1,519 research outputs found
Holography and Variable Cosmological Constant
An effective local quantum field theory with UV and IR cutoffs correlated in
accordance with holographic entropy bounds is capable of rendering the
cosmological constant (CC) stable against quantum corrections. By setting an IR
cutoff to length scales relevant to cosmology, one easily obtains the currently
observed rho_Lambda ~ 10^{-47} GeV^4, thus alleviating the CC problem. It is
argued that scaling behavior of the CC in these scenarios implies an
interaction of the CC with matter sector or a time-dependent gravitational
constant, to accommodate the observational data.Comment: 7 pages, final version accepted by PR
Bayesian analysis of Friedmannless cosmologies
Assuming only a homogeneous and isotropic universe and using both the 'Gold'
Supernova Type Ia sample of Riess et al. and the results from the Supernova
Legacy Survey, we calculate the Bayesian evidence of a range of different
parameterizations of the deceleration parameter. We consider both spatially
flat and curved models. Our results show that although there is strong evidence
in the data for an accelerating universe, there is little evidence that the
deceleration parameter varies with redshift.Comment: 7 pages, 3 figure
Could the cosmic acceleration be transient? A cosmographic evaluation
A possible slowing down of the cosmic expansion is investigated through a
cosmographic approach. By expanding the luminosity distance to fourth order and
fitting the SN Ia data from the most recent compilations (Union, Constitution
and Union 2), the marginal likelihood distributions for the deceleration
parameter today suggest a recent reduction of the cosmic acceleration and
indicate that there is a considerable probability for . Also in contrast
to the prediction of the CDM model, the cosmographic
reconstruction permits a cosmic expansion history where the cosmic acceleration
could already have peaked and be presently slowing down, which would imply that
the recent accelerated expansion of the Universe is a transient phenomenon. It
is also shown that to describe a transient acceleration the luminosity distance
needs to be expanded at least to fourth order. The present cosmographic results
depend neither on the validity of general relativity nor on the matter-energy
contents of the Universe.Comment: 11 pages, 4 figures. Accepted for publication in Classical and
Quantum Gravit
Cosmic Acceleration, Dark Energy and Fundamental Physics
A web of interlocking observations has established that the expansion of the
Universe is speeding up and not slowing, revealing the presence of some form of
repulsive gravity. Within the context of general relativity the cause of cosmic
acceleration is a highly elastic (p\sim -rho), very smooth form of energy
called ``dark energy'' accounting for about 75% of the Universe. The
``simplest'' explanation for dark energy is the zero-point energy density
associated with the quantum vacuum; however, all estimates for its value are
many orders-of-magnitude too large. Other ideas for dark energy include a very
light scalar field or a tangled network of topological defects. An alternate
explanation invokes gravitational physics beyond general relativity.
Observations and experiments underway and more precise cosmological
measurements and laboratory experiments planned for the next decade will test
whether or not dark energy is the quantum energy of the vacuum or something
more exotic, and whether or not general relativity can self consistently
explain cosmic acceleration. Dark energy is the most conspicuous example of
physics beyond the standard model and perhaps the most profound mystery in all
of science.Comment: 10 pages, 8 figures, invited review for Journal of the Physical
Society of Japan, in pres
A model of the Universe including Dark Energy accounted for by both a Quintessence Field and a (negative) Cosmological Constant
In this work we present a model of the universe in which dark energy is
modelled explicitely with both a dynamical quintessence field and a
cosmological constant. Our results confirm the possibility of a future
collapsing universe (for a given region of the parameter space), which is
necessary for a consistent formulation of string theory and quantum field
theory. We have also reproduced the measurements of modulus distance from
supernovae with good accuracy.Comment: 11 pages, 4 figures, only the results for the single exponential
potential are preserved. One author added. Some changes in the reference
section. Submitted to Physical Review
A note on light velocity anisotropy
It is proved that in experiments on or near the Earth, no anisotropy in the
one-way velocity of light may be detected. The very accurate experiments which
have been performed to detect such an effect are to be considered significant
tests of both special relativity and the equivalence principleComment: 8 pages, LaTex, Gen. Relat. Grav. accepte
Bayesian Analysis and Constraints on Kinematic Models from Union SNIa
The kinematic expansion history of the universe is investigated by using the
307 supernovae type Ia from the Union Compilation set. Three simple model
parameterizations for the deceleration parameter (constant, linear and abrupt
transition) and two different models that are explicitly parametrized by the
cosmic jerk parameter (constant and variable) are considered. Likelihood and
Bayesian analyses are employed to find best fit parameters and compare models
among themselves and with the flat CDM model. Analytical expressions
and estimates for the deceleration and cosmic jerk parameters today ( and
) and for the transition redshift () between a past phase of cosmic
deceleration to a current phase of acceleration are given. All models
characterize an accelerated expansion for the universe today and largely
indicate that it was decelerating in the past, having a transition redshift
around 0.5. The cosmic jerk is not strongly constrained by the present
supernovae data. For the most realistic kinematic models the
confidence limits imply the following ranges of values: ,
and , which are compatible with the
CDM predictions, , and .
