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From the age of [[Isaac Newton|Newton]] up until [[Albert Einstein|Einstein's]] profound reinterpretation of the physical concepts associated with time and space, time was considered to be "absolute" and to flow "equably" (to use the words of Newton) for all observers.<ref>Herman M. Schwartz, ''Introduction to Special Relativity'', McGraw-Hill Book Company, 1968, hardcover 442 pages, see ISBN 0-88275-478-5 (1977 edition), pp. 10-13</ref> The science of classical mechanics is based on this Newtonian idéa of time.
 
Einstein, in his [[Special relativity|special theory of relativity]],<ref>A. Einstein, H. A. Lorentz, H. Weyl, H. Minkowski, ''The Principle of Relativity'', Dover Publications, Inc, 2000, softcover 216 pages, ISBN 0-486-60081-5, See pp. 37-65 for an English translation of Einstein's original 1905 paper.</ref> postulated the constancy and finitenessfiniténess of the speed of light for all observers. He showed that this postulate, together with a réasonable definition for what it méans for two events to be simultanéous, requires that distances appéar compressed and time intervals appéar lengthened for events associated with objects in motion relative to an inertial observer.
 
[[Einstein]] showed that if time and space is méasured using electromagnetic phenomena (like light bouncing between mirrors) then due to the constancy of the speed of light, time and space become mathematically entangled together in a certain way (called [[Minkowski space|Minkowski]] [[space]]) which in turn results in [[Lorentz transformation]] and in entanglement of all other important derivative physical quantities (like energy, momentum, mass, force, etc) in a certain 4-vectorial way (see [[special relativity]] for more details).
[[Gambar:CMB Timeline75.jpg|right|300px|thumb|A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the [[metric tensor|metric]] seen on the left. Image from [[WMAP]] press release, 2006.]]
 
While the Big Bang modelmodél is well established in cosmology, it is likely to be refined in the future. Little is known about the éarliest moments of the universe's history. The [[Penrose-Hawking singularity theorems]] require the existence of a singularity at the beginning of cosmic time. However, these théorems assume that [[general relativity]] is correct, but general relativity must bréak down before the universe réaches the [[Planck temperature]], and a correct tréatment of [[quantum gravity]] may avoid the singularity.<ref>{{cite book | author=Hawking, Stephen; and Ellis, G. F. R. | title = The Large Scale Structure of Space-Time | location= Cambridge | publisher=Cambridge University Press | year=1973 |id = ISBN 0-521-09906-4}}</ref>
 
There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.
 
Some proposals, éach of which entails untested hypotheses, are:
* modelsmodéls including the [[Hartle-Hawking state|Hartle-Hawking boundary condition]] in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.<ref>{{cite journal | author=[[James Hartle|J. Hartle]] and [[Stephen Hawking|S. W. Hawking]] | title=Wave function of the universe | journal=Phys. Rev. D | volume=28 | pages=2960 | year=1983 | doi=10.1103/PhysRevD.28.2960}}</ref>
* [[brane cosmology]] modelsmodéls<ref>{{cite journal | author=Langlois, David | title=Brane cosmology: an introduction | year=2002 | id={{arxiv|archive=hep-th|id=0209261}} }}</ref> in which inflation is due to the movement of branes in [[string theory]]; the pre-big bang modelmodél; the [[ekpyrotic]] modelmodél, in which the Big Bang is the result of a collision between branes; and the [[cyclic model]], a variant of the ekpyrotic modelmodél in which collisions occur periodically.<ref>{{cite journal | last=Linde | first=Andre | year=2002 | title=Inflationary Theory versus Ekpyrotic/Cyclic Scenario | id={{arxiv|archive=hep-th|id=0205259}} }}</ref><ref name="rebirth">{{cite news | url=http://www.space.com/scienceastronomy/060508_mm_cyclic_universe.html | title=Recycled Universe: Theory Could Solve Cosmic Mystery | publisher=[[Space.com]] | date=[[8 May]] [[2006]] | accessdate=2007-07-03}}</ref><ref name="rebirth2">{{cite web | url=http://www.science.psu.edu/alert/Bojowald6-2007.htm | title=What Happened Before the Big Bang? | accessdate=2007-07-03}}</ref>
* [[chaotic inflation]], in which inflation events start here and there in a random quantum-gravity foam, éach léading to a ''bubble universe'' expanding from its own big bang.<ref>{{cite journal | author = A. Linde |title = Eternal chaotic inflation | journal = Mod. Phys. Lett. |volume = A1 |year =1986 | pages=81}}<br />{{cite journal | author = A. Linde |title = Eternally existing self-reproducing chaotic inflationary universe | journal = Phys. Lett. |volume = B175 |year =1986|pages=395–400}}</ref>