2 research outputs found
The Accelerated Acceleration of the Universe
We present a simple mechanism which can mimic dark energy with an equation of
state w < -1 as deduced from the supernova data. We imagine that the universe
is accelerating under the control of a quintessence field, which is moving up a
very gently sloping potential. As a result, the potential energy and hence the
acceleration increases at lower redshifts. Fitting this behavior with a dark
energy model with constant w would require w<-1. In fact we find that the
choice of parameters which improves the fit to the SNe mimics w = -1.4 at low
redshifts. Running up the potential in fact provides the best fit to the SN
data for a generic quintessence model. However, unlike models with phantoms,
our model does not have negative energies or negative norm states. Future
searches for supernovae at low redshifts 0.1 < z < 0.5 and at high redshifts
z>1 may be a useful probe of our proposal.Comment: 14 pages, 5 figure
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
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