We find that even very simple kinematic models are equally good to describe the
data compared to the concordance CDM model, and that the current
observations are not powerful enough to discriminate among all of them.Comment: 13 pages. Matches published versio
The Laser Astrometric Test of Relativity Mission
This paper discusses new fundamental physics experiment to test relativistic
gravity at the accuracy better than the effects of the 2nd order in the
gravitational field strength. The Laser Astrometric Test Of Relativity (LATOR)
mission uses laser interferometry between two micro-spacecraft whose lines of
sight pass close by the Sun to accurately measure deflection of light in the
solar gravity. The key element of the experimental design is a redundant
geometry optical truss provided by a long-baseline (100 m) multi-channel
stellar optical interferometer placed on the International Space Station. The
geometric redundancy enables LATOR to measure the departure from Euclidean
geometry caused by the solar gravity field to a very high accuracy. LATOR will
not only improve the value of the parameterized post-Newtonian (PPN) parameter
gamma to unprecedented levels of accuracy of 1 part in 1e8, it will also reach
ability to measure effects of the next post-Newtonian order (1/c^4) of light
deflection resulting from gravity's intrinsic non-linearity. The solar
quadrupole moment parameter, J2, will be measured with high precision, as well
as a variety of other relativistic. LATOR will lead to very robust advances in
the tests of fundamental physics: this mission could discover a violation or
extension of general relativity, or reveal the presence of an additional long
range interaction in the physical law. There are no analogs to the LATOR
experiment; it is unique and is a natural culmination of solar system gravity
experiments.Comment: 8 pages, 2 figures, invited talk given at the Second International
Conference on Particle and Fundamental Physics in Space (SpacePart'03), 10-12
December 2003, Washington, D
Model- and calibration-independent test of cosmic acceleration
We present a calibration-independent test of the accelerated expansion of the
universe using supernova type Ia data. The test is also model-independent in
the sense that no assumptions about the content of the universe or about the
parameterization of the deceleration parameter are made and that it does not
assume any dynamical equations of motion. Yet, the test assumes the universe
and the distribution of supernovae to be statistically homogeneous and
isotropic. A significant reduction of systematic effects, as compared to our
previous, calibration-dependent test, is achieved. Accelerated expansion is
detected at significant level (4.3 sigma in the 2007 Gold sample, 7.2 sigma in
the 2008 Union sample) if the universe is spatially flat. This result depends,
however, crucially on supernovae with a redshift smaller than 0.1, for which
the assumption of statistical isotropy and homogeneity is less well
established.Comment: 13 pages, 2 figures, major change
Gravitational Radiation from the Coalescence of Binary Neutron Stars: Effects Due to the Equation of State, Spin, and Mass Ratio
We calculate the gravitational radiation produced by the coalescence of
inspiraling binary neutron stars in the Newtonian regime using 3-dimensional
numerical simulations. The stars are modeled as polytropes and start out in the
point-mass regime at wide separation. The hydrodynamic integration is performed
using smooth particle hydrodynamics (SPH) with Newtonian gravity, and the
gravitational radiation is calculated using the quadrupole approximation. We
have run a number of simulations varying the neutron star radii, equations of
state, spins, and mass ratio. The resulting gravitational waveforms and spectra
are rich in information about the hydrodynamics of coalescence, and show
characteristic dependence on GM/Rc^2, the equation of state, and the mass
ratio.Comment: 39 pages, uses Latex 2.09. To be published in the Dec. 15, 1996 issue
of Physical Review D. 16 Figures (bitmapped). Originals available in
compressed Postscript format at ftp://zonker.drexel.edu/papers/PAPER2
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