THE birth of the first stars may have been triggered by cosmic strings 鈥� giant threads of energy that lashed about chaotically in the early universe.
This peculiar idea, put forward by two physicists in the US, is an attempt to solve the cosmological puzzle posed by the widespread existence of ionised hydrogen 鈥� in the form of protons and electrons 鈥� in today鈥檚 universe. The ions that filled the universe immediately after the big bang cooled enough to form neutral hydrogen atoms about 300,000 years later, but the hydrogen has again been largely reionised.
Theorists have suggested that the reionisation was caused by intense ultraviolet radiation from the first stars. But observations of the microwave radiation left over from the big bang indicate that the reionisation occurred when the universe was about 200 million years old. Some cosmologists consider this too early for stars to have formed, if they are assumed simply to have coalesced out of the cooling debris of the big bang.
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Levon Pogosian and Alexander Vilenkin of Tufts University in Medford, Massachusetts, now suggest that the stars did form that early, but their genesis was triggered by giant cosmic strings. The idea of cosmic strings being responsible for stars was first mooted by Martin Rees of the University of Cambridge and others in the mid-1980s. They proposed that these strings were super-massive 鈥渢opological defects鈥� in the fabric of space-time that seeded great clusters of galaxies. But this idea was eventually discredited by experimental data.
The strings being proposed by Pogosian and Vilenkin are different and are supported by superstring theory, which says that matter is made of tiny strings vibrating in 10 dimensions of space-time. The theory permits the existence of higher-dimensional objects called 鈥渂ranes鈥� and 鈥渁nti-branes鈥�, and it proposes that a collision between these objects would have generated gigantic strings that spanned the universe (New 杏吧原创, 3 July, p 12).
鈥淎s the strings lashed about, they created regions of high density in the cooling gas left over from the big bang鈥�
As the strings lashed about, they created regions of high density in the cooling gas left over from the big bang. Eventually, the gravity of these regions pulled in dark matter and ordinary matter to form the universe鈥檚 first objects, the precursors of today鈥檚 galaxies, within which the first stars would have formed. The researchers have discovered that star formation triggered in this way could indeed have re-ionised the universe around 200 million years ago ().