Magazine article Science News

Cosmological Paradox in the Dark of Night

Magazine article Science News

Cosmological Paradox in the Dark of Night

Article excerpt

Cosmological paradox in the dark of night

Why is the sky dark at night? At first glance, there seems nothing remarkable about the fact that the night sky appears basically dark, sprinkled with pinpricks of light from a scattering of planets, stars and galaxies. But this simple observaton has important cosmological implications.

Early in the 19th century, German physician and amateur astronomer Heinrich Wilhelm Olbers argued that if stars were evenly distributed throughout space in an infinite universe, an observer looking far enough in any direction ought to see a star. Consequently, the entire sky should glow with starlight. The fact that this argument doesn't work and that the night sky is dark became known as Olbers' paradox, although Olbers wasn't the first to ponder the mystery.

Astronomers in the 1950s and 1960s, armed with observations of remote galaxies and intergalactic background radiation, zeroed in on two possible explanations. They argued that the finite age of galaxies, combined with the finite speed of light, limits the amount of light that galaxies have produced and how far out an observer can see. In addition, the expansion of the universe increases the volume of space and shifts to longer wavelengths any light emitted by stars.

Both the age and expansion factors contribute to the darkness of the night sky, but which factor has the greater influence?

In the Feb. 1 ASTROPHYSICAL JOURNAL, astrophysicist Paul S. Wesson of the University of Waterloo in Ontario presents new calculations demonstrating that the finite age of the galaxies--rather than the expansion of the universe--chiefly accounts for the resolution of Olbers' paradox. …

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