Gravitational Trapping Research Articles

Self-Shielding Enhanced Organics Synthesis in an Early Reduced Earth's Atmosphere.

Earth is expected to have acquired a reduced proto-atmosphere enriched in H2 and CH4 through the accretion of building blocks that contain metallic Fe and/or the gravitational trapping of surrounding nebula gas. Such an early, wet, reduced atmosphere that covers a proto-ocean would then ultimately evolve toward oxidized chemical compositions through photochemical processes that involve reactions with H2O-derived oxidant radicals and the selective escape of hydrogen to space. During this time, atmospheric CH4 could be photochemically reprocessed to generate not only C-bearing oxides but also organics. However, the branching ratio between organic matter formation and oxidation remains unknown despite its significance on the abiotic chemical evolution of early Earth. Here, we show via numerical analyses that UV absorptions by gaseous hydrocarbons such as C2H2 and C3H4 significantly suppress H2O photolysis and subsequent CH4 oxidation during the photochemical evolution of a wet proto-atmosphere enriched in H2 and CH4. As a result, nearly half of the initial CH4 converted to heavier organics along with the deposition of prebiotically essential molecules such as HCN and H2CO on the surface of a primordial ocean for a geological timescale order of 10-100 Myr. Our results suggest that the accumulation of organics and prebiotically important molecules in the proto-ocean could produce a soup enriched in various organics, which might have eventually led to the emergence of living organisms.

Light behaviors outside a black hole surrounded by dark matter

Here we show the light behaviors outside a Schwarzschild black hole that is surrounded by a dark matter model proposed by Konoplya (see Konoplya R., Phys. Lett. B, 795 (2019) 1). It is worth mentioning that dark matter is studied for the first time in an optical system by transformation optics theory. Different from other works of mimicking curved spaces with geodesic equations, we calculate ray trajectories using the equivalent anisotropic material tensor, which can also be used to simulate field patterns for wave optics. The results from geometric optics and wave optics agree with each other perfectly, showing the gravitational lensing effects and light trapping properties, revealing the influences of dark matter on the black hole shadow and gravity, which can give some hints for future observations.

Radiation hydrodynamic simulations of massive star formation via gravitationally trapped H ii regions – spherically symmetric ionized accretion flows

This paper investigates the gravitational trapping of HII regions predicted by steady-state analysis using radiation hydrodynamical simulations. We present idealised spherically symmetric radiation hydrodynamical simulations of the early evolution of HII regions including the gravity of the central source. As with analytic steady state solutions of spherically symmetric ionised Bondi accretion flows, we find gravitationally trapped HII regions with accretion through the ionisation front onto the source. We found that, for a constant ionising luminosity, fluctuations in the ionisation front are unstable. This instability only occurs in this spherically symmetric accretion geometry. In the context of massive star formation, the ionising luminosity increases with time as the source accretes mass. The maximum radius of the recurring HII region increases on the accretion timescale until it reaches the sonic radius, where the infall velocity equals the sound speed of the ionised gas, after which it enters a pressure-driven expansion phase. This expansion prevents accretion of gas through the ionisation front, the accretion rate onto the star decreases to zero, and it stops growing from accretion. Because of the time required for any significant change in stellar mass and luminosity through accretion our simulations keep both mass and luminosity constant and follow the evolution from trapped to expanding in a piecewise manner. Implications of this evolution of HII regions include a continuation of accretion of material onto forming stars for a period after the star starts to emit ionising radiation, and an extension of the lifetime of ultracompact HII regions.

Heterogeneity-assisted carbon dioxide storage in marine sediments

Global climate change is a pressing problem caused by the accumulation of anthropogenic greenhouse gas emissions in the atmosphere. Carbon dioxide (CO2) capture and storage is a promising component of a portfolio of options to stabilize atmospheric CO2 concentrations. Meaningful capture and storage requires the permanent isolation of enormous amounts of CO2 away from the atmosphere. We investigate the effectiveness of heterogeneity-induced trapping mechanism, in potential synergy with a self-sealing gravitational trapping mechanism, for secure CO2 storage in marine sediments. We conduct the first comprehensive study on heterogeneous marine sediments with various thicknesses at various ocean depths. Prior studies of gravitational trapping have assumed homogeneous (deep-sea) sediments, but numerous studies suggest reservoir heterogeneity may enhance CO2 trapping. Heterogeneity can deter the upward migration of CO2 and prevent leakage through the seafloor into the seawater. Using geostatistically-based Monte Carlo simulations of CO2 transport in heterogeneous sediment, we show that strong spatial variability in permeability is a dominant physical mechanism for secure CO2 storage in marine sediments below 1.2 km water depth (less than half of the depth needed for the gravitational trapping). We identify thresholds for sediment thickness, mean permeability and porosity, and their relationships to meaningful injection rates. Our results for the U.S. Gulf of Mexico suggest that heterogeneity-assisted trapping has a greater areal extent with more than three times the CO2 storage capacity for secure offshore CO2 storage than with gravitational trapping. These characteristics offer CO2 storage opportunities that are closer to coasts, more accessible, and likely to be less costly.

