Gravitational Scenarios Research Articles

Gravity theories with local energy-momentum exchange: a closer look at Rastall-like gravity

Einstein’s famous equivalence principle is certainly one of the most striking features of the gravitational interaction. In a strict reading, it states that the effects of gravity can be made to disappear locally by a convenient choice of reference frame. As a consequence, no covariantly-defined net gravitational (point-wise) force should exist and energy-momentum of all matter and interaction fields combined, with gravity excluded, should be locally conserved. Although elegant, this represents a strong constraint on the dynamics of a gravitating system and it is only logical to question its naturality and observational basis. This is the purpose of the present work. For concreteness sake, we analyze, in the context of metric (torsion-free) theories of gravity, the simplest phenomenological model which allows for local energy-momentum exchange between the spacetime and matter/interaction fields while preserving the seemingly more natural principle of general covariance. This concrete model turns out to be a generalization of the socalled Rastall’s theory, with one important advantage: criticisms made to the latter, which are often used to dismiss it as a viable or interesting model, do not apply to the former in a Universe containing ‘dark’ ingredients, such as ours—a connection which seems to have been overlooked thus far. We conclude by exploring the consequences of our Rastall-like model to standard (astrophysical and cosmological) gravitational scenarios.

Non-linear effective field theory simulators in two-fluid interfaces

Analogue gravity offers an approach for testing the universality and robustness of quantum field theories in curved spacetimes and validating them using down-to-earth, laboratory-based experiments. Fluid interfaces are a promising framework for creating these gravity simulators and have successfully replicated phenomena such as Hawking radiation and black hole superradiance. Recent work has shown that hydrodynamical instabilities on the interface between two fluids can capture features of the post-inflationary thermalisation of the Early Universe. In this study, we extend fluid dynamics methods to develop an effective field theory for the interface between two fluids, demonstrating the equivalence between the governing equations and a relativistic scalar field in an analogue spacetime. We also show that the interfacial height field serves as the analogue relativistic field even in a nonlinear, interacting field theory. We propose that these mathematical equivalences can be extrapolated to probe regimes where calculations are challenging or impractical. Our work provides a new framework for simulating far-from-equilibrium cosmological and gravitational scenarios in the laboratory.

Study of imbibition in various geometries using phase field method

Phase field method has been widely utilized to study multiphase flow problems, but has seldom been applied to the study of imbibition. Previous methods used to simulate imbibition, such as moving mesh method, need to specify capillary pressure as a boundary condition a priori, whereas phase field method can calculate capillary pressure automatically for various geometries. Therefore, phase field method would be a versatile tool for the study of imbibition in various geometries. In this paper, phase field method is employed to solve dynamical imbibition problem in various geometries, including straight tube, conical tube and structures in which the topology changes. The variation of the imbibition height with respect to time from phase field simulation is verified with theoretical predictions from Lucas-Washburn law in a straight capillary tube with three gravitational scenarios. In addition, the capillary pressure and velocity field are found to be consistent with Laplace-Young equation and Hagen-Poiseuille equation in various geometries. The applicability and accuracy of the phase field method for the study of imbibition in structures with changing topology are also discussed. Cited as : Xiao, J., Luo, Y., Niu, M., Wang, Q., Wu, J., Liu, X., Xu, J. Study of imbibition in various geometries using phase field method. Capillarity, 2019, 2(4): 57-65, doi: 10.26804/capi.2019.04.01

