17 research outputs found
An indirect limit on the amplitude of primordial Gravitational Wave Background from CMB-Galaxy Cross Correlation
While large scale cosmic microwave background (CMB) anisotropies involve a
combination of the scalar and tensor fluctuations, the scalar amplitude can be
independently determined through the CMB-galaxy cross-correlation. Using
recently measured cross-correlation amplitudes, arising from the
cross-correlation between galaxies and the Integrated Sachs Wolfe effect in CMB
anisotropies, we obtain a constraint r < 0.5 at 68% confidence level on the
tensor-to-scalar fluctuation amplitude ratio. The data also allow us to exclude
gravity waves at a level of a few percent, relative to the density field, in a
low - Lambda dominated universe(Omega_Lambda~0.5). In future, joining
cross-correlation ISW measurements, which captures cosmological parameter
information, with independent determinations of the matter density and CMB
anisotropy power spectrum, may constrain the tensor-to-scalar ratio to a level
above 0.05. This value is the ultimate limit on tensor-to-scalar ratio from
temperature anisotropy maps when all other cosmological parameters except for
the tensor amplitude are known and the combination with CMB-galaxy correlation
allows this limit to be reached easily by accounting for degeneracies in
certain cosmological parameters.Comment: 5 Pages, 1 Figure, revised discussion on cosmic variance limits on
the tensor-to-scalar ratio from CMB, matches PRD accepted versio
Scalar field exact solutions for non-flat FLRW cosmology: A technique from non-linear Schr\"odinger-type formulation
We report a method of solving for canonical scalar field exact solution in a
non-flat FLRW universe with barotropic fluid using non-linear Schr\"{o}dinger
(NLS)-type formulation in comparison to the method in the standard Friedmann
framework. We consider phantom and non-phantom scalar field cases with
exponential and power-law accelerating expansion. Analysis on effective
equation of state to both cases of expansion is also performed. We speculate
and comment on some advantage and disadvantage of using the NLS formulation in
solving for the exact solution.Comment: 12 pages, GERG format, Reference added. accepted by Gen. Relativ. and
Gra
Probing Dark Energy with Supernovae : Bias from the time evolution of the equation of state
Observation of thousands of type Ia supernovae should offer the most direct
approach to probe the dark energy content of the universe. This will be
undertaken by future large ground-based surveys followed by a space mission
(SNAP/JDEM). We address the problem of extracting the cosmological parameters
from the future data in a model independent approach, with minimal assumptions
on the prior knowledge of some parameters. We concentrate on the comparison
between a fiducial model and the fitting function and adress in particular the
effect of neglecting (or not) the time evolution of the equation of state. We
present a quantitative analysis of the bias which can be introduced by the
fitting procedure. Such bias cannot be ignored as soon as the statistical
errors from present data are drastically improved.Comment: 22 pages, 10 figures, submitted to Phys. Rev.
Comparison of Standard Ruler and Standard Candle constraints on Dark Energy Models
We compare the dark energy model constraints obtained by using recent
standard ruler data (Baryon Acoustic Oscillations (BAO) at z=0.2 and z=0.35 and
Cosmic Microwave Background (CMB) shift parameters R and l_a) with the
corresponding constraints obtained by using recent Type Ia Supernovae (SnIa)
standard candle data (ESSENCE+SNLS+HST from Davis et. al.). We find that, even
though both classes of data are consistent with LCDM at the 2\sigma level,
there is a systematic difference between the two classes of data. In
particular, we find that for practically all values of the parameters
(\Omega_0m,\Omega_b) in the 2\sigma range of the the 3-year WMAP data (WMAP3)
best fit, LCDM is significantly more consistent with the SnIa data than with
the CMB+BAO data. For example for (\Omega_0m,\Omega_b)=(0.24,0.042)
corresponding to the best fit values of WMAP3, the dark energy equation of
state parametrization w(z)=w_0 + w_1 (z/(1+z)) best fit is at a 0.5\sigma
distance from LCDM (w_0=-1,w_1=0) using the SnIa data and 1.7\sigma away from
LCDM using the CMB+BAO data. There is a similar trend in the earlier data (SNLS
vs CMB+BAO at z=0.35). This trend is such that the standard ruler CMB+BAO data
show a mild preference for crossing of the phantom divide line w=-1, while the
recent SnIa data favor LCDM. Despite of this mild difference in trends, we find
no statistically significant evidence for violation of the cosmic distance
duality relation \eta \equiv d_L(z)/(d_A(z) (1+z)^2)=1. For example, using a
prior of \Omega_0m=0.24, we find \eta=0.95 \pm 0.025 in the redshift range
0<z<2, which is consistent with distance duality at the 2\sigma level.Comment: References added. 9 pages, 7 figures. The Mathematica files with the
numerical analysis of the paper can be found at
http://leandros.physics.uoi.gr/rulcand/rulcand.ht
Dark matter and structure formation a review
This paper provides a review of the variants of dark matter which are thought
to be fundamental components of the universe and their role in origin and
evolution of structures and some new original results concerning improvements
to the spherical collapse model. In particular, I show how the spherical
collapse model is modified when we take into account dynamical friction and
tidal torques
Fitting Type Ia supernovae with coupled dark energy
We discuss the possible consistency of the recently discovered Type Ia
supernovae at z>1 with models in which dark energy is strongly coupled to a
significant fraction of dark matter, and in which an (asymptotic) accelerated
phase exists where dark matter and dark energy scale in the same way. Such a
coupling has been suggested for a possible solution of the coincidence problem,
and is also motivated by string cosmology models of "late time" dilaton
interactions. Our analysis shows that, for coupled dark energy models, the
recent data are still consistent with acceleration starting as early as at
(to within 90% c.l.), although at the price of a large "non-universality"
of the dark energy coupling to different matter fields. Also, as opposed to
uncoupled models which seem to prefer a ``phantom'' dark energy, we find that a
large amount of coupled dark matter is compatible with present data only if the
dark energy field has a conventional equation of state w>-1.Comment: 13 pages, 6 figures. Final version, accepted for publication in JCA
Two Loop Scalar Self-Mass during Inflation
We work in the locally de Sitter background of an inflating universe and
consider a massless, minimally coupled scalar with a quartic self-interaction.