Injectivity Evaluation for Offshore CO2 Sequestration in Marine Sediments

Global and regional climate change caused by greenhouse gases emissions has stimulated interest in developing various technologies (such as carbon dioxide (CO2) geologic sequestration in brine reservoirs) to reduce the concentrations of CO2 in the atmosphere. This study develops a statistical framework to identify gravitational CO2 trapping processes and to quantitatively evaluate both CO2 injectivity (or storage capacity) and leakage potential from marine sediments which exhibit heterogeneous permeability and variable thicknesses. We focus on sets of geostatistically-based heterogeneous models populated with fluid flow parameters from several reservoir sites in the U.S. Gulf of Mexico (GOM). A computationally efficient uncertainty quantification study was conducted with results suggesting that permeability heterogeneity and anisotropy, seawater depth, and sediment thickness can all significantly impact CO2 flow and trapping. Large permeability/porosity heterogeneity can enhance gravitational, capillary, and dissolution trapping, which acts to deter CO2 upward migration and subsequent leakage onto the seafloor. When log permeability variance is 5, self-sealing with heterogeneity-enhanced gravitation trapping can be achieved even when water depth is 1.2km. This extends the previously identified self-sealing condition that water depth be greater than 2.7km. Our results have yielded valuable insight into the conditions under which safe storage of CO2 can be achieved in offshore environments. The developed statistical framework is general and can be adapted to study other offshore sites worldwide.

DO MAGNETIC FIELDS DESTROY BLACK HOLE ACCRETION DISK g-MODES?

Diskoseismology, the theoretical study of normal mode oscillations in geometrically thin, optically thick accretion disks, is a strong candidate to explain some QPOs in the power spectra of many black hole X-ray binary systems. The existence of g-modes, presumably the most robust and visible of the modes, depends on general relativistic gravitational trapping in the hottest part of the disk. As the existence of the required cavity in the presence of magnetic fields has been put into doubt by theoretical calculations, we will explore in greater generality what the inclusion of magnetic fields has to say on the existence of g-modes. We use an analytical perturbative approach on the equations of MHD to assess the impact of such effects. Our main conclusion is that there appears to be no compelling reason to discard g-modes. In particular, the inclusion of a non-zero {\it radial} component of the magnetic field enables a broader scenario for cavity non-destruction, especially taking into account recent simulations' saturation values for the magnetic field.

Geological storage of CO2 within the oceanic crust by gravitational trapping

AbstractThe rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a key driver of anthropogenic climate change. Mitigation strategies include improved efficiency, using renewable energy, and capture and long‐term sequestration of CO2. Most sequestration research considers CO2 injection into deep saline aquifers or depleted hydrocarbon reservoirs. Unconventional suggestions include CO2 storage in the porous volcanic lavas of uppermost oceanic crust. Here we test the feasibility of injecting CO2 into deep‐sea basalts and identify sites where CO2 should be both physically and gravitationally trapped. We use global databases to estimate pressure and temperature, hence density of CO2 and seawater at the sediment‐basement interface. At previously suggested sites on the Juan de Fuca Plate and in the eastern equatorial Pacific Ocean, CO2 is gravitationally unstable. However, we identify five sediment‐covered regions where CO2 is denser than seawater, each sufficient for several centuries of anthropogenic CO2 emissions.

Gravitational trapping of carbon dioxide in deep ocean sediments: hydraulic fracturing and mechanical stability

Gravitational trapping of carbon dioxide in deep ocean sediments is attractive both for the long term stability provided by gravity as well as the large volume and hence storage capacity of deep ocean sediments at necessary depths. Unfortunately, most pelagic sediments suffer from extremely low permeability and are not expected to have an overlying mechanical seal, making emplacement of CO2 contingent upon large scale hydraulic fracturing and some mechanism of arresting fracture growth before reaching the seafloor. An experimental design is presented with the capability of testing a variety of proposed fracture arrest mechanisms.

