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Seeing giant celestial arcs - or only cosmic mirages

WOW! It's the greatest, biggest, grandest, most massive (pick an adjective) cosmic structure ever known! Many such findings have made news in recent years. But initial hype has often yielded to less prominently reported uncertainty. Occasionally, there's even been a retraction. That happened last week when astronomers at Stanford University and the Kitt Peak National Observatory discounted their ``discovery'' of giant luminous arcs embracing distant galaxies, extending like luminous ropes for some 18 trillion miles. As reported last January, they appeared to be the largest visible cosmic structures yet found.

Now the astronomers say in effect, ``Sorry folks. They're only a mirage.''

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Light passing by a massive object, such as a large galaxy, bends as though it went through a giant lens. When the lensing mass lies in the line of sight from Earth to the light source, it forms a ring-shaped image of that source. The image becomes an arc if the lens is slightly out of the sight line.

C.Roger Lynds of Kitt Peak and Stanford astronomer Vahe Petrosian have convinced themselves that this is what they have been seeing. The giant arcs associated with the galaxy clusters Abell 370 and Cluster 2242-02 are most likely gravitationally produced images of more distant galaxies.

This is a significant discovery in its own right. The strong lensing suggests the galaxy clusters are more massive than suspected. It challenges astronomers to find out what that mass may be. But the finding also warns the rest of us to be skeptical of superlatives in astronomical news.

New devices detect fainter structures than ever before, as happened with the luminous arcs. Computer processing gives new meaning to old data.

University of Hawaii astronomer R. Brent Tully recently described a network of galaxies that appears to stretch one-tenth of the way across the universe. He found it by making a three-dimensional computer map using information from a galaxy catalog compiled by the late George Abell three decades ago.

Thanks to such technical advances, new glimpses of cosmic structures are coming so fast they are bound to need later correction. Astronomers usually point this out.

Lynds and Petrosian said, in the initial announcement, that they didn't really know what the luminous arcs might be. Tully acknowledges he isn't certain he has found that enormous galactic network. But news reports of the discoveries don't usually feature the caveats.

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Our knowledge of the cosmos is changing rapidly. It's exciting to follow the developments. Yet we should beware of astronomical hype. The ``biggest'' and the ``grandest'' may turn out to be a mirage.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.

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