THE first lights in the sky of the early universe didn鈥檛 switch on suddenly. Instead, the brightening of the cosmos was much more tentative, unfolding gradually or perhaps in successive waves, according to scientists studying the light from distant quasars. The discovery is 鈥渜uite surprising鈥�, says Abraham Loeb, one of the scientists who reached the conclusion.
When the heat of the big bang subsided, the primordial plasma of ions and electrons cooled enough to form clouds of hydrogen and helium atoms that absorbed ultraviolet light. During the subsequent era, dubbed the 鈥渃osmic dark ages鈥�, the glow from the hot primordial gas had faded and the first stars had not yet switched on, so no light penetrated anywhere in the universe.
It took the violent lives and deaths of the first stars to bring light back into this darkness. When they lit up their light was initially blocked by the surrounding gas. But later their own radiation, especially when they exploded into brilliant supernovae at the end of their lives, heated up and ionised the gas around them, making it once again transparent to their light.
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It had been assumed that this 鈥渞eionisation鈥� happened quite suddenly. Last year, observations made by the WMAP satellite revealed a detailed view of the earliest time after the big bang when stars lit up and began to clear the fog of neutral gas. A preliminary interpretation of the data found that the transition from darkness to light happened about 200 million years after the big bang (快猫短视频, 15 Feb 2003, p 13).
Now Loeb, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleague Stuart Wyithe of the University of Melbourne in Australia say the story is not so simple.
Wyithe and Loeb examined the spectra of the two most distant quasars known, which formed a billion years after the big bang. They conclude that the density of neutral hydrogen around the quasars when they formed was high compared to today, suggesting that although reionisation began at 200 million years, it was still far from complete even after a billion years (Nature, vol 427, p 815). That means either that the gas ionised extremely slowly, or that it ionised repeatedly in successive phases, punctuated by periods of darkness as the gas cooled again.
Loeb鈥檚 hunch is that star formation may have ceased for a while after the first generation of stars, thanks to their violent demise. 鈥淚t鈥檚 quite possible, because the first generation of stars were quite different,鈥� he says. Unlike all subsequent generations of stars they were made of pure hydrogen and helium, with no heavier elements, so they were much larger and shorter-lived than most stars we see today. 鈥淚t was a suicidal population,鈥� Loeb says.
He speculates that those dying stars released so much energy and excited the surrounding gases so much that they suppressed further star formation for a while. However, he says it is also possible that the interstellar gas simply ionised much more slowly than scientists originally thought.