244 research outputs found
Scales of the Extra Dimensions and their Gravitational Wave Backgrounds
Circumstances are described in which symmetry breaking during the formation
of our three-dimensional brane within a higher-dimensional space in the early
universe excites mesoscopic classical radion or brane-displacement degrees of
freedom and produces a detectable stochastic background of gravitational
radiation. The spectrum of the background is related to the unification energy
scale and the the sizes and numbers of large extra dimensions. It is shown that
properties of the background observable by gravitational-wave observatories at
frequencies Hz to Hz contain information about
unification on energy scales from 1 to TeV, gravity propagating
through extra-dimension sizes from 1 mm to mm, and the dynamical
history and stabilization of from one to seven extra dimensions.Comment: 6 pages, Latex, 1 figure, submitted to Phys. Re
Noise characterization for LISA
We consider the general problem of estimating the inflight LISA noise power
spectra and cross-spectra, which are needed for detecting and estimating the
gravitational wave signals present in the LISA data. For the LISA baseline
design and in the long wavelength limit, we bound the error on all spectrum
estimators that rely on the use of the fully symmetric Sagnac combination
(). This procedure avoids biases in the estimation that would otherwise
be introduced by the presence of a strong galactic background in the LISA data.
We specialize our discussion to the detection and study of the galactic white
dwarf-white dwarf binary stochastic signal.Comment: 9 figure
Age Constraints on Brane Models of Dark Energy
Inspired by recent developments in particle physics, the so-called brane
world cosmology seems to provide an alternative explanation for the present
dark energy problem. In this paper, we use the estimated age of high-
objects to constrain the value of the cosmological parameters in some
particular scenarios based on this large scale modification of gravity. We show
that such models are compatible with these observations for values of the
crossover distance between the 4 and 5 dimensions of the order of .Comment: 4 pages, 2 figures, 1 table, to appear in Phys. Rev.
Revisit of the Interaction between Holographic Dark Energy and Dark Matter
In this paper we investigate the possible direct, non-gravitational
interaction between holographic dark energy (HDE) and dark matter. Firstly, we
start with two simple models with the interaction terms
and , and then we move on to the general form . The cosmological constraints of the models are
obtained from the joint analysis of the present Union2.1+BAO+CMB+ data. We
find that the data slightly favor an energy flow from dark matter to dark
energy, although the original HDE model still lies in the 95.4% confidence
level (CL) region. For all models we find at the 95.4% CL. We show that
compared with the cosmic expansion, the effect of interaction on the evolution
of and is smaller, and the relative increment
(decrement) amount of the energy in the dark matter component is constrained to
be less than 9% (15%) at the 95.4% CL. By introducing the interaction, we find
that even when the big rip still can be avoided due to the existence of a
de Sitter solution at . We show that this solution can not be
accomplished in the two simple models, while for the general model such a
solution can be achieved with a large , and the big rip may be avoided
at the 95.4% CL.Comment: 26 pages, 9 figures, version accepted for publication in JCA
Brane World Cosmologies and Statistical Properties of Gravitational Lenses
Brane world cosmologies seem to provide an alternative explanation for the
present accelerated stage of the Universe with no need to invoke either a
cosmological constant or an exotic \emph{quintessence} component. In this paper
we investigate statistical properties of gravitational lenses for some
particular scenarios based on this large scale modification of gravity. We show
that a large class of such models are compatible with the current lensing data
for values of the matter density parameter
(). If one fixes to be , as suggested by
most of the dynamical estimates of the quantity of matter in the Universe, the
predicted number of lensed quasars requires a slightly open universe with a
crossover distance between the 4 and 5-dimensional gravities of the order of
.Comment: 6 pages, 3 figures, revte
Detection methods for non-Gaussian gravitational wave stochastic backgrounds
We address the issue of finding an optimal detection method for a
discontinuous or intermittent gravitational wave stochastic background. Such a
signal might sound something like popcorn popping. We derive an appropriate
version of the maximum likelihood detection statistic, and compare its
performance to that of the standard cross-correlation statistic both
