Gravity In Space Research Articles

Quantum surface holonomies for loop quantum gravity and their application to black hole horizons

In this work we define a new type of flux operators on the Hilbert space of loop quantum gravity. We use them to solve an equation of the form $F(A)=c\,\Sigma$ in loop quantum gravity. This equation, which relates the curvature of a connection $A$ with its canonical conjugate $\Sigma=*E$, plays an important role for spherically symmetric isolated horizons, and, more generally, for maximally symmetric geometries and for the Kodama state. If the equation holds, the new flux operators can be interpreted as a quantization of surface holonomies from higher gauge theory. Also, they represent a kind of quantum deformation of SU(2). We investigate their properties and discuss how they can be used to define states that satisfy the isolated horizon boundary condition in the quantum theory.

A new formula for conserved charges of Lovelock gravity in AdS space–times and its generalization

Within the framework of the Lovelock gravity theory, we propose a new rank-four divergenceless tensor consisting of the Riemann curvature tensor and inheriting its algebraic symmetry characters. Such a tensor can be adopted to define conserved charges of the Lovelock gravity theory in asymptotically anti-de Sitter (AdS) space–times. Besides, inspired with the case of the Lovelock gravity, we put forward another general fourth-rank tensor in the context of an arbitrary diffeomorphism invariant theory of gravity described by the Lagrangian constructed out of the curvature tensor. On basis of the newly-constructed tensor, we further suggest a Komar-like formula for the conserved charges of this generic gravity theory.

Asymptotic shear and the intrinsic conformal geometry of null-infinity

In this article, we propose a new geometrization of the radiative phase space of asymptotically flat space-times: we show that the geometry induced on null-infinity by the presence of gravitational waves can be understood to be a generalization of the tractor calculus of conformal manifolds adapted to the case of degenerate conformal metrics. It follows that the whole formalism is, by construction, manifestly conformally invariant. We first show that a choice of asymptotic shear amounts to a choice of linear differential operator of order 2 on the bundle of scales of null-infinity. We refer to these operators as Poincaré operators. We then show that Poincaré operators are in one-to-one correspondence with a particular class of tractor connections, which we call “null-normal” (they generalize the normal tractor connection of conformal geometry). The tractor curvature encodes the presence of gravitational waves, and the non-uniqueness of flat null-normal tractor connections corresponds to the “degeneracy of gravity vacua” that has been extensively discussed in the literature. This work thus brings back the investigation of the radiative phase space of gravity to the study of (Cartan) connections and associated bundles. This should allow us, in particular, to proliferate invariants of the phase space.

Fifth force induced by a chameleon field on nested cylinders

This article investigates the properties of a scalar fifth force that arises in a scalar tensor-theory with a chameleon screening mechanism in the context of gravity space missions like the MICROSCOPE experiment. In such an experiment, the propagation of the chameleon field inside the nested cylinders of the experiment causes a fifth force when the cylinders are not perfectly co-axial. We propose a semi-analytic method to compute the field distribution and the induced fifth force and compare it to a full numerical simulation, in settings where the cylindrical symmetry is broken. The scaling of the fifth force with both the parameters of the model and the geometry of the experiment is discussed. We show that the fifth force is repulsive, hence adds a destabilizing stiffness that should be included in the force budget acting on the detector. This opens the way to a new method to constrain a scalar fifth force in screened models.

Generalized BMS charge algebra

It has been argued that the symmetries of gravity at null infinity should include a Diff$(S^2)$ factor associated to diffeomorphisms on the celestial sphere. However, the standard phase space of gravity does not support the action of such transformations. Building on earlier work by Laddha and one of the authors, we present an extension of the phase space of gravity at null infinity on which Diff$(S^2)$ acts canonically. The Poisson brackets of supertranslation and Diff$(S^2)$ charges reproduce the generalized BMS algebra introduced in arXiv:1408.2228 .

