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Expanding Universe

Albert Einstein's General Theory of Relativity, which established the relationship between matter, space, time and gravity, governs modern cosmology's view of the universe. But when Einstein began to apply his theory to the structure of the universe, he was dismayed to find that it predicted either an expanding or contracting universe--something entirely incompatible with the prevailing notion of a static universe. In what he would later call "the greatest blunder of my life," Einstein added a term called the cosmological constant to his equations that would make his calculations consistent with a static universe.

Einstein admitted his mistake in 1929 when Edwin Hubble showed that distant galaxies were, indeed, receding from the earth, and the further away they were,the faster they were moving. That discovery changed cosmology.

Enter Hubble's Law.

The familiar sound of a train whistle as it recedes into the distance is a consequence of the Doppler Effect. As the train moves away from the listener, the crests of the sound waves are stretched out or shifted, resulting in a lower pitch. The faster the train recedes, the more stretched out the waves become. The same holds true for any wave-emitting object--whether they be sound waves, light waves, or radio waves. Conversely, the wavelength of objects that are moving toward us are shorter than those emitted by an object at rest.

Atoms emit or absorb light in characteristic wavelengths: hydrogen, helium, and all the other atomic elements have their own spectrum signatures. In the early part of this century, Vesto Slipher was studying the spectra of light emitted from nearby galaxies. He noticed that the light coming from many galaxies was shifted toward the red, or longer wavelength, end of the spectrum. The simplest interpretation of this "redshift" was that the galaxies were moving away from us.

Hubbles's Law

Hubble, who had been the first to establish that the universe included many other galaxies outside of our own, noticed something else: the galaxies were receding from us at a velocity proportional to their distance. The more distant the galaxy, the greater its redshift, and therefore the higher the velocity, a relation known as Hubble's Law.
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The velocity v could be determined by multiplying the distance R by H, the Hubble constant, given by the slope of the line in the above graph, in units of kilometers per second per million light years. The Hubble constant describes the universe's rate of expansion.

The apparent linearity of Hubble's Law implies that the universe is uniformly expanding. What does that actually mean?

For one thing, it means that no matter which galaxy we happen to be in, virtually all of the other galaxies are moving away from us (the exceptions are at the local level: gravitational attraction pulls neighboring galaxies, such as Andromeda and the Milky Way, closer together). In other words, it's not as though we here on earth are at the center of the universe and everything else is receding from us. The universe has no "edge" as such.

Cosmic Expansion
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It also means that the galaxies are not moving away through space, they are moving away with space, as space itself expands. Think of a loaf of unbaked raisin bread you've set in a warm place to rise. The raisins are like galaxies or clusters of galaxies, and the dough, space. As the dough rises, the raisins move farther apart, but they've moved with the dough, not through the dough.

How old is the universe?

Determining the Hubble Constant is something of a Holy Grail for cosmologists, because it holds the key to the age of the universe. Imagine running a film of cosmic expansion backwards to the Big Bang--in

other words, a contracting universe instead of an expanding universe. Because the Hubble Constant is a measure of how much space is expanding in units of distance per second, it's possible to estimate how long it would take, rolling the movie backwards, for the most distant galaxies to collide with each other and finally collapse in the Big Bang.

Unfortunately, it's not so easy to determine the Hubble Constant. While cosmologists have mastered the trick of determining a galaxy's redshift, and therefore its velocity, determining the distance to far-off objects is quite another matter. We don't have any yardsticks that long.

Instead, cosmologists use standard candles, bright beacons that serve as reference points. One kind of standard candle are the Cepheid variables (the North Star is one), so called because they blink at a rate that is precisely related to their brightness. Because the brightness of individual stars is proportional to their distance from us, cosmologists compare nearby Cepheids (to which we know the precise distance) to those farther away. A Cepheid that is four times fainter than a nearby Cepheid is estimated to be twice as far away. Cosmologists use an entire ladder of distance indicators that are calibrated using the lower (nearest) rungs.

Until just recently, most estimates of the Hubble Constant have hovered around 50, which implies that the universe is about 20 billion years old. However, this provides only an upper limit to the age of the universe, and is based on the present rate of expansion, as observed by the recession of distant galaxies. It's likely that this rate was greater in earlier epochs of cosmic evolution. As galaxies tugged at each other through their gravitation, the expansion slowed down.

Cepheid Variable in M100

The Hubble Telescope was designed, in part, to find Cepheid variables and other standard candles even farther away than those detectable by ground-based telescopes. Cosmologists hoped that these objects, not influenced by the gravitational pull of the Milky Way, would yield more accurate information about the expansion of the universe.
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One team using the Hubble Telescope found a number of Cepheids in the Virgo cluster, which allowed them to estimate the distance to the far-off Coma cluster.
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The team estimated the Hubble Constant to be 80, which would make the universe eight to twelve billion years old. Separate, ground-based observations of another galaxy within Virgo yield an even higher value of 87.
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M100 Cepheid Variable Credits

Other groups using another kind of standard candle called supernovae --massive stars that have collapsed and exploded--come up with lower Hubble Constants, either 73 or 50.

On the other hand, astronomers who study the chemistry and life cycles of stars are quite certain that the oldest stars in the Milky Way are about 14 billion years old. Clearly, cosmologists are facing a paradox: you can't have stars that are older than the universe!

All of the galaxies studied are only in the region of 50 million light years from Earth, too close to get a more truly "global" value for the Hubble Constant. Studies are now underway at several observatories worldwide, and with the Hubble Telescope, to probe much further out and find redshifts corresponding to times when the universe was one fourth or less than its present size.

Clearly the pressure is on to find a correct value for the Hubble Constant. Cosmologists hope that better instrumentation, earth-bound and space-born, will provide the means to do so.

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Copyright 1995, The Board of Trustees of the University of Illinois

NCSA. Last modified 11/1/95.