The correct answer may not be as obvious as you think.
Why is the nighttime sky dark? Such a question seems ridiculously simple to answer. The sun, which illuminates our day, sets in the west each evening as our spinning earth carries us away from its rays. As this occurs, the earth’s shadow rises in the east and slowly engulfs us in darkness. As the last remaining glitter of reddened sunlight dissolves into the western horizon, the stars sparkle majestically over-head. And since the stars are too tiny and faint to illuminate the nighttime sky, it appears dark.
Sounds very simple doesn’t it? But if astronomer’s age-old beliefs about the universe are correct, our nighttime sky shouldn’t appear dark at all. In fact, both our daytime and nighttime sky should actually be ablaze with light.
This conclusion actually dates back to the beliefs of the earliest sky watchers thousands of years ago. They believed that the universe extended outward forever and contained an infinite number of stars that also extended outward forever. But if there exists an infinite number of stars—no matter how distant or faint—their combined light should be enough to light up the nighttime sky.
To understand this reasoning, imagine that the universe consists of shells of similar stars that surround the earth, much like the layers of an onion. We can examine the starlight from any shell and measure the total light created by its stars. Now if we examine a shell that is just two times larger, an interesting phenomenon immediately becomes apparent.
Because each star on this shell is twice as distant as are stars from the first shell, each must appear four times fainter. But wait! This shell has four times the surface area as the first and, therefore, contains four times as many stars. Their combined light produces exactly the same amount of light as the first shell! What about a shell that is five times farther out? Each star here appears 25 times fainter, but the shell holds 25 times more stars. It, too, appears the same brightness. In fact, no matter how far into the universe one probes, the stars will appear increasingly fainter, but we should count many more of them. And if the universe contains an infinite number of stars extending forever, then our gaze outward in any direction should eventually encounter the surface of a star. And our sky should appear ablaze with starlight—each part as bright as the sun itself!
Now it really doesn’t take an Albert Einstein sized brain to see that this is really not the case. Our nighttime sky is dark. This contradiction between reason and observation is known today as “Olbers’ Paradox, named after Wilhelm Olbers, the nineteenth-century physician and astronomer who eloquently stated the problem.
Perhaps the suggestion of the problem first came from Sir Edmund Halley, known primarily for the great comet that bears his name. In 1721 A.D. Halley presented the problem to the Royal Society in London. Much the same arguments were proposed by Jean-Phillipe Loys de Cheseaux in 1744. But it was not until Olbers wrote a complete paper on the problem in 1826 that it finally received the attention it deserved.
Olbers explored the puzzle from several different angles. His first thought was the universe was filled with clouds of dust in every direction. This dust, he argued, would block and absorb light from distant stars and cause our nighttime sky to appear dark. Not much dust would have been needed either. He calculated that the material could be millions of times more transparent than water and still work just fine.
While this idea settled the issue for awhile, another problem soon surfaced. The new field of thermodynamics showed convincingly that if such dust existed everywhere in the universe, it would indeed absorb starlight—just a Olbers had predicted. But it would also eventually heat up and begin to glow on its own. The universe again would be ablaze with light! Obviously another solution was needed.
In the early part of the 20th century, astronomers stumbled upon another possible answer. Edwin Hubble, observing the farthest reaches of our universe, with the giant telescope on top of Mount Wilson, made a startling discovery. All of the galaxies in the universe seemed to be speeding outward from each other as if from a titanic explosion some 15 or 20 billion years ago. As they recede, the color of their light is altered, much as the sound of a train whistle changes pitch as the train races past us. The amount of energy coming to us from these distant galaxies was shown to be less the faster that were receding. So if the stars and galaxies do go on forever, their combined light wouldn’t be enough to illuminate our sky.
Today some scientists think that the radiant energy from distant galaxies isn’t “thinned out” enough to cause the effect of a darkened sky and they have suggested a more fundamental explanation. It relies on the fact that light takes time to travel from place to place.
Light moves at the incredible speed of 186,000 miles every second. In our everyday world this is virtually instantaneous. But when traveling the vast expanses of outer space, light takes considerably longer to reach our eyes. For example, light from the nearest star beyond the sun (Proxima Centauri) takes more than four years to get here…From one of the nearest galaxies—The Great Spiral in Andromeda—it takes two million years. And from the farthest objects in the visible universe light has journeyed for some 10 or 20 billion years. When observing the distant galaxies, astronomers use ancient light waves to peer back into time and see objects as they were long ago.
But there is a limit as to how far these astronomers can actually see. They have seen a point beyond which no more stars or galaxies appear—a time before the stars and galaxies were even born. This means that, while space itself may go on forever, stars and galaxies do not. And those in the visible part of our universe do not produce enough light to brighten the nighttime sky.
How far back into time would stars need to exist to create this effect? Professor E. R. Harrison of the University of Massachusetts has determined that we would need to see stars as they were about one septillion years ago (1 followed by 24 zeroes). Even the slowest burning stars can live only billions of years. In fact, the entire universe isn’t even that old!
With the simple observation of the nighttime sky and the powers of the human mind, we have deduced that the universe of stars and galaxies does not go on, nor has it lived forever. The next time you watch the sun set and the first stars of the evening come out, think of the amazing story the dark nighttime sky has told us about structure, the size, and the age of the universe in which we live. Oh the wonder of it all.
Peace and Love to All of You…………………….Poppa Bear