How do Astronomers know that the universe is expanding by studying light waves?

The universe is blowing up.

Earth is but a particle of debris hurtling through space from an awesome explosion that took place 20 billion years ago, which generated temperatures of trillions of degrees, and marked the Beginning.

Ever since the “Big Bang”, stars, planets, gases, and intergalactic dust have been moving away from one another like so much shrapnel.

From our vantage point on Earth, the stars don’t seem to be moving. Indeed, their positions seem for us to be the most solidly predictable phenomenon we know. In Shakespeare’s Julius Caesar, Caesar describes himself as a model of statesmanlike consistency and forthrightness:

For I am constant as the northern star, Of whose true fixed and resting quality There is no fellow in the firmament.

Caesar would have been surprised to learn that the North Star, Polaris, of which he speaks is flying toward us through space at 10 miles per second. Astronomers assure us, however, that we and Polaris are not on a collision course.

In general, the stars are moving away from us, especially stars outside our own galaxy. Astronomers can tell that we are all moving away from the Big Bang. The key is that light waves reaching us from the distant stars and galaxies exhibit a “red shift.” To understand this, one must understand something about waves.

When a fire engine blowing its siren is coming toward you, the pitch of the siren seems to rise; as it passes and moves on into the distance, the pitch seems to descend. Sound, like light, is a form of energy, which travels in waves.

Sound consists of waves of compression in the air, which bump against our eardrums in rapid succession. The diaphragm in the siren creates the pattern of waves by vibrating, sending off a pulse of densely packed air every time it is pushed out, followed by a space of loosely packed air created each time it is sucked back. The compression pattern sent through the air by one complete out and in cycle of the diaphragm is one wave, or pulse. The number of pulses generated in a second is called the frequency of the sound; the more waves per second our ears pick up, the higher the frequency we hear.

As the fire engine approaches us, sending out hundreds of sound waves each second, each wave “catches up” a little with the next, the next pulse is emitted closer to us than the last. Each succeeding pulse strikes our ears, with less and less time between pulses; therefore the frequency of the sound waves increases, and the sound seems higher. As the engine moves away, the distance between pulses is increasingly stretched out, since the siren emits each one a little farther from us; the number reaching our ear every second is less, and thus the pitch seems to go down.

Light is another form of wave energy, and it too has different frequencies. Unlike sound, which needs a medium such as air or water through which to travel, light can travel in the vacuum of space.

An astronomer analyzes starlight using a prism attached to his telescope; the prism spreads the point of “white” light from a single star, which contains many different frequencies, into a long colored band or spectrum, in which the light is arranged by frequency from left to right. The higher frequencies, which look bluish or violet, are toward the left of the band; the blue fades into green, yellow, orange, and finally red as the frequencies get lower toward the right.

When any element is burned, on Earth or in space, it gives off a certain frequency or group of frequencies of light energy. If you spread out that light with a prism, you can see the “signature” of the element as a pattern of lines in the colored wave band the prism produces.

The pattern for the element calcium, for example, which is burned on the fiery surfaces of many stars, is a pair of dark lines near the high frequency or “blue” end of the spectrum. By watching for that “signature” we can tell how a star is moving: If a star is moving away from us, the waves of light we receive from it will seem to be of a lower frequency, just as the pitch of the fire siren sounds lower as the vehicle recedes in the distance. The two lines for calcium still appear on our spectral band, but not where they should be. They are shifted to the right, toward the red or lower frequency end of the spectrum.

The farther toward the red the shift of these two calcium lines is, the faster the star or galaxy is moving away from us. With such a tool for analyzing starlight, astronomers can map out the motions of all the stars and galaxies.

The results of this mapping of the universe show that all the other stars and galaxies, except for a few relatively near ones, are moving away from ours at scores, hundreds, or thousands of miles per second. The stars that are farthest from us are moving away the fastest. The pattern is like the flight of debris spreading out from an explosion. Probably the universe will expand forever, as every star in it gets more distant from every other, there is no limit to the universe’s size.

There is another possibility, however, generally considered remote, which is that in 60 to 100 billion years the force of gravity will slow down the expansion, and the stars will reverse directions and speed toward one another until they meet in another Big Bang. As it looks now, however, this second Bang is not possible, since it would take ten times as much matter as the universe now contains to generate sufficient gravitational pull to force a halt to the expansion.

A hundred billion years is pretty far down the pike as far as we on Earth are concerned. Life here in its simplest form is only 4 billion years old; man, a million or so at the most. A mere 6 billion years from now our own sun will expand into a Red Giant and burn the entire solar system into vapor.

By then our descendants, if we have left any, will long before have moved on to inhabit other solar systems.