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The Night Sky
| Article
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13164 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
11 / 1987 |
1,011 Words |
| Author
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Robert Jastrow
Robert Jastrow is founder and past director of NASA's Goddard
Institute for Space Studies and recipient of the NASA Medal
for Exceptional Scientific Achievement. He is currently
professor of earth sciences at Dartmouth College. |
I teach an introductory science course on the history of the universe that carries the story of cosmic evolution from the Big Bang to the appearance of man on the earth. The students are mainly nonscience majors--some three to four hundred of the "unwashed"--who tend to feel inhibited about asking questions. I try to encourage them at the beginning of the term by pointing out that a child's questions--such as, "Why is the sky dark at night?"--are the best kind and often offer deep insights into fundamental problems.
The question of why the sky is dark at night is simple, and the answer seems obvious: The sun has gone down. But here is why the question is actually quite profound: If the universe is infinite and contains an infinite number of stars, then, when you look up into the sky at night, you will see the entire sphere of the heavens covered with stars. Some are near; some are far. And since their number is infinite, in every direction you look, your line of sight should intersect a star somewhere. But if your line of sight did intersect a star in every direction, then the entire night sky would be blazingly bright--as bright as the surface of the sun. So the sky should not be dark at night, as, in fact, it is. Rather, it should blaze with the brightness of an infinite number of stars. That is Wilhelm Olber's Paradox.
The resolution of the paradox turns out to be connected with one of the greatest discoveries ever made in the history of astronomy. For 60 years we have known that we live in an expanding universe, in which all the galaxies around us are rushing away from us and one another at enormous speeds, as if in the aftermath of a cosmic explosion. Further, for 20 years we have known that the cause of the expansion is, in fact, a great explosion that occurred billions of years ago.
As best as we can tell, the explosion took place 15 billion years ago, give or take a few billion years. The important thing is not exactly when it happened, but that the great event took place in an instant. The material universe exploded into being abruptly. The seed of every star, planet, and living thing in the cosmos was planted in that moment. It was, in a material sense, the movement of creation.
So astronomy has proven that the universe has not existed forever. It is, in fact, 15 billion years old; although it is older than the Bible suggests, it is nevertheless not infinitely old.
As soon as we realize that the universe
...
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