Cosmological paradox in the dark of night - why the sky is dark at night

Science News, Feb 23, 1991

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. Wesson tackled the question after discovering that a number of textbooks still erroneously attribute most or all of the effect to the expansion of the universe. "Unfortunately, this topic has a long history of confusion which still persists," he says.

Using various models of expanding and static universes, Wesson calculated the expected intensity of the extragalactic radiation field at different wavelenghts. Those calculations enabled him to separate the influence of the expansion of the universe from that of the age of galaxies. He found that for most reasonable cosmological models, expansion reduces the light intensity to small a factor to account for the night sky's darkness. His work confirms previous calculations suggesting that the darkness of intergalactic space primarily results from the finite age of galaxies.

Wesson concludes that the intensity of intergalactic radiation would still be low even if the universe were static, and expansion reduces that intensity by only a relatively modest amount. In other words, the universe isn't old enough yet to have permitted light from any of the more distant stars and galaxies to reach Earth. This is also implies that there was a time when the early universe lacked galaxies resembling those now observed, and that there has been no opportunity for filing the universe with visible radiation.