Sir Martin Rees, Britain's astronomer royal, named the long interval between these two enlightenments the cosmic "Dark Age". The name describes not only the poorly lit conditions, but also the ignorance of astronomers about that period. Nobody knows exactly when the first stars formed or how they organised themselves into galaxies or even whether stars were the first luminous objects. They may have been preceded by quasars, which are mysterious, bright spots found at the centres of some galaxies.
Now two independent group of astronomers, one led by Robert Becker of the University of California, Davis and the other by George Djorgovski of the Caltech, claim to have peered far enough into space with their telescopes (and therefore backwards enough in time) to observe the closing days of the Dark Age.
The main problem that plagued previous efforts to study the Dark Age was not the lack of suitable telescopes but rather the lack of suitable things at which to point them. Because these events took place over 18 billion years ago, if astronomers are to have any hope of unraveling them they study objects that are at least 13 billion light years away. The best prospects are quasars, because they are so bright and compact that they can be seen across vast stretches of space. The energy source that powers a quasars is unknown although it is suspected to be the intense gravity of a giant black hole. However at the distances required for the study of Dark Age, even quasars are extremely rare and faint.
Recently some members of Dr. Becker's steam announced their discovery of the four most distant quasars known. All the new quasars are terribly faint, a challenge that both teams overcame by peering at them through one of the twin Keck telescopes in Hawaii. These are the world's largest and can therefore collect most light. The new work by Dr. Becker's team analysed the light from all four quasars. Three of them appeared to be similar to ordinary, less distant quasars. However, the fourth and the most distant, unlike any other quasars ever seen, showed unmistakable signs of being shrouded in a fog of hydrogen gas. This gas is left over material from the Big Bang that did not condense into stars or quasars. It acts like fog because new born stars and quasars emit mainly ultraviolet light, and hydrogen gas is opaque to ultraviolet. Seeing this fog had been the goal of would be Dark Age astronomers since 1965, when James Gunn and Bruce Peterson spelled out the techniques for using quasars for backlighting becomes to observe the fog's ultraviolet shadow.
The fog prolonged the period of darkness until the heat from the first stars and quasars had the chance to ionise the hydrogen (breaking it into its constituent parts, protons and electrons). Ionised hydrogen is transparent to ultraviolet radiation, so at that moment the fog lifted and the universe became the well-lit place it is today. For this reason, the end of the dark age is called the "Epoch of Re-ioninsation". Because the ultraviolet shadow is visible only in the most distant of four quasars, Dr. Becker's team concluded that the fog has dissipated completely by the time the universe was about 900 million years old, and one-seventh of its current size.
1. In the passage, the Dark Age refers to :
(a) the period when the universe became cold after the Big Bang.
(b) a period about which astronomers know very little.
(c) the medieval period when cultural activity seemed to have come to an end.
(d) the time that the universe took to heat up after the Big Bang.
2. Astronomers find it difficult to study the Dark Age because :
(a) suitable telescopes are few.
(b) the associated events took place aeons ago.
(c) the energy source that powers a quasar is unknown.
(d) their best chance is to study quasars, which are faint objects to begin with.
3. The four distant quasars discovered recently :
(a) could only be seen with the help of large telescopes.
(b) appear to be similar to ordinary quasars.
(c) appear to be shrouded in a fog of hydrogen gas.
(d) have been sought to be discovered by Dark Age astronomers since 1965.
4. The fog of hydrogen gas seen through the telescopes :
(a) is transparent to ultraviolet radiation from stars and quasars in all states.
(b) was lifted after heat from stars and quasars ionised it.
(c) is material which eventually became stars and quasars.
(d) is broken into constituent elements when stars and quasars are formed.