As we have seen, the baryon and Cold Dark Matter densities as well as curvature and cosmological constant can leave some imprint on the CMB anisotropies.However, a number of more specific properties of matter can also be tested by the study of CMB anisotropies, and/or by combining it with other astrophysical tests such as supernovae luminosity data and large scale structure.

Cosmological constant / Quintessence

The main effects of a cosmological constant is to reduce the growth rate of density fluctuations and to modify the distance of the last scattering surface. These two effect have to do with the fact that cosmological constant modifies the expansion history of the universe at late times. What is seen in the CMB is some sort of integrated effect along the whole history of the universe: roughly speaking what we can measure is the distance to the last scattering surface and the integrated variation of the gravitational potential. By considering another type of dark energy, such as quintessence, which has a different (and usually time-varying) equation of state, it is likely that it can modify these two observables. However, the richness of quintessence models (i.e., the richness of possible history of equation of state for dark energy) is larger than the number of dark energy related observable quantities in the CMB. One therefore expects degeneracies between cosmological constant and various other dark energy models. Such degeneracies exist because CMB only allows to see some integrated effect through the history of the universe.

On the other hand, by observing large scale structure at different redshift, one can measure the history of the growth rate of fluctuations. Equivalently, by measuring the supernovae luminosity as a function of the redshift, one can reconstruct the expansion history of the universe at low redshift. Hence, it is possible to break some of the degeneracies that CMB alone exhibits. In particular, it is in principle possible to distinguish between a pure cosmological constant and quintessence models.

Dark Matter

So far, we only addressed Dark Matter issues in the hypothesis where it is "cold" in the sense that it is non relativistic during all the epochs of interest. Although this hypothesis is well-motivated (the most popular candidate comes from supersymmetry, and, not having been detected yet in accelerator is likely to have a mass greater than 100 GeV), it is possible that Dark Matter is made of several species, in particular massive neutrinos. The photon number density being well known, the neutrino number density also is, so that there is a direct relation between the hypothetic massive neutrino density parameter and their mass. This puts a very strong upper limit on their mass, around 30 eV if all the neutrinos are massive, and three times more if only one species is. The temperature of the universe at recombination is 3000 K, or 0.3 eV, therefore, massive neutrinos were probably relativistic not long before recombination.