WASTE WATER TREATMENT IN SPACE CONDITIONS

Purpose. The main purpose of the article is to develop a technology for wastewater treatment in space. Methodology. The studies were performed based on the analysis of Ukrainian and foreign scientific sources and reporting data on the specifics of water use at space stations and the treatment methods of the used waters (wastewaters). In addition to international experience, our own research was used to develop a technology for wastewater treatment in space. Findings. A comprehensive review of the issues related to wastewater treatment in space, allows us to conclude that it is necessary to regenerate water at International space stations (ISS). Indeed, to ensure the life support of the astronauts, a colossal amount of water is required, and its delivery to the ISS from the Earth is expensive. Originality. The authors of the article analyzed the operation of existing wastewater treatment facilities in space and made recommendations for their use at the ISS. The developed technology for the treatment of wastewater and drinking water in zero-gravity (space) is based on the use of various reactors. They can be made of various materials (metal, plastic, etc.); they do not contain non-standard equipment that requires factory manufacturing. Compactness, complete tightness and small dimensions of bio- and physicochemical reactors allow them to be installed within the ISS. The cleaning process is easy to manage and can be fully automated. Practical value. Water problems are central to the whole world, including in space. The ISS should have a system for the wastewater treatment and their closed use, since the supply of new water to stations significantly increases the cost of space exploration. Quality water is the health and well-being of people in space. Since there is no gravity in space, centrifugal forces (centrifuges) must be used to separate suspended particles from water.

Charged black holes in AdS spaces in 4D Einstein Gauss-Bonnet gravity

Recently a non-trivial 4-dimensional theory of gravity that claims to circumvent Lovelock's theorem and avoid Ostrogradsky instability was formulated in Glavan and Lin (2020) [1]. This theory, named “4D Einstein Gauss-Bonnet gravity”, presents several novel predictions for cosmology and black hole physics. In this paper, we generalize the vacuum black hole solution of Glavan & Lin to include electric charge in an anti-de Sitter space and explore some properties of this solution such as the asymptotics, properties of the horizons, the general relativity limit and thermodynamics.

Fixed-time terminal sliding mode control of spinning tether system for artificial gravity environment in high eccentricity orbit

In this paper a new strategy is introduced to produce an artificial gravity environment by spinning tether system (STS) in high eccentricity transfer orbit. Compared with other common artificial gravity environment methods, spinning tether system has simple structure, light weight and suitable spin velocity, which is more favorable for the spacecraft to generate the artificial gravity in the weightless space. The mathematical model of the spinning tether system is established by the second type of Lagrangian equation firstly. Then, the tension control law and the tangential thrust control law of tether system are designed. Through the mutual coordination of both control law, a method of Variable Moment-of-Inertia and Angular Momentum is realized which makes the tether system spin along the expected spinning velocity in most of the processes of uniform velocity development by constant tangential thrust. With the end of development, the artificial gravity environment level on the spacecraft also met the expected requirements, and the nominal trajectory curve of STS under the high-eccentricity transfer orbit was obtained. Finally, considering the rapidity of convergence time, a fixed time terminal sliding mode controller is designed to control the closed-loop trajectory curve of the tether system.

Energy transfer and thermodynamics of quantum gravity computation

In a quantum gravity environment, the processes and events are causally non-separable because the term of time and the time-steps have no interpretable meaning in a non-fixed causality structure. Here, we study the energy transfer and thermodynamics of quantum gravity computations. We show that a non-fixed causality stimulates entropy transfer between the quantum gravity environment and the independent local systems of the quantum gravity space. We prove that the entropy transfer reduces the entropies of the contributing local systems and increases the entropy of the quantum gravity environment. We reveal on a smooth Cauchy slice that the space-time geometry of the quantum gravity environment dynamically adapts to the vanishing causality. We define the corresponding Hamiltonians and the causal development of the quantum gravity environment.

Relativistic stars in mass-varying massive gravity

Mass-varying massive gravity allows the graviton mass to vary according to different environments. We investigate neutron star and white dwarf solutions in this theory and find that the graviton mass can become very large near the compact stars and settle down quickly to small cosmological values away the stars, similar to that of black holes in the theory. It is found that there exists a tower of compact star solutions where the graviton mass decreases radially to zero non-trivially. We compute the massive graviton effects on the mass-radius relations of the compact stars, and also compare the relative strengths between neutron stars and white dwarfs in constraining the parameter space of mass-varying massive gravity.

