2,318 research outputs found
Study of the Magnitude-Redshift Relation for type Ia Supernovae in a Model resulting from a Ricci-Symmetry
Models with a dynamic cosmological term \Lambda (t) are becoming popular as
they solve the cosmological constant problem in a natural way. Instead of
considering any ad-hoc assumption for the variation of \Lambda, we consider a
particular symmetry, the contracted Ricci-collineation along the fluid flow, in
Einstein's theory. We show that apart from having interesting properties, this
symmetry does require \Lambda to be a function of the scale factor of the
Robertson-Walker metric. In order to test the consistency of the resulting
model with observations, we study the magnitude-redshift relation for the type
Ia supernovae data from Perlmutter et al. The data fit the model very well and
require a positive non-zero \Lambda and a negative deceleration parameter. The
best-fitting flat model is obtained as \Omega_0 \approx 0.5 with q_0 \approx
-0.2.Comment: 14 pages including 1 figure, accepted for publication in Gen.
Relativ. Gra
Einstein-de Sitter model re-examined for the newly discovered SNe Ia
Consistency of Einstein-de Sitter model with the recently observed SNe Ia by
the Hubble Space Telescope is examined. The model shows a reasonable fit to the
observation, if one takes into account the extinction of SNe light by the
intergalactic metallic dust ejected from the SNe explosions. Though the fit to
the new data is worsened considerably compared with the earlier data, it can
still be regarded acceptable. We should wait for more accurate observations at
higher redshifts (as expected from the coming space missions such as SNAP and
JWST) in order to rule out a model, which seems to explain all the other
existing observations well (some even better than the favoured CDM
model), is consistent with beautiful theoretical ideas like inflation and cold
dark matter, and is not as speculative as the models of dark energy.Comment: 14 pages including 2 figures, to appear in the Mon. Not. Roy. Astron.
So
A Machian Model of Dark Energy
Einstein believed that Mach's principle should play a major role in finding a
meaningful spacetime geometry, though it was discovered later that his field
equations gave some solutions which were not Machian. It is shown, in this
essay, that the kinematical models, which are invoked to solve the
cosmological constant problem, are in fact consistent with Mach's ideas. One
particular model in this category is described which results from the
microstructure of spacetime and seems to explain the current observations
successfully and also has some benefits over the conventional models. This
forces one to think whether the Mach's ideas and the cosmological constant are
interrelated in some way.Comment: Received an Honorable mention in the Essay Contest-2002 sponsored by
the Gravity Research Foundation; A paragraph added on how the model can
explain the CMB anisotropy observations; To appear in the Classical and
Quantum Gravit
Modeling Repulsive Gravity with Creation
There is a growing interest in the cosmologists for theories with negative
energy scalar fields and creation, in order to model a repulsive gravity. The
classical steady state cosmology proposed by Bondi, Gold and Hoyle in 1948, was
the first such theory which used a negative kinetic energy creation field to
invoke creation of matter. We emphasize that creation plays very crucial role
in cosmology and provides a natural explanation to the various explosive
phenomena occurring in local (z<0.1) and extra galactic universe. We exemplify
this point of view by considering the resurrected version of this theory - the
quasi-steady state theory, which tries to relate creation events directly to
the large scale dynamics of the universe and supplies more natural explanations
of the observed phenomena. Although the theory predicts a decelerating universe
at the present era, it explains successfully the recent SNe Ia observations
(which require an accelerating universe in the standard cosmology), as we show
in this paper by performing a Bayesian analysis of the data.Comment: The paper uses an old SNeIa dataset. With the new improved data, for
example the updated gold sample (Riess et al, astro-ph/0611572), the fit
improves considerably (\chi^2/DoF=197/180 and a probability of
goodness-of-fit=18%
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