473 research outputs found
Dark Energy and Some Alternatives: a Brief Overview
The high-quality cosmological data, which became available in the last
decade, have thrusted upon us a rather preposterous composition for the
universe which poses one of the greatest challenges theoretical physics has
ever faced: the so-called dark energy. By focusing our attention on specific
examples of dark energy scenarios, we discuss three different candidates for
this dark component, namely, a decaying vacuum energy or time-varying
cosmological constant [], a rolling homogeneous quintessence field
(), and modifications in gravity due to extra spatial dimensions. As
discussed, all these candidates [along with the vacuum energy or cosmological
constant ()] seem somewhat to be able to explain the current
observational results, which hampers any definitive conclusion on the actual
nature of the dark energy.Comment: 8 pages, 6 figures, Invited talk at XXVI Brazilian National Meeting
on Particles and Fields, Sao Lourenco, Brazil. To appear in Brazilian Journal
of Physics (2006
Energy Conditions and Segre Classification of Phantom Fields
Recent discoveries in the field of observational cosmology have provided
increasing evidence that the Universe is undergoing a late time acceleration,
which has also stimulated speculations on the nature of the dark component
responsible for such a phenomenon. Among several candidates discussed in the
current literature, phantom fields, an exotic scalar field with a negative
kinetic term and that violates most of the classical energy conditions, appear
as a real possibility according to recent observational analysis. In this paper
we examine the invariant characterization for the energy-momentum tensor of
phantom fields through the Segre algebraic classification in the framework of
general relativity. We also discuss some constraints which are imposed on the
values of from the classical energy conditions.Comment: 4 pages, 1 figure, to appear in Phys. Lett.
Deflationary cosmology: constraints from angular size and ages of globular clusters
Observational constraints to a large class of decaying vacuum cosmologies are
derived using the angular size data of compact radio sources and the latest age
estimates of globular clusters. For this class of deflationary
models, the present value of the vacuum energy density is quantified by a
positive parameter smaller than unity. In the case of milliarcsecond
compact radio-sources, we find that the allowed intervals for and the
matter density parameter are heavily dependent on the value of the
mean projected linear size . For pc, the best
fit occurs for , , and , , respectively. This analysis shows that if
one minimizes for the free parameters , and
, the best fit for these angular size data corresponds to a decaying
with and
pc. Constraints from age estimates of globular clusters and old high redshift
galaxies are not so restrictive, thereby suggesting that there is no age crisis
for this kind of cosmologies.Comment: 6 pages, 3 figures, revised version to appear in Phys. Rev.
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