensors monitoring the modulated laser power, which reduced relative calibration errors between elements of the detector network.
Abstract: As sensitivities improve and more detectors are added to the global network of gravitational wave observatories, calibration accuracy and precision are becoming increasingly important. Photon calibrators, relying on power-modulated auxiliary laser beams reflecting from suspended interferometer optics, enable continuous calibration by generating displacement fiducials proportional to the modulated laser power. Developments in the propagation of laser power calibration via transfer standards to on-line power sensors monitoring the modulated laser power have enabled generation of length fiducials with improved accuracy. Estimated uncertainties are almost a factor of two smaller than the lowest values previously reported. This is partly due to improvements in methodology that have increased confidence in the results reported. Referencing the laser power calibration standards for each observatory to a single transfer standard enables reducing relative calibration errors between elements of the detector network. Efforts within the national metrology institute community to realize improved laser power sensor calibration accuracy are ongoing.
TL;DR: In this article, the design and estimated sensitivity of co-located twin table-top 3D interferometers being built to measure or constrain quantum gravity is described, and the experiment is estimated to be sensitive to displacements in a frequency band between 1 and 250 MHz.
Abstract: Theories of quantum gravity based on the holographic principle predict the existence of quantum fluctuations of distance measurements that accumulate and exhibit correlations over macroscopic distances. This paper models an expected signal due to this phenomenology, and details the design and estimated sensitivity of co-located twin table-top 3D interferometers being built to measure or constrain it. The experiment is estimated to be sensitive to displacements $\sim10^{-19}\,\rm{m}/\sqrt{\rm{Hz}}$ in a frequency band between 1 and 250 MHz, surpassing previous experiments and enabling the possible observation of quantum gravity phenomena. The experiment will also be sensitive to MHz gravitational waves and various dark matter candidates.