## 4. Discussion and conclusionIn CS we identified a subclass of the LTB models with a Big-Bang of "delayed" type which solves the standard horizon problem without need for any inflationary phase. In this preliminary approach, the observer was assumed located sufficiently near the symmetry center of the model as to justify the "centered earth" approximation. Here, we report a further analysis of the properties of the DBB model to show that this model solves the horizon problem even with an off-center observer. The model is thus relieved of a prescription that could be considered as "unnatural". The model is also provided with a new free parameter, the spatial location of the earth in the universe, which accounts for the large scale inhomogeneities observed in the CMBR temperature anisotropies. The measured dipole and quadrupole moments of these anisotropies set bounds on the correlated values of this parameter and of the local deviation of the model from homogeneity, accounted for by the slope of the Big-Bang function. A possible cosmological part of these large scale features seen in the CMBR, if once observationally identified, would select an even narrower curve in the parameter space of the model, as shown in SC. It is of the utmost importance to stress that, as was the case with
a centered observer, these results hold for any universe arbitrarily
A point worth discussing here is the validity of this claim as
regards the almost Ehlers-Geren-Sachs (AEGS) theorem (Stoegger et al.
1995). This theorem states that "if all fundamental observers measure
the cosmic background radiation to be almost isotropic in an expanding
universe region, then that univere is locally almost spatially
homogeneous and isotropic in that region." The U region considered by
the AEGS authors is "the region within and near our past light cone
from decoupling to the present day". It is easy to see that small
It is also interesting to note that, contrary to the inflationary assumption which restores causality between the different points seen in the CMBR, but only temporarily, the DBB model provides a permanent solution to the horizon problem, whatever the position of the observer on his world line. In the prospect of future observational tests of the large scale
(in)homogeneity of the universe, the development of other interesting
inhomogeneous models must be regarded as an important issue. However,
this presented result is only a first improvement in the release of
the simplifying assumptions (retained in CS) for a preliminary study
of the properties induced by a "delayed Big-Bang". Other analyses are
still needed, among which the release of the spatial spherical
symmetry of the model and of the dust approximation should be
considered as priorities. © European Southern Observatory (ESO) 2000 Online publication: October 30, 2000 |