De Sitter duality and logarithmic decay of dark energy

We investigate infrared dynamics of four-dimensional Einstein gravity in de Sitter space. We set up a general framework to investigate dynamical scaling relations in quantum/classical gravitational theories. The conformal mode dependence of Einstein gravity is renormalized to the extent that general covariance is not manifest. We point out that the introduction of an inflaton is necessary as a counterterm. We observe and postulate a duality between quantum effects in Einstein gravity and classical evolutions in an inflation (or quintessence) model. The effective action of Einstein gravity can be constructed as an inflation model with manifest general covariance. We show that $g=G_N H^2/\pi$: the only dimensionless coupling of the Hubble parameter $H^2$ and the Newton's coupling $G_N$ in Einstein gravity is screened by the infrared fluctuations of the conformal mode. We evaluate the one-loop $\beta$ function of $g$ with respect to the cosmic time $\log Ht$ as $\beta(g)=-(1/2)g^2$, i.e., $g$ is asymptotically free toward the future. The exact $\beta$ function with the backreaction of $g$ reveals the existence of the ultraviolet fixed point. It indicates that the de Sitter expansion stared at the Planck scale with a minimal entropy $S=2$. We have identified the de Sitter entropy $1/g$ with the von Neumann entropy of the conformal zero mode. The former evolves according to the screening of $g$ and the Gibbons-Hawking formula. The latter is found to increase by diffusion in the stochastic process at the horizon in a consistent way. Our Universe is located very close to the fixed point $g=0$ with a large entropy. We discuss possible physical implications of our results such as logarithmic decay of dark energy.

Correlation measure equivalence in dynamic causal structures of quantum gravity

We prove an equivalence transformation between the correlation measure functions of the causally-unbiased quantum gravity space and the causally-biased standard space. The theory of quantum gravity fuses the dynamic (nonfixed) causal structure of general relativity and the quantum uncertainty of quantum mechanics. In a quantum gravity space, the events are causally nonseparable and all time bias vanishes, which makes it no possible to use the standard causally-biased entropy and the correlation measure functions. Since a corrected causally-unbiased entropy function leads to an undefined, obscure mathematical structure, in our approach the correction is made in the data representation of the causally-unbiased space. Here we prove that the standard causally-biased entropy function with a data correction can be used to identify correlations in dynamic causal structures. As a corollary, all mathematical properties of the causally-biased correlation measure functions are preserved in the causally-unbiased space. The equivalence transformation allows us to measure correlations in a quantum gravity space with the stable, well-defined mathematical background and apparatus of the causally-biased functions of quantum Shannon theory.

On the Quantization of Time, Space and Gravity

We combine the de Broglie Matter Wave Equation with the Heisenberg Uncertainty Principle to derive an equation for time as a wave. This happens to be the first time that these two statements have been combined in this manner to derive an equation for time. The result is astounding. Time turns out to be a minuscule blob of quantum electromagnetic energy in perpetual angular momentum. From this time equation, we derive an equation for space which turns out to also predict a string (like the string of string theory). We then combine the time equation with the space equation to derive an equation for the inverse of quantum gravity which is also surprisingly electromagnetic in nature. This last statement implies that space is multidimensional and gravity in multidimensional space is not quantized, but its inverse (which is single-dimensional) is.

Long-term digital frequency-stabilized laser source for large-scale passive laser gyroscopes.

