From Grolier's The New Book of Knowledge

Big Bang

This artist's concept represents crucial periods in the development of the Universe according to one theory. It begins with a tiny fraction of a second after the Big Bang and goes through the way it looks today-15 billion years later. (NASA Goddard Space Flight Center)

How and when did the universe begin? No other scientific question is more fundamental or provokes such spirited debate among researchers. After all, no one was around when the universe began, so who can say what really happened? The best that scientists can do is work out the most foolproof theory, backed up by observations of the universe. The trouble is, so far, no one has come up with an absolutely indisputable explanation of how the cosmos came to be.

The Big Bang

Since the early part of the 1900s, one explanation of the origin and fate of the universe, the Big Bang theory, has dominated the discussion. Proponents of the Big Bang maintain that, between 13 billion and 15 billion years ago, all the matter and energy in the known cosmos was crammed into a tiny, compact point. In fact, according to this theory, matter and energy back then were the same thing, and it was impossible to distinguish one from the other.

Adherents of the Big Bang believe that this small but incredibly dense point of primitive matter/energy exploded. Within seconds the fireball ejected matter/energy at velocities approaching the speed of light. At some later time—maybe seconds later, maybe years later—energy and matter began to split apart and become separate entities. All of the different elements in the universe today developed from what spewed out of this original explosion.

Big Bang theorists claim that all of the galaxies, stars, and planets still retain the explosive motion of the moment of creation and are moving away from each other at great speed. This supposition came from an unusual finding about our neighboring galaxies. In 1929 astronomer Edwin Hubble, working at the Mount Wilson Observatory in California, announced that all of the galaxies he had observed were receding from us, and from each other, at speeds of up to several thousand miles per second.

The Redshift

To clock the speeds of these galaxies, Hubble took advantage of the Doppler effect. This phenomenon occurs when a source of waves, such as light or sound, is moving with respect to an observer or listener. If the source of sound or light is moving toward you, you perceive the waves as rising in frequency: sound becomes higher in pitch, whereas light becomes shifted toward the blue end of the visible spectrum. If the source is moving away from you, the waves drop in frequency: sound becomes lower in pitch, and light tends to shift toward the red end of the spectrum. You may have noticed the Doppler effect when you listen to an ambulance siren: the sound rises in pitch as the vehicle approaches, and falls in pitch as the vehicle races away.

To examine the light from the galaxies, Hubble used a spectroscope, a device that analyzes the different frequencies present in light. He discovered that the light from galaxies far off in space was shifted down toward the red end of the spectrum. Where in the sky each galaxy lay didn't matter—all were redshifted. Hubble explained this shift by concluding that the galaxies were in motion, whizzing away from Earth. The greater the redshift, Hubble assumed, the greater the galaxy's speed.

Some galaxies showed just a slight redshift. But light from others was shifted far past red into the infrared, even down into microwaves. Fainter, more distant galaxies seemed to have the greatest red shifts, meaning they were traveling fastest of all.

An Expanding Universe

So if all the galaxies are moving away from Earth, does that mean Earth is at the center of the universe? The very vortex of the Big Bang? At first glance, it would seem so. But astrophysicists use a clever analogy to explain why it isn't. Imagine the universe as a cake full of raisins sitting in an oven. As the cake is baked and rises, it expands. The raisins inside begin to spread apart from each other. If you could select one raisin from which to look at the others, you'd notice that they were all moving away from your special raisin. It wouldn't matter which raisin you picked, because all the raisins are getting farther apart from each other as the cake expands. What's more, the raisins farthest away would be moving away the fastest, because there'd be more cake to expand between your raisin and these distant ones.

That's how it is with the universe, say Big Bang theorists. Since the Big Bang explosion, they reason, the universe has been expanding. Space itself is expanding, just as the cake expanded between the raisins in their analogy. No matter whether you're looking from Earth or from an alien planet billions of miles away, all other galaxies are moving away from you as space expands. Galaxies farther from you move faster away from you, because there's more space expanding between you and those galaxies. That's how Big Bang theorists explain why light from the more distant galaxies is shifted farther to the red end of the spectrum. In fact, most astronomers now use this rule, known as Hubble's law, to measure the distance of an object from Earth—the bigger the redshift, the more distant the object.

In 1965 two scientists made a blockbuster discovery that solidified the Big Bang theory. Arno Penzias and Robert Wilson of Bell Telephone Laboratories detected faint microwave radiation that came from all points of the sky. They and other physicists theorized that they were seeing the afterglow from the Big Bang's explosion. Since the Big Bang affected the entire universe at the same moment in time, the afterglow should permeate the entire universe and could be detected no matter what direction you looked. This afterglow is called the cosmic background radiation. Its wavelength and uniformity fit nicely with other astronomers' mathematical calculations about the Big Bang.

How Lumpy Do You Like Your Universe?

The Big Bang model is not uniformly accepted, however. One problem with the theory is that it predicts a smooth universe. That is, the distribution of matter, on a large scale, should be roughly the same wherever you look. No place in the universe should be unduly lumpy.

But in 2001, astronomers announced the discovery of a group of galaxies and quasars that fills more than 125 million million cubic light-years of space, and is presently the largest structure in the universe. Instead of an even distribution of matter, the universe seems to contain great empty spaces punctuated by densely packed streaks of matter.

