Standard GR Research Articles

Introduction to loop quantum gravity. The Holst’s action and the covariant formalism

We review Holst formalism and dynamical equivalence with standard GR (in dimension [Formula: see text]). Holst formalism is written for a spin coframe field [Formula: see text] and a Spin(3,1)-connection [Formula: see text] on spacetime [Formula: see text] and it depends on the Holst parameter [Formula: see text]. We show the model is dynamically equivalent to standard GR, in the sense that up to a pointwise Spin(3,1)-gauge transformation acting on (uppercase Latin) frame indices, solutions of the two models are in one-to-one correspondence. Hence, the two models are classically equivalent. One can also introduce new variables by splitting the spin connection into a pair of a Spin(3)-connection [Formula: see text] and a Spin(3)-valued 1-form [Formula: see text]. The construction of these new variables relies on a particular algebraic structure, called a reductive splitting. A weaker structure than requiring that the gauge group splits as the products of two sub-groups, as it happens in Euclidean signature in the selfdual formulation originally introduced in this context by Ashtekar, and it still allows to deal with the Lorentzian signature without resorting to complexifications. The reductive splitting of [Formula: see text] is not unique and it is parameterized by a real parameter [Formula: see text] which is called the Immirzi parameter. The splitting is here done on spacetime, not on space as it is usually done in the literature, to obtain a Spin(3)-connection [Formula: see text], which is called the Barbero–Immirzi connection on spacetime. One obtains a covariant model depending on the fields [Formula: see text] which is again dynamically equivalent to standard GR. Usually in the literature one sets [Formula: see text] for the sake of simplicity. Here, we keep the Holst and Immirzi parameters distinct to show that eventually only [Formula: see text] will survive in boundary field equations.

Cosmological evolution in bimetric gravity: observational constraints and LSS signatures

Bimetric gravity is an interesting alternative to standard GR given its potential to provide a concrete theoretical framework for a ghost-free massive gravity theory. Here we investigate a class of Bimetric gravity models for their cosmological implications. We study the background expansion as well as the growth of matter perturbations at linear and second order. We use low-redshift observations from SnIa (Pantheon+ and SH0ES), Baryon Acoustic Oscillations (BAO), the growth (fsigma _{8}) measurements and the measurement from Megamaser Cosmology Project to constrain the Bimetric model. We find that the Bimetric models are consistent with the present data alongside the Lambda CDM model. We reconstructed the “ effective dark energy equation of state”(omega _{de}) and “Skewness”(S_{3}) parameters for the Bimetric model from the observational constraints and show that the current low-redshift data allow significant deviations in omega _{de} and S_{3} parameters with respect to the Lambda CDM behaviour. We also look at the ISW effect via galaxy-temperature correlations and find that the best fit Bimetric model behaves similarly to Lambda CDM in this regard.

Wormhole solutions in [formula omitted] gravity

In this work, we intend to explore wormhole geometries in the framework of f(R,Lm) gravity. We derive the field equations for the generic f(R,Lm) function by assuming the static and spherically symmetric Morris–Thorne wormhole metric. Then we consider two non-linear f(R,Lm) model, specifically, f(R,Lm)=R2+Lmα and f(R,Lm)=R2+(1+λR)Lm, where α and λ are free model parameters. We obtain the wormhole solutions by assuming three cases, namely, a linear barotropic EoS, anisotropic EoS, and isotropic EoS corresponding to model I. We observe that for both barotropic and anisotropic cases, the corresponding wormhole solutions obey the flaring-out condition under asymptotic background, while for the isotropic case, the shape function does not follow the flatness condition. Also, we find that the null energy condition exhibits negative behavior in the vicinity of the throat. Further, we consider two different shape functions to investigate the behavior of model II. We find some constraints on the model parameter for which the violation of the null energy condition exhibits. Finally, we employ the volume integral quantifier to calculate the amount of exotic matter required near the wormhole throat for both models. We conclude that the modification of standard GR can efficiently minimize the use of exotic matter and provide stable traversable wormhole solutions.

