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Any data that we have on the Big Bang comes from the cosmic microwave background (CMB) which was created about 380,000 years after the Big Bang. From there we have been able to calculate what the Universe was like prior to the CMB's creation by applying the laws of physics, up until the Planck era, in which our model simply doesn't work any more.

Then how do we know (some things about) what happened during/before the Planck era? How do we know that the Universe started out as an infinitesimal point and not a very small, but not infinitesimally small, region of space? If everything we know, including this, was calculated backwards from the CMB and our calculations stop working at the Planck era, how did we deduce this?

Kyle Oman
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Annonymus
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    Can you link to your source for these claims, e.g. that we know that the universe started out as a point? – David Z Jul 05 '16 at 10:36
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    I can't actually remember where I picked that up for the first time, but a simple google search finds plenty of sources which state this, like http://why-sci.com/big-bang/, http://www.big-bang-theory.com/, this book: https://play.google.com/store/books/details?id=8mbBAwAAQBAJ&rdid=book-8mbBAwAAQBAJ – Annonymus Jul 05 '16 at 10:44
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    "... in Hawking's original formulation, it guaranteed that the Big Bang has infinite density. Hawking later revised his position in A Brief History of Time (1988) where he stated that "there was in fact no singularity at the beginning of the universe" (p50). This revision followed from quantum mechanics, in which general relativity must break down at times less than the Planck time. Hence general relativity cannot be used to show a singularity." – lemon Jul 05 '16 at 10:57
  • I think it is a logical deduction from the fact that an incredibly violent outward explosion occurred, and that that explosion and the fact that matter seems to have been originated from the same point both result from one and the same event. It could be wrong of course, but a theory claiming that all matter was globbed together and then accelerated away at the speed of light for no apparent reason is no better. I don't believe we have any direct evidence of this though. It's a bit like assuming the earth is round purely on the basis of the fact that it has shown to curve. – Neil Jul 05 '16 at 11:15
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    Well, scientists such as Lemaitre were able to calculate the singularity from Einstein's equations of general relativity...though I guess @lemon is saying that doesn't really count. However, aside from that, I've read some of Hawking's books and he uses imaginary time to calculate his "no boundaries" proposal. When he converts it back to normal time (i.e., something we can use in the real world) the singularity reappears. – auden Jul 05 '16 at 12:10
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    @Neil As a cosmologist, I agree that such a theory claiming matter to have all been at the same point and then have exploded outward at the speed of light is nonsense and probably wrong. Thankfully, that is not at all similar to the actual Big Bang theory. Phew! We really dodged a bullet on that one. – Jim Jul 05 '16 at 12:34
  • A lot of it is theoretical and comes from all fields of physics. The CMB and distribution and dynamics of galaxies and clusters in the universe offer the primary experimental evidence – user122066 Jul 05 '16 at 12:46
  • Also note our theories can only ever suggest what happened AFTER the big bang. – user122066 Jul 05 '16 at 12:47
  • I consider the big bang invalid and violating basic physics. There is no need of such thing for matter creation. – Overmind Jul 05 '16 at 12:48
  • Annonymus, I'm trying to think of the best way to answer this. Until I can figure out a good, post-worthy answer, let me mention a few things that might clear stuff up in the meantime. The old big bang theory is what said the universe was a singularity at the beginning. The observational evidence was that the rate of expansion