A global assessment of deep-sea basalt sites for carbon sequestration

AbstractIn recent years, the debate over the most effective means to stabilize greenhouse gas concentrations in the atmosphere has endorsed multiple approaches and a variety of technologies. Assuring secure storage of anthropogenic carbon dioxide is one of our most pressing global scientific challenges that may contribute to achieving a stable solution over the next several decades. Geological sequestration by injection into deep-sea basalt formations provides unique and significant advantages over other potential storage options, including: (a) vast reservoir capacities with high porosity and permeability, sufficient to accommodate centuries-long U.S. production of fossil fuel CO~2~ at locations within a few hundreds of kilometers of populated areas; (b) chemical reactivity of CO~2~ with basalt and in situ fluids to produce stable, non-toxic carbonates; and (c) significant risk reduction for post-injection leakage by geological, gravitational, and mineral trapping mechanisms. We compare independent trapping mechanisms available in deep-sea basalts to those in saline aquifers, which have also been proposed as potential storage environments for anthropogenic carbon dioxide. We suggest that deep-sea basalts offer significant advantages over saline aquifers, in terms of reduced risk of post-injection leakage and storage capacity. Using a global site assessment strategy to highlight the most secure oceanic basalt sites that provide all trapping mechanisms, we initially identify potential target regions that occur in deep-sea basalt and calculate the potential injection volume for each. The largest volumes and most secure basalt sites occur in regions adjacent to intermediate- to fast-spreading seismic ridges as well as deep aseismic ridges. We then use site-specific criteria, such as abundance of ODP and IODP drill sites with basement penetration, permeability and/or porosity data, to refine volume calculations and to prioritize these target regions as promising locations to securely accommodate carbon dioxide injection. Pilot injection studies in deep-sea basalts are necessary to establish the viability of these reservoirs for future CO~2~ sequestration. We suggest that basaltic crust at deep ocean sites offers vast capacity and potential for permanent sequestration of carbon dioxide to mitigate atmospheric build-up of this greenhouse gas.

GRAVITATIONAL FIELD OF SPHERICAL BRANES

The warped solution of Einstein's equations corresponding to the spherical brane in five-dimensional AdS is considered. This metric represents interiors of black holes on both sides of the brane and can provide gravitational trapping of physical fields on the shell. It is found that the analytic form of the coordinate transformations from the Schwarzschild to co-moving frame that exists only in five dimensions. It is shown that in the static coordinates active gravitational mass of the spherical brane, in agreement with Tolman's formula, is negative, i.e. such objects are gravitationally repulsive.

Time Variation in G24.78+0.08 A1: Evidence for an Accreting Hypercompact H ii Region?

Over a timescale of a few years, an observed change in the optically thick radio continuum flux can indicate whether an unresolved H II region around a newly formed massive star is changing in size. In this Letter we report on a study of archival VLA observations of the hypercompact H II region G24.78+0.08 A1 that shows a decrease of ~ 45 % in the 6-cm flux over a five year period. Such a decrease indicates a contraction of ~ 25 % in the ionized radius and could be caused by an increase in the ionized gas density if the size of the H II region is determined by a balance between photoionization and recombination. This finding is not compatible with continuous expansion of the H II region after the end of accretion onto the ionizing star, but is consistent with the hypothesis of gravitational trapping and ionized accretion flows if the mass-accretion rate is not steady.

Gravitational trapping of carbon dioxide in deep sea sediments: Permeability, buoyancy, and geomechanical analysis

Liquid carbon dioxide injected in deep‐sea sediments at km depths and near freezing temperatures is denser than surrounding pore water and will be trapped by gravitational forces. Storage capacity for CO2 in such formations below the ocean floor is shown to vary with seafloor depth, geothermal gradient, porosity, and pore water salinity. The formation permeability, or the successful engineering of such permeability through hydraulic fracturing, will determine the capacity for gravitational trapping in deep‐sea geological formations. We conclude that most ocean sediments at appropriate depth will lack the required permeability and that conventional hydraulic fracturing would only be possible in carefully selected sites.

