Atom Formation During Recombination
During the first few hours of the Universe, atoms did not exist. It was simply too hot. Heat causes things to agitate and the Universe was so hot during its early history that the negatively charged electrons, which form the outer part of atoms, would have had so much motion as to escape from the nuclei, which are the heavy positively charged central cores of atoms. Atoms are held together by the electric force but, in this extremely hot environment, the force was not sufficiently strong as to be able to bind electrons to nuclei. Thus the state of the Universe was a plasma. (A plasma is a gas in which negative and positive charges move around independently).
As the Universe expanded, however, it cooled and eventually the temperature became sufficiently low enough (about 4,000 degrees Celsius or 7,000 degrees Fahrenheit) as to allow for atom formation. This process began when the Universe was about 300,000 years old and the event is called "recombination" by cosmologists. During recombination, electrons moved more slowly than previously, and when they encountered a nucleus, they were captured by it. The electrons became bound or tied to nuclei thereby forming atoms. When an electron is captured by a nucleus, it tumbles into a quantum energy level giving off light. Since enormous numbers of atoms formed during recombination, profuse amounts of light were given off.
After all atoms had formed at around 500,000 years, the constituents in the Universe were neutral and the light that had been emitted by electrons propagated through the Universe in all directions with little interaction. Initially, this light
was in the visible region of the electromagnetic spectrum. But during the next 13 billion years or so, the Universe expanded about a thousand-fold. As the light moved in this expanding space, its wavelength stretched also. First it became infrared radiation and eventually it became microwaves. Atomic physics plus the Standard Model of cosmology thus predicts that today there should be microwave radiation propagating everywhere that is left over from the days of recombination. This ancient relic is known as the cosmic microwave background radiation. You have actually probably seen some of it: a few percent of the "snow" that you see when your TV is tuned to a blank station is due to cosmic microwave background radiation.
Thus theory predicted the presence of this cosmic radiation. In 1965, two Bell Laboratory scientists, Arno Penzias and Robert Wilson, detected noise in a large radio disk. Unable to determine its source, they sought help from Princeton University astrophysicists, Robert Dicke, James Peebles, Peter Roll and David Wilkinson, who realized that the noise was due to the cosmic microwave background radiation. The spectral nature of the radiation nicely matched theoretical predictions. This was an extraordinary development for which Penzias and Wilson would receive the 1978 Nobel prize in physics. In 1965, a crude image of the Universe when it was a few hundred-thousand years had been obtained.
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