Thus, by measuring the relative heights of the peaks in the fluctuation spectrum, we should be able to determine the number density of protons and electrons relative to that of photons. 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. The cosmic background radiation propogating from the surface of last scatter to our experiment is also affected by gravitational fluctuations along its path.
Photons falling into the gravitational potential wells of clusters gain energy.
They lose this energy when they climb out of the potential wells. If the universe is flat and composed entirely of matter, then these two effects cancel exactly and the matter along the photon path has no net gravitational effect. If there is a cosmological constant, then the depth of gravitational potential wells decay with time. Thus, a photon which falls into a deep potential well gets to climb out of a slightly shallower well. The net effect leads to a slight increase in photon energy along this path.