The cosmic reionization of hydrogen is measured to be finished by z=6 but its time evolution is unclear. It is commonly thought that reionization is caused by the ionizing radiation of massive stars in the first galaxies.
By confirming the non-detections of two galaxies at z=7.7 (600 million years after Big Bang), we were able to put important constraints on the time evolution of reionization. While 100% ionized hydrogen at z=6, we find only 30% to be ionized at z=8, which suggests a very fast reionization of the universe in only about 500 million years.
June 2014