82 research outputs found
Phantom Dark Energy Models with Negative Kinetic Term
We examine phantom dark energy models derived from a scalar field with a
negative kinetic term for which V(phi) approaches infinity asymptotically. All
such models can be divided into three classes, corresponding to an equation of
state parameter w with asymptotic behavior w -> -1, w -> w_0
infinity. We derive the conditions on the potential V(phi) which lead to each
of these three types of behavior. For models with w -> -1, we derive the
conditions on V(phi) which determine whether or not such models produce a
future big rip. Observational constraints are derived on two classes of these
models: power-law potentials with V(phi) = lambda phi^alpha (with alpha
positive or negative) and exponential potentials of the form V(phi) = beta
e^{lambda phi^alpha}. It is shown that these models spend more time in a state
with Omega_m ~ Omega_phi than do corresponding models with a constant value of
w, thus providing a more satisfactory solution to the coincidence problem.Comment: 9 pages, 11 figures, minor clarifications added, to appear in Phys.
Rev.
Cosmic Mimicry: Is LCDM a Braneworld in Disguise ?
For a broad range of parameter values, braneworld models display a remarkable
property which we call cosmic mimicry. Cosmic mimicry is characterized by the
fact that, at low redshifts, the Hubble parameter in the braneworld model is
virtually indistinguishable from that in the LCDM cosmology. An important point
to note is that the \Omega_m parameters in the braneworld model and in the LCDM
cosmology can nevertheless be quite different. Thus, at high redshifts (early
times), the braneworld asymptotically expands like a matter-dominated universe
with the value of \Omega_m inferred from the observations of the local matter
density. At low redshifts (late times), the braneworld model behaves almost
exactly like the LCDM model but with a renormalized value of the cosmological
density parameter \Omega_m^{LCDM}. The redshift which characterizes cosmic
mimicry is related to the parameters in the higher-dimensional braneworld
Lagrangian. Cosmic mimicry is a natural consequence of the scale-dependence of
gravity in braneworld models. The change in the value of the cosmological
density parameter is shown to be related to the spatial dependence of the
effective gravitational constant in braneworld theory. A subclass of mimicry
models lead to an older age of the universe and also predict a redshift of
reionization which is lower than z_{reion} \simeq 17 in the LCDM cosmology.
These models might therefore provide a background cosmology which is in better
agreement both with the observed quasar abundance at z \gsim 4 and with the
large optical depth to reionization measured by the Wilkinson Microwave
Anisotropy Probe.Comment: 22 pages, 4 figures. A subsection and references added; main results
remain unchanged. Accepted for publication in JCA
Is the Universe Inflating? Dark Energy and the Future of the Universe
We consider the fate of the observable universe in the light of the discovery
of a dark energy component to the cosmic energy budget. We extend results for a
cosmological constant to a general dark energy component and examine the
constraints on phenomena that may prevent the eternal acceleration of our patch
of the universe. We find that the period of accelerated cosmic expansion has
not lasted long enough for observations to confirm that we are undergoing
inflation; such an observation will be possible when the dark energy density
has risen to between 90% and 95% of the critical. The best we can do is make
cosmological observations in order to verify the continued presence of dark
energy to some high redshift. Having done that, the only possibility that could
spoil the conclusion that we are inflating would be the existence of a
disturbance (the surface of a true vacuum bubble, for example) that is moving
toward us with sufficiently high velocity, but is too far away to be currently
observable. Such a disturbance would have to move toward us with speed greater
than about 0.8c in order to spoil the late-time inflation of our patch of the
universe and yet avoid being detectable.Comment: 7 pages, 7 figure
Alternatives to Quintessence Model Building
We discuss the issue of toy model building for the dark energy component of
the universe. Specifically, we consider two generic toy models recently
proposed as alternatives to quintessence models, known as Cardassian expansion
and the Chaplygin gas. We show that the former is enteriely equivalent to a
class of quintessence models. We determine the observational constraints on the
latter, coming from recent supernovae results and from the shape of the matter
power spectrum. As expected, these restrict the model to a behaviour that
closely matches that of a standard cosmological constant .Comment: RevTex4; 7 pages, 4 figures. v2: Improved discussion of constraints
on Chaplygin gas models. Other clarifications added. Phys Rev. D (in press
Transition Redshift: New Kinematic Constraints from Supernovae
The transition redshift (deceleration/acceleration) is discussed by expanding
the deceleration parameter to first order around its present value. A detailed
study is carried out by considering two different parameterizations: and , and the associated free parameters () are constrained by 3 different supernova samples. The previous analysis
by Riess {\it{et al.}} [ApJ 607, 665, 2004] using the first expansion is
slightly improved and confirmed in light of their recent data ({\emph{Gold}}07
sample). However, by fitting the model with the Supernova Legacy Survey (SNLS)
type Ia sample we find that the best fit to the redshift transition is instead of as derived by the High-z Supernovae Search
(HZSNS) team. This result based in the SNLS sample is also in good agreement
with the Davis {\it{et al.}} sample, ().
