20,861 research outputs found
Gauge Invariance and Fractional Statistics
We present a new -dimensional field theory showing exotic statistics
and fractional spin. This theory is achieved through a redefinition of the
gauge field . New properties are found. Another way to implement the
field redefinition is used with the same results obtained.Comment: 5 page
Accelerating Cold Dark Matter Cosmology ()
A new kind of accelerating flat model with no dark energy that is fully
dominated by cold dark matter (CDM) is investigated. The number of CDM
particles is not conserved and the present accelerating stage is a consequence
of the negative pressure describing the irreversible process of gravitational
particle creation. A related work involving accelerating CDM cosmology has been
discussed before the SNe observations [Lima, Abramo & Germano, Phys. Rev. D53,
4287 (1996)]. However, in order to have a transition from a decelerating to an
accelerating regime at low redshifts, the matter creation rate proposed here
includes a constant term of the order of the Hubble parameter. In this case,
does not need to be small in order to solve the age problem and the
transition happens even if the matter creation is negligible during the
radiation and part of the matter dominated phase. Therefore, instead of the
vacuum dominance at redshifts of the order of a few, the present accelerating
stage in this sort of Einstein-de Sitter CDM cosmology is a consequence of the
gravitational particle creation process. As an extra bonus, in the present
scenario does not exist the coincidence problem that plagues models with
dominance of dark energy. The model is able to harmonize a CDM picture with the
present age of the universe, the latest measurements of the Hubble parameter
and the Supernovae observations.Comment: 9 pages, 6 figures, typos corrected, references added, discussion in
Appendix B extende
Features of collisionless turbulence in the intracluster medium from simulated Faraday Rotation maps
Observations of the intracluster medium (ICM) in galaxy clusters suggest for
the presence of turbulence and the magnetic fields existence has been proved
through observations of Faraday Rotation and synchrotron emission. The ICM is
also known to be filled by a rarefied weakly collisional plasma. In this work
we study the possible signatures left on Faraday Rotation maps by collisionless
instabilities. For this purpose we use a numerical approach to investigate the
dynamics of the turbulence in collisionless plasmas based on an
magnetohydrodynamical (MHD) formalism taking into account different levels of
pressure anisotropy. We consider models covering the sub/super-Alfv\'enic and
trans/supersonic regimes, one of them representing the fiducial conditions
corresponding to the ICM. From the simulated models we compute Faraday Rotation
maps and analyze several statistical indicators in order to characterize the
magnetic field structure and compare the results obtained with the
collisionless model to those obtained using standard collisional MHD framework.
We find that important imprints of the pressure anisotropy prevails in the
magnetic field and also manifest in the associated Faraday Rotation maps which
evidence smaller correlation lengths in the collisionless MHD case. These
points are remarkably noticeable for the case mimicking the conditions
prevailing in ICM. Nevertheless, in this study we have neglected the decrease
of pressure anisotropy due to the feedback of the instabilities that naturally
arise in collisionless plasmas at small scales. This decrease may not affect
the statistical imprint differences described above, but should be examined
elsewhere.Comment: 24 pages, 15 figures, MNRAS accepte
Procedimentos corretos para a prática do pesque-e-solte.
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Gravity-induced vacuum dominance
It has been widely believed that, except in very extreme situations, the
influence of gravity on quantum fields should amount to just small,
sub-dominant contributions. This view seemed to be endorsed by the seminal
results obtained over the last decades in the context of renormalization of
quantum fields in curved spacetimes. Here, however, we argue that this belief
is false by showing that there exist well-behaved spacetime evolutions where
the vacuum energy density of free quantum fields is forced, by the very same
background spacetime, to become dominant over any classical energy-density
component. This semiclassical gravity effect finds its roots in the infrared
behavior of fields on curved spacetimes. By estimating the time scale for the
vacuum energy density to become dominant, and therefore for backreaction on the
background spacetime to become important, we argue that this vacuum dominance
may bear unexpected astrophysical and cosmological implications.Comment: To appear in Phys. Rev. Lett
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