General teleparallel modifications of Schwarzschild geometry

Teleparallel theories of gravity are described in terms of the tetrad of a metric and a flat connection with torsion. In this paper, we study spherical symmetry in a modified teleparallel theory of gravity which is based on an arbitrary function of the five possible scalars constructed from the irreducible parts of torsion. This theory is a generalization of the so-called New General Relativity theory. We find that only two scalars are different to zero in spherical symmetry and we solve the corresponding field equations analytically for conformal Teleparallel gravity, and then perturbatively around Schwarzschild geometry for the general perturbative theory around GR. Finally, we compute phenomenological effects from the perturbed solutions such as the photon sphere, perihelion shift, Shapiro delay, and the light deflection. We find their correspondent correction to the standard GR contribution and their dependence on the three model parameters.

Prediction of dressing percentage, carcass characteristics and meat yield of goats, and implications for live assessment and carcass-grading systems

Abstract Context Dressing percentage (DP) and meat yield (MY) predictions using live assessments and carcass measurements enable objective valuation of animals and their carcasses. We hypothesised that distribution of goat carcass tissues affects predictive value of live body condition scoring (CS) methods and carcass measurements for these traits. Aims The present paper aimed to assess the value of CS methods for prediction of DP and MY and of carcass measurements for prediction of MY. Methods Correlation and regression analyses from a dataset (n = 1014 goats) highly heterogeneous for factors influencing DP and MY were used to assess (1) the value of live-goat assessments and classifications, including five CS methods, age (dentition), liveweight (LW), sex, fleece characteristics and breed or genotype to predict DP and MY, and (2) the value of hot standard carcass weight (HSCW) and carcass GR (soft tissue over the 12th rib) tissue depth, eye-muscle depth and eye-muscle area to predict MY. Key results Among kids, LW accounted for 1% (residual standard deviation of 2.6%) of variation in DP, 22% (2.3%) in MY (% LW) and 34% (2.5%) in MY (% HSCW). LW plus the best CS method accounted for 24% (2.3%) of variation in DP, 58% (1.7%) in MY (% LW) and 61% (2.0%) in MY (% HSCW). Among all goats, LW plus CS accounted for up to 21% (3.2%), 39% (2.1%) and 45% (2.2%) of variation in these traits. Regression models that included age, sex, fleece type, breed or genotype, LW and CS accounted for 67% (2.5%), 72% (1.9%) and 72% (2.1%) of variation in DP, MY (% LW) and MY (% HSCW). Among carcass measurements, HSCW plus eye-muscle depth had best predictive value, accounting for 61% (2.3%) of variation in MY (% HSCW) for kids and 40% (2.9%) for all goats. Conclusions The body condition-score methods that best relate to DP and MY (% LW or % HSCW) assessed the shape of M. longissimus lumborum (eye muscle) in the lumbar region, which relates to muscularity of goats, rather than subcutaneous fat depth such as assessed at the GR-site. Implications The results guide potential targets for future developments in live-goat assessment, carcass classification and grading, and trading languages underpinned by value-based marketing.

The Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: measurement of the BAO and growth rate of structure of the luminous red galaxy sample from the anisotropic power spectrum between redshifts 0.6 and 1.0

ABSTRACT We analyse the clustering of the Sloan Digital Sky Survey IV extended Baryon Oscillation Spectroscopic Survey Data Release 16 luminous red galaxy sample (DR16 eBOSS LRG) in combination with the high redshift tail of the Sloan Digital Sky Survey III Baryon Oscillation Spectroscopic Survey Data Release 12 (DR12 BOSS CMASS). We measure the redshift space distortions (RSD) and also extract the longitudinal and transverse baryonic acoustic oscillation (BAO) scale from the anisotropic power spectrum signal inferred from 377 458 galaxies between redshifts 0.6 and 1.0, with the effective redshift of zeff = 0.698 and effective comoving volume of $2.72\, {\rm Gpc}^3$. After applying reconstruction, we measure the BAO scale and infer DH(zeff)/rdrag = 19.30 ± 0.56 and DM(zeff)/rdrag = 17.86 ± 0.37. When we perform an RSD analysis on the pre-reconstructed catalogue on the monopole, quadrupole, and hexadecapole we find, DH(zeff)/rdrag = 20.18 ± 0.78, DM(zeff)/rdrag = 17.49 ± 0.52 and fσ8(zeff) = 0.454 ± 0.046. We combine both sets of results along with the measurements in configuration space and report the following consensus values: DH(zeff)/rdrag = 19.77 ± 0.47, DM(zeff)/rdrag = 17.65 ± 0.30 and fσ8(zeff) = 0.473 ± 0.044, which are in full agreement with the standard ΛCDM and GR predictions. These results represent the most precise measurements within the redshift range 0.6 ≤ z ≤ 1.0 and are the culmination of more than 8 yr of SDSS observations.

