Four realms of Mechanics

1.Classical Mechanics: When we talk of macroscopic objects we are actually dealing with classical mechanics. Here the speed (v) of objects considered is small as compared to velocity of light (c), i.e. v <<< c.

2.Quantum Mechanics: For the microscopic objects (of the order of size of an atom) we deal with quantum mechanics.

3.Relativistic Mechanics: For objects that travel very fast (at speeds comparable to c), classical mechanics transforms into relativistic mechanics.

4.Quantum Field Theory: For objects which are microscopic as well as travel with the speed of light, we require the theory which incorporates relativity and quantum principles and thereby we have theory entitled 'The Quantum Field Theory'.

Source:Introduction to elementary particles by David Griffiths

Origin Of The Universe

Big Bang Hypothesis
Big Bang is one of the many attempts to explain the origin of the universe. According to this theory universe originated from something infinitely hot , infinitely dense but having infinitesimal volume i.e. a 'singularity'... & here the laws of Physics have got no meaning because according to the experts prior to the singularity nothing existed, no space, time, matter, or energy - nothing. This infinitesimally small volume/singularity expanded (& is still expanding) and has reached its present temperature, density & volume/size...and now we are part of this vast expanse which we call the universe.
The strongest proof for the 'Big Bang hypothesis' is existence of the microwave background radiation. In 1965, Radioastronomers Arno Penzias and Robert Wilson discovered a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant of
infinite heat the initial singularity engulfed.

References:
http://www.big-bang-theory.com/
http://www.kheper.net/cosmos/universe/Big_Bang.htm

// This post will continually be updated, each time I learn something more about the subject