83 research outputs found
Isocurvature modes and Baryon Acoustic Oscillations
The measurement of Baryonic Acoustic Oscillations from galaxy surveys is well
known to be a robust and powerful tool to constrain dark energy. This method
relies on the knowledge of the size of the acoustic horizon at radiation drag
derived from Cosmic Microwave Background Anisotropy measurements. In this paper
we quantify the effect of non-standard initial conditions in the form of an
isocurvature component on the determination of dark energy parameters from
future BAO surveys. In particular, if there is an isocurvature component (at a
level still allowed by present data) but it is ignored in the CMB analysis, the
sound horizon and cosmological parameters determination is biased, and, as a
consequence, future surveys may incorrectly suggest deviations from a
cosmological constant. In order to recover an unbiased determination of the
sound horizon and dark energy parameters, a component of isocurvature
perturbations must be included in the model when analyzing CMB data.
Fortunately, doing so does not increase parameter errors significantly.Comment: 23 pages, 3 figure
Can Structure Formation Influence the Cosmological Evolution?
The backreaction of structure formation influences the cosmological evolution
equation for the homogenous and isotropic average metric. In a cold dark matter
universe this effect leads only to small corrections unless a substantial
fraction of matter is located in regions where strong gravitational fields
evolve in time. A``cosmic virial theorem'' states that the sum of gravitational
and matter pressure vanishes and therefore relates the average kinetic energy
to a suitable average of the Newtonian potential. In presence of a scalar
``cosmon'' field mediating quintessence, however, cosmology could be modified
if local cosmon fluctuations grow large. We speculate that this may trigger the
accelerated expansion of the universe after the formation of structure.Comment: new "cosmic virial theorem",new references,LaTex,11 page
Some FRW Models of Accelerating Universe with Dark Energy
The paper deals with a spatially homogeneous and isotropic FRW space-time
filled with perfect fluid and dark energy components. The two sources are
assumed to interact minimally, and therefore their energy momentum tensors are
conserved separately. A special law of variation for the Hubble parameter
proposed by Berman (1983) has been utilized to solve the field equations. The
Berman's law yields two explicit forms of the scale factor governing the FRW
space-time and constant values of deceleration parameter. The role of dark
energy with variable equation of state parameter has been studied in detail in
the evolution of FRW universe. It has been found that dark energy dominates the
universe at the present epoch, which is consistent with the observations. The
physical behavior of the universe is discussed in detail.Comment: 10 pages, 5 figure
Quintessence arising from exponential potentials
We demonstrate how exponential potentials that could arise in the early
Universe as a result of Kaluza-Klein type compactifications of string theory,
can lead to cosmological solutions which correspond to the currently observed
accelerating Universe. The idea is simple, relying solely on the known scaling
properties associated with exponential potentials. In particular we show that
the existence of stable attractor solutions implies that the results hold for a
wide range of coupling constants and initial conditions.Comment: 4 pages, 3 figures, published versio
Interacting Dark Energy in Ho\v{r}ava-Lifshitz Cosmology
In the usual Ho\v{r}ava-Lifshitz cosmological models, the scalar field is
responsible for dark matter. Using an additional scalar field, Saridakis
\cite{sari} has formulated Ho\v{r}ava-Lifshitz cosmology with an effective dark
energy sector. In the paper \cite{sari} the scalar fields do not interact with
each other, here we extend this work to the interacting case, where matter
scalar field interact with dark energy scalar field . We will
show that in contrast with \cite{sari}, where -filed is absent, we can
obtain , that is we result to an effective dark energy
presenting phantom behaviour. This behaviour is pure effect of the interaction.Comment: 10 pages, no figur
Conformal Invariance, Dark Energy, and CMB Non-Gaussianity
In addition to simple scale invariance, a universe dominated by dark energy
naturally gives rise to correlation functions possessing full conformal
invariance. This is due to the mathematical isomorphism between the conformal
group of certain 3 dimensional slices of de Sitter space and the de Sitter
isometry group SO(4,1). In the standard homogeneous isotropic cosmological
model in which primordial density perturbations are generated during a long
vacuum energy dominated de Sitter phase, the embedding of flat spatial sections
in de Sitter space induces a conformal invariant perturbation spectrum and
definite prediction for the shape of the non-Gaussian CMB bispectrum. In the
case in which the density fluctuations are generated instead on the de Sitter
horizon, conformal invariance of the horizon embedding implies a different but
also quite definite prediction for the angular correlations of CMB
non-Gaussianity on the sky. Each of these forms for the bispectrum is intrinsic
to the symmetries of de Sitter space and in that sense, independent of specific
model assumptions. Each is different from the predictions of single field slow
roll inflation models which rely on the breaking of de Sitter invariance. We
propose a quantum origin for the CMB fluctuations in the scalar gravitational
sector from the conformal anomaly that could give rise to these
non-Gaussianities without a slow roll inflaton field, and argue that conformal
