6 research outputs found
Type Ia supernovae tests of fractal bubble universe with no cosmic acceleration
The unexpected dimness of Type Ia supernovae at redshifts z >~ 1 has over the
past 7 years been seen as an indication that the expansion of the universe is
accelerating. A new model cosmology, the "fractal bubble model", has been
proposed by one of us [gr-qc/0503099], based on the idea that our observed
universe resides in an underdense bubble remnant from a primordial epoch of
cosmic inflation, together with a new solution for averaging in an
inhomogeneous universe. Although there is no cosmic acceleration, it is claimed
that the luminosity distance of type Ia supernovae data will nonetheless fit
the new model, since it mimics a Milne universe at low redshifts. In this paper
the hypothesis is tested statistically against the available type Ia supernovae
data by both chi-square and Bayesian methods. While the standard model with
cosmological constant Omega_Lambda = 1-Omega_m is favoured by a Bayesian
analysis with wide priors, the comparison depends strongly on the priors chosen
for the density parameter, Omega_m. The fractal bubble model gives better
agreement generally for Omega_m<0.2. It also gives reasonably good fits for all
the range, Omega_m=0.01-0.55, allowing the possibility of a viable cosmology
with just baryonic matter, or alternatively with both baryonic matter and
additional cold dark matter.Comment: 10 pages, 5 figures, aastex. v3: Corrected volume factor changes
parameter estimates and discussion, figures redrawn, references adde
How strong is the evidence for accelerated expansion?
We test the present expansion of the universe using supernova type Ia data
without making any assumptions about the matter and energy content of the
universe or about the parameterization of the deceleration parameter. We assume
the cosmological principle to apply in a strict sense. The result strongly
depends on the data set, the light-curve fitting method and the calibration of
the absolute magnitude used for the test, indicating strong systematic errors.
Nevertheless, in a spatially flat universe there is at least a 5 sigma evidence
for acceleration which drops to 1.8 sigma in an open universe.Comment: 16 pages, 3 figure
Can a dust dominated universe have accelerated expansion?
Recently, there has been suggestions that the apparent accelerated expansion
of the universe is due not to a cosmological constant, but rather to
inhomogeneities in the distribution of matter. In this work, we investigate a
specific class of inhomogeneous models that can be solved analytically, namely
the dust-dominated Lemaitre-Tolman-Bondi universe models. We show that they do
not permit accelerated cosmic expansion.Comment: 9 pages, 1 figure. v3: Paper shortened and updated. References added.
v4: Minor LATEX problem fixed. Submitted to JCA