THE young universe was precocious. It had formed a vast supercluster of galaxies by the time it was just 3 billion years old 鈥� much earlier than astronomers thought possible.
The string of galaxies, at least 300 million light years long, was reported last week at an American Astronomical Society meeting in Atlanta, Georgia. Another group announced that galaxies in the early universe look unexpectedly mature. Astronomers are now struggling to explain how these ancient structures could have formed so fast.
Povilas Palunas from the University of Texas at Austin and his group detected the supercluster with the 4-metre Blanco Telescope in Chile. Using a filter to block out most of the light from nearby sources, they were able to pick out the red-shifted emission from galaxies 11 billion light years away.
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The astronomers expected to see galaxies spread quite evenly over their field of view. Instead, they saw one coherent string of 37 galaxies, with emptiness elsewhere. 鈥淭hat may not seem like a whole lot, but we鈥檙e only seeing the brightest galaxies,鈥� says Palunas. If this pattern extends to dimmer galaxies, there must be many thousands of them in the supercluster.
The sheer number is surprising, given current thinking. Astrophysicists believe that structures in the universe are seeded by tiny variations in the density of matter coming out of the big bang. Most of it is thought to be invisible 鈥渃old dark matter鈥�, which clumps together under its own gravity. Gas then gets dragged into these gravitational pits to form stars and galaxies.
But while it may take individual galaxies just a billion years to grow, in computer simulations larger structures take much longer. After only 3 billion years, there should be nothing as huge as this supercluster.
Other signs of early star and galaxy formation have emerged over the past year, including news from NASA鈥檚 Wilkinson Microwave Anisotropy Probe that the first stars shone out only 200 million years after the big bang. At the conference, a team from the University of Toronto reported their observations of galaxies between 8 and 11 billion years ago. They expected to see small galaxies in a state of frantic flux; instead there were fat, sedate galaxies much like today鈥檚.
鈥淚t鈥檚 possible we are seeing the first hint of something fundamental,鈥� says Harry Teplitz of Caltech in Pasadena, a member of Palunas鈥檚 group. If matter is clumping faster than expected, the theory of cold dark matter could be in trouble. But models of how gas forms stars and galaxies are more likely to be responsible.
Patrick McCarthy of the Carnegie Institution in Washington DC suggests a slightly more interesting possibility: that massive black holes may have helped seed early galaxies. This is still a speculative idea, but Volker Bromm of Harvard University, also speaking in Atlanta, described how gas clouds might have collapsed to form black holes a million times as massive as the sun when the universe was less than a billion years old.