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).