Abstract:
Recent measurements of the polarization of the cosmic microwave background
(CMB) on scales of several degrees and larger have found a surprisingly large
signal. This signal is thought to be caused by the first stars forming at
z=17. Such early star formation puts some stringent constraints on the nature
of the first stars, and such a large optical depth to Thomson scattering
should lead to some unique signatures in future CMB measurements. These
signatures are entirely complementary to absorption studies, since the CMB is
sensitive only to free electrons while absorption studies are limited to
neutral material.