The Doppler Foothills

An important feature of models involving the gravitational growth of modes of a primordial spectrum is the sub-peaks after the first peak. These are the result of shorter wavelength modes which have achieved 1, $1\frac{1}{2}$, 2, $2\frac{1}{2}$ ...oscillations at re-combination. Measurement of these would not only validate these models, but offer the prospect of determining the most important cosmological constants ($\Omega_0$, $\Omega_B$, H₀, $\Lambda$ and etc. to extraordinary accuracy due to wealth of information these peaks contain. For instance, the total matter density can be better determined from the inter-peak separation than the position of the first peak.

The decay of the peaks as one goes to higher $\ell$ is due to the photon mean free path being longer than the scale size of these modes at re-combination. That is to say that these perturbations are optically thin and that the last scattering surface is not a sharp boundary, so several modes are seen through so reducing their contribution.