Origins of the Big Bang Theory and Evidence For It

That the universe is expanding was news to Einstein. But the possibility that the galaxies are moving away from each other had been proposed a few years before Hubble's papers on theoretical grounds arising from Einstein's own equations. In 1922, Russian physicist and mathematician Alexander Friedmann investigated what would happen in a model universe based upon two assumptions that greatly simplified the mathematics: that the universe looks identical in every direction, and that it looks that way from every observation point. We know that Friedmann's first assumption is not exactly true—the universe fortunately is not uniform everywhere! If we gaze upward in one direction, we might see the sun; in another, the moon or a colony of migrating vampire bats. But the universe does appear to be roughly the same in every direction when viewed on a scale that is far larger—larger even than the distance between galaxies. It is something like looking down at a forest. If you are close enough, you can make out individual leaves, or at least trees, and the spaces between them. But if you are so high up that if you hold out your thumb it covers a square mile of trees, the forest will appear to be a uniform shade of green. We would say that, on that scale, the forest is uniform.

Based on his assumptions Friedmann was able to discover a solution to Einstein's equations in which the universe expanded in the manner that Hubble would soon discover to be true. In particular, Friedmann's model universe begins with zero size and expands until gravitational attraction slows it down, and eventually causes it to collapse in upon itself (There are, it turns out, two other types of solutions to Einstein's equations that also satisfy the assumptions of Friedmann's model, one corresponding to a universe in which the expansion continues forever, though it does slow a bit, and another to a universe in which the rate of expansion slows toward zero, but never quite reaches it.) Friedmann died a few years after producing this work, and his ideas remained largely unknown until after Hubble's discovery. But in 1927 a professor of physics and Roman Catholic priest named Georges Lemaitre proposed a similar idea: If you trace the history of the universe backward into the past, it gets tinier and tinier until you come upon a creation event—what we now call the big bang.

Not everyone liked the big bang picture. In fact, the term "big bang" was coined in 1949 by Cambridge astrophysicist Fred Hoyle, who believed in a universe that expanded forever, and meant the term as a derisive description. The first direct observations supporting the idea didn't come until 1965, with the discovery that there is a faint background of microwaves throughout space. This cosmic microwave background radiation, or CMBR, is the same as that in your microwave oven, but much less powerful. You can observe the CMBR yourself by tuning your television to an unused channel—a few percent of the snow you see on the screen will be caused by it. The radiation was discovered by accident by two Bell Labs scientists trying to eliminate such static from their microwave antenna. At first they thought the static might be coming from the droppings of pigeons roosting in their apparatus, but it turned out their problem had a more interesting origin—the CMBR is radiation left over from the very hot and dense early universe that would have existed shortly after the big bang. As the universe expanded, it cooled until the radiation became just the faint remnant we now observe. At present these microwaves could heat your food to only about -270 degrees Centigrade —3 degrees above absolute zero, and not very useful for popping corn.

Astronomers have also found other fingerprints supporting the big bang picture of a hot, tiny early universe. For example, during the first minute or so, the universe would have been hotter than the center of a typical star. During that period the entire universe would have acted as a nuclear fusion reactor. The reactions would have ceased when the universe expanded and cooled suficiently. but the theory predicts that this should have left a universe composed mainly of hydrogen, but also about 23 percent helium, with traces of lithium (all heavier elements were made later, inside stars). The calculation is in good accordance with the amounts of helium, hydrogen, and lithium we observe.

The logical conclusion drawn by a Catholic Priest and the Cosmic Background Radiation (CMBR) we see left over from the event.