6,643 research outputs found
The cosmological dark sector as a scalar -meson field
Previous quantum field estimations of the QCD vacuum in the expanding
space-time lead to a dark energy component scaling linearly with the Hubble
parameter, which gives the correct figure for the observed cosmological term.
Here we show that this behaviour also appears at the classical level, as a
result of the chiral symmetry breaking in a low energy, effective
-model. The dark sector is described in a unified way by the
condensate and its fluctuations, giving rise to a decaying dark energy and a
homogeneous creation of non-relativistic dark particles. The creation rate and
the future asymptotic de Sitter horizon are both determined by the
mass scale.Comment: A typo was fixed in Eq. (19)-(20), and a reference adde
Probability Amplitudes for Charge-Monopole Scattering
In this letter we quantize a previously proposed non-local lagrangean for the
classical dual electrodynamics (Phys.Lett.B 384(1996)197), showing how it can
be used to construct probability amplitudes. Our results are shown to agree
with those obtained in the context of Schwinger and Zwanzinger formulations,
but without necessity of introducing strings.Comment: To appear in The Journal of High Energy Physics (JHEP), Latex, 11
pages, new results adde
On the vacuum entropy and the cosmological constant
It is generally accepted that the entropy of an asymptotically de Sitter
universe is bounded by the area, in Planck units, of the de Sitter horizon.
Based on an analysis of the entropy associated to the vacuum quantum
fluctuations, we suggest that the existence of such a holographic bound
constitutes a possible explanation for the observed value of the cosmological
constant, theoretically justifying a relation proposed 35 years ago by
Zel'dovich.Comment: Version to appear in the GRF2003 Special Issue of IJMP
The role of Weyl symmetry in hydrodynamics
This article is dedicated to the analysis of Weyl symmetry in the context of
relativistic hydrodynamics. Here is discussed how this symmetry is properly
implemented using the prescription of minimal coupling: . It is shown that this prescription has no problem to deal
with curvature since it gives the correct expressions for the commutator of
covariant derivatives.
In the hydrodynamics the Weyl gauge connection emerges from the degrees of
freedom of the fluid: it is a combination of the expansion and entropy
gradient. The remaining degrees of the fluid and the metric tensor are see in
this context as charged fields under the Weyl gauge connection. The gauge
nature of conformal hydrodynamics is emphasized and a charge for the Weyl
connection is defined. A notion of local charge and current densities are
considered and a local charge conservation law is reached.Comment: 13 page
Probing AdS/QCD backgrounds with semi-classical strings
New AdS/QCD backgrounds have been proposed to describe the spectrum of heavy
vector mesons via the implementation of additional energy scales on the bulk
geometry of the soft wall model. The extra energy scales are needed to include
the decay constants of hadronic states when describing the radial excitations
of the heavy meson. Here we analyze one model that introduces an ultraviolet
cutoff on Anti de-Sitter space and a model that consider a dilaton profile
modified by the addition of an extra term and no cutoff. For each one of these
two models we consider the presence of a semi-classical string in the bulk that
is dual to a static and infinitely heavy meson. We compute the expected value
of the Wilson loop operator using the holographic dictionary and obtain the
dual potential for the static pair. For the model with modified
dilaton profile the on-shell string action presents a peculiar ultraviolet
divergence, a compatible regularization is discussed and a new subtraction
scheme is used. We consider the case of finite temperature and determine how
the dissociation temperature of the heavy meson is affected by the additional
energy scales.Comment: 17 pages, 12 figures (3 new
Open cosmologies with rotation
We study a rotating and expanding, Godel type metric, originally considered
by Korotkii and Obukhov, showing that, in the limit of large times and nearby
distances, it reduces to the open metric of Friedmann. In the epochs when
radiation or dust matter dominate the energy density, our solutions are similar
to the isotropic ones and, in what concerns processes occurring at small times,
the rotation leads only to higher order corrections. At large times, the
solution is dominated by a decaying positive cosmological term, with negative
pressure, and necessarily describes a quasi-flat universe if the energy
conditions have to be satisfied. The absence of closed time-like curves
requires a superior limit for the global angular velocity, which appears as a
natural explanation for the observed smallness of the present rotation. The
conclusion is that the introduction of a global rotation, in addition to be
compatible with observation, can enrich the standard model of the Universe,
explaining issues like the origin of galaxies rotation and the quasi-flatness
problem.Comment: Final version, accepted for publication in General Relativity and
Gravitatio
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