946 research outputs found
Hydrostatic Equilibrium of a Perfect Fluid Sphere with Exterior Higher-Dimensional Schwarzschild Spacetime
We discuss the question of how the number of dimensions of space and time can
influence the equilibrium configurations of stars. We find that dimensionality
does increase the effect of mass but not the contribution of the pressure,
which is the same in any dimension. In the presence of a (positive)
cosmological constant the condition of hydrostatic equilibrium imposes a lower
limit on mass and matter density. We show how this limit depends on the number
of dimensions and suggest that is more effective in 4D than in
higher dimensions. We obtain a general limit for the degree of compactification
(gravitational potential on the boundary) of perfect fluid stars in
-dimensions. We argue that the effects of gravity are stronger in 4D than in
any other number of dimensions. The generality of the results is also
discussed
Stellar models with Schwarzschild and non-Schwarzschild vacuum exteriors
A striking characteristic of non-Schwarzschild vacuum exteriors is that they
contain not only the total gravitational mass of the source, but also an {\it
arbitrary} constant. In this work, we show that the constants appearing in the
"temporal Schwarzschild", "spatial Schwarzschild" and
"Reissner-Nordstr{\"o}m-like" exteriors are not arbitrary but are completely
determined by star's parameters, like the equation of state and the
gravitational potential. Consequently, in the braneworld scenario the
gravitational field outside of a star is no longer determined by the total mass
alone, but also depends on the details of the internal structure of the source.
We show that the general relativistic upper bound on the gravitational
potential , for perfect fluid stars, is significantly increased in
these exteriors. Namely, , and for the
temporal Schwarzschild, spatial Schwarzschild and Reissner-Nordstr{\"o}m-like
exteriors, respectively. Regarding the surface gravitational redshift, we find
that the general relativistic Schwarzschild exterior as well as the braneworld
spatial Schwarzschild exterior lead to the same upper bound, viz., .
However, when the external spacetime is the temporal Schwarzschild metric or
the Reissner-Nordstr{\"o}m-like exterior there is no such constraint: . This infinite difference in the limiting value of is because for
these exteriors the effective pressure at the surface is negative. The results
of our work are potentially observable and can be used to test the theory.Comment: 19 pages, 3 figures and caption
Transition from decelerated to accelerated cosmic expansion in braneworld universes
Braneworld theory provides a natural setting to treat, at a classical level,
the cosmological effects of vacuum energy. Non-static extra dimensions can
generally lead to a variable vacuum energy, which in turn may explain the
present accelerated cosmic expansion. We concentrate our attention in models
where the vacuum energy decreases as an inverse power law of the scale factor.
These models agree with the observed accelerating universe, while fitting
simultaneously the observational data for the density and deceleration
parameter. The redshift at which the vacuum energy can start to dominate
depends on the mass density of ordinary matter. For Omega = 0.3, the transition
from decelerated to accelerated cosmic expansion occurs at z approx 0.48 +/-
0.20, which is compatible with SNe data. We set a lower bound on the
deceleration parameter today, namely q > - 1 + 3 Omega/2, i.e., q > - 0.55 for
Omega = 0.3. The future evolution of the universe crucially depends on the time
when vacuum starts to dominate over ordinary matter. If it dominates only
recently, at an epoch z < 0.64, then the universe is accelerating today and
will continue that way forever. If vacuum dominates earlier, at z > 0.64, then
the deceleration comes back and the universe recollapses at some point in the
distant future. In the first case, quintessence and Cardassian expansion can be
formally interpreted as the low energy limit of our model, although they are
entirely different in philosophy. In the second case there is no correspondence
between these models and ours.Comment: In V2 typos are corrected and one reference is added for section 1.