Black hole echoes

We consider a very simple model for gravitational wave echoes from black hole merger ringdowns which may arise from local Lorentz symmetry violations that modify graviton dispersion relations. If the corrections are sufficiently soft so they do not remove the horizon, the reflection of the infalling waves which trigger the echoes is very weak. As an example, we look at the dispersion relation of a test scalar field corrected by roton-like operators depending only on spatial momenta, in Gullstrand-Painlev\'e coordinates. The near-horizon regions of a black hole do become reflective, but only very weakly. The resulting ``bounces" of infalling waves can yield repetitive gravity wave emissions but their power is very small. This implies that to see any echoes from black holes we really need an egregious departure from either standard GR or effective field theory, or both. One possibility to realize such strong echoes is the recently proposed classical firewalls which replace black hole horizons with material shells surrounding timelike singularities.

Physical properties of class I compact star model for linear and Starobinsky−f(R,T) functions

We have explored exact solutions free from any physical and geometrical singularities, as well as the existence of compact stellar systems throughout linear and Starobinsky- f ( R , T ) − gravity theory. As we generally well-known, the general exact solutions of the altered Einstein Field Equations (EFEs) in the background of this gravity theory are one of the most complex tasks. To acquire simpler solution of the altered field equations, we consider linear and Starobinsky shape of the algebraic function as f ( R , T ) = R + 2 χ T and f ( R , T ) = R + ξ R 2 + 2 χ T (where R is scalar curvature, T is the trace of the stress–energy tensor and χ & ξ denotes a coupling constants). In this regards, we propose metric potentials e λ , and obtain the other metric potentials ( e ν ) under the embedding class one condition. We then compare the cases when ξ = χ = 0 [GR], ξ = 0 , χ = 0 . 5 [ f L ( R , T ) ], ξ = 0 . 5 , χ = 0 [ f S ( R , T ) ] and ξ = χ = 0 . 5 [ f S + L ( R , T ) ]. The obtained solution is well-behaved in all physical and mathematical points of view. Further, we provided a detailed physical acceptability of the solution by exploring main salient characteristics under different physical analysis in the linear f ( R , T ) − gravity. Moreover, the generalizing M − R and M − I curves from our solution are well fitted with observational data of the three compact objects viz., PSR J1614-2230, Vela X-1 and 4U 1820-30. We also found the beautiful results that the equation of state (EoS) is stiffest in χ = 0 than χ ≠ 0 , and the sensitivity in EoS is better in I − M graph than in M − R graph. Finally, we have successfully represented the effects of all the physical requirements in the arena of f ( R , T ) − gravity and we compared them with the standard GR results which can be recovered at χ = 0 .

First-order formulation of teleparallel gravity and dual loop gravity

There are at least two ways to encode gravity into geometry: Einstein’s general theory of relativity (GR) for the metric tensor, and teleparallel gravity, where torsion as opposed to curvature encodes the dynamics of the gravitational degrees of freedom. The main purpose of the paper is to explore the relation between loop gravity and teleparallel gravity. We argue that these two formulations of gravity are related to two different discretizations of the Einstein–Cartan action, which were studied recently in the literature. The first discretization leads to the loop gravity kinematical phase space where the zero torsion condition is enforced first and the other is the dual loop gravity kinematical phase space where curvature is imposed to vanish first. Our argument is based on the observation that the GR first-order Einstein–Cartan action can also be seen as a first-order action for teleparallel gravity up to a boundary term. The results of our paper suggest that the dual loop gravity framework is a natural discretization of teleparallel gravity, whereas loop gravity is naturally related to the standard GR metric description.