Big Bang proponents maintain that their theory is not flawed. They argue that gravity from huge, undetected objects in space (clouds of cold, dark matter we can't see with telescopes, or so-called cosmic strings) attracts matter into clumps. Other astronomers, still reluctant to believe in invisible objects just to solve an inexplicable problem, continue to question fundamental aspects of the Big Bang theory.

In spite of its problems, the Big Bang is still considered by most astronomers to be the best theory we have. As with any scientific hypothesis, however, more observation and experimentation are needed to determine its credibility. Advances ranging from more-sensitive telescopes to experiments in physics should add more fuel to the cosmological debate during the coming decades.

The Steady State Theory

But the Big Bang is not the only proposed theory concerning our universe's origin. In the 1940s a competing hypothesis arose, called the Steady State theory. Some astronomers turned to this idea simply because, at the time, there wasn't enough information to test the Big Bang. British astrophysicist Fred Hoyle and others argued that the universe was not only uniform in space—an idea called the cosmological principle—but also unchanging in time, a concept called the perfect cosmological principle. This theory didn't depend on a specific event like the Big Bang. Under the Steady State theory, stars and galaxies may change, but on the whole the universe has always looked the way it does now, and it always will.

The Big Bang predicts that as galaxies recede from one another, space becomes progressively emptier. The Steady State theorists admit that the universe is expanding, but predict that new matter continually comes to life in the spaces between the receding galaxies. Astronomers propose that this new material is made up of atoms of hydrogen, which slowly coalesce in open space to form new stars.

Naturally, continuous creation of matter from empty space has met with criticism. How can you get something from nothing? The idea violates a fundamental law of physics: the conservation of matter. According to this law, matter can neither be created nor destroyed, but only converted into other forms of matter, or into energy. But skeptical astronomers have found it hard to directly disprove the continuous creation of matter, because the amount of matter formed under the Steady State theory is so very tiny: about one atom every billion years for every several cubic feet of space.

The Steady State theory fails, however, in one important way. If matter is continuously created everywhere, then the average age of stars in any section of the universe should be the same. But astronomers have found that not to be true.

Astronomers can figure out how old a galaxy or star is by measuring its distance from Earth. The farther away from Earth an object is, the longer it has taken light from the object to travel across space and reach Earth. That means that the most distant objects we can see are also the oldest.

For example, take quasars, the small points of light that give off enormous amounts of radio energy. Because the light from quasars is shifted so far to the red end of the spectrum, astronomers use Hubble's law to calculate that these powerhouses lie at a great distance from Earth, and hence are very old. But quasars exist only at these great distances—none are found nearer. If the Steady State theory were true, there ought to be both young and old quasars. Since astronomers haven't found quasars that formed recently, they conclude the universe must have changed over time. The discovery of quasars has put the Steady State theory on unsteady ground.

The Plasma Universe and Little Bangs

Not happy with either the Big Bang or the Steady State theory? A minority of astronomers are formulating other views of the creation of the universe. One model comes from the mind of Nobel laureate Hannes Alfvén, a Swedish plasma physicist. Called the Plasma Universe, his model starts by noting that 99 percent of the observable universe (including the stars) is made of plasma. Plasma, an ionized gas that conducts electricity, is sometimes called the fourth state of matter. This theory states that the Big Bang never happened, and that the universe is crisscrossed by gigantic electric currents and huge magnetic fields.

Under this view the universe has existed forever, chiefly under the influence of an electromagnetic force. Such a universe has no distinct beginning and no predictable end. In the Plasma Universe, galaxies come together slowly over a much greater time span than in the Big Bang theory, perhaps taking as long as 100 billion years.

Little of the evidence for the Plasma Universe comes from direct observations of the sky. Instead, it comes from laboratory experiments. Computer simulations of plasmas subjected to high-energy fields reveal patterns that look like simulated galaxies. Using actual electromagnetic fields in the laboratory, researchers have also been able to replicate the plasma patterns seen in galaxies. While still a minority view, the Plasma Universe is gaining favor with younger, more laboratory-minded astronomers who value hard empirical evidence over mathematical proofs.

Meanwhile, another group of astronomers is developing a steady-state theory that actually conforms to astronomical observations. Like its predecessor, this steady-state theory proposes a universe with no beginning and no end. Rather, matter is continuously created via a succession of "Little Bangs," perhaps associated with mysterious quasars. In this new theory, galaxies would form at a rate determined by the pace at which the universe expands. These theorists can even account for the cosmic background radiation: they maintain that the microwaves are actually coming from a cloud of tiny iron particles—and are not the residual effects of some primordial explosion.

The End of the Universe

Will the universe continue expanding? Will it just stop or even begin to contract? The answer depends on the amount of mass that the universe contains. If the universe's mass exceeds a certain crucial value, then gravity should eventually stop everything from flying away from everything else.

With enough mass, the universe will eventually succumb to the overpowering force of gravity and collapse again into a single point—a theory often called the Big Crunch. But without enough mass, the universe will continue to expand. As of 2001, many scientists concluded that the latter hypothesis appears to be the most likely.

In 1998, astronomers found an even more remarkable puzzle: the universe seems to be accelerating while expanding, as if being pulled by some kind of "antigravity" force. Other astronomers have since corroborated this finding using a variety of methods, and have all but confirmed the existence of this mysterious "dark energy."

David Fishman