Such results are in line with some independent analyzes and accommodates more
easily the concordance flat model (CDM). For both parameterizations,
the three SNe type Ia samples considered favor recent acceleration and past
deceleration with a high degree of statistical confidence level. All the
kinematic results presented here depend neither on the validity of general
relativity nor the matter-energy contents of the Universe.Comment: 19 pages, 15 figures, 1 table, revised version accepted for
publication in MNRA
Further Evidence for Cosmological Evolution of the Fine Structure Constant
We describe the results of a search for time variability of the fine
structure constant, alpha, using absorption systems in the spectra of distant
quasars. Three large optical datasets and two 21cm/mm absorption systems
provide four independent samples, spanning 23% to 87% of the age of the
universe. Each sample yields a smaller alpha in the past and the optical sample
shows a 4-sigma deviation: da/a = -0.72 +/- 0.18 x 10^{-5} over the redshift
range 0.5 < z < 3.5. We find no systematic effects which can explain our
results. The only potentially significant systematic effects push da/a towards
positive values, i.e. our results would become more significant were we to
correct for them.Comment: 5 pages, 1 figure. Published in Phys. Rev. Lett. Small changes to
discussion, added an acknowledgement and a referenc
Asymptotic behavior of w in general quintom model
For the quintom models with arbitrary potential , the
asymptotic value of equation of state parameter w is obtained by a new method.
In this method, w of stable attractors are calculated by using the ratio (d ln
V)/(d ln a) in asymptotic region. All the known results, have been obtained by
other methods, are reproduced by this method as specific examples.Comment: 8 pages, one example is added, accepted for publication in Gen. Rel.
Gra
Linear and non-linear perturbations in dark energy models
I review the linear and second-order perturbation theory in dark energy
models with explicit interaction to matter in view of applications to N-body
simulations and non-linear phenomena. Several new or generalized results are
obtained: the general equations for the linear perturbation growth; an
analytical expression for the bias induced by a species-dependent interaction;
the Yukawa correction to the gravitational potential due to dark energy
interaction; the second-order perturbation equations in coupled dark energy and
their Newtonian limit. I also show that a density-dependent effective dark
energy mass arises if the dark energy coupling is varying.Comment: 12 pages, submitted to Phys. Rev; v2: added a ref. and corrected a
typ
Constraining the dark energy with galaxy clusters X-ray data
The equation of state characterizing the dark energy component is constrained
by combining Chandra observations of the X-ray luminosity of galaxy clusters
with independent measurements of the baryonic matter density and the latest
measurements of the Hubble parameter as given by the HST key project. By
assuming a spatially flat scenario driven by a "quintessence" component with an
equation of state we place the following limits on the
cosmological parameters and : (i) and (1) if the
equation of state of the dark energy is restricted to the interval (\emph{usual} quintessence) and (ii) and
() if violates the null energy condition and assume values (\emph{extended} quintessence or ``phantom'' energy). These results are in
good agreement with independent studies based on supernovae observations,
large-scale structure and the anisotropies of the cosmic background radiation.Comment: 6 pages, 4 figures, LaTe
Scenario of Accelerating Universe from the Phenomenological \Lambda- Models
Dark matter, the major component of the matter content of the Universe,
played a significant role at early stages during structure formation. But at
present the Universe is dark energy dominated as well as accelerating. Here,
the presence of dark energy has been established by including a time-dependent
term in the Einstein's field equations. This model is compatible with
the idea of an accelerating Universe so far as the value of the deceleration
parameter is concerned. Possibility of a change in sign of the deceleration
parameter is also discussed. The impact of considering the speed of light as
variable in the field equations has also been investigated by using a well
known time-dependent model.Comment: Latex, 9 pages, Major change
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