7,809 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
Latent solitons, black strings, black branes, and equations of state in Kaluza-Klein models
In Kaluza-Klein models with an arbitrary number of toroidal internal spaces,
we investigate soliton solutions which describe the gravitational field of a
massive compact object. We single out the physically interesting solution
corresponding to a point-like mass. For the general solution we obtain
equations of state in the external and internal spaces. These equations
demonstrate that the point-like mass soliton has dust-like equations of state
in all spaces. We also obtain the PPN parameters, which give the possibility to
obtain the formulas for perihelion shift, deflection of light and time delay of
radar echoes. Additionally, the gravitational experiments lead to a strong
restriction on the parameter of the model: . The point-like mass solution contradicts this restriction. The
condition satisfies the experimental limitation and defines a new
class of solutions which are indistinguishable from general relativity. We call
such solutions latent solitons. Black strings and black branes belong to this
class. Moreover, the condition of stability of the internal spaces singles out
black strings/branes from the latent solitons and leads uniquely to the black
string/brane equations of state , in the internal spaces and
to the number of the external dimensions . The investigation of
multidimensional static spherically symmetric perfect fluid with dust-like
equation of state in the external space confirms the above results.Comment: 8 pages, Revtex4, no figures, minor changes adde
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
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
Kaluza-Klein models: can we construct a viable example?
In Kaluza-Klein models, we investigate soliton solutions of Einstein
equation. We obtain the formulas for perihelion shift, deflection of light,
time delay of radar echoes and PPN parameters. We find that the solitonic
parameter k should be very big: |k|\geq 2.3\times10^4. We define a soliton
solution which corresponds to a point-like mass source. In this case the
soliton parameter k=2, which is clearly contrary to this restriction. Similar
problem with the observations takes place for static spherically symmetric
perfect fluid with the dust-like equation of state in all dimensions. The
common for both of these models is the same equations of state in our three
dimensions and in the extra dimensions. All dimensions are treated at equal
footing. To be in agreement with observations, it is necessary to break the
symmetry between the external/our and internal spaces. It takes place for black
strings which are particular examples of solitons with k\to \infty. For such k,
black strings are in concordance with the observations. Moreover, we show that
they are the only solitons which are at the same level of agreement with the
observations as in general relativity. Black strings can be treated as perfect
fluid with dust-like equation of state p_0=0 in the external/our space and very
specific equation of state p_1=-(1/2)\epsilon in the internal space. The latter
equation is due to negative tension in the extra dimension. We also demonstrate
that dimension 3 for the external space is a special one. Only in this case we
get the latter equation of state. We show that the black string equations of
state satisfy the necessary condition of the internal space stabilization.
Therefore, black strings are good candidates for a viable model of
astrophysical objects (e.g., Sun) if we can provide a satisfactory explanation
of negative tension for particles constituting these objects.Comment: 11 pages, Revtex4, no figures, appendix and references adde
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