Big bang to big crunch

Most scientists now believe that we live in a finiteexpanding universewhich has not existed forever, and that all thematter,energyand space in theuniversewas once squeezed into an infinitesimally small volume, which erupted in a cataclysmic"explosion" which has become known as theBig Bang.Thus, space, time,energyandmatterall came into being at an infinitely dense, infinitely hotgravitational singularity, and began expanding everywhere at once. Current best estimates are that this occurred some 13.7 billion years ago, although you may sometimes see estimates of anywhere between 11 and 18 billion years.TheBig Bangis usually considered to be a theory of the birth of theuniverse, although technically it does not exactly describe the origin of theuniverse, but rather attempts to explain how theuniversedeveloped from a very tiny, dense state into what it is today. It is just a model to convey what happened and not a description of an actual explosion, and theBig Bangwas neither Big (in the beginning theuniversewas incomparably smaller than the size of a singleproton), nor a Bang (it was more of a snap or a sudden inflation).
In fact, 揺xplosion� is really just an often-used analogy and is slightly misleading in that it conveys the image that theBig Bangwas triggered in some way at some particular centre. In reality, however, the same pattern of expansion would be observed from anywhere in theuniverse, so there is no particular location in our present universe which could claim to be the origin.It really describes a very rapid expansion or stretching of space itself rather than an explosion in pre-existing space. Perhaps a better analogy sometimes used to describe the even expansion of galaxies throughout the universe is that of raisins baked in a cake becoming more distant from each other as the cake rises and expands, or alternatively of a balloon inflating.Neither does it attempt to explain what initiated the creation of the universe, or what came before theBig Bang, or even what lies outside the universe. All of this is generally considered to be outside the remit of physics, and more the concern of philosophy. Given that time and space as we understand it began with theBig Bang, the phase 揵efore theBig Bang� is as meaningless as 搉orth of the North Pole�.Therefore, to those who claim that the very idea of a Big Bang violates theFirst Law of Thermodynamics(also known as theLaw of Conservation of Energy) that matter and energy cannot becreated or destroyed, proponents respond that theBig Bangdoes not address the creation of theuniverse, only its evolution, and that, as the laws of science break down anyway as we approach the creation of the universe, there is no reason to believe that theFirst Law of Thermodynamics would apply.
The Second Law of Thermodynamics, on the other hand, lends theoretical (albeit inconclusive) support to the idea of a finite universe originating in a Big Bang type event. If disorder andentropyin the universe as a whole is constantly increasing until it reaches thermodynamic equilibrium, as the Law suggests, then it follows that theuniversecannot have existed forever, otherwise it would have reached its equilibrium end state an infinite time ago, our Sun would have exhausted its fuel reserves and died long ago, and the constant cycle of death and rebirth ofstarswould have ground to a halt after an eternity of dissipation ofenergy, losses of material toblack holes, etc.TheBig Bangmodel rests on two main theoretical pillars: theGeneral Theory of Relativity(Albert Einstein抯 generalization of Sir Isaac Newton抯 original theory ofgravity) and theCosmological Principle(the assumption that thematterin theuniverseis uniformly distributed on the large scales, that theuniverseis homogeneous and isotropic).TheBig Bang(a phrase coined, incidentally, by the English astronomerFred Hoyleduring a 1949 radio broadcast as a derisive description of a theory he disagreed with) is currently considered by most scientists as by far the most likely scenario for the birth ofuniverse. However, this has not always been the case, as thefollowing discussion illustrates.