297 research outputs found
Curvature Dependence of Peaks in the Cosmic Microwave Background Distribution
The widely cited formula for the multipole
number of the first Doppler peak is not even a crude approximation in the case
of greatest current interest, in which the cosmic mass density is less than the
vacuum energy density. For instance, with fixed at 0.3, the position
of any Doppler peak varies as near .Comment: 7 pages, Late
An Interacting Two-Fluid Scenario for Dark Energy in FRW Universe
We study the evolution of the dark energy parameter within the scope of a
spatially flat and isotropic Friedmann-Robertson-Walker (FRW) model filled with
barotropic fluid and dark energy. To obtain the deterministic solution we
choose the scale factor which yields a time dependent
deceleration parameter (DP). In doing so we consider the case minimally coupled
with dark energy to the perfect fluid as well as direct interaction with it.Comment: 11 pages, accepted for publication in Chinese Physics Letters. Unlike
the previous version the new one contains the time depending deceleration
paramete
Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications
The ultimate fate of the universe, infinite expansion or a big crunch, can be
determined by measuring the redshifts, apparent brightnesses, and intrinsic
luminosities of very distant supernovae. Recent developments have provided
tools that make such a program practicable: (1) Studies of relatively nearby
Type Ia supernovae (SNe Ia) have shown that their intrinsic luminosities can be
accurately determined; (2) New research techniques have made it possible to
schedule the discovery and follow-up observations of distant supernovae,
producing well over 50 very distant (z = 0.3 -- 0.7) SNe Ia to date. These
distant supernovae provide a record of changes in the expansion rate over the
past several billion years. By making precise measurements of supernovae at
still greater distances, and thus extending this expansion history back far
enough in time, we can distinguish the slowing caused by the gravitational
attraction of the universe's mass density Omega_M from the effect of a possibly
inflationary pressure caused by a cosmological constant Lambda. We report here
the first such measurements, with our discovery of a Type Ia supernova (SN
1997ap) at z = 0.83. Measurements at the Keck II 10-m telescope make this the
most distant spectroscopically confirmed supernova. Over two months of
photometry of SN 1997ap with the Hubble Space Telescope and ground-based
telescopes, when combined with previous measurements of nearer SNe Ia, suggests
that we may live in a low mass-density universe. Further supernovae at
comparable distances are currently scheduled for ground and space-based
observations.Comment: 12 pages and 4 figures (figure 4 is repeated in color and black and
white) Nature, scheduled for publication in the 1 January, 1998 issue. Also
available at http://www-supernova.lbl.go
The Age-Redshift Relation for Standard Cosmology
We present compact, analytic expressions for the age-redshift relation
for standard Friedmann-Lema\^ \itre-Robertson-Walker (FLRW)
cosmology. The new expressions are given in terms of incomplete Legendre
elliptic integrals and evaluate much faster than by direct numerical
integration.Comment: 13 pages, 3 figure
Perturbation evolution with a non-minimally coupled scalar field
We recently proposed a simple dilaton-derived quintessence model in which the
scalar field was non-minimally coupled to cold dark matter, but not to
`visible' matter. Such couplings can be attributed to the dilaton in the low
energy limit of string theory, beyond tree level. In this paper we discuss the
implications of such a model on structure formation, looking at its impact on
matter perturbations and CMB anisotropies. We find that the model only deviates
from CDM and minimally coupled theories at late times, and is well
fitted to current observational data. The signature left by the coupling, when
it breaks degeneracy at late times, presents a valuable opportunity to
constrain non-minimal couplings given the wealth of new observational data
promised in the near future.Comment: Version appearing in Physical Review D. 10 pages, 9 figs. Comparison
with SN1a and projected MAP results, and appendix adde
Support of dS/CFT correspondence from space-time perturbations
We analyse the spectrum of perturbations of the de Sitter space on the one
hand, while on the other hand we compute the location of the poles in the
Conformal Field Theory (CFT) propagator at the border. The coincidence is
striking, supporting a dS/CFT correspondence. We show that the spectrum of
thermal excitations of the CFT at the past boundary together with that
spectrum at the future boundary is contained in the quasi-normal mode
spectrum of the de Sitter space in the bulk.Comment: Modified version, appearing in Phys. Rev. D66 (2002) 10401
Cosmological model with macroscopic spin fluid
We consider a Friedmann-Robertson-Walker cosmological model with some exotic
perfect fluid with spin known as the Weyssenhoff fluid. The possibility that
the dark energy may be described in part by the Weyssenhoff fluid is discussed.
The observational constraint coming from supernovae type Ia observations is
established. This result indicates that, whereas the cosmological constant is
still needed to explain current observations, the model with spin fluid is
admissible. For high redshifts the differences between the model with
spin fluid and the cold dark matter model with a cosmological constant become
detectable observationally for the flat case with .
From the maximum likelihood method we obtain the value of
. This gives us the limit
at the level. While the model with
``brane effects'' is preferred by the supernovae Ia data, the model with spin
fluid is statistically admissible. For comparison, the limit on the spin fluid
coming from cosmic microwave background anisotropies is also obtained. The
uncertainties in the location of a first peak give the interval . From big bang nucleosynthesis we
obtain the strongest limit . The
interconnection between the model considered and brane models is also pointed
out.Comment: RevTeX4, 15 pages, 10 figures; some minor change
Quantum driven Bounce of the future Universe
It is demonstrated that due to back-reaction of quantum effects, expansion of
the universe stops at its maximum and takes a turnaround. Later on, it
contracts to a very small size in finite future time. This phenomenon is
followed by a " bounce" with re-birth of an exponentially expanding
non-singular universe
Current constraints on the dark energy equation of state
We combine complementary datasets from Cosmic Microwave Background (CMB)
anisotropy measurements, high redshift supernovae (SN-Ia) observations and data
from local cluster abundances and galaxy clustering (LSS) to constrain the dark
energy equation of state parameterized by a constant pressure-to-density ratio
. Under the assumption of flatness, we find at 68% c.l.,
providing no significant evidence for quintessential behaviour different from
that of a cosmological constant. We then generalise our result to show that the
constraints placed on a constant can be safely extended to dynamical
theories. We consider a variety of quintessential dynamical models based on
inverse power law, exponential and oscillatory scaling potentials. We find that
SN1a observations are `numbed' to dynamical shifts in the equation of state,
making the prospect of reconstructing , a challenging one indeed.Comment: 6 pages, 6 figures. Version accepted for publication in PR
Deformed special relativity with an invariant minimum speed and its cosmological implications
The paper aims to introduce a new symmetry principle in the space-time
geometry through the elimination of the classical idea of rest and by including
a universal minimum limit of speed in the subatomic world. Such a limit,
unattainable by particles, represents a preferred reference frame associated
with a universal background field that breaks Lorentz symmetry. Thus the
structure of space-time is extended due to the presence of a vacuum energy
density, which leads to a negative pressure at cosmological scales. The tiny
values of the cosmological constant and the vacuum energy density shall be
successfully obtained, being in good agreement with current observational
results.Comment: 7 pages, 3 figures. arXiv admin note: substantial text overlap with
arXiv:0705.4315, arXiv:0709.172
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