Saturated imbibition under the influence of gravity and geometry

HypothesisThe effect of gravity was generally neglected in the classical imbibition law for one dimensional geometries. Following researches complemented the classical “Lucas-Washburn law” with consideration of gravity, but no examination of geometries under influence of gravity has been done, while geometry was shown to yield different scaling law for the imbibition process. Hence, it is possible to discover new time exponents for imbibition length in two dimensional and three dimensional imbibition process under gravity. MethodsThrough theoretical analysis and numerical simulations, the size of wetted region under three gravitational scenarios (zero gravity, acceleration and deceleration) in three geometries (one dimensional, two dimensional radial and three dimensional radial) are determined quantitatively. FindingsNew time exponents other than classic 1/2 are identified under different directions of gravity in two dimensional radial and three dimensional radial imbibition, and symmetry of time exponents due to different directions of gravity is discovered. A new time exponent of 1 for the acceleration case in one dimensional imbibition is found. The flow field in the wetted region is also determined from simulations. Discoveries in this paper show that new physical laws for imbibition length exist at the intersection of gravity and geometry.

Ngravs: Distinct gravitational interactions in gadget-2

We discuss an extension of the massively parallel cosmological simulation code gadget-2, which now enables investigation of multiple and distinct gravitational force laws, provided they are dominated by a constant scaling of the Newtonian force. In addition to simplifying investigations of a universally modified force law, the ngravs extension provides a foundation for state-of-the-art collisionless cosmological simulations of exotic gravitational scenarios. We briefly review the algorithms used by gadget-2, and present our extension to multiple gravities, highlighting additional features that facilitate consideration of exotic force laws. We discuss the accuracy and performance of the ngravs extension, both internally and with an unaltered gadget-2, in the relevant operational modes. The ngravs extension is publicly released to the research community. Program summaryProgram title: Gadget-2.0.7-ngravsCatalogue identifier: AFAV_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AFAV_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU GPL v2No. of lines in distributed program, including test data, etc.: 36504No. of bytes in distributed program, including test data, etc.: 3919508Distribution format: tar.gzProgramming language: C.Computer: Commodity.Operating system: Unix.RAM: 256MB+Classification: 1.9, 4.12.External routines: GSL, FFTW3, HDF5 (optional)Nature of problem:N-body cosmological codes are traditionally designed to investigate a single gravitating species interacting via the Newtonian force law. There exist viable extensions to General Relativity [1], however, which predict weakfield, slow-motion limits featuring distinct gravitational force laws between distinct particle species. To enable investigation and constraint of these theories with available astrophysical data, a necessary first step is to extend an N-body simulator to handle distinct gravitating species.Solution method:The massively parallel Barnes–Hut tree, Fast Fourier Transform, and sorting routines of the versatile and well-vetted N-body simulator [2] gadget-2 were extended to support D distinct gravitationally interacting species. The tree implementation now vectorizes over each species’ monopole masses and positions, the Fourier routines now handle active and passive gravitational masses separately, and the sorting routines now group all particle data by type. An additional file was introduced allowing the user to specify all D2 gravitational interactions: real space, Fourier space, and lattice summation corrections. To improve monopole approximations in scenarios where the scale of the gravitational interaction depends on the mass itself, an optional tracking of the number of bodies contributing to any particular monopole approximation has been written.Restrictions:Force laws with mass-dependent scale lengths are not amenable to the implemented Fourier methods (or even the traditional [3] Fourier approach). Nodes containing highly heterogeneous collections of particles with different mass-dependent scale lengths may not be well-approximated, even with the additional tracking introduced. The collisional “gas” species can only interact via a single gravitational force law.Unusual features:The extension allows consideration of quite general and exotic interactions between bodies, and will serve as a common platform for model-dependent adjustments to the cosmological background evolution.Additional comments:Data file format is identical to that of gadget-2. Configuration file format is unchanged, save for the addition of required bindings between particle species and gravitational type.Running time:Typical running times are ≤2Dx those of gadget-2, where D is an integer between 1 and 6.

Holographic Path to the Turbulent Side of Gravity

We study the dynamics of a 2+1 dimensional relativistic viscous conformal fluid in Minkowski spacetime. Such fluid solutions arise as duals, under the "gravity/fluid correspondence", to 3+1 dimensional asymptotically anti-de Sitter (AAdS) black brane solutions to the Einstein equation. We examine stability properties of shear flows, which correspond to hydrodynamic quasinormal modes of the black brane. We find that, for sufficiently high Reynolds number, the solution undergoes an inverse turbulent cascade to long wavelength modes. We then map this fluid solution, via the gravity/fluid duality, into a bulk metric. This suggests a new and interesting feature of the behavior of perturbed AAdS black holes and black branes, which is not readily captured by a standard quasinormal mode analysis. Namely, for sufficiently large perturbed black objects (with long-lived quasinormal modes), nonlinear effects transfer energy from short to long wavelength modes via a turbulent cascade within the metric perturbation. As long wavelength modes have slower decay, this lengthens the overall lifetime of the perturbation. We also discuss various implications of this behavior, including expectations for higher dimensions, and the possibility of predicting turbulence in more general gravitational scenarios.

Phantom without ghost

The Nine-Year WMAP results combined with other cosmological data seem to indicate an enhanced favor for the phantom regime, comparing to previous analyses. This behavior, unless reversed by future observational data, suggests to consider the phantom regime more thoroughly. In this work we provide three modified gravitational scenarios in which we obtain the phantom realization without the appearance of ghosts degrees of freedom, which plague the naive approaches on the subject, namely the Brans-Dicke type gravity, the scalar-Einstein-Gauss-Bonnet gravity, and the $F(R)$ gravity, which are moreover free of perturbative instabilities. The phantom regime seems to favor the gravitational modification instead of the universe-content alteration.

Recreating Fundamental Effects in the Laboratory?

This article provides a brief (non-exhaustive) overview of some possibilities for recreating fundamental effects which are relevant for black holes (and other gravitational scenarios) in the laboratory. Via suitable condensed matter analogues and other laboratory systems, it might be possible to model the Penrose process (superradiant scattering), the Unruh effect, Hawking radiation, the Eardley instability, black-hole lasers, cosmological particle creation, the Gibbons-Hawking effect, and the Schwinger mechanism. Apart from an experimental verification of these yet unobserved phenomena, the study of these laboratory systems might shed light onto the underlying ideas and problems and should therefore be interesting from a (quantum) gravity point of view as well.

NewVRMagnification Ratios of QSO 0957+561

We present VR magnification ratios of QSO 0957+561 that are inferred from the GLITP light curves of Q0957+561A and new frames taken with the 2.56 m Nordic Optical Telescope about 14 months after the GLITP monitoring. To extract the fluxes of the two close components, two different photometric techniques are used, pho2comC and psfphot. From the two photometric approaches and a reasonable range for the time delay in the system (415-430 days), we do not obtain achromatic optical continuum ratios, but ratios depending on the wavelength. Our final global measurements ΔmAB(λV) = 0.077 ± 0.023 mag and ΔmAB(λR) = 0.022 ± 0.013 mag (1 σ intervals) are in agreement with the Oslo group results (using the same telescope in the same seasons but another photometric task and only one time delay of about 416 days). These new measurements are consistent with differential extinction in the lens galaxy, the Lyman limit system, the damped Lyα system, or the host galaxy of the QSO. The possible values for the differential extinction and the ratio of total to selective extinction in the V band are reasonable. Moreover, crude probability arguments suggest that the ray paths of the two components cross a similar dusty environment, including a network of compact dust clouds and compact dust voids. As an alternative (in fact, the usual interpretation of the old ratios), we also try to explain the new ratios as being caused by gravitational microlensing in the deflector. From magnification maps for each of the gravitationally lensed images, using different fractions of the surface mass density represented by the microlenses as well as different sizes and profiles of the V-band and R-band sources, several synthetic distributions of [ΔmAB(λV), ΔmAB(λR)] pairs are derived. In some gravitational scenarios, there is an apparent disagreement between the observed pair of ratios and the simulated distributions. However, several microlensing pictures work well. To decide between either extinction, microlensing, or a mixed scenario (extinction + microlensing), new observational and interpretation efforts are required.