Quasar clumps dim cosmological theory.
A research satellite launched last June has allowed astronomers to peer deeper into space with X-ray eyes than ever before. And the distant vistas they've glimpsed show evidence of what appear to be giant clusters of quasars eight to 12 billion light-years from Earth -- more than halfway to the edge of the observable universe. These clusters suggest the universe began getting lumpy earlier -- and on a larger scale -- than previous sky maps had indicated, researches reported last week.
Combined with other recent observations of large-scale slumping (SN: 1/12/91, p.22), cosmologists say the perplexing new finding, if confirmed, may force them to abandon a standard theory -- known as the cold-dark-matter model -- of how the universe evolved. "This may be the start of the death knell for the cold-dark-matter theory," says physicist Paul Steinhardt, at the University of Pennsylvania in Philadelphia.
Scientists proposed the theory as a way of reconciling two snapshots of the universe -- one representing the distant past and another of the present. Several measurements indicate that the ubiquitous background microwave radiation, believed a remnant from the Big Bang, has the same intensity everywhere in space. This suggests the universe began as an incredibly smooth and uniform soup of matter and energy (SN: 1/20/90, p.36). The conundrum is how it evolved into its current lumpy collection of stars and galaxies.
Because the observed mass in the cosmos is too small to gravitationally bind large objects, cosmologists at first suspected that the universe contains a large quantity of invisible, ordinary dark matter to supply the needed gravitational glue.
But the garden-variety dark matter -- composed of protons, neutrons or other familiar building blocks -- interacts strongly with light. During the early history of the universe, this interaction would have suppressed gravity's tug and prevented tiny lumps or fluctuations in the density of primordial matter from growing fast enough to account for the present large-scale structures.
To solve the problem, theorists turned to the concept of cold dark matter, which postulates a type of hidden material that interacts weakly with light and has more time to expand the size of primordial lumps. But the simplest form of this model -- in which all hidden matter is cold -- still cannot explain the picture of the universe emerging from recent observations, Steinhardt and others say.
Next month, the U.S.-European Roentgen Satellite (RPSAT) will complete its first sky survey. Up to this point, it has resolved X-ray sources in greater detail and detected sources about 2.5 times fainter than other X-ray-probing satellites.
One of ROSAT's detectors imaged very dence, fuzzy patches of low-energy X-rays from dimmer sources about 8 to 12 billion light-years from Earth, Guenther Hasinger of the Max Planck institute for Extraterrestrial Physics in Garching, Germany, reported last week at a meeting of the American Astronomical Society in Philadelphia. Based on this intensity, the rough shape of the fuzzy images and their proximity to individual quasars, he speculates that the X-ray emissions originate from clusters of quasars with a diameter of 15 to 75 million light-years.
Hasinger adds that further ROSAT observations with the same detector, expected to resume this August, may confirm the validity of the quasar-cluster interpretation. Stephen S. Holt of the NASA Goddard Space Flight Center in Greenbelt, MD., notes that ROSAT offers a prime tool to discern quasars, since many radiate 100 times more intensely in X-ray than in visible light.
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|Title Annotation:||cold-dark-matter model|
|Date:||Jan 26, 1991|
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