Gravitational trapping near domain walls and stable solitons

In this work, the behavior of test particles near a domain wall of a stable false vacuum bubble is studied. It is shown that matter is naturally trapped in the vicinity of a static domain wall, and also, that there is a discontinuity in the test particle's velocity when crossing the domain wall. The latter is unexpected as it stands in contrast to Newtonian theory, where infinite forces are not allowed. The weak field limit is defined in order to show that there is no conflict with the nonrelativistic behavior of gravitational fields and particle motions under these conditions.

Gravitational trapping potential with arbitrary extra dimensions

We extend a recently discovered, nonsingular 6-dimensional brane, solution to $D=4+n$ dimensions. As with the previous 6D solution the present solution provides a gravitational trapping mechanism for fields of spin 0, $\frac{1}{2}$, 1, and 2. There is an important distinction between two extra dimensions and $n$ extra dimensions that makes this more than a trivial extension. In contrast to gravity in $n>2$ dimensions, gravity in $n=2$ dimensions is conformally flat. The stress-energy tensor required by this solution has reasonable physically properties, and for $n=2$ and $n=3$ can be made to asymptotically go to zero as one moves away from the brane.

Brane in 6D with an increasing gravitational trapping potential

A new solution to Einstein equations in (1+5)-spacetime with an embedded (1+3) brane is given. This solution localizes the zero modes of all kinds of matter fields and 4-gravity on the (1+3) brane by an increasing, transverse gravitational potential. This localization occurs despite the fact that the gravitational potential is not a decreasing exponential, and asymptotically approaches a finite value rather than zero.

Gravitational localization of all local fields on the brane

We present a new (p−1)-brane solution to Einstein's equations in a general spacetime dimension. This solution is a natural generalization of the stringlike defect solution with codimension 2 in 6 spacetime dimensions, which has been recently discovered by Gogberashvili and Singleton, to a general (p−1)-brane solution with codimension n in general D=p+n spacetime dimensions. It is shown that all the local fields are localized on the brane only through the gravitational interaction although this solution does not have a warp factor and takes a finite value in the radial infinity. Thus, this solution is a solution in an arbitrary spacetime dimension realizing the idea of “gravitational trapping” of the whole bulk fields on the brane within the framework of a local field theory. Some problems associated with this solution and localization are pointed out.

Brane-Universe in six dimensions

Growing nonsingular solution of 6-dimensional Einstein equations for the 4-brane in infinite transversal 2-space is found. This solution provides gravitational trapping of matter and 4-dimensional gravity on the brane without extra delta-like source. The suitable solution exists in the case of the (2+4)-space and not exists for the (1+5)-signature.

Wormhole surgery and cosmology on the brane: The world is not enough

We show that the 5-dimensional model introduced by Randall and Sundrum is (half of) a wormhole, and that this is a general result in models of the RS type. We also discuss the gravitational trapping of a scalar particle in 5-d spacetimes. Finally, we present a simple model of brane-world cosmology in which the background is a static anti-de Sitter manifold, and the location of the two 3-branes is determined by the technique of ``surgical grafting''.

Neutrino Pair Annihilation in the Gravitation of Gamma‐Ray Burst Sources

We study semianalytically the gravitational effects on neutrino pair annihilation near the neutrinosphere and around the thin accretion disk. For the disk case, we assume that the accretion disk is isothermal and that the gravitational field is dominated by the Schwarzschild black hole. General relativistic effects are studied only near the rotation axis. The energy deposition rate is enhanced by the effect of orbital bending toward the center. However, the effects of the redshift and gravitational trapping of the deposited energy reduce the effective energy of the gamma-ray burst's source. Although each effect is substantial, the effects partly cancel one another. As a result, the gravitational effects do not substantially change the energy deposition rate for either the spherically symmetric case or the disk case.

Trajectories of zero rest-mass particles and gravitational trapping in interior geometries

Trajectories of zero rest-mass particles (ZRP) are quite well studied in Schwarzschild geometry but not in interior geometries. In this paper the trajectories of ZRP have been obtained and discussed for Tolman's IV solution. It is found that the inward moving ZRP approaches the centre up to a minimum distance and then draws away from it if the angle of emittance is less than that for the ‘cone of avoidance’. If, however, the angle of emittance is greater than the angle of ‘cone of avoidance’, the ZRP moves inwards up to a certain minimum distance from the centre and then stays at this distance in a circular orbit. In this regard four different kinds of orbits have been identified. Calculations have also been done for the number of ZRP that will escape such massive configuration.