Predictions of the pseudo-complex theory of Gravity for EHT observations – II: theory and predictions

Abstract We present a resumé on the modified theory of gravity, called pseudo-complex General Relativity (pc-GR). It is the second in a series of papers, where the first one (Boller et al. 2019; referred to as Paper I) discussed the observational consequences of pc-GR. In this paper, we concentrate on the underlying theory. PC-GR involves an algebraic extension of the standard theory of GR and it depends on two phenomenological parameters. An element included in pc-GR that is not present in standard GR is the energy–momentum tensor corresponding to an anisotropic ideal fluid, which we call dark energy. The two parameters are related to the coupling of mass to the dark energy and its fall-off as a function of r. The consequences and predictions of this theory will be discussed in the context of the observational results of the Even Horizon Telescope, expected soon. Our main result is that due to the accumulation of dark energy near a large mass, the modified theory predicts a dark ring followed by a bright ring in the emission profile of the accretion disc. We also discuss the light ring in the equatorial plane.

Predictions of the pseudo-complex theory of gravity for EHT observations – I. Observational tests

Abstract A modified theory of gravity, avoiding singularities in the standard theory of gravitation, has been developed by Hess & Greiner, known as the pseudo-complex theory of gravitation. The pseudo-complex General Relativity (pc-GR) theory shows remarkable observational differences with respect to standard GR. The intensity profiles are significantly different between both theories, which is a rare phenomenon in astrophysics. This will allow robust tests of both theories using Event Horizon Telescope observations of the Galactic Centre. We also predict the time evolution of orbiting matter. In this paper we summarize the observational tests we have developed to date. The theory is described in the second paper of this series (referred to as paper II hereafter).

No evidence for modifications of gravity from galaxy motions on cosmological scales

The recent discovery of gravitational waves marks the culmination of a sequence of successful tests of the general theory of relativity (GR) since its formulation in 1915. Yet these tests remain confined to the scale of stellar systems or the strong gravity regime. A departure from GR on larger, cosmological scales has been advocated by the proponents of modified gravity theories as an alternative to the Cosmological Constant to account for the observed cosmic expansion history. While indistinguishable in these terms by construction, such models on the other hand yield distinct values for the linear growth rate of density perturbations and, as a consequence, for the associated galaxy peculiar velocity field. Measurements of the resulting anisotropy of galaxy clustering, when spectroscopic redshifts are used to derive distances, have thus been proposed as a powerful probe of the validity of GR on cosmological scales. However, despite significant effort in modelling such redshift space distortions, systematic errors remain comparable to current statistical uncertainties. Here, we present the results of a different forward-modelling approach, which fully exploits the sensitivity of the galaxy velocity field to modifications of GR. We use state-of-the-art, high-resolution N-body simulations of a standard GR and a compelling f(R) model, one of GR's simplest variants, to build simulated catalogues of stellar-mass-selected galaxies through a robust match to the Sloan Digital Sky Survey observations. We find that, well within the uncertainty of this technique, f(R) fails to reproduce the observed redshift-space clustering on scales 1-10 Mpc/h. Instead, the standard LCDM GR model agrees impressively well with the data. This result provides a strong confirmation, on cosmological scales, of the robustness of Einstein's general theory of relativity.

An \xe9tude on global vacuum energy sequester

Recently two of the authors proposed a mechanism of vacuum energy sequester as a means of protecting the observable cosmological constant from quantum radiative corrections. The original proposal was based on using global Lagrange multipliers, but later a local formulation was provided. Subsequently other interesting claims of a different non-local approach to the cosmological constant problem were made, based again on global Lagrange multipliers. We examine some of these proposals and find their mutual relationship. We explain that the proposals which do not treat the cosmological constant counterterm as a dynamical variable require fine tunings to have acceptable solutions. Furthermore, the counterterm often needs to be retuned at every order in the loop expansion to cancel the radiative corrections to the cosmological constant, just like in standard GR. These observations are an important reminder of just how the proposal of vacuum energy sequester avoids such problems.