We use dimensional regularization to compute the fully renormalized scalar
self-mass-squared at one and two loop order for a state which is released in
Bunch-Davies vacuum at t=0. Although the field strength and coupling constant
renormalizations are identical to those of lfat space, the geometry induces a
non-zero mass renormalization. The finite part also shows a sort of growing
mass that competes with the classical force in eventually turning off this
system's super-acceleration.Comment: 31 pages, 5 figures, revtex4, revised for publication with extended
list of reference
Models of quintessence coupled to the electromagnetic field and the cosmological evolution of alpha
We study the change of the effective fine structure constant in the
cosmological models of a scalar field with a non-vanishing coupling to the
electromagnetic field. Combining cosmological data and terrestrial observations
we place empirical constraints on the size of the possible coupling and explore
a large class of models that exhibit tracking behavior. The change of the fine
structure constant implied by the quasar absorption spectra together with the
requirement of tracking behavior impose a lower bound of the size of this
coupling. Furthermore, the transition to the quintessence regime implies a
narrow window for this coupling around in units of the inverse Planck
mass. We also propose a non-minimal coupling between electromagnetism and
quintessence which has the effect of leading only to changes of alpha
determined from atomic physics phenomena, but leaving no observable
consequences through nuclear physics effects. In doing so we are able to
reconcile the claimed cosmological evidence for a changing fine structure
constant with the tight constraints emerging from the Oklo natural nuclear
reactor.Comment: 13 pages, 10 figures, RevTex, new references adde
Cosmological constraints on the dark energy equation of state and its evolution
We have calculated constraints on the evolution of the equation of state of
the dark energy, w(z), from a joint analysis of data from the cosmic microwave
background, large scale structure and type-Ia supernovae. In order to probe the
time-evolution of w we propose a new, simple parametrization of w, which has
the advantage of being transparent and simple to extend to more parameters as
better data becomes available. Furthermore it is well behaved in all asymptotic
limits. Based on this parametrization we find that w(z=0)=-1.43^{+0.16}_{-0.38}
and dw/dz(z=0) = 1.0^{+1.0}_{-0.8}. For a constant w we find that -1.34 < w <
-0.79 at 95% C.L. Thus, allowing for a time-varying w shifts the best fit
present day value of w down. However, even though models with time variation in
w yield a lower chi^2 than pure LambdaCDM models, they do not have a better
goodness-of-fit. Rank correlation tests on SNI-a data also do not show any need
for a time-varying w.Comment: 19 pages, 11 figures, JCAP format, typos corrected, references
update
Dark energy cosmology with generalized linear equation of state
Dark energy with the usually used equation of state , where
is hydrodynamically unstable. To overcome this drawback we consider
the cosmology of a perfect fluid with a linear equation of state of a more
general form , where the constants and
are free parameters. This non-homogeneous linear equation of state provides the
description of both hydrodynamically stable () and unstable
() fluids. In particular, the considered cosmological model describes
the hydrodynamically stable dark (and phantom) energy. The possible types of
cosmological scenarios in this model are determined and classified in terms of
attractors and unstable points by the using of phase trajectories analysis. For
the dark energy case there are possible some distinctive types of cosmological
scenarios: (i) the universe with the de Sitter attractor at late times, (ii)
the bouncing universe, (iii) the universe with the Big Rip and with the
anti-Big Rip. In the framework of a linear equation of state the universe
filled with an phantom energy, , may have either the de Sitter attractor
or the Big Rip.Comment: 12 pages, 11 figures, typos corrected, references adde