137 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
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
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
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
The cosmic coincidence in Brans-Dicke cosmologies
Among the suggested solutions to the cosmological constant problem, we find
the idea of a dynamic vacuum, with an energy density decaying with the universe
expansion. We investigate the possibility of a variation in the gravitational
constant as well, induced, at the cosmological scale, by the vacuum decay. We
consider an effective Brans-Dicke theory in the spatially flat FLRW spacetime,
finding late time solutions characterized by a constant ratio between the
matter and vacuum energy densities. By using the observed limits for the
universe age, we fix the only free parameter of our solutions, obtaining a
relative matter density in the range 0.25-0.4. In particular, for Ht = 1 we
obtain a relative matter density equals to 1/3. This constitutes a possible
explanation for another problem related to the cosmological term, the cosmic
coincidence problem.Comment: This essay received an "honorable mention" in the 2005 Essay
Competition of the Gravity Research Foundatio
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