The tau of physics
Starring: Fritiof Capra
If the Universe was reioninized long ago by quasars or
early stars, then there is a non-zero optical depth tau.
This means that there is a non-zero probability that
a CMB photon arriving in our detector once Thomson scattered off
of a free electron, and in fact originated from a somewhat different
direction than we thought. This effect smears out small-scale features
in the CMB power spectrum (top), suppressing all accoustic peaks by a constant
factor exp(-tau), while leaving the power on the largest scales
unaffected. As you can see, this has no effect at all on the galaxy power
spectrum (bottom).