Cosmic timeline 11
The Cosmic Dark Ages
In brief, the cosmic “dark ages” refers to the period after recombination occurred, about 380,000 years after the big bang, creating the cosmic microwave background (CMB), up to and partly including the time that the first stars had formed, perhaps as early as four hundred million years later, and caused the reionization of much of the neutral hydrogen in the universe.
About one million years after the Big Bang and in this early period of these dark ages, the Primordial Era ends and the Stelliferous Era begins.
During this period, before what cosmologists call decoupling occurs, most of the photons in the universe are interacting with electrons and protons in the photon-baryon fluid. This photon-baryon fluid makes the whole universe appear opaque or “foggy” as a result. There is light but not light we could observe through telescopes. At this point the only radiation emitted is the 21 cm spin line of neutral hydrogen.
This is the radio signal that neutral hydrogen sends once every million years when it flips its spin.
There is currently an observational effort underway to detect this faint radiation, as it is in principle an even more powerful tool than the cosmic microwave background for studying the early universe .
The hydrogen line, 21 centimeter line or HI line refers to the spectral line created by changes in the energy state of neutral hydrogen. This line falls within the radio region of the electromagnetic spectrum and is used in radio astronomy, since it can penetrate dust clouds that are opaque to visible wavelengths.
The most distant object ever observed in space has provided scientists with an unprecedented insight into the “cosmic dark ages” following the birth of the Universe some 13.7 billion years ago. A gigantic explosion on the edge of the known Universe has been confirmed as the furthermost object in the cosmos. It occurred nearly 700 million years after the Big Bang and its radiation has taken some 13 billion years to reach Earth – making it 13 billion light years away. The explosion is one of many thousands of gamma-ray bursts that scientists have detected since they were first discovered more than 40 years ago by spy satellites designed to monitor the radiation emitted by man-made nuclear explosions. This particular gamma-ray burst, named 090423, occurred on 23 April 2009 and its afterglow lasted for about 10 seconds before it died out. This was long enough for the Swift satellite operated by Nasa to identify its location so that other telescopes on the ground could analyse the explosion in more detail.
Gamma-ray bursts are the most energetic events known to scientists. In just a couple of seconds these massively powerful explosions in space release as much energy as the Sun would release in its entire lifecycle of 10 billion years. Finding a gamma-ray burst that is 13 billion light years away means that it must have taken place within the period known as the “cosmic dark ages”, a timespan of about 900 million years that separates the Big Bang from the formation of the earliest stars and galaxies.
“This observation allows us to begin exploring the last blank space on our map of the Universe. It’s the first time that we’ve seen an object within this period of the Universe’s dark ages,” said Nial Tanvir, of Leicester University, who led the study published in the journal Nature and as reported in the press last year.
“We’re beginning to peer back to the era of the very first structures in the Universe. It’s the last unexplained era because in broad-brush terms we have a reasonably good idea of what happened during the rest of the life of the Universe,” he added.
Gamma-ray bursts, named after the intense amounts of gamma radiation they release, are believed to occur when massive stars some 50 or 100 times bigger than the Sun are swallowed up by a newly formed black hole, which results in the instant conversion of matter into energy.
Astronomers believe that as the black hole swallows up the dead star, intense jets of gas punch their way through the stellar material, forming interactions with other stellar gases previously shed by the dying star, causing it to heat up and release short-lived after-glows of radiation, which can be detected and measured by astronomers on Earth.
Andrew Levan, of Warwick University, another member of the international research team, said that the very ancient age of the gamma-ray burst detected in April meant that the dying star that formed it must be one of the first to be created in the early Universe.
“We’re looking back into the Universe when it was very, very young and we’re seeing objects that formed in the very early Universe – it was one of the first objects to form after the Big Bang,” said Dr Levan.
“We thought that there were possibly stars there during this epoch but we’ve not until now seen them before. These early stars only lived for a few million years. They lived fast and died young,” he added.
The explosion of gamma-ray burst 090423 occurred when the Universe was less than 5 per cent of its present age and a tenth of its present size.