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17.4 The Big Bang Model

One of the most fundamental questions of humanity is the question of origins: Where did we come from? This question runs deeper than the origins of any one person, and delves into questions of where did life, our planet, and the entire universe come from. Many cultures have avoided the notion of an origin to the universe by placing their creation stories in a cycle. Buddhist and Hindu origins stories measure the birth, death, and rebirth of the universe in units of 4 trillion years, which is a day in the life of Brahma. At night, all matter is absorbed into the spirit of the sleeping Brahma. At dawn, when Brahma awakes, matter reappears and the cycle continues. The Greek Stoics saw the universe as being created from fire, only to be destroyed by fire, and so on. Cosmological cycles are also found in the cultures of the Maya and the Aztec.

The modern theory of the origin of the universe starts with the idea of expanding space. Georges Lemaître was a priest and a mathematician, an unassuming man who had beaten the giants of physics to the punch in deducing that the universe could be expanding. In 1929, Lemaître was the first to hypothesize a revolutionary idea: at one time, the universe might have been as small as an atomic nucleus. He proposed that the universe derived from a cosmic singularity, "a day without a yesterday" when the universe was infinitely small and infinitely curved, and all matter and energy were concentrated in a single point. Many astrophysicists found the idea bizarre and distasteful and English theorist Fred Hoyle disparaged the idea with the name "big bang." The label stuck, and scientists continue to call the description of the creation of the universe the big bang model.

The big bang model forces us to consider the idea of cosmic evolution. The universal recession of galaxies implies that the universe is evolving, and it has not always been in the same state. Our observations of nearby galaxies represent a single frame in a movie that has been playing for billions of years. We can wind the tape backwards, seeing snapshots of a progressively older universe, by observing sets of galaxies at progressively larger distances, and thus at progressively larger look back times. (Since light takes time to travel, we see objects far away as they looked when the light was released rather than as they are now.)

As we look back into the early universe, we can see the galaxies move closer together. Observationally, we can't see all the way back to the first moments, but we can see times when galaxies were just forming, and the large scale structures — the cosmic walls and voids — were just starting to form. If we could look further back, we'd see that as the universe gets smaller, its volume contracts until all the mass is smashed together in a tiny universe. The mass that formed galaxies and stars breaks down into a seething hot gas, as the universe tends toward a state of infinite temperature and density. Lemaître described the big bang model lyrically in one popular account: "The evolution of the world could be compared to a display of fireworks just ended — some few red wisps, ashes, and smoke. Standing on a well-cooled cinder we see the slow fading of the suns and we try to recall the vanished brilliance of the origin of the worlds." Thinking of the big bang as an explosion is tempting, but it is also misleading. In an explosion on Earth, debris flies through space. In the big bang, the initial singularity contains all space and matter. Time itself begins with the big bang. The evolution of the universe is the unfolding of time and space from a condition of incredible heat and density to a cold and enormous state billions of years later. This bang had no center, it had no past, it simply was the sudden beginning of everything in an expanding geometry that may or may not be finite.

The scientific story of creation says that everything — you and the Earth and the Sun and the Milky Way and all the billions of galaxies — emerged from a tiny dense dot of energy and matter that unfolded into the universe we see now. It sounds as fantastic as any of the creation myths of older cultures. How do we know that the big bang actually occurred? There are three primary pieces of evidence:

• Galaxies are taking part in a universal expansion as indicated by the linear relation between distance and red shift (the Hubble relation).

• The abundance of the lightest elements can be explained by fusion in the universe when it was young and dense and hot (cosmic nucleo synthesis).

• Space is filled with the radiation from the early hot phase, now diluted and reduced in energy to the level of microwaves (the cosmic microwave background).

Astropedia Image
Sir Fred Hoyle. Click here for original source URL.

Astropedia Image
Georges Lemaitre. Click here for original source URL.