111 research outputs found
Brane in 6D with 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.Comment: Revised paper. 6 pages, revtex 4. to be published in PR
Testing Relativity at High Energies Using Spaceborne Detectors
(ABRIDGED) The Gamma-ray Large Area Space Telescope (GLAST) will measure the
spectra of distant extragalactic sources of high energy gamma-rays. GLAST can
look for energy dependent propagation effects from such sources as a signal of
Lorentz invariance violation (LIV). Such sources should also exhibit high
energy spectral cutoffs from pair production interactions with low energy
photons. The properties of such cutoffs can also be used to test LIV. Detectors
to measure gamma-ray polarization can look for the depolarizing effect of
space-time birefingence predicted by loop quantum gravity. A spaceborne
detector array looking down on Earth to study extensive air showers produced by
ultrahigh energy cosmic rays can study their spectral properties and look for a
possible deviation from the predicted GZK effect as another signal of LIV.Comment: 14 pages, Text of invitated talk presented at the "From Quantum to
Cosmos: Fundamental Physics Studies from Space" meeting. More references
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Thermodynamic Gravity and the Schrodinger Equation
We adopt a 'thermodynamical' formulation of Mach's principle that the rest
mass of a particle in the Universe is a measure of its long-range collective
interactions with all other particles inside the horizon. We consider all
particles in the Universe as a 'gravitationally entangled' statistical ensemble
and apply the approach of classical statistical mechanics to it. It is shown
that both the Schrodinger equation and the Planck constant can be derived
within this Machian model of the universe. The appearance of probabilities,
complex wave functions, and quantization conditions is related to the
discreetness and finiteness of the Machian ensemble.Comment: Minor corrections, the version accepted by Int. J. Theor. Phy
Big bang of the brane universe
Big bang of the Friedmann-Robertson-Walker (FRW)-brane universe is studied.
In contrast to the spacelike initial singularity of the usual FRW universe, the
initial singularity of the FRW-brane universe is point-like from the viewpoint
of causality including gravitational waves propagating in the bulk. Existence
of null singularities (seam singuralities) is also shown in the flat and open
FRW-brane universe models.Comment: LaTeX, 11 pages, 3 EPS figure
Gravitating global monopoles in extra dimensions and the brane world concept
Multidimensional configurations with Minkowski external space-time and a
spherical global monopole in extra dimensions are discussed in the context of
the brane world concept. The monopole is formed with a hedgehog-like set of
scalar fields \phi^i with a symmetry-breaking potential V depending on the
magnitude \phi^2 = \phi^i \phi^i. All possible kinds of globally regular
configurations are singled out without specifying the shape of V(\phi). These
variants are governed by the maximum value \phi_m of the scalar field,
characterizing the energy scale of symmetry breaking. If \phi_m < \phi_cr
(where \phi_cr is a critical value of \phi related to the multidimensional
Planck scale), the monopole reaches infinite radii while in the ``strong field
regime'', when \phi_m\geq \phi_cr, the monopole may end with a cylinder of
finite radius or possess two regular centers. The warp factors of monopoles
with both infinite and finite radii may either exponentially grow or tend to
finite constant values far from the center. All such configurations are shown
to be able to trap test scalar matter, in striking contrast to RS2 type 5D
models. The monopole structures obtained analytically are also found
numerically for the Mexican hat potential with an additional parameter acting
as a cosmological constant.Comment: 21 pages, 6 figures, latex, gc styl
Signature Change on the Brane
We explore the possibility of having a good description of classical
signature change in the brane scenario.Comment: RevTeX, 4 pages, 1 figure. Uses epsfig package. Slightly shorter
version to match the published version. Reference adde
Cosmological Expansion in the Randall-Sundrum Brane World Scenario
The cosmology of the Randall-Sundrum scenario for a positive tension brane in
a 5-D Universe with localized gravity has been studied previously. In the
radiation-dominated Universe, it was suggested that there are two solutions for
the cosmic scale factor a(t) : the standard solution , and a
solution , which is incompatible with standard big bang
nucleosynthesis. In this note, we reconsider expansion of the Universe in this
scenario. We derive and solve a first order, linear differential equation for
H^2, the square of the expansion rate of the Universe, as a function of a. The
differences between our equation for H^2 and the relationship found in standard
cosmology are (i) there is a term proportional to density squared (a fact
already known), which is small when the density is small compared to the brane
tension, and (ii) there is a contribution which acts like a relativistic fluid.
We show that this second contribution is due to gravitational degrees of
freedom in the bulk. Thus, we find that there need not be any conflict between
cosmology of the Randall-Sundrum scenario and the standard model of cosmology.
We discuss how reheating at the end of inflation leads to the correct
relationship between matter density and expansion rate, , and the conditions that must be met for the expansion rate of the
Universe to be close to its standard model value around the epoch of
cosmological nucleosynthesis.Comment: 14 pages, 1 figure. Error in interpretation corrected, references
adde
Dark matter in the framework of shell-universe
We show that the shell-universe model, used to explain the observed expansion
rate of the universe without a dark energy component, provides also a natural
mechanism for local increasing of the shell's tension leading to the modified
Newton's law alternative to galactic dark matter.Comment: 8 pages, minor corrections, version to appear in GR
Brane cosmological perturbations
We address the question of cosmological perturbations in the context of brane
cosmology, where our Universe is a three-brane where matter is confined,
whereas gravity lives in a higher dimensional spacetime. The equations
governing the bulk perturbations are computed in the case of a general warped
universe. The results are then specialized to the case of a five-dimensional
spacetime, scenario which has recently attracted a lot of attention. In this
context, we decompose the perturbations into `scalar', `vector' and `tensor'
modes, which are familiar in the standard theory of cosmological perturbations.
The junction conditions, which relate the metric perturbations to the matter
perturbations in the brane, are then computed.Comment: 14 pages, Latex; no figur
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