## 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 power spectrum, suppressing all accoustic peaks by a constant
factor exp(-tau), while leaving the power on the largest scales
unaffected.