http://arxiv.org/abs/astro-ph/0304325
We compare the WMAP temperature power spectrum and SNIa data to models with a generalized Chaplygin gas as dark energy. The generalized Chaplygin gas is a component with an exotic equation of state, p_X=-A/\rho^\alpha_X (a polytropic gas with negative constant and exponent). Our main result is that, restricting to a flat universe and to adiabatic pressure perturbations for the generalized Chaplygin gas, the constraints at 95% CL to the present equation of state w_X = p_X / \rho_X and to the parameter \alpha are -1\leq w_X < -0.8, 0 \leq \alpha <0.2, respectively. Moreover, we show that a Chaplygin gas (\alpha =1) as a candidate for dark energy is ruled out by our analysis at more than the 99.99% CL. A generalized Chaplygin gas as a unified dark matter candidate (\Omega_{CDM}=0) appears much less likely than as a dark energy model, although its \chi^2 is only two sigma away from the expected value.
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http://arxiv.org/PS_cache/astro-ph/pdf/0402/0402228.pdf
Unified dark energy models : a phenomenological approach
V.F. Cardone, * A. Troisi, and S. Capozziello
Dipartimento di Fisica “E.R. Caianiello”, Universit`
a di Salerno and INFN, Sez. di Napoli,
Gruppo Coll. di Salerno, via S. Allende, 84081 -Baronissi (Salerno), Italy
A phenomenological approach is proposed to the problem of universe accelerated expansion and of the dark energy nature. A general class of models is introduced whose energy density depends
on the redshift z in such a way that a smooth transition among the three main phases of the universe evolution (radiation era, matter domination, asymptotical de Sitter state) is naturally
achieved. We use the estimated age of the universe, the Hubble diagram of Type Ia Supernovae and the angular size -redshift relation for compact and ultracompact radio structures to test whether
the model is in agreement with astrophysical observation and to constrain its main parameters.
Although phenomenologically motivated, the model may be straightforwardly interpreted as a two fluids scenario in which the quintessence is generated by a suitably chosen scalar field potential. On the other hand, the same model may also be read in the context of unified dark energy models or in the framework of modified Friedmann equation theories.
PACS numbers: 98.80.-k, 98.80.Es, 97.60.Bw, 98.70.Dk
I. INTRODUCTION
In the last few years, an increasing bulk of data has
been accumulated leading to the emergence of a new
cosmological scenario. The Hubble diagram of type Ia
Supernovae (SNeIa) first indicated that the universe expansion
is today accelerating [1, 2]. The precise determination
of first and second peaks in the anisotropy
spectrum of cosmic microwave background radiation
(CMBR) by the BOOMERanG and MAXIMA collaborations
[3] strongly suggested that the geometry of the
universe is spatially flat. When combined with the data
on the matter density parameter
M, these results lead to the conclusion that the contribution
X of dark energy is the dominant one, being M . 0.3,
X . 0.7.
This picture has been strenghtened by the recent determination
of CMBR spectrum measured by the WMAP team
According to the standard recipe, pressureless cold
darkmatter anda homogenously distributedcosmic fluid
with negative pressure, referred to as dark energy, fill the
universe making up of order 95% of its energy budget.
What is the nature of this dark energy still remains an
open and fascinating problem. The simplest explanation
claims for the cosmological constant thus leading to
the so called CDM model Although being the best
fit to most of the available astrophysical data the
CDM model is also plagued by many problems on different
scales. If interpreted as vacuum energy, is up
to 120 orders of magnitudes smaller than the predicted
value. Furthermore, one should also solve the coincidece
problem, i.e. the nearly equivalence of the matter and
contribution to the total energy density.
As a response to these problems, much interest has
been devoted to models with dynamical vacuum energy,
*
Corresponding author, email:
winny@na.infn.it
dubbed quintessence [6]. These models typically involve
scalar fields with a particular class of potentials, allowing
the vacuum energy to become dominant only recently
(see [7, 8] for comprehensive reviews). Altough
quintessence by a scalar field is the most studied candidate
for dark energy, it generally does not avoid ad
hoc fine tuning to solve the coincidence problem. On
the other hand, a quintessential behaviour may also be
recovered without the need of scalar fields, but simply
by taking into account the effective contribution to cosmology of some (usually neglected aspects)of fundamental
physics A first tentative were undertaken showing
that a universe with a non vanishing torsion field
is consistent with SNeIa Hubble diagram and Sunyaev -
Zel’dovich data on clusters of galaxies [10]. The same
quintessential framework can be obtained with the extension
of Einstein gravity to higher order curvature invariants
leading to a model which is in good agreement
with the SNeIa Hubble diagram and the estimated age
of the universe [11]. It is worth noting that these alternative
schemes provide naturally a cosmological component with negative
pressure whose origin is simply related
to the geometry of the universe itself thus overcoming
the problems linked to the physical significance of scalar
fields.
Despite the broad interest in darkmatter and darkenergy,
their physical properties are still poorly understood
at a fundamental level and, indeed, it has never been
shown that the two are in fact two different ingredients.
(it kinda combined the ones I discuss below, together with estimates of what SNAP would do to the error zones).
