Light is part of the spectrum of electromagnetic energy. Electromagnetic energy travels as waves at the speed of light. Electromagnetic energy can be of varying wavelength (distance between one wave crest to the next) or conversely frequency (how many wave crests pass per second). Electromagnetic energy ranges from radio waves at the longest wavelengths (lowest frequencies) to gamma rays at the shortest wavelengths (highest frequencies). Red light is longer wavelength than violet light.
Light coming from the sun appears white to us, but passing this light through a prism spreads the light into its constituent colors, the colors of the rainbow. The prism simply bends or refracts the light; short wavelengths are refracted by a greater amount than longer wavelengths.
Astronomers during the nineteenth century discovered that when sunlight is passed through a prism dark lines at specific wavelengths appear on the rainbow of visible colors. The dark lines were determined to be caused by the absorption of certain wavelengths of light by atoms lying between the light source and the observer. This is because electrons orbiting the nucleus of an atom may absorb only specific wavelengths of light to supply just exactly the necessary energy to make the jump from a lower to a higher energy level. The electrons in atoms of a given element (hydrogen for example) may only make certain discrete energy jumps. There is a unique "fingerprint" of absorption lines for each element. Similarly, the spectrum of light from a heated, glowing gas of a given element gives off bright emission lines of the same wavelengths as the absorption lines for that element., The emission lines are the result of excited electrons giving up discrete wavelengths of energy to go down to a lower energy level. Spectral analysis, identification of absorption and emission spectra, of sunlight, starlight, and light reflected from planet, asteroid, and comet surfaces can be used to identify elements found in the atmospheres and on the surfaces of those bodies. From spectral analysis we know that hydrogen (H) is by far the most abundant element in our solar system and in the universe. Helium (He) is a distant second.
An astonishing observation was made in the late 1920's by analyzing light coming from stars in other galaxies (clusters of billions of stars beyond our own Milky Way Galaxy). The characteristic spectral lines for hydrogen are shifted to lower frequencies (toward the red end of the visible spectrum). This is similiar to the way the pitch of a horn on a passing train or car shifts to a lower pitch as the blaring horn moves quickly away. This is called a Doppler shift. When an observer is moving away from the transmitter of a wave (light or sound) the wave crests arrive at the observer more slowly than expected. Edwin Hubble (1929) interpreted the frequency shift of extra-galactic starlight to mean that distant galaxies are moving away from us. In fact, all of the observed galaxies are moving away from our own Milky Way Galaxy and from one another. And the farther away they are, the faster they are moving away from us. This is exactly like a gigantic explosion, or better yet, the expansion of a balloon, from exceedingly small size to the current size of the universe. All of the parts move outward away from the origin and away from one another as the balloon inflates and the skin stretches. This theoretical, great cosmic expansion was dubbed "The Big Bang." In what has become the standard theory, space and time and all the matter and energy now in the universe were formed in the Big Bang. Based on the rate of expansion and the distances between galaxies, the Big Bang must have occurred some 13.5 to 14 billion years ago.
The Early Universe
If the universe is expanding from such an event, what would the conditions have been like in the first moments of the universe when all of the matter and energy in the universe was concentrated in a very small point? Theoretical work shows that near the instant of creation, when the universe was 10-32 seconds old, the pressures and temperatures were too great for normal matter to exist. Only quarks (constituents of protons and neutrons) and smaller particles including the nearly massless electrons and neutrinos existed. Protons and neutrons, which account for almost all of the mass of normal matter, formed as the universe rapidly expanded and the temperatures and pressures decreased. By 13.8 seconds the temperature would have decreased to about 3 billion Kelvin at which point protons and neutrons could start to form atomic nuclei of hydrogen and helium which continued until the universe was about 30 minutes old.
The Microwave Background Radiation
By the time the universe was approximately 700,000 years old the temperature would have decreased to about 3000 Kelvin, finally cool enough for electrons to be captured by the hydrogen nuclei to form complete atoms. Prior to this time, the radiation that filled the universe was scattered by roaming electrons. After the electrons were captured by hydrogen and helium nuclei the universe became transparent. Radiation, including light, could now travel through the universe without being scattered. It was like the fog lifting to show a clear blue sky. The remnant scattered 3000 K radiation from 700,000 years after the Big Bang was predicted by the Big Bang Theory to be detectable in all directions, however at a longer wavelength (lower temperature) in the microwave range due to the expansion of the universe. In 1965, Arno Penzias and Robert Wilson of Bell Labs accidentally discovered this background radiation as noise in a sensitive microwave receiver they were building in Holmdel, New Jersey. Contact was eventually made with Robert Dicke of Princeton University who was studying the early universe and whose student was trying to detect the background radiation. This discovery of an effect previously predicted by the Big Bang Theory is a very powerful support for the theory.
The conditions in the rapidly expanding and cooling universe following the Big Bang were such that only simple matter was formed. The universe was filled with hydrogen (H) and small amounts of helium (He). As the universe expanded, galaxies formed in areas of higher concentrations of H and He and stars formed within the galaxies in areas of highest concentrations of H and He.