After Hubble’s discoveries, it was clear that the universe was expanding. This new knowledge led to new questions. It is always this way in science. Scientists then wondered, “What would cause the universe to expand?”
Some scientists (notably Georges Lemaître) proposed an idea that the universe could have begun with a large explosion. Matter and energy, they reasoned, would have been blasted outward in all directions by the explosion. They hypothesized that over a long time, this matter collected and formed into stars, galaxies, planets, and all the other things we find in the universe. These scientists argued that this idea would explain the outward expansion of the universe. It would also explain the background radiation and the lumpiness of the universe, which we’ll talk about.
Other astronomers (notably Fred Hoyle) thought that the idea of the universe beginning as a big explosion sounded ridiculous. With contempt, they argued that the universe could not just have begun with some “big bang.” To them, the idea was ridiculous. It is a quirk of fate that these astronomers, who were trying to belittle the theory, actually gave it its name. The name stuck and today we call this idea the big bang theory.
A new scientific theory is subjected to rigorous testing. As a general rule, scientists always try to disprove a theory or hypothesis. It is not really possible for science to absolutely prove something, but it is possible to disprove it. Those who sought to disprove the big bang theory noted that if the universe started with an explosion, then scientists should be able to detect the leftover effects or radiation of that explosion. They pointed out that the universe should be cooling down. Any explosion will cool off as it expands outward. The big bang explosion should be no different. No such effect had been observed, so they had a good point. Where was this leftover, or background, radiation?
In the 1960s, radio astronomers were using giant radio telescopes to chart the radio energy coming from space. These astronomers noticed that there was always some static in the radio signals they received. They thought that there was some faulty equipment, so they checked and rechecked the receivers and computers.
All was well. Then they noticed a bird’s nest in the dish antenna they were using. At last they thought they had found the source of the static. After clearing out the nest, they checked their equipment again. The static was still there! Eventually the scientists realized that their equipment was working perfectly and that this pesky static was coming from outer space. The static was the type of radio signal one would expect from radiation at a temperature of 3°C above absolute zero. What they accidentally discovered was the leftover radiation from the big bang, or background radiation.
The discovery of background radiation helped confirm the possibility of the universe starting with a bang. If the big bang theory was correct, the universe should have a temperature of 3°C above absolute zero. This was exactly what Wilson and Penzias found.
A hallmark of any good theory is its ability to make predictions. In explosions, matter and energy are not spread uniformly. There is lumpiness in the exploding blast. If the universe began in a big explosion, there should be lumpiness in the distribution of matter throughout our universe. Until a few decades ago, no one had mapped enough of the universe to know whether or not galaxies were spread uniformly across the sky or if they were clustered together. So, mapping projects were undertaken. As scientists gathered data and prepared maps, they discovered that the matter in our universe is, indeed, lumpy. Galaxies are not spread equally in all directions; they exist in groups and clusters. The big bang theory had passed another test by neatly explaining these groups and clusters of galaxies.
The heat from an exploding fireball is not uniform. In the 1980s, a satellite was placed in orbit to more accurately measure the background radiation of the universe. The big bang theory predicted that the radiation should not be the same everywhere. Until that time, data indicated that it was the same. The more sensitive instruments on the satellite found that the background radiation did in fact vary from one part of the universe to another. Some parts were slightly warmer than 3°C above absolute zero and other places were slightly colder. In other words, the radiation was not uniform. The big bang theory had predicted this unevenness of the background radiation, and the prediction was found to be true.
Today the big bang theory is the most accepted scientific theory for the origin of the universe because it explains measured facts. It explains the expansion of galaxies. It explains the 3°C above absolute zero background radiation, and it explains the lumpy distribution of galaxies. Its predictions have been verified.