Newtonian potential and geodesic completeness in infinite derivative gravity

Recent study has shown that a non-singular oscillating potential, a feature of Infinite Derivative Gravity (IDG) theories, matches current experimental data better than the standard GR potential. In this work we show that this non-singular oscillating potential can be given by a wider class of theories which allows the defocusing of null rays, and therefore geodesic completeness. We consolidate the conditions whereby null geodesic congruences may be made past-complete, via the Raychaudhuri Equation, with the requirement of a non-singular Newtonian potential in an IDG theory. In so doing, we examine a class of Newtonian potentials characterised by an additional degree of freedom in the scalar propagator, which returns the familiar potential of General Relativity at large distances.

Low-energy electromagnetic radiation as an indirect probe of black-hole evaporation

We study the influence of black-hole evaporation on light propagation. The framework employed is based on the non-linear QED effective action at one-loop level. We show that the light-cone condition is modified for low-energy radiation due to black-hole evaporation. We discuss conditions under which the phase velocity of this low-energy radiation is greater than c. We also compute the modified light-deflection angle, which turns out to be significantly different from the standard GR value for black-hole masses in the range MPl≪M≲1019MPl.

Signatures of Horndeski gravity on the dark matter bispectrum

We present a detailed study of second-order matter perturbations for the general Horndeski class of models. Being the most general scalar-tensor theory having second-order equations of motion, it includes many known gravity and dark energy theories and General Relativity with a cosmological constant as a specific case. This enables us to estimate the leading order dark matter bispectrum generated at late-times by gravitational instability. We parametrize the evolution of the first and second-order equations of motion as proposed by Bellini and Sawicki (2014), where the free functions of the theory are assumed to be proportional to the dark energy density. We show that it is unnatural to have large ≳ 10% (≳ 1%) deviations of the bispectrum introducing even larger ∼ 30% (∼ 5%) deviations in the linear growth rate. Considering that measurements of the linear growth rate have much higher signal-to-noise than bispectrum measurements, this indicates that for Horndeski models which reproduce the expansion history and the linear growth rate as predicted by GR the dark matter bispectrum kernel can be effectively modelled as the standard GR one. On the other hand, an observation of a large bispectrum deviation that can not be explained in terms of bias would imply either that the evolution of perturbations is strongly different than the evolution predicted by GR or that the theory of gravity is exotic (e.g., breaks the weak equivalence principle) and/or fine-tuned.

A novel concept for the control of parasitic weeds by decomposing germination stimulants prior to action

A new concept for controlling parasitic weeds is described. By decomposing germination stimulants prior to action no germination of seeds can take place anymore. Ethanol fractions of the strigolactone (SL) analogues viz., the standard synthetic analogues GR 24 and Nijmegen-1, and analogues derived from tetralone and coumarine, were added to an aqueous buffer with a pH ranging from 6 to 8 and the half lifes (t1/2) of the hydrolysis were measured. Nijmegen-1 hydrolysed faster than GR 24 and the analogue from tetralone was the most stable one at all pH's. It was found that the aqueous solutions of either borax or thiourea rapidly decompose typical SL analogues, including GR 24 and Nijmegen-1, within an hour. The hydrolysis of SLs by borax was monitored with UV spectroscopy and for thiourea gas chromatography was used. This decomposition of SLs by either borax or thiourea in natural conditions would deprive the seeds of the parasitic weeds of the essential germination stimulants and as a consequence not allow them to germinate. Hence, conditions for an effective weed control are fulfilled.

