Initial singularity
{{Short description|Time period of seeming infinite density just after the Big Bang}}
The initial singularity is a singularity predicted{{dubious|reason=reliable referencnes do not agree|date=April 2025}} by some models of the Big Bang theory to have existed before the Big Bang.{{cite news|url=http://www.space.com/13347-big-bang-origins-universe-birth.html|title=The Big Bang: What Really Happened at Our Universe's Birth?|author=Wall, Mike|date=21 October 2011|work=The History & Future of the Cosmos|accessdate=April 16, 2012|agency=Space.com}} The instant immediately following the initial singularity is part of the Planck epoch, the earliest period of time in the history of our universe.
Traditional models of our universe
File:CMB Timeline300 no WMAP.jpg
The use of only general relativity to predict what happened in the beginnings of the universe has been heavily criticized, as quantum mechanics becomes a significant factor in the high-energy environment of the earliest stage of the universe, and general relativity on its own fails to make accurate predictions.{{cite news | url=https://www.sciencedaily.com/releases/2007/07/070702084231.htm | title=What Happened Before The Big Bang? | date=2 July 2007 | agency=ScienceDaily | accessdate=April 16, 2012 | author=Penn State}} In response to the inaccuracy of considering only general relativity, as in the traditional model of the Big Bang, alternative theoretical formulations for the beginning of the universe have been proposed, including a string theory-based model in which two branes, enormous membranes much larger than the universe, collided, creating mass and energy.{{cite news | url=http://science.howstuffworks.com/dictionary/astronomy-terms/before-big-bang1.htm | title=Branes, Crunches, and Other Big Ideas | work=What existed before the big bang? | date=12 May 2010 | agency=HowStuffWorks | accessdate=April 16, 2012 | author=Lamb, Robert}}
The cosmic microwave background is evidence that the universe expanded from a very hot, dense state, possibly a singularity of infinite density.{{cite web|title=What If the Big Bang Wasn't the Beginning? New Study Proposes Alternative|url=https://www.space.com/38982-no-big-bang-bouncing-cosmology-theory.html|website=Space.com|date=5 December 2017 |accessdate=2 April 2018}}
Alternatives to the singularity
Various new models of what preceded and caused the Big Bang have been proposed as a result of the problems created by quantum mechanics. One model, using loop quantum gravity, aims to explain the beginnings of the universe through a series of Big Bounces, in which quantum fluctuations cause the universe to expand. This use of loop quantum gravity also predicts a cyclic model of universes, with a new universe being created after an old one is destroyed, each with different physical constants. These proposals have been criticized as inconsistent with the Borde–Guth–Vilenkin theorem, however their modifications with only one bounce (as opposed to cyclic series of bounces) circumvent this problem (particularly if the contracting phase is empty, i.e. compactified Milne, and (2+1)-dimensional, due to the inherent stabilizing rigidity of vacuum in this case).
Another possibility based on M-theory and observations of the cosmic microwave background (CMB) states that the universe is but one of many in a multiverse, and has budded off from another universe (e.g., one that macroscopically looks like static empty space) as a result of quantum fluctuations such as quantum foam, as opposed to our universe being all that exists.{{cite news | url=http://www.universetoday.com/15051/thinking-about-time-before-the-big-bang/ | title=Thinking About Time Before the Big Bang | work=Universe Today | date=13 June 2008 | agency=Universe Today | accessdate=April 16, 2012 | author=Atkinson, Nancy}}
Non-quantum classical models without singularities have also been proposed.{{Cite journal |last=Veklych |first=Bogdan |date=2025-04-08 |title=Is a quantum gravity era necessary? |url=https://linkinghub.elsevier.com/retrieve/pii/S2405428325000152 |journal=Reviews in Physics |pages=100116 |doi=10.1016/j.revip.2025.100116 |issn=2405-4283|arxiv=2310.02338 }}
References
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