Let us imagine for the sake of this section, that `cosmology' is synonymous with the search for the values of a number of parameters which describe the properties of the Universe.
Figure 3 appears to show that
avoiding any detailed statistical arguments here, and just sticking to round numbers (and remembering that there are tight upper limits at higher values of l, so that the power spectrum really does have to come down again). Since the standard CDM model has the main peak at l 220, and it is pushed to smaller scales in open models, then it is hard for tot to be less than, say, 0.3. Rigorous studies (e.g. ) arrive at similar conclusions.
The height of the peak is somewhat higher than predicted for sCDM, but entirely consistent with several variants (2) Currently popular models with a cosmological constant tend to provide perfectly good fits to the CMB (in addition to large-scale clustering of galaxies and the supernovae results ). The curve plotted in Figure 3, shows one such flat model with = 0.6 and a Hubble constant of 70 km s-1 Mpc-1 .
Since the height of the first peak depends on a combination of parameters, then exactly what quantities are constrained depends on the parameter space being searched, as well as on the choice of additional constraints. Currently it is possible to constrain the matter density MM to ~ ± 0.1 from the peak height, but that depends sensitively on the assumptions used. All this is expected to change as better data come in.
The basic thing to take away here is that models with adiabatic-type (i.e. where you perturb the matter and radiation at the same time in order to keep the entropy fixed) perturbations have the right kind of character. On the other hand isocurvature-type models (where the matter and radiation get equal and opposite perturbations, so that the local curvature is unperturbed) tend to look poor - generically they have a `shoulder' rather than a first peak, and then the highest peak is at much smaller scale (see e.g. ). While there are some loop-holes, it seems difficult to get isocurvature models to fit the current data.
2 Rather than vanilla CDM you can have a slightly different flavour, or some chocolate sprinkles, or maybe a cherry on top. Back.