146 Cosmological models
in another 50 Hubble times), so late-time anisotropic modes which are presently
negligble could come to dominate (cf the discussion in the section on evolution
of Bianchi models above). Thus we can observationally support the supposition
of spatial homegeneity and isotropy within the domainV, but not too far outside
of it.
3.8.6 Importance of consistency checks
Because we have no knock-out observational proof of spatial homogeneity, it is
important to consider all the possible observationally based consistency checks
on the standard model geometry. The most important are as follows:
(1) Ages. This has been one of the oldest worries for expanding universe models:
the requirement that the age of the universe must be greater than the ages of
all objects in it. However with present estimates of the ages of stars on
the one hand, and of the value of the Hubble constant on the other, this
no longer seems problematic, particularly if current evidence for a positive
cosmological constant turn out to be correct.
(2) Anisotropic number counts. If our interpretation of the CBR dipole as
due to our motion relative to the FL model is correct, then this must also
be accompanied by a dipole in all cosmological number counts at the 2%
level [38]. Observationally verifying that this is so is a difficult task, but it is
a crucial check on the validity of the standard model of cosmology.
(3) High-z observations. The best check on spatial homogeneity is to try to
check the physical state of the universe at high redshifts and hence at great
distances from us, and to compare the observations with theory. This can
be done in particular (a) for theCBR, whose temperature can be measured
via excited states of particular molecules; this can then be compared with the
predicted temperatureT=T 0 ( 1 +z),whereT 0 is the present day temperature
of 2.75 K. It can also be done (b) for element abundances in distant objects,
specifically helium abundances. This is particularly useful as it tests the
thermal history of the universe at very early times of regions that are far out
from us [34].3.9 Explaining homogeneity and structure
This is the unique core business of physical cosmology: explaining both why
the universe has the very improbable high-symmetry FL geometry on the largest
scales, and how structures come into existence on all smaller scales. Clearly
only cosmology itself can ask the first question; and it uniquely sets the initial
conditions underlying the astrophysical and physical processes that are the key to
the second, underlying all studies of origins.There is a creative tension between
two aims: smoothing processes, on the one hand, and structure growth, on the
other. Present day cosmology handles this tension by suggesting a change of