To appear in General Relativity and Gravitatio
Mass and Charge in Brane-World and Non-Compact Kaluza-Klein Theories in 5 Dim
In classical Kaluza-Klein theory, with compactified extra dimensions and
without scalar field, the rest mass as well as the electric charge of test
particles are constants of motion. We show that in the case of a large extra
dimension this is no longer so. We propose the Hamilton-Jacobi formalism,
instead of the geodesic equation, for the study of test particles moving in a
five-dimensional background metric. This formalism has a number of advantages:
(i) it provides a clear and invariant definition of rest mass, without the
ambiguities associated with the choice of the parameters used along the motion
in 5D and 4D, (ii) the electromagnetic field can be easily incorporated in the
discussion, and (iii) we avoid the difficulties associated with the "splitting"
of the geodesic equation. For particles moving in a general 5D metric, we show
how the effective rest mass, as measured by an observer in 4D, varies as a
consequence of the large extra dimension. Also, the fifth component of the
momentum changes along the motion. This component can be identified with the
electric charge of test particles. With this interpretation, both the rest mass
and the charge vary along the trajectory. The constant of motion is now a
combination of these quantities. We study the cosmological variations of charge
and rest mass in a five-dimensional bulk metric which is used to embed the
standard k = 0 FRW universes. The time variations in the fine structure
"constant" and the Thomson cross section are also discussed.Comment: V2: References added, discussion extended. V3 is identical to V2,
references updated. To appear in General Relativity and Gravitatio
Accelerated expansion from braneworld models with variable vacuum energy
In braneworld models a variable vacuum energy may appear if the size of the
extra dimension changes during the evolution of the universe. In this scenario
the acceleration of the universe is related not only to the variation of the
cosmological term, but also to the time evolution of and, possibly, to the
variation of other fundamental "constants" as well. This is because the
expansion rate of the extra dimension appears in different contexts, notably in
expressions concerning the variation of rest mass and electric charge. We
concentrate our attention on spatially-flat, homogeneous and isotropic,
brane-universes where the matter density decreases as an inverse power of the
scale factor, similar (but at different rate) to the power law in FRW-universes
of general relativity.
We show that these braneworld cosmologies are consistent with the observed
accelerating universe and other observational requirements. In particular,
becomes constant and asymptotically in
time. Another important feature is that the models contain no "adjustable"
parameters. All the quantities, even the five-dimensional ones, can be
evaluated by means of measurements in 4D. We provide precise constrains on the
cosmological parameters and demonstrate that the "effective" equation of state
of the universe can, in principle, be determined by measurements of the
deceleration parameter alone. We give an explicit expression relating the
density parameters , and the deceleration
parameter . These results constitute concrete predictions that may help in
observations for an experimental/observational test of the model.Comment: References added, typos correcte
Late time cosmic acceleration from vacuum Brans-Dicke theory in 5D
We show that the scalar-vacuum Brans-Dicke equations in 5D are equivalent to
Brans-Dicke theory in 4D with a self interacting potential and an effective
matter field. The cosmological implication, in the context of FRW models, is
that the observed accelerated expansion of the universe comes naturally from
the condition that the scalar field is not a ghost, i.e., . We
find an effective matter-dominated 4D universe which shows accelerated
expansion if . We study the question of whether
accelerated expansion can be made compatible with large values of ,
within the framework of a 5D scalar-vacuum Brans-Dicke theory with variable,
instead of constant, parameter . In this framework, and based on a
general class of solutions of the field equations, we demonstrate that
accelerated expansion is incompatible with large values of .Comment: In V2 the summary section is expanded. To be published in Classical
and Quantum Gravity
Oxidation of the borohydride Ion at silver nanoparticles on a glassy carbon electrode (GCE) using pulsed potential techniques
Direct oxidation borohydride fuel cells are very attractive energy conversion devices. Silver has been reported as one of the few materials which can catalyze an 8-electron oxidation. Potential step amperometric pulse techniques to synthesize nanostructured silver material on flat glassy carbon electrodes is reported and significant differences with bulk silver deposit have been observed. The oxidation of borohydride ion on the silver particles occurs at -0.025 V vs. SCE and the potential decreases towards negative values at longer cycle times. The oxidation current also decreases with the number of cycles, suggesting that the silver active sites become partially blocked by oxidation products of borohydride. The electroactive area per unit electrode area of silver was relatively low for particles deposited using potential step amperometric techniques on glassy carbon (0.002 cm2 per cm-2) compared with the area found at a polycrystalline silver electrode (0.103 cm2 per cm-2
Effective spacetime from multi-dimensional gravity
We study the effective spacetimes in lower dimensions that can be extracted
from a multidimensional generalization of the Schwarzschild-Tangherlini
spacetimes derived by Fadeev, Ivashchuk and Melnikov ({\it Phys. Lett,} {\bf A
161} (1991) 98). The higher-dimensional spacetime has
dimensions, where and are the number of "internal" and "external" extra
dimensions, respectively. We analyze the effective spacetime obtained
after dimensional reduction of the external dimensions. We find that when
the extra dimensions are compact (i) the physics in lower dimensions is
independent of and the character of the singularities in higher dimensions,
and (ii) the total gravitational mass of the effective matter distribution
is less than the Schwarzshild mass. In contrast, when the extra dimensions
are large this is not so; the physics in does explicitly depend on
, as well as on the nature of the singularities in high dimensions, and the
mass of the effective matter distribution (with the exception of wormhole-like
distributions) is bigger than the Schwarzshild mass. These results may be
relevant to observations for an experimental/observational test of the theory.Comment: A typo in Eq. (24) is fixe
- …