I have heard theories for the big bang like: of another universe existing before the Big Bang, or an unstable white hole that blew up, or our universe detaching from our mother universe, or our universe coming out of a black hole. Most of these theories don't make sense, and I want to know the most-likely-to-be-true theory for how the singularity was made.

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    $\begingroup$ @mark-olsen made a fair attempt to answer this informatively, but I voted to close because what's "likely to be true" is purely a matter for opinion and such question are, under the rules, not allowed. $\endgroup$ – StephenG Jul 6 '18 at 22:09
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    $\begingroup$ @StephenG I think the question and Mark's answer are both good and valid, but perhaps the title should be something like "What are some explanations of the origin of Big Bang?". I mean, the explanations that Mark lists are not just random thoughts, but are physically founded. $\endgroup$ – pela Jul 7 '18 at 10:48

No one knows what came before the Big Bang if, indeed, anything did. Theories include:

  • The Ekpyrotic universe theory where the BB was the result of the collision of branes.
  • Various oscillating universe theories where the expansion of the universe ultimately reverses into a Big Crunch
  • Quantum fluctuation models where a zero-net-energy universe is a quantum fluctuation from nothing.
  • Various multiverse models where the universe we see is only an infinitesimal bubble in a much, much, much larger universe.
  • The Hartle-Hawking idea where spacetime gets a bit scrambled and there is no before. (Really: time turns into a space dimension if you go far enough back, and there is no time.)
  • Stringy stuff. Very difficult math.

I've only scratched the surface -- making models like this is an avocation to some physicists, while others have discovered that it sells books and generates publicity. All of the theories have one thing in common: There is no experimental evidence for any of them. Additionally, many of the theories have been shown to contradict currently-understood physical law. (Especially the 2nd Law of Thermodynamics.) The authors generally just shrug and write another paper.

(On the religious side, speculation about what happened before Creation has been around as far back as written history, with roughly equal success. In the 4th century St. Augustine, after being pushed to answer that question retorted that before the Creation, God was creating a Hell for people who ask such questions. That didn't stop the speculation either.)

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    $\begingroup$ A brave attempt to answer a very open ended opinion based question. $\endgroup$ – StephenG Jul 6 '18 at 22:10
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    $\begingroup$ I'll give a plus one, even with the word "theories" written twice. These are not theories. They are at best hypotheses. An even more appropriate term is conjecture (aka "wild ass guess"). A scientific theory represents the pinnacle of science. Evolution, general relativity, quantum mechanics, Maxwell's electrodynamics, and Newtonian mechanics: These are all theories. They are well-developed and have scads of evidence underlying them. (I listed Maxwell's electrodynamics and Newtonian mechanics because they remain valid in restricted domains.) Please don't denigrate the word "theory". $\endgroup$ – David Hammen Jul 7 '18 at 12:14
  • $\begingroup$ @David Hammen I think you're being a bit too picky here. I understand "theory" to be more inclusive than you seem to -- as I understand the term, "theories" don't have to be well-supported and widely believed! (OTOH, theories do have to be testable, and most of these are not testable in any plausible way.) I might better have used "speculation" as a description. But "wild ass guess" really is a bit unfair, since most of the ideas are at least reasonable extrapolations of plausible physics. In any event, I share your desire to make it clear that none of this is well-grounded science. $\endgroup$ – Mark Olson Jul 7 '18 at 12:47
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    $\begingroup$ It's not a problem! "Wild-ass guess" is a common term among scientists and engineers to indicate something below "guess" which is below "estimate" which is below "informed estimate" which is below actual knowledge... It's not uncommon for people to self-describe an idea they're throwing out as a WAG. $\endgroup$ – Mark Olson Jul 7 '18 at 19:23
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    $\begingroup$ @LeoPan -- Mean? I gave an upvote to this answer. Regarding the use of "ass", the phrase "wild ass guess" has become quite common in the US; see en.wikipedia.org/wiki/Scientific_wild-ass_guess. $\endgroup$ – David Hammen Jul 7 '18 at 21:28

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