invariance also leads to the expectation for the relation n_S-1=n_T between the
spectral indices of the scalar and tensor power spectrum. Confirmation of this
prediction or detection of non-Gaussian correlations in the CMB of one of the
bispectral shape functions predicted by conformal invariance can be used both
to establish the physical origins of primordial density fluctuations and
distinguish between different dynamical models of cosmological vacuum dark
energy.Comment: 73 pages, 9 figures. Final Version published in JCAP. New Section 4
added on linearized scalar gravitational potentials; New Section 8 added on
gravitational wave tensor perturbations and relation of spectral indices n_T
= n_S -1; Table of Contents added; Eqs. (3.14) and (3.15) added to clarify
relationship of bispectrum plotted to CMB measurements; Some other minor
modification
On exact solutions for quintessential (inflationary) cosmological models with exponential potentials
We first study dark energy models with a minimally-coupled scalar field and
exponential potentials, admitting exact solutions for the cosmological
equations: actually, it turns out that for this class of potentials the
Einstein field equations exhibit alternative Lagrangians, and are completely
integrable and separable (i.e. it is possible to integrate the system
analytically, at least by quadratures). We analyze such solutions, especially
discussing when they are compatible with a late time quintessential expansion
of the universe. As a further issue, we discuss how such quintessential scalar
fields can be connected to the inflationary phase, building up, for this class
of potentials, a quintessential inflationary scenario: actually, it turns out
that the transition from inflation toward late-time exponential quintessential
tail admits a kination period, which is an indispensable ingredient of this
kind of theoretical models. All such considerations have also been done by
including radiation into the model.Comment: Revtex4, 10 figure
A Planck-scale axion and SU(2) Yang-Mills dynamics: Present acceleration and the fate of the photon
From the time of CMB decoupling onwards we investigate cosmological evolution
subject to a strongly interacting SU(2) gauge theory of Yang-Mills scale
eV (masquerading as the factor of the SM at
present). The viability of this postulate is discussed in view of cosmological
and (astro)particle physics bounds. The gauge theory is coupled to a spatially
homogeneous and ultra-light (Planck-scale) axion field. As first pointed out by
Frieman et al., such an axion is a viable candidate for quintessence, i.e.
dynamical dark energy, being associated with today's cosmological acceleration.
A prediction of an upper limit for the duration of the
epoch stretching from the present to the point where the photon starts to be
Meissner massive is obtained: billion years.Comment: v3: consequences of an error in evolution equation for coupling
rectified, only a minimal change in physics results, two refs. adde
Global Fluctuation Spectra in Big Crunch/Big Bang String Vacua
We study Big Crunch/Big Bang cosmologies that correspond to exact world-sheet
superconformal field theories of type II strings. The string theory spacetime
contains a Big Crunch and a Big Bang cosmology, as well as additional
``whisker'' asymptotic and intermediate regions. Within the context of free
string theory, we compute, unambiguously, the scalar fluctuation spectrum in
all regions of spacetime. Generically, the Big Crunch fluctuation spectrum is
altered while passing through the bounce singularity. The change in the
spectrum is characterized by a function , which is momentum and
time-dependent. We compute explicitly and demonstrate that it arises
from the whisker regions. The whiskers are also shown to lead to
``entanglement'' entropy in the Big Bang region. Finally, in the Milne orbifold
limit of our superconformal vacua, we show that and, hence, the
fluctuation spectrum is unaltered by the Big Crunch/Big Bang singularity. We
comment on, but do not attempt to resolve, subtleties related to gravitational
backreaction and light winding modes when interactions are taken into account.Comment: 68 pages, 1 figure; typos correcte
The Covariant Entropy Bound, Brane Cosmology, and the Null Energy Condition
In discussions of Bousso's Covariant Entropy Bound, the Null Energy Condition
is always assumed, as a sufficient {\em but not necessary} condition which
helps to ensure that the entropy on any lightsheet shall necessarily be finite.
The spectacular failure of the Strong Energy Condition in cosmology has,
however, led many astrophysicists and cosmologists to consider models of dark
energy which violate {\em all} of the energy conditions, and indeed the current
data do not completely rule out such models. The NEC also has a questionable
status in brane cosmology: it is probably necessary to violate the NEC in the
bulk in order to obtain a "self-tuning" theory of the cosmological constant. In
order to investigate these proposals, we modify the Karch-Randall model by
introducing NEC-violating matter into in such a way that the brane
cosmological constant relaxes to zero. The entropy on lightsheets remains
finite. However, we still find that the spacetime is fundamentally incompatible
with the Covariant Entropy Bound machinery, in the sense that it fails the
Bousso-Randall consistency condition. We argue that holography probably forbids
all {\em cosmological} violations of the NEC, and that holography is in fact
the fundamental physical principle underlying the cosmological version of the
NEC.Comment: 21 pages, 3 figures, version 2:corrected and greatly improved
discussion of the Bousso-Randall consistency check, references added;
version3: more references added, JHEP versio
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