Quantum Physics is a branch of science that deals with discrete, indivisible units of energy called quanta as described by the quantum theory. In physics, a quantum (plural: quanta) is the minimum unit of any physical entity involved in an interaction. In other words, there is a limit to how small something can be, and that limit is a quantum.
Quantum Theory is a way of explaining the nature of matter, and how it behaves on the atomic, and even the subatomic, level. In other words, quantum theory is essentially a description of this microscopic world. Quantum theory has made the modern world possible, giving us lasers and computers and iPod nanos, not to mention explaining how the sun shines and why the ground is solid.
The Big Bang model or theory is the prevailing cosmological theory of the early development of the universe. According to the Big Bang model, the universe was originally in an extremely hot and dense state that expanded rapidly. This expansion caused the universe to cool and resulted in the present diluted state that continues to expand today. Based on the best available measurements as of 2010, the original state of the universe existed around 13.7 billion years ago, which is often referred to as the time when the Big Bang occurred. The theory is the most comprehensive and accurate explanation supported by scientific evidence and observations.