4,327 research outputs found
On the Dirac field in the Palatini form of 1/R gravity
In recent papers (astro-ph/0306630, gr-qc/0312041) I have argued that the
observed cosmological acceleration can be accounted for by the inclusion of a
1/R term in the gravitational action in the Palatini formalism. Subsequently,
Flanagan (astro-ph/0308111, gr-qc/0403063) argued that this theory is
equivalent to a scalar-tensor theory which produces corrections to the standard
model that are ruled out experimentally.
In this article I examine the Dirac field coupled to 1/R gravity. The Dirac
action contains the connection which was taken to be the Christoffel symbol,
not an independent quantity, in the papers by Flanagan. Since the metric and
connection are taken to be independent in the Palatini approach it is natural
to allow the connection that appears in the Dirac action to be an independent
quantity. This is the approach that is taken in this paper. The resulting
theory is very different and much more complicated than the one discussed in
Flanagan's papers.Comment: 6 pages, LaTe
Opportunities for future supernova studies of cosmic acceleration
We investigate the potential of a future supernova dataset, as might be
obtained by the proposed SNAP satellite, to discriminate among different ``dark
energy'' theories that describe an accelerating Universe. We find that many
such models can be distinguished with a fit to the effective
pressure-to-density ratio, , of this energy. More models can be
distinguished when the effective slope, , of a changing is also fit,
but only if our knowledge of the current mass density, , is improved.
We investigate the use of ``fitting functions'' to interpret luminosity
distance data from supernova searches, and argue in favor of a particular
preferred method, which we use in our analysis.Comment: Four pages including figures. Final published version. No significant
changes from v
Phantom Energy Accretion by Stringy Charged Black Hole
We investigate the dynamical behavior of phantom energy near stringy
magnetically charged black hole. For this purpose, we derive equations of
motion for steady-state spherically symmetric flow of phantom energy onto the
stringy magnetically charged black hole. It is found that phantom energy
accreting onto black hole decreases its mass. Further, the location of critical
points of accretion is explored, which yields mass to charge ratio. This ratio
implies that accretion process cannot transform a black hole into an extremal
black hole or a naked singularity, hence cosmic censorship hypothesis remains
valid here.Comment: 7 pages, no figur
A Critique of Drexler Dark Matter
Drexler dark matter is an alternate approach to dark matter that assumes that
highly relativistic protons trapped in the halo of the galaxies could account
for the missing mass. We look at various energetics involved in such a scenario
such as the energy required to produce such particles and the corresponding
lifetimes. Also we look at the energy losses from synchrotron and inverse
Compton scattering and their signatures. The Coulomb repulsive instability due
to the excess charge around the galaxies is also calculated. The above results
lead us to conclude that such a model for DM is unfeasible.Comment: 4 pages, 10 equation
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