A&A 429, 807-818 (2005)

DOI: 10.1051/0004-6361:20041168

## Cosmological parameters from supernova observations: A critical comparison of three data sets

**T. R. Choudhury**

^{1}and T. Padmanabhan^{2}^{1}SISSA/ISAS, via Beirut 2-4, 34014 Trieste, Italy

e-mail: chou@sissa.it

^{2}IUCAA, Ganeshkhind, Pune, 411 007, India

e-mail: nabhan@iucaa.ernet.in

(Received 27 April 2004 / Accepted 15 September 2004 )

** Abstract **

We extend our previous analysis of cosmological supernova type Ia data
(Padmanabhan & Choudhury 2003) to include three recent compilation of data sets.
Our analysis ignores the possible correlations and systematic effects
present in the data and concentrates mostly on some key theoretical
issues.
Among the three data sets, the first set
consists of 194 points obtained from various
observations while the second
discards some of the points from the
first one because of large uncertainties
and
thus consists of 142 points. The third data set is obtained
from the second by adding the latest 14 points observed through HST.
A careful comparison of these different data sets help us to draw
the following conclusions: (i) All the three data sets strongly rule out
non-accelerating models.
Interestingly, the first and the second data sets favour a closed
universe; if
, then the
probability of obtaining models with
is
0.97.
Hence these data sets are in
mild disagreement with the "concordance" flat model. However, this
disagreement is reduced (the probability of obtaining models with
being
0.9) for the third data set, which
includes the most recent points observed by HST around
1 < *z* < 1.6.
(ii) When the first data set is divided into two separate
subsets consisting of low (
*z* < 0.34) and high (
*z* > 0.34)
redshift supernova, it
turns out that these two subsets, individually, admit
non-accelerating models with
zero dark energy because of different magnitude zero-point values
for the different subsets.
This can also be seen when the data is analysed while allowing
for possibly different
magnitude zero-points for the two redshift subsets.
However, the non-accelerating models
seem to be ruled out using *only* the low redshift data for
the other two data sets, which have
less uncertainties.
(iii) We have also
found that it is quite difficult to measure the
evolution of the dark energy equation of state
*w*_{X}(*z*) though
its present value can be constrained quite well.
The best-fit value seems to *mildly* favour a dark energy component
with current equation of state
*w*_{X} < -1, thus opening
the possibility of existence of more exotic forms of matter. However,
the data is still consistent with the
the standard cosmological constant at 99 per cent confidence level
for
.

**Key words:**supernovae: general

**--**cosmology: miscellaneous

**--**cosmological parameters

**©**

*ESO 2005*