21 research outputs found
Which cosmological models -- with dark energy or modified FRW dynamics?
Recent measurements of distant type Ia supernovae (SNIa) as well as other
observations indicate that our universe is in accelerating phase of expansion.
In principle there are two alternative explanation for such an acceleration.
While in the first approach an unknown form of energy violating the strong
energy condition is postulated, in second one some modification of FRW dynamics
is postulated. The both approaches are in well agreement with present day
observations which is the manifestation of the degeneracy problem appearing in
observational cosmology. We use the Akaike (AIC) and Bayesian (BIC) information
criteria of model selection to overcome this degeneracy and to determine a
model with such a set of parameters which gives the most preferred fit to the
SNIa data. We consider five representative evolutional scenarios in each of
groups. Among dark energy proposal the CDM model, CDM model with
phantom field, CDM model with topological defect, model with Chaplygin gas, and
the model with the linear dynamical equation of state parameter. As an
alternative prototype scenarios we consider: brane world Dvali Gabadadze
Porrati scenario, brane models in Randall-Sundrum scenario, Cardassian models
with dust matter and radiation, bouncing model with the cosmological constant
and metric-affine gravity (MAG) inspired cosmological models. Applying the
model selection criteria we show that both AIC and BIC indicates that
additional contribution arises from nonstandard FRW dynamics are not necessary
to explain SNIa. Adopting the model selection information criteria we show that
the AIC indicates the flat phantom model while BIC indicates both flat phantom
and flat CDM models.Comment: 17 pages 6 figure
Some observational aspects of the orientation of galaxies
We investigated the sample of galaxies belonging to the Tully groups of
galaxies. We analyzed the orientation of galaxies inside the group. We did not
found significant deviation from isotropy both in orientation of position
angles and angles and giving the spatial orientation of
galaxy planes. Moreover we analyzed consequences of different approximation of
"true shape" of galaxies and showed possible influence of this problem for
investigation of spatial orientation of galaxies. Implications of the obtained
results for theory of galaxy formation was discussed as well.Comment: Acta Physica Polonica B (accepted), references correcte
Can the Stephani model be an alternative to FRW accelerating models?
A class of Stephani cosmological models as a prototype of non-homogeneous
universe is considered. The non-homogeneity can lead to accelerated evolution
which is now observed from the SNIa data. Three samples of type Ia supernovae
obtained by Perlmutter at al., Tonry et al. and Knop et al. are taken into
account. Different statistical methods (best fits as well as maximum likelihood
method) to obtain estimates of the model parameters are used. Stephani model is
considered as an alternative to the concordance of Lambda-CDM model in the
explanation of the present acceleration of the universe. The model explains the
acceleration of the universe at the same level of accuracy as the Lambda-CDM
model. From the best fit analysis it follows that the Stephani model is
characterized by higher value of nonrelativistic matter density parameter than
the Lambda-CDM model. It is also shown that the obtained results are consistent
with location of CMB peaks.Comment: 25 pages, 12 figure
Problems of Clustering of Radiogalaxies
We present the preliminary analysis of clustering of a sample of 1157
radio-identified galaxies from Machalski & Condon (1999). We found that for
separations Mpc their redshift space autocorrelation function
can be approximated by the power law with the correlation length Mpc and slope . The correlation length for
radiogalaxies is found to be lower and the slope steeper than the corresponding
parameters of the control sample of optically observed galaxies. Analysis the
projected correlation function displays possible differences in the
clustering properties between active galactic nuclei (AGN) and starburst (SB)
galaxies.Comment: Submitted: Proceedings of IAUS 290 "Feeding Compact Objects:
Accretion on All Scales", C. M. Zhang, T. Belloni, M. Mendez & S. N. Zhang
(eds.