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The quest for the golden lens: a perfect alignment of massive objects would offer clues to the rate of cosmic expansion
Natural History, Sept, 2003 by Charles Liu
Long ago, Greek bards sang of Jason, a prince denied his throne unless he could provide his usurper with the Golden Fleece. Accompanied by a crew of young heroes, Jason set off on the great ship Argo to find this mysterious treasure, known to be in the land of Colchis, at the far eastern end of the Black Sea. The ancient Greeks immortalized their heroes in the stars, and many of the names that still designate stars and constellations today bring to mind Jason's mythical voyage: Carina, Puppis, and Vela represent the keel, stern, and sail of the Argo; Castor and Pollux, who accompanied Jason, became Gemini; and Hercules was one of Jason's mates.
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Few, if any, modern astronomers lead the adventurous lives of Jason and his Argonauts, yet many of us are on a quest for something golden. The object we seek, of course, is no fanciful animal skin, but the rarest of cosmic coincidences: a "golden lens." Such a lens, created by the interaction of a quasar and a galaxy perfectly aligned with Earth, would enable astronomers to deduce one of the holy grails of astronomy: the Hubble constant, or the expansion rate of the universe.
Among the cosmic mariners seeking that prize is Somak Raychaudhury, an astronomer at the University of Birmingham in England. Observing in the X-ray part of the spectrum, he and his collaborators have been studying one promising celestial candidate, known as B1422+231. Despite its unglamorous name, the object would surely be elevated to the stuff of legend if, indeed, its lens were golden.
Einstein's general theory of relativity--the unification of space, time, and gravity--predicts the existence of gravitational lenses, including golden ones. Massive objects bend space-time around them, creating a dimple in space-time akin to the depression made by a bowling ball on a trampoline. Any light passing through such a dimple follows a curved path.
Now imagine a massive object that lies between a shining beacon and an observer. The massive object bends the light streaming at it from the beacon; if the alignment is right, the bent light can focus or magnify the original image of the beacon for the observer, perhaps to many times its original brightness. Hence the intervening object acts as a lens--not because it's made of glass or plastic, but because its gravity bends light.
Astronomers love gravitational lenses. They are sheer cosmic serendipity, but they provide us, free of charge, with a powerful telescope. Of course, you get what you pay for. First, they're quite rare; the Earth, the lens, and the light source have to line up just about exactly to give rise to a measurable lensing effect. If they deviate from a straight line by less than a thousandth of a degree of arc--about the width of a penny 3,000 feet away--the lens splits the magnified image into two or more irregularly spaced patches of differing brightness.
A second, and worse, problem with gravitational lenses is their blotchiness--they aren't created by smooth, regular massive objects, but rather by complex, asymmetric ones such as galaxies and clusters, with scattered dense and sparse spots. Such a lens distorts as well as magnifies the light that comes through, sometimes creating multiple, twisted images of the objects behind it. It's more like looking through the thick glass bottom of a bottle of lemon soda than through a good magnifying lens.
From such lemon bottles, though, astronomers have mixed excellent lemonade. The distortions themselves carry information about the universe. Imagine what happens if the light source changes its appearance--if, say, a quasar suddenly brightens with a new burst of energy. Each distorted multiple image of the quasar represents a different path taken by light through the dimpled space-time surrounding the lens, and some of those paths are longer than others. So first one image brightens--the one with the shortest path--then the one with the next-shortest path, and so on. The time between brightenings, it turns out, depends on two factors: the structure of the lens and the expansion rate of the universe--the Hubble constant. So all we need for a solid measurement of the Hubble constant, independent of the usual redshift of receding galaxies, is to identify a gravitationally lensed image of a flickering source with a near-perfect alignment, a readily measurable time delay, and a smooth, uncomplicated intervening mass. That's what gilds a gravitational lens.
With such stringent requirements, it's little wonder that Raychaudhury and his colleagues could have embarked on a quest that others had envisioned many years before their time but left still unfulfilled. Within a few years of looking, though, they thought they'd found a good candidate golden lens. The light from quasar B1422+231, which lies some 11 billion light-years from Earth, passes through a lens created by an intervening mass about three billion light-years from Earth. The lens gives rise to four detectable images of the quasar [see photograph on next page]. Furthermore, other astronomers had recently reported measuring a time delay in the brightening of two of those images.
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