Evidence for the Big Bang
The best evidence for the Big Bang theory is the existence of cosmic microwave background radiation. This is very faint radiation that appears to come from all parts of the Universe. When George Gamow (1904-68) developed the Big Bang model of the Universe, he predicted that if the Universe had originated in a Big Bang then some residual radiation would still be reaching us from the farthest parts of the Universe (those parts that had rushed away from the center at the greatest speeds following the Big Bang ). Such radiation would appear to come from all parts of the Universe, and would have a long wavelength (similar to that of the radiation emitted by a body with the very low temperature of 3K). The wavelength predicted of this residual radiation indicates how the Universe has cooled during its expansion after the Big Bang . The characteristics of residual radiation predicted by Gamow exactly match those observed of the cosmic microwave background radiation.
Further evidence that appears to support the Big Bang theory and contradicts the alternative Steady State theory is provided by the study of distant galaxies. Some of these galaxies are 13 billion light-years away, which means that the light we see from them has taken 13 billion years to reach us. These galaxies appear to be more tightly packed than closer galaxies, which would indicate that the Universe is expanding in volume as it gets older and was once much smaller and denser. If the Steady State theory was correct the density of galaxies throughout the Universe should look the same, however far one looks back in time.
Certain types of objects in the Universe, notably quasars, are observable only at very great distances, indicating that they are objects from the remote past. The fact that such objects are found only at great distances and are not observable within our more immediate vicinity again suggests that the constitution of the Universe was very different billions of years ago than it is today. Again, this is evidence that supports the Big Bang theory and contradicts the Steady State theory.