Science: Astronomers' double whammy rocks cosmology

日期:2019-03-02 08:20:09 作者:侴倚讶 阅读:

By JEFF HECHT in BOSTON The detection of a cluster of galaxies that existed only a billion years after the big bang could upset current ideas about how the Universe evolved, because no one can explain how it could have formed so early. Accepted ideas about cosmology face attack from another direction, too, as equally remote quasars have been found that contain more iron than the Sun. This is hard to explain, because iron is made by nuclear reactions inside stars as the Universe evolves, so ancient quasars would be expected to contain far less iron than the Sun. The galaxy cluster was found by Charles Steidel of the Massachusetts Institute of Technology and Donald Hamilton of the California Institute of Technology in Pasadena. They were analysing the spectrum of a distant quasar, a type of object that is known to lie at the centres of large, unseen galaxies, looking for the absorption fingerprint that might belong to an intervening galaxy too faint to see directly. The astronomers found that a part of the absorption spectrum called the Lyman a lpha line, which is normally in the ultraviolet, had been red-shifted by a factor of 3.390 so that it appeared at visible wavelengths. They knew that this Lyman alpha line was caused by a galaxy because it matched the equivalent line in a faint, visible galaxy near to the quasar. The astronomers found 15 more galaxies in the same area, with similar spectra and probable red shifts of between 3.0 and 3.5. Independently, Duccio Macchetto and Mauro Giavalisco of the Space Telescope Science Institute in Baltimore carried out a search for faint galaxies that emit Lyman alpha lines very strongly, a characteristic of young galaxies with many hot, bright stars. They failed to detect such emission from the galaxy found by Steidel and Hamilton near the quasar, but they did detect it in a nearby galaxy that was among the 16 cluster candidates, and had a red shift of 3.428. The difference in red shift corresponds to a difference of 2600 kilometres per second in the velocities at which the galaxies are receding from Earth. This is small enough for the galaxies to be in the same cluster, says Giavalisco. Though the spread of velocities within galaxy clusters is now no more than about half this, it may have been different in the remote past, he says. The astronomers say there is a 98.8 per cent chance that the galaxies are part of a real cluster, and not simply a chance superposition in the sky. The discovery of such a cluster spells trouble for theories of cold dark matter, which assume that a large fraction of the mass of the Universe is in cold, dark objects such as planets or black holes. The theories predict that material in the early Universe clumped together from the ‘bottom up’, so that galaxies formed first, then only later clumped to form clusters. Giavalisco believes it might just be possible to explain the birth of the first galaxy cluster at a red shift of 3.4 by fine-tuning the cold dark matter theory. But he adds a warning. ‘If you found ten clusters at red shift 3.5, it would kill cold dark matter theories.’ The levels of iron in two quasars found by Gary Hill of the University of Texas at Austin and his colleagues pose other problems (Nature, vol 367, p 250). Astronomers believe most iron comes from type 1A supernovas, in which one star in a binary explodes. However, the binaries need a billion years to evolve to this stage. ‘For some brands of cosmology, the Universe just isn’t old enough to have synthesised iron at a red shift of 3.4,’ says Hill. He says that a red shift of 3.4 corresponds to an age of only 700 million years, given that the Hubble constant (a measure of the current expansion rate of the Universe) is 100 kilometres per second per megaparsec, and the cosmic deceleration parameter (q0), a measure of how fast the expansion is slowing, is 0.5. But if either the Hubble constant or q0 were smaller,