analytically and with Monte Carlo simulations. The maximum likelihood statistic
performs better than the cross-correlation statistic when the background is
sufficiently non-Gaussian. For both ground and space based detectors, this
results in a gain factor, ranging roughly from 1 to 3, in the minimum
gravitational-wave energy density necessary for detection, depending on the
duty cycle of the background. Our analysis is exploratory, as we assume that
the time structure of the events cannot be resolved, and we assume white,
Gaussian noise in two collocated, aligned detectors. Before this detection
method can be used in practice with real detector data, further work is
required to generalize our analysis to accommodate separated, misaligned
detectors with realistic, colored, non-Gaussian noise.Comment: 25 pages, 12 figures, submitted to physical review D, added revisions
in response to reviewers comment
Evolution of cosmic string configurations
We extend and develop our previous work on the evolution of a network of
cosmic strings. The new treatment is based on an analysis of the probability
distribution of the end-to-end distance of a randomly chosen segment of
left-moving string of given length. The description involves three distinct
length scales: , related to the overall string density, , the
persistence length along the string, and , describing the small-scale
structure, which is an important feature of the numerical simulations that have
been done of this problem. An evolution equation is derived describing how the
distribution develops in time due to the combined effects of the universal
expansion, of intercommuting and loop formation, and of gravitational
radiation. With plausible assumptions about the unknown parameters in the
model, we confirm the conclusions of our previous study, that if gravitational
radiation and small-scale structure effects are neglected, the two dominant
length scales both scale in proportion to the horizon size. When the extra
effects are included, we find that while and grow,
initially does not. Eventually, however, it does appear to scale, at a much
lower level, due to the effects of gravitational back-reaction.Comment: 61 pages, requires RevTex v3.0, SUSSEX-TH-93/3-4,
IMPERIAL/TP/92-93/4
The Deformable Universe
The concept of smooth deformations of a Riemannian manifolds, recently
evidenced by the solution of the Poincar\'e conjecture, is applied to
Einstein's gravitational theory and in particular to the standard FLRW
cosmology. We present a brief review of the deformation of Riemannian geometry,
showing how such deformations can be derived from the Einstein-Hilbert
dynamical principle. We show that such deformations of space-times of general
relativity produce observable effects that can be measured by four-dimensional
observers. In the case of the FLRW cosmology, one such observable effect is
shown to be consistent with the accelerated expansion of the universe.Comment: 20 pages, LaTeX, 3 figure
Komar energy and Smarr formula for noncommutative Schwarzschild black hole
We calculate the Komar energy for a noncommutative Schwarzschild black
hole. A deformation from the conventional identity is found in the
next to leading order computation in the noncommutative parameter
(i.e. ) which is also consistent
with the fact that the area law now breaks down. This deformation yields a
nonvanishing Komar energy at the extremal point of these black holes.
We then work out the Smarr formula, clearly elaborating the differences from
the standard result , where the mass () of the black hole is
identified with the asymptotic limit of the Komar energy. Similar conclusions
are also shown to hold for a deSitter--Schwarzschild geometry.Comment: 5 pages Late
Spatial gradients in the cosmological constant
It is possible that there may be differences in the fundamental physical
parameters from one side of the observed universe to the other. I show that the
cosmological constant is likely to be the most sensitive of the physical
parameters to possible spatial variation, because a small variation in any of
the other parameters produces a huge variation of the cosmological constant. It
therefore provides a very powerful {\em indirect} evidence against spatial
gradients or temporal variation in the other fundamental physical parameters,
at least 40 orders of magnitude more powerful than direct experimental
constraints. Moreover, a gradient may potentially appear in theories where the
variability of the cosmological constant is connected to an anthropic selection
mechanism, invoked to explain the smallness of this parameter. In the Hubble
damping mechanism for anthropic selection, I calculate the possible gradient.
While this mechanism demonstrates the existence of this effect, it is too small
to be seen experimentally, except possibly if inflation happens around the
Planck scale.Comment: 12 page
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