1.1. Cosmological Density Parameters
As we have heard at this meeting, there is abundant evidence for the
dominance of dark matter and dark energy on the largest distance scales.
The cosmological microwave background (CMB) radiation
[1] tells us
that the total energy density of the Universe,
tot, is
very close to the critical value marking the boundary between open and
closed universes
[2].
This information is provided, in particular, by the value of the multipole
~ 210 at which the first
acoustic peak appears in the CMB,
as seen in Fig. 1.
This tells us, in effect, the relative sizes of the Universe today and
when the nuclei and free electrons in the primordial plasma combined to
form neutral atoms. There are now indications for a second and even a
third acoustic peak in the CMB at higher
[1], but these are
not yet securely established. However, the magnitudes of the fluctuations
T / T at
these larger values of
already tell us that the overall baryon density
b
<< 1, agreeing to within ~ 50 % with the
value estimated on the basis of Big Bang nucleosynthesis calculations.
Other information about the large-scale geometry of the Universe for
redshifts
z 
1 is
provided by data on high-z supernovae
[3], which
constrain a combination of the matter density
m and the
vacuum energy density
. Combining the
CMB and high-z supernova
data, one finds fairly accurate values for the cosmological density
parameters
[2]:
| (1) |
where h is the present-day Hubble expansion rate in units of
100 km/s/Mpc, consistent with the `concordance model':
tot ~ 1,
/
m ~ 2
3 with
b
small, as seen in Fig. 2.
These data may also be used to constrain neutrino degeneracy
[5].
|
Figure 1. Recent compilation of data on the
cosmic microwave background
(CMB), exhibiting clearly the first acoustic peak, whose location fixes
|
|
Figure 2. The combination of cosmological
data favours the `concordance' model with
|
There are excellent prospects for significant progress in improving the
CMB constraints using data from the MAP and Planck satellites
[6].
One of the open issues concerns the amount of information likely to be
obtained at large
[7], particularly
from polarization measurements
[6].
These must contend with weak lensing effects
that must be subtracted in order to extract useful cosmological information.