98 research outputs found
Possible evidence for "dark radiation" from Big Bang Nucleosynthesis Data
We address the emerging discrepancy between the Big Bang Nucleosynthesis data
and standard cosmology, which asks for a bit longer evolution time. If this
effect is real, one possible implication (in a framework of brane cosmology
model) is that there is a ``dark radiation'' component which is negative and
makes few percents of ordinary matter density. If so, all scales of this model
can be fixed, provided brane-to-bulk leakage problem is solved.Comment: We found that references to some nhumbers from unpublished ref.3 in
v1 lead to confusion of some readers: we decided to removed those in v
4D gravity localized in non Z_2-symmetric thick branes
We present a comparative analysis of localization of 4D gravity on a non
Z_2-symmetric scalar thick brane in both a 5-dimensional Riemannian space time
and a pure geometric Weyl integrable manifold. This work was mainly motivated
by the hypothesis which claims that Weyl geometries mimic quantum behaviour
classically. We start by obtaining a classical 4-dimensional Poincare invariant
thick brane solution which does not respect Z_2-symmetry along the
(non-)compact extra dimension. The scalar energy density of our field
configuration represents several series of thick branes with positive and
negative energy densities centered at y_0. The only qualitative difference we
have encountered when comparing both frames is that the scalar curvature of the
Riemannian manifold turns out to be singular for the found solution, whereas
its Weylian counterpart presents a regular behaviour. By studying the
transverse traceless modes of the fluctuations of the classical backgrounds, we
recast their equations into a Schroedinger's equation form with a volcano
potential of finite bottom (in both frames). By solving the Schroedinger
equation for the massless zero mode m^2=0 we obtain a single bound state which
represents a stable 4-dimensional graviton in both frames. We also get a
continuum gapless spectrum of KK states with positive m^2>0 that are suppressed
at y_0, turning into continuum plane wave modes as "y" approaches spatial
infinity. We show that for the considered solution to our setup, the potential
is always bounded and cannot adopt the form of a well with infinite walls;
thus, we do not get a discrete spectrum of KK states, and we conclude that the
claim that Weylian structures mimic, classically, quantum behaviour does not
constitute a generic feature of these geometric manifolds.Comment: 13 pages, 4 figures, JHEP forma
Volume stabilization in a warped flux compactification model
We investigate the stability of the extra dimensions in a warped, codimension
two braneworld that is based upon an Einstein-Maxwell-dilaton theory with a
non-vanishing scalar field potential. The braneworld solution has two 3-branes,
which are located at the positions of the conical singularities. For this type
of brane solution the relative positions of the branes (the shape modulus) is
determined via the tension-deficit relations, if the brane tensions are fixed.
However, the volume of the extra dimensions (the volume modulus) is not fixed
in the context of the classical theory, implying we should take quantum
corrections into account. Hence, we discuss the one-loop effective potential of
the volume modulus for a massless, minimally coupled scalar field.Comment: 25 pages, 8 figures, typos correcte
Spectrum from the warped compactifications with the de Sitter universe
We discuss the spectrum of the tensor metric perturbations and the stability
of warped compactifications with the de Sitter spacetime in the
higher-dimensional gravity. The spacetime structure is given in terms of the
warped product of the non-compact direction, the spherical internal dimensions
and the four-dimensional de Sitter spacetime. To realize a finite bulk volume,
we construct the brane world model, using the cut-copy-paste method. Then, we
compactify the spherical directions on the brane. In any case, we show the
existence of the massless zero mode and the mass gap of it with massive
Kaluza-Klein modes. Although the brane involves the spherical dimensions, no
light massive mode is excited. We also investigate the scalar perturbations,
and show that the model is unstable due to the existence of a tachyonic bound
state, which seems to have the universal negative mass square, irrespective of
the number of spacetime dimensions.Comment: Journal version (JHEP
Vacuum densities for a thick brane in AdS spacetime
For a massive scalar field with general curvature coupling parameter we
evaluate Wightman function, vacuum expectation values of the field square and
the energy-momentum tensor induced by a -symmetric brane with finite
thickness located on -dimensional AdS bulk. For the general case of
static plane symmetric interior structure the expectation values in the region
outside the brane are presented as the sum of free AdS and brane induced parts.
