Parameters of Cosmology: Counting the Electrons in the Universe

The behavior of the early universe fluid will also depend upon the relative number densities of electrons and photons. The more electrons (and protons), the stronger their response to the gravitational potential wells. This leads to an enhancement of the first and third acoustic peaks, which are produced by the fluid falling into the potential wells and a suppression of the second acoustic peak, which is produced by the fluid moving outward. Thus, by measuring the relative heights of the peaks in the fluctuation spectrum, we should be able to determine the relative number density of protons and electrons.

Increasing the ratio of electrons to photons also has the effect of decreasing the sound speed of the fluid. Since the fluid moves more slowly, the secondary oscillations occur at larger angular scales. This effect shifts the location of the latter peaks in the fluctuation spectrum.