THE Universe is destined to expand forever, says an astronomer who has
surveyed clusters of galaxies in deep space. His study found that there were
many more clusters when the Universe was half its current age than astronomers
had thought. This implies that the Universe is too light for gravity to stop it
expanding.
Harald Ebeling of the University of Hawaii and his team have so far
discovered 101 massive galaxy clusters that are more than 5 billion light years
away. 鈥淭heir number is much higher than people had expected,鈥� Ebeling told the
meeting in Honolulu last week. And there are probably more to come, as Ebeling鈥檚
Massive Cluster Survey (MACS) is currently only 75 per cent complete.
The distant clusters show the Universe as it was billions of years ago. Some
of them contain thousands of galaxies, making them some of the most massive
systems in the Universe to be held together by gravity.
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Ebeling found the new clusters by sifting through old data from the now
retired German X-ray satellite ROSAT. The tenuous hot gas that fills the space
between the galaxies in a cluster emits energetic X-rays. But because of the
immense distances of these clusters, ROSAT could only pick up the faintest
signal: on average 30 X-ray photons from each of them, says Ebeling.
鈥淚t鈥檚 marvellous what you can do with just 30 little photons,鈥� says Bill
Forman of the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts. Ebeling鈥檚 team is doing 鈥渁 very hard job鈥�, he says.
If the Universe is as massive as most people think, it should have had far
fewer massive clusters in the past than it does now, explains team member
Patrick Henry. With so much matter, clusters would have formed slowly over the
life of the Universe, and would still be forming now, he says.
In a low-density Universe, by contrast, the sparse matter would have formed
into clusters quickly and then settled down. This means that the number of
clusters in the past would be comparable to the present number鈥攚hich is
what MACS is finding. Earlier surveys have either found massive clusters at
smaller distances, or distant clusters with a lower mass. Both are less
important for constraining cosmological theories, says Ebeling.
The mass density of the Universe is key to cosmological theories about its
fate. If the density is low, the Universe will expand forever; if it is high the
expansion will eventually stop and the Universe may contract towards a 鈥渂ig
肠谤耻苍肠丑鈥�.
Although the MACS survey is incomplete, Ebeling says he is confident that it
will show that the mass density must be less than half that needed to stop the
cosmic expansion. 鈥淭hirty per cent [of the critical density] appears to be in
the right ballpark,鈥� he says. That would put the cluster results in line with
other recent cosmological evidence.
The MACS results are very promising, says Aaron Lewis of the University of
Colorado. 鈥淐luster observations will be one of a few solid legs on which the
determination of the mass density of the Universe rests.鈥