5,454 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
Spintessence: a possible candidate as a driver of the late time cosmic acceleration
In this paper, it is shown completely analytically that a spintessence model
can very well serve the purpose of providing an early deceleration and the
present day acceleration.Comment: 5 pages, no figure. Accepted for publication in Astrophysics and
Space Scienc
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
Dynamics of Massive Scalar Fields in dS Space and the dS/CFT Correspondence
Global geometric properties of dS space are presented explicitly in various
coordinates. A Robertson-Walker like metric is deduced, which is convenient to
be used in study of dynamics in dS space. Singularities of wavefunctions of
massive scalar fields at boundary are demonstrated. A bulk-boundary propagator
is constructed by making use of the solutions of equations of motion. The
dS/CFT correspondence and the Strominger's mass bound is shown.Comment: latex, 14 pages and 3 figure
Geodesics on Lie groups: Euler equations and totally geodesic subgroup
The geodesic motion on a Lie group equipped with a left or right invariant Riemannian
metric is governed by the Euler-Arnold equation. This paper investigates conditions on the
metric in order for a given subgroup to be totally geodesic. Results on the construction
and characterisation of such metrics are given. The setting works both in the classical nite
dimensional case, and in the category of in nite dimensional Fr echet Lie groups, in which
di eomorphism groups are included. Using the framework we give new examples of both nite
and in nite dimensional totally geodesic subgroups. In particular, based on the cross helicity,
we construct right invariant metrics such that a given subgroup of exact volume preserving
di eomorphisms is totally geodesic.
The paper also gives a general framework for the representation of Euler-Arnold equations
in arbitrary choice of dual pairing
Symplectic Reduction for Semidirect Products and Central Extensions
This paper proves a symplectic reduction by stages theorem in the context of geometric mechanics on symplectic manifolds with symmetry groups that are group extensions. We relate the work to the semidirect product reduction theory developed in the 1980's by Marsden, Ratiu, Weinstein, Guillemin and Sternberg as well as some more recent results and we recall how semidirect product reduction finds use in examples, such as the dynamics of an underwater vehicle.
We shall start with the classical cases of commuting reduction (first appearing in Marsden and Weinstein, 1974) and present a new proof and approach to semidirect product theory. We shall then give an idea of how the more general theory of group extensions proceeds (the details of which are given in Marsden, Misiołek, Perlmutter and Ratiu, 1998). The case of central extensions is illustrated in this paper with the example of the Heisenberg group. The theory, however, applies to many other interesting examples such as the Bott-Virasoro group and the KdV equation
Speaking in Code, Mental Time Travel and Mind Reading: an exploration of the study of animal intelligence
Many humans believe they are separate from nature based on “unique” characteristics. For many centuries, our advanced tool use was believed to separate us from other animals. However, discoveries made by Jane Goodall in the 1960’s demonstrated that humans are not unique for this capacity.1 In light of these discoveries, the definition of human was altered to suggest that our intelligence sets us apart from other animals. This definition has been used in order to justify our exploitation of other animals by suggesting that we are unique, and therefore, superior due to this “unique” intelligence. However, humans are not alone in our capacity for complex thought. Other animals exhibit three major features of intelligence: language, episodic memory, and theory of mind. However, homocentric methodology has denied these animals the opportunity to demonstrate their intelligence by confining them to human-specific standards. Only through species-specific testing can we reveal the true intellectual capacities of other animals. Through the recognition of these commonalities, the distinction between “human” and “other animals” can be blurred, thus aiding in conservation efforts as humans recognize that we are not superior to animals and therefore are not more deserving of our Earth’s resources
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
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