The Big Bang is one of the best known theories in cosmology, which is the study of the universe as a whole and as to what happened in its past and what will happen in the future. It has many concepts which are hard to grasp.
This is the idea that our universe started out much hotter and denser than it is now and has been expanding since then. This theory is based on observations of our universe. Scientists say that external galaxies are receding in such a way that their recessional speeds are proportional to the distance they are away from us.
This observation is explained well by a uniform expansion of the universe. If the universe is expanding, it must have started out very small some time far in the past. It is this point which has been called the beginning of the universe or the “Big Bang.”
When we observe the night sky we see an excess of radiation which is called the CMB radiation (cosmic microwave background radiation). It is a perfect black body with a temperature of 3 Kelvin. Taken with the expansion of the universe, this radiation says that the universe must have been much hotter in the past and also opaque to radiation. It turns out that the CMB radiation fits in perfectly with being from the first photons to escape after the universe became transparent.
As the last century came to a close and the first few years of this century passed us by, cosmologists have been beginning to feel like they are getting close to knowing what the basic parameters of the universe are. A recent talk by a cosmologist at Cornell started with the question “Is cosmology complete?” The answer, by the toss of a coin was no! The talk then went on to explain why in some sense the answer is no, and in others it is yes.
Recent observations (in particular from WMAP) have solved several long-standing problems in cosmology. Evidence strongly suggests that the Hubble Constant is around 71 km/s/Mpc (accurate to about 5 percent) and that the universe is geometrically flat, but that it is probably dominated by some weird form of energy called “dark energy.” “Ordinary” matter also seems to be dominated by “dark matter,” which cannot be the same as the matter (called baryonic matter) that makes up humans.
Overall, it’s a strange universe, but one that we are beginning to understand in greater detail than ever before!