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

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

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

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

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, $w$, of this energy. More models can be distinguished when the effective slope, $dw/dz$, of a changing $w$ is also fit, but only if our knowledge of the current mass density, $\Omega_m$, 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

Modified Chaplygin Gas and Constraints on its B parameter from CDM and UDME Cosmological models

We study Modified Chaplygin Gas (MCG) as a candidate for dark energy and predict the values of parameters of the gas for a physically viable cosmological model. The equation of state of MCG ($p=B \rho - \frac {A}{\rho^\alpha}$) involves three parameters: $B$, $A$ and $\alpha$. The permitted values of these parameters are determined with the help of dimensionless age parameter ($H_{o}t_{o}$) and $H(z)-z$ Data. Specifically we study the allowed ranges of values of B parameter in terms of $\alpha$ and $A_{s}$ ($A_{s}$ is defined in terms of the constants in the theory). We explore the constraints of the parameters in Cold Dark Matter(CDM) model and UDME(Unified Dark Matter Energy) model respectively.Comment: 5 pages, 10 fig

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

Nonvanishing Cosmological Constant of Flat Universe in Brane-World Scenario

The finite temperature effect is examined in Randall-Sundrum brane-world scenario with inclusion of the matter fields on the brane. At zero temperature it is found that the theory on the brane is conformally invariant, which guarantees $AdS$/CFT. At 4d effective action we derived a temperature-dependent nonvanishing cosmological constant at the flat spacetime limit of brane worldvolume. At the cosmological temperature $3 {\bf K}$ the cosmological constant is roughly $(0.0004 eV)^4$ which is within the upper bound of the recent experimental value $(0.01 eV)^4$Comment: 10 pages, 1 figure; V2 12 pages, figure removed, the contribution of matter to the cosmological constant is added, will appear in PL