A gamma-ray burst detected in April by NASA’s Swift orbiter has a higher redshift (z
= 8.26 ± 0.08) than any other celestial entity for which a redshift has
been measured—except for the cosmic microwave background (CMB) at z
≈ 1100. That means the massive star whose collapse to a black hole the
GRB is presumed to manifest was significantly more distant than any
star or galaxy yet observed. Its demise provides a glimpse of the
cosmos just 625 million years after the Big Bang. Beyond revealing that
such stars already existed back then and providing a first
approximation to their formation rate, the discovery adds a potentially
powerful new probe to the search for the first generation of stars and
the investigation of how UV radiation from early stars reionized the
intergalactic medium. After the first moment of cosmic transparency,
signaled by the CMB, and before there were stars, almost all the
primordial hydrogen and helium was unionized. To reconstruct the
history of cosmic reionization, one seeks to measure the absorption by
neutral atomic hydrogen of light arriving from sources at various very
high redshifts. Such observations with quasars have revealed that
cosmic reionization was essentially complete by z = 6 (950
Myr after the Big Bang). But high-redshift GRBs seem to be essential
for tracing its earlier stages. GRBs are briefly luminous enough to be
seen at much greater distances than quasars. (N. R. Tanvir et al., http://arxiv.org/abs/0906.1577; R. Salvaterra et al., http://arxiv.org/abs/0906.1578.)—Bertram Schwarzschild
