THE UNIVERSAL EXPANSION: Edwin Hubble observations of universal expansion provides the first strong evidence for the big bang theory. If the universe began from a compact source and has been expanding, then you would expect to see the expansion going on today. The fact that we do see such an expansion is taken as evidence for a big bang event in the past. It is not, however, conclusive evidence. Many other theories of the universe have incorporated an expansion, but not a specific beginning of time. During the 1940's for example, scientists proposed a steady-state universe. Galaxies in this model move away from each other, but the new galaxies are constantly being formed in the spaces that are being vacated. Thus the steady-state model describes a universe that is constantly expanding and forming new galaxies, but with no trace of a beginning. The possibility of this kind of theory, the universal expansion, in and of itself, does not compel us to accept the big bang theory.
THE COSMIC MICROWAVE BACKGROUND: In 1964 Arno Penzias and Robert W. Wilson , two scientists working at the Bell laboratories in New Jersey, used a primitive radio receiver to scan the skies for radio signals. There motivation was a simple one. There worked during the early days of satellite broadcasting, and they were measuring microwave radiation to document the kinds of background signals that might interfere with radio transmission, There found that whichever way they pointed their receiver they heard a faint hiss in their apparatus. They seemed to be a microwave radiation falling on the earth from all directions. We now call that radiation the cosmic microwave background radiation
At first they suspected that this background noise might be an artifact---a fault in their electronics, or even interference caused by droppings from a pair of pigeons that had nested inside the funnel-shaped microwave antenna. However, a thorough testing and cleaning made no difference in the odd results. A constant influx of microwave radiation of wavelength 7.35 centimeters flooded the Earth from every direction in space. The scientists asked: where is this radiation coming from?
In order to understand the answer to their questions, you need to remember that every object in the universe that is above the temperature of absolute zero emits some sort of radiation. An example of this is coal on a fire. It may first glow white hot and emit the complete spectrum of visible Electro magnetic radiation. As the fire cools it no longer glows with a visible light, you can tell that the coal is giving off radiation by holding your hand to it and sensing the infrared or heat radiation that still pours from the dying embers. As the coal cools still more, it will give off wavelengths of longer and longer radiation. Therefore, one way to think about cosmic background radiation, then is to imagine that you are inside a cooling coal of fire. No matter which way you look, you will see radiation coming toward you, and that radiation will shift from white to orange to red light and eventually all the way down to microwave as the coal cools.
In 1964, a group of theorists at Princeton University (not far from Bell Laboratories) pointed out that if the universe had indeed begun at some time in the past, then today it would still be giving off electromagnetic radiation in the microwave range. In fact, the best calculation at the time indicated that the radiation would be characteristic of an object at a few degrees above absolute zero. When Penzias and Wilson contacted these theorists, the reason that they could not get rid of the microwave signal became obvious. Not only was it a real signal, it was evidence for the big bang it self. For their discovery the two scientists won the Noble Prize for physics in 1978---not a bad outcome for a measurement designed to do something entirely different.
We said before that it impossible to imagine theories, such as the steady state theory, in which the universe is expanding but has no beginning. However, it is impossible to imagine a universe that does not have a beginning but that produces that kind of microwave background were are talking about. The Penzias and Wilson discovery put an end to that steady state theory. In 1989 an exact measurement by the Cosmic Background Explorer satellite put it at 2.7 K. This finding reaffirmed the validity of the big bang theory in the minds of scientists.
THE ABUNDANCE OF LIGHT ELEMENTS: The third importance piece of the evidence for the big bang theory comes from the study of the abundance of light nuclei in the universe. For a short period in the early history of the universe atomic, nuclei could form from elementary particles. Cosmologists believe that the only nuclei that could have formed in the big bang are isotopes of hydrogen, helium and lithium. These are the first three elements with one, two , and three protons in their nuclei respectively. All elements heavier than lithium were formed later in the starts.
The conditions necessary for the formation of light elements were two-fold. First matter had to be packed together densely enough to allow enough collisions to produce a fusion reaction. Second, the temperature had to be high enough for those reactions to happen. However, not so high that nuclei created by fusion would be broken up in subsequent collisions. In an expanding universe, the density of matter will decrease rapidly because of the expansion. Thus nuclei will form only in a very narrow window of opportunity. Calculations based on density and collision frequency, together with known nuclear reaction rates, make rather specific predictions about how much of each isotope could have been made before matter spread too thinly. Thus the cosmic abundance of elements such as deuterium (the hydrogen isotope with one proton and one neutron in it nucleus) Helium-3 (The isotope with two protons and one neutron) and Helium-4 (with two protons and two neutrons) comprise another test of our theories about the origin of the universe.
In fact studies, of the abundance of these isotopes find that they agree quite well with the predictions made in this way. The prediction for the primordial abundance of helium-4 in the universe, for example, is that it cannot have exceeded 25%. Observations of helium abundance are quite close to this prediction. If the abundance of helium differed by more that a few percent from this value, the theory would be in serious trouble.