Planar AdS black holes in Lovelock gravity with a nonminimal scalar field

In arbitrary dimension D, we consider a self-interacting scalar field nonminimally coupled with a gravity theory given by a particular Lovelock action indexed by an integer k. To be more precise, the coefficients appearing in the Lovelock expansion are fixed by requiring the theory to have a unique AdS vacuum with a fixed value of the cosmological constant. This yields to k = 1, 2,⋯, $ \left[ {\frac{D-1 }{2}} \right] $ inequivalent possible gravity theories; here the case k = 1 corresponds to the standard Einstein-Hilbert Lagrangian. For each par (D, k), we derive two classes of AdS black hole solutions with planar event horizon topology for particular values of the nonminimal coupling parameter. The first family of solutions depends on a unique constant and is valid only for k ≥ 2. In fact, its GR counterpart k = 1 reduces to the pure AdS metric with a vanishing scalar field. The second family of solutions involves two independent constants and corresponds to a stealth black hole configuration; that is a nontrivial scalar field together with a black hole metric such that both side of the Einstein equations (gravity and matter parts) vanishes identically. In this case, the standard GR case k = 1 reduces to the Schwarzschild-AdS-Tangherlini black hole metric with a trivial scalar field. We show that the two-parametric stealth solution defined in D dimension can be promoted to the uniparametric black hole solution in (D + 1) dimension by fixing one of the two constants in term of the other and by adding a transversal coordinate. In both cases, the existence of these solutions is strongly inherent of the presence of the higher order curvature terms k ≥ 2 of the Lovelock gravity. We also establish that these solutions emerge from a stealth configuration defined on the pure AdS metric through a Kerr-Schild transformation. Finally, in the last part, we include multiple exact (D − 1) − forms homogenously distributed and coupled to the scalar field. For a specific coupling, we obtain black hole solutions for arbitrary value of the nonminimal coupling parameter generalizing those obtained in the pure scalar field case.

Galaxy And Mass Assembly (GAMA): improved cosmic growth measurements using multiple tracers of large-scale structure

We present the first application of a "multiple-tracer" redshift-space distortion (RSD) analysis to an observational galaxy sample, using data from the Galaxy and Mass Assembly survey (GAMA). Our dataset is an r < 19.8 magnitude-limited sample of 178,579 galaxies covering redshift interval z < 0.5 and area 180 deg^2. We obtain improvements of 10-20% in measurements of the gravitational growth rate compared to a single-tracer analysis, deriving from the correlated sample variance imprinted in the distributions of the overlapping galaxy populations. We present new expressions for the covariances between the auto-power and cross-power spectra of galaxy samples that are valid for a general survey selection function and weighting scheme. We find no evidence for a systematic dependence of the measured growth rate on the galaxy tracer used, justifying the RSD modelling assumptions, and validate our results using mock catalogues from N-body simulations. For multiple tracers selected by galaxy colour, we measure normalized growth rates in two independent redshift bins f*sigma_8(z=0.18) = 0.36 +/- 0.09 and f*sigma_8(z=0.38) = 0.44 +/- 0.06, in agreement with standard GR gravity and other galaxy surveys at similar redshifts.

ISOTHERMAL DISTRIBUTIONS IN MONDian GRAVITY AS A SIMPLE UNIFYING EXPLANATION FOR THE UBIQUITOUSρ∝r–3DENSITY PROFILES IN TENUOUS STELLAR HALOS

That the stellar halo of the Milky Way has a density profile which to first approximation satisfies $\rho \propto r^{-3}$ has been known for a long time. More recently, it has become clear that M31 also has such an extended stellar halo, which approximately follows the same radial scaling. Studies of distant galaxies have revealed the same phenomenology. Also, we now know that the density profiles of the globular cluster systems of our Galaxy and Andromeda to first approximation follow $\rho \propto r^{-3}$, $\Sigma \propto R^{-2}$ in projection. Recently, diffuse populations of stars have been detected spherically surrounding a number of Galactic globular clusters, extending much beyond the Newtonian tidal radii, often without showing any evidence of tidal features. Within the standard Newtonian and GR scenario, numerous and diverse particular explanations have been suggested, individually tailored to each of the different classes of systems described above. Here we show that in a MONDian gravity scenario, any isothermal tenuous halo of tracer particles forming a small perturbation surrounding a spherically symmetric mass distribution, will have an equilibrium configuration which to first approximation satisfies a $\rho \propto r^{-3}$ scaling.