For a conformally coupled massless scalar the brane induced part in the vacuum
energy-momentum tensor vanishes. In the limit of strong gravitational fields
the brane induced parts are exponentially suppressed for points not too close
to the brane boundary. As an application of general results a special model is
considered in which the geometry inside the brane is a slice of the Minkowski
spacetime orbifolded along the direction perpendicular to the brane. For this
model the Wightman function, vacuum expectation values of the field square and
the energy-momentum tensor inside the brane are evaluated. It is shown that for
both minimally and conformally coupled scalar fields the interior vacuum forces
acting on the brane boundaries tend to decrease the brane thickness.Comment: 12 pages, 2 figures, talk presented at QFEXT07, Leipzig, September
17-21, 200
Thermodynamics of extremal rotating thin shells in an extremal BTZ spacetime and the extremal black hole entropy
© 2017 American Physical Society. In a (2+1)-dimensional spacetime with a negative cosmological constant, the thermodynamics and the entropy of an extremal rotating thin shell, i.e., an extremal rotating ring, are investigated. The outer and inner regions with respect to the shell are taken to be the Bañados-Teitelbom-Zanelli (BTZ) spacetime and the vacuum ground state anti-de Sitter spacetime, respectively. By applying the first law of thermodynamics to the extremal thin shell, one shows that the entropy of the shell is an arbitrary well-behaved function of the gravitational area A+ alone, S=S(A+). When the thin shell approaches its own gravitational radius r+ and turns into an extremal rotating BTZ black hole, it is found that the entropy of the spacetime remains such a function of A+, both when the local temperature of the shell at the gravitational radius is zero and nonzero. It is thus vindicated by this analysis that extremal black holes, here extremal BTZ black holes, have different properties from the corresponding nonextremal black holes, which have a definite entropy, the Bekenstein-Hawking entropy S(A+)=A+4G, where G is the gravitational constant. It is argued that for extremal black holes, in particular for extremal BTZ black holes, one should set 0â€S(A+)â€A+4G; i.e., the extremal black hole entropy has values in between zero and the maximum Bekenstein-Hawking entropy A+4G. Thus, rather than having just two entropies for extremal black holes, as previous results have debated, namely, 0 and A+4G, it is shown here that extremal black holes, in particular extremal BTZ black holes, may have a continuous range of entropies, limited by precisely those two entropies. Surely, the entropy that a particular extremal black hole picks must depend on past processes, notably on how it was formed. A remarkable relation between the third law of thermodynamics and the impossibility for a massive body to reach the velocity of light is also fo und. In addition, in the procedure, it becomes clear that there are two distinct angular velocities for the shell, the mechanical and thermodynamic angular velocities. We comment on the relationship between these two velocities. In passing, we clarify, for a static spacetime with a thermal shell, the meaning of the Tolman temperature formula at a generic radius and at the shell
Thermodynamics of extremal rotating thin shells in an extremal BTZ spacetime and the extremal black hole entropy
© 2017 American Physical Society. In a (2+1)-dimensional spacetime with a negative cosmological constant, the thermodynamics and the entropy of an extremal rotating thin shell, i.e., an extremal rotating ring, are investigated. The outer and inner regions with respect to the shell are taken to be the Bañados-Teitelbom-Zanelli (BTZ) spacetime and the vacuum ground state anti-de Sitter spacetime, respectively. By applying the first law of thermodynamics to the extremal thin shell, one shows that the entropy of the shell is an arbitrary well-behaved function of the gravitational area A+ alone, S=S(A+). When the thin shell approaches its own gravitational radius r+ and turns into an extremal rotating BTZ black hole, it is found that the entropy of the spacetime remains such a function of A+, both when the local temperature of the shell at the gravitational radius is zero and nonzero. It is thus vindicated by this analysis that extremal black holes, here extremal BTZ black holes, have different properties from the corresponding nonextremal black holes, which have a definite entropy, the Bekenstein-Hawking entropy S(A+)=A+4G, where G is the gravitational constant. It is argued that for extremal black holes, in particular for extremal BTZ black holes, one should set 0â€S(A+)â€A+4G; i.e., the extremal black hole entropy has values in between zero and the maximum Bekenstein-Hawking entropy A+4G. Thus, rather than having just two entropies for extremal black holes, as previous results have debated, namely, 0 and A+4G, it is shown here that extremal black holes, in particular extremal BTZ black holes, may have a continuous range of entropies, limited by precisely those two entropies. Surely, the entropy that a particular extremal black hole picks must depend on past processes, notably on how it was formed. A remarkable relation between the third law of thermodynamics and the impossibility for a massive body to reach the velocity of light is also fo und. In addition, in the procedure, it becomes clear that there are two distinct angular velocities for the shell, the mechanical and thermodynamic angular velocities. We comment on the relationship between these two velocities. In passing, we clarify, for a static spacetime with a thermal shell, the meaning of the Tolman temperature formula at a generic radius and at the shell
Effective Monopoles within Thick Branes
The monopole mass is revealed to be considerably modified in the thick
braneworld paradigm, and depends on the position of the monopole in the brane
as well. Accordingly, the monopole radius continuously increases, leading to an
unacceptable setting that can be circumvented when the brane thickness has an
upper limit. Despite such peculiar behavior, the quantum corrections accrued --
involving the classical monopole solution -- are shown to be still under
control. We analyze the monopole's peculiarities also taking into account the
localization of the gauge fields. Furthermore, some additional analysis in the
thick braneworld context and the similar behavior evinced by the topological
string are investigated.Comment: 7 pages, 1 figur
Stability of the de Sitter spacetime in Horava-Lifshitz theory
The stability of de Sitter spacetime in Horava-Lifshitz theory of gravity
with projectability but without detailed balance condition is studied. It is
found that, in contrast to the case of the Minkowski background, the spin-0
graviton now is stable for any given , and free of ghost for
in the infrared limit, where is the dynamical coupling constant.Comment: Mod. Phys. Lett. A25, 2267-2279 (2010
A doubly covariant formula of deficit angle and its application to six-dimensional braneworld
We reformulate boundary conditions for axisymmetric codimension-2 braneworlds
in a way which is applicable to linear perturbation with various gauge
conditions. Our interest is in the thin brane limit and thus this scheme
assumes that the perturbations are also axisymmetric and that the surface
energy-momentum tensor of the brane is proportional to its induced metric. An
advantage of our scheme is that it allows much more freedom for convenient
coordinate choices than the other methods. This is because in our scheme, the
coordinate system in the bulk and that on the brane are completely
disentangled. Therefore, the latter does not need to be a subset of the former
and the brane does not need to stay at a fixed bulk coordinate position. The
boundary condition is manifestly doubly covariant: it is invariant under gauge
transformations in the bulk and at the same time covariant under those on the
brane. We take advantage of the double covariance when we analyze the linear
perturbation of a particular model of six-dimensional braneworld with warped
flux compactification.Comment: 25 pages, REVTeX4; published in CQ
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