We report on the development of a digitally controlled long-term frequency stabilized ultrastable laser source, which serves as an injection laser to stabilize the perimeter of a 3 m × 3 m heterolithic passive resonant gyroscope. We operate the gyroscope at two different cavity modes to reduce back-scattering coupling disturbance for gyroscope locking. This scheme increases the requirement for the injection laser frequency stability since we are using the wavelength of the laser as the length standard for the heterolithic gyroscope structure. The laser source is digitally locked to an ultrastable high-finesse Fabry-Perot cavity and a femtosecond optical frequency comb referenced to an active hydrogen maser simultaneously. The fractional frequency stability of the locked laser is better than 1.2 × 10-14 for averaging times from 0.1 s to 10 000 s. The short-term frequency stability is limited by the stability of the Fabry-Perot cavity, and the long-term frequency stability is limited by the stability of the frequency comb. The digital locking system enables the laser to run autonomously for weeks and can quickly relock itself within seconds to ensure continuous running of the gyroscope. The digital frequency stabilization technique can also fulfill the requirements of space gravitational waves detection and the next generation space gravity recovery mission.

Methods of metrological support of space gravity gradiometers

An analysis is made of the known methods for calibrating and evaluating the parameters of implemented space gradiometers. There are 4 main stages: laboratory assessment of amendments, assessment of amendments during operation, assessment of amendments from independent data, calibration. A description of each step is provided. Two methods for calibrating space gradiometers are proposed: based on a complex of calibration sites and an onboard stand. The principles of building elements of a complex of calibration sites are investigated and their structure is formed. The analysis of the possibility of using the onboard mass on board the spacecraft for calibrating the space gradiometer is carried out. The main parameters of the onboard stand are highlighted. Scope: determination of metrological characteristics of space gravitational gradiometers.

A baseline Antarctic GIA correction for space gravimetry

Within the past decade, newly collected GPS data and geochronological constraints have resulted in refinement of glacial isostatic adjustment (GIA) models for Antarctica. These are critical to estimating and understanding ice mass changes at present-day. A correction needs to be made when using space gravity for ice mass balance assessments due to the fact that any vertical movements of the solid Earth masquerade as changes in ice mass, and these must be carefully removed. The main upshot of the new Antarctic GIA models is a downward revision of negative ice mass trends deduced from the Gravity Recovery and Climate Experiment (GRACE), resulting from a reduced GIA correction. This revision places GRACE inferred trend in mass balance within the one sigma uncertainty of mass balance deduced by altimetry. Because uncertainties in Holocene ice history and the low viscosity rheology beneath the West Antarctic Ice Sheet (WAIS) continue to vex further improvement in predictions of present-day GIA gravity rate, more emphasis has been given to regional-scale GIA models. Here we use a Bayesian method to explore the gravimetric GIA trend over Antarctica, both with and without the impact of a late Pleistocene Antarctic ice loads, along with the contribution of oceanic loads. We call this model without loads associated with Antarctica a baseline for regional GIA models to build upon. We consider variations of the radial mantle viscosity profile and the volume of continental-scale ice sheets during the last glacial cycle. The modeled baseline GIA is mainly controlled by the lower mantle viscosity and continental levering caused by ocean loading. We find that the predicted baseline GIA correction weakly depends on the ice history. This correction averages to +28.4 [16.5–41.9, 95% confidence] Gt/yr, of which +23.6±5.3 are contributed by East Antarctica. In contrast, with late-Pleistocene Antarctic-proximal ice included, the total modeled mass trend due to GIA is +73.7 [30.1–114.7] Gt/yr. A baseline GIA correction of 28.4 Gt/yr is of order 50% of the mean net mass trend measured during the period 1992-2017. The statistical analysis provides tools for properly synthesizing any regional Antarctic GIA model with a self-consistent far-field component. This may prove important for accounting for both global and regional 3-D variations in mantle viscosity.

Opposed-Flow Flame Spread and Extinction in Electric Wires: The Effects of Gravity, External Radiant Heat Flux, and Wire Characteristics on Wire Flammability

