Paper: astro-ph/0512622
Date: Tue, 27 Dec 2005 22:36:21 GMT (74kb)

Title: "Expansion" around the vacuum: how far can we go from Lambda?

Authors: J.S. Alcaniz, H. Stefancic

Comments: 7 pages, 3 figures, LaTeX
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The cosmological constant ($\Lambda$), i.e., the energy density stored in the
true vacuum state of all existing fields in the Universe, is the simplest and
the most natural possibility to describe the current cosmic acceleration.
However, despite its observational successes, such a possibility exacerbates
the well known $\Lambda$ problem, requiring a natural explanation for its
small, but nonzero, value. In this paper we discuss how different our Universe
may be from the $\Lambda$CDM model by studying observational aspects of a kind
of "expansion" around the vacuum given by the equation of (EOS)
$p_{d}=-\rho_{d} - A \rho_{d}^{\alpha}$. In different parameter regimes such a
parametrization is capable of describing both quintessence-like and phantom
like dark energy, transient acceleration, and various (non)singular
possibilities for the final destiny of the Universe, including singularities at
finite values of the scale factor, the so-called "Big Rip", as well as sudden
future singularities. By using some of the most recent cosmological
observations we show that if the functional form of the dark energy EOS has
additional parameters very little can be said about their values from the
current observational results, which postpones, until the arrival of more
precise observational data, a definitive answer to the question posed above.

\\ ( http://arXiv.org/abs/astro-ph/0512622 , 74kb)