“The Epoch of Reionization (EOR) refers to the period in the history of the universe during which the predominantly neutral intergalactic medium was ionized by the emergence of the first luminous sources. These sources may have been stars, galaxies, quasars, or some combination of the above. Reionization was complete about 1 billion years after the Big Bang, corresponding to a redshift of about 6.5.” “Observations of the 21-centimetre line of atomic hydrogen in the early Universe directly probe the history of the reionization of the gas between galaxies. If reionization happened rapidly, there should be a characteristic signature visible against the smooth foreground in an all-sky spectrum.” Judd D. Bowman, Alan E. E. Rogers, “A lower limit of Δz > 0.06 for the duration of the reionization epoch,” Nature 468: 796–798, 09 December 2010, “report an all-sky spectrum between 100 and 200 MHz, corresponding to the redshift range 6 < z < 13 for the 21-centimetre line. The data exclude a rapid reionization timescale of Δz < 0.06 at the 95% confidence level.”
“After the Big Bang, the Universe was a hot, but quickly cooling soup of fundamental particles. After a few hundred thousand years, things cooled enough so that protons and electrons could combine to form neutral hydrogen. This was a rather sudden event, and allowed the thermal glow of the fireball plasma, to radiate throughout the universe in all directions, resulting in the Cosmic Microwave Background (CMB) .
After the Universe became neutral, it became unobservable across much of the electromagnetic spectrum during a long interval known as the Dark Ages. Eventually the first stars, galaxies and quasars started to form. Gradually, energetic radiation emitted by the first sources caused local heating, and then ionization of the hydrogen in the Universe. It will have started with “bubbles” of ionized plasma surrounding the most energetic sources. As the bubbles grew and became more numerous, they started to overlap, and more and more of the neutral medium became exposed to the harsh ionizing radiation, which travels unimpeded through ionized regions. The final phase of reionization of the Universe may have occurred swiftly. As soon as the bulk of the Universe was reionized, light at many wavelengths could escape from the early galaxies and quasars, revealing the distant Universe that we see today with optical and infrared telescopes.”
“The global 21-cm signal is challenging to observe in practice because the low-frequency radio sky is dominated by intense synchrotron emission from our own Galaxy that is more than four orders of magnitude brighter than the signal. Bowman and Rogers deployed a custom-built, high-dynamic-range broadband radio spectrometer, called EDGES, at the Murchison Radio-astronomy Observatory in Western Australia, to measure the radio spectrum between 100 and 200 MHz. The instrument observed continuously for three months.
By assuming that reionization was equally likely to have occurred at any redshift between 6 < z < 13 and treating each frequency trial as an independent measurement, the observations exclude reionization histories shorter than Δz < 0.19 at 68% statistical confidence. Combining statistical and systematic uncertainties in derivative space in quadrature yields our final confidence bounds of Δz < 0.13 at 68% combined confidence and Δz < 0.06 at 95% confidence. The result extends findings by WMAP, which ruled out at a fiducial instantaneous transition at z < 7 and yielded a best-fit z = 10.5 ± 1.2.”