The Steady-State Theory Disproved
The Big Bang theory, however, wasn't believed by everyone. In the 1940s and 1950s, the other main competing theory was the steady-state theory, proposed by Fred Hoyle (1915—2001). Hoyle's theory was that instead of having an origin around 15 billion years ago in the Big Bang, the universe had been around forever and was in a steady state. This idea is distinct from the critical density condition, because Hoyle did not believe that the universe originated in the Big Bang.
Hoyle did agree that the universe was expanding, but instead of having galaxies moving away from each other, he proposed that space was constantly being created in between galaxies, giving them the appearance of moving away. He also suggested that in this new space, matter was also created to keep the average density of the universe constant. This aspect of the theory would allow new galaxies to be created in between old ones.
Over the years, however, two pieces of evidence were found that supported the expanding universe theory and disproved the steady-state theory. First, astronomers measured the abundances of various elements in the sun, planets, and other stars, and they came up with a list of the most abundant elements. These abundances matched the amounts in which elements were created in the Big Bang theory very closely, and the steady-state theory was unable to explain these proportions.
Cosmic Microwave Background Radiation
The most compelling piece of evidence in favor of the Big Bang theory, however, is the cosmic microwave background radiation, discovered in 1965. This radiation is a kind of fossilized remnant from the earliest moments of the universe. As the universe formed in a primordial fireball, it gave off radiation. However, as the universe has expanded over time, the radiation has been stretched out to longer and longer wavelengths.
Scientists predicted that the radiation from the beginning of the universe, if still observable, would be at a wavelength of about 7 centimeters. Sure enough, an omnidirectional radiation source was found in 1965 at just this wavelength! The cosmic microwave background radiation was found to come from all directions in the sky, and there are subtle variations in the radiation (as mapped out by the COBE satellite, launched in 1989) that could correspond to the first density fluctuations in the early universe, from which arose matter concentrations and eventually stars and galaxies.
The cosmic microwave background radiation is at a wavelength of 7 centimeters, which is in the microwave range of the electromagnetic spectrum. This wavelength corresponds to a temperature of about 3 degrees Kelvin. This is an extremely cold temperature, only 3 degrees above absolute zero (the coldest possible temperature, at which all motion stops), corresponding to 454 degrees below zero Fahrenheit!