Combustion of electric wires is the most probable cause of fire in human space activities. Therefore, the fire performance of electric wires in microgravity conditions must be thoroughly analyzed. This study investigates the opposed-flow flame spread and its limits in electric wires preheated by external radiation, under both normal gravity and microgravity, to understand their fire performance when exposed to external heat sources in such gravity conditions. The experiments were performed on low-density polyethylene (LDPE)-insulated copper (Cu) wires having an outer diameter of 4 mm and differing in core diameter (2.5 and 0.7 mm, corresponding to insulation thicknesses of 0.75 and 1.65 mm, respectively). Both standard and black LDPE insulations were used to study the effect of radiation absorption on the wire preheating and subsequent flame spread. The comparison of the flame spread limits revealed that the wire with the thicker Cu core was less flammable under both normal gravity and microgravity; in particular, its flammability further decreased in the case of microgravity, in contrast with thinner electric wires (~ 1 mm outer diameter), which exhibited higher flammability in the same gravity condition. These results suggest that different mechanisms, for thicker and thinner wires, determining the critical conditions to sustain flame spread under microgravity. This study provides valuable information about the fire performance of electric wires in space gravity.

Theory of quantum gravity information processing

The theory of quantum gravity is aimed to fuse general relativity with quantum theory into a more fundamental framework. The space of quantum gravity provides both the non-fixed causality of general relativity and the quantum uncertainty of quantum mechanics. In a quantum gravity scenario, the causal structure is indefinite and the processes are causally non-separable. Here, we provide a model for the information processing structure of quantum gravity. We show that the quantum gravity environment is an information resource-pool from which valuable information can be extracted. We analyze the structure of the quantum gravity space and the entanglement of the space-time geometry. We study the information transfer capabilities of quantum gravity space and define the quantum gravity channel. We reveal that the quantum gravity space acts as a background noise on the local environment states. We characterize the properties of the noise of the quantum gravity space and show that it allows the separate local parties to simulate remote outputs from the local environment state, through the process of remote simulation.

Media Optimization, Strain Compatibility, and Low-Shear Modeled Microgravity Exposure of Synthetic Microbial Communities for Urine Nitrification in Regenerative Life-Support Systems.

Urine is a major waste product of human metabolism and contains essential macro- and micronutrients to produce edible microorganisms and crops. Its biological conversion into a stable form can be obtained through urea hydrolysis, subsequent nitrification, and organics removal, to recover a nitrate-enriched stream, free of oxygen demand. In this study, the utilization of a microbial community for urine nitrification was optimized with the focus for space application. To assess the role of selected parameters that can impact ureolysis in urine, the activity of six ureolytic heterotrophs (Acidovorax delafieldii, Comamonas testosteroni, Cupriavidus necator, Delftia acidovorans, Pseudomonas fluorescens, and Vibrio campbellii) was tested at different salinities, urea, and amino acid concentrations. The interaction of the ureolytic heterotrophs with a nitrifying consortium (Nitrosomonas europaea ATCC 19718 and Nitrobacter winogradskyi ATCC 25931) was also tested. Lastly, microgravity was simulated in a clinostat utilizing hardware for in-flight experiments with active microbial cultures. The results indicate salt inhibition of the ureolysis at 30 mS cm-1, while amino acid nitrogen inhibits ureolysis in a strain-dependent manner. The combination of the nitrifiers with C. necator and V. campbellii resulted in a complete halt of the urea hydrolysis process, while in the case of A. delafieldii incomplete nitrification was observed, and nitrite was not oxidized further to nitrate. Nitrate production was confirmed in all the other communities; however, the other heterotrophic strains most likely induced oxygen competition in the test setup, and nitrite accumulation was observed. Samples exposed to low-shear modeled microgravity through clinorotation behaved similarly to the static controls. Overall, nitrate production from urea was successfully demonstrated with synthetic microbial communities under terrestrial and simulated space gravity conditions, corroborating the application of this process in space.

Concept study and preliminary design of a cold atom interferometer for space gravity gradiometry

We study a space-based gravity gradiometer based on cold atom interferometry and its potential for the Earth’s gravitational field mapping. The instrument architecture has been proposed in Carraz et al (2014 Microgravity Sci. Technol. 26 139) and enables high-sensitivity measurements of gravity gradients by using atom interferometers in a differential accelerometer configuration. We present the design of the instrument including its subsystems and analyze the mission scenario, for which we derive the expected instrument performances, the requirements on the sensor and its key subsystems, and the expected impact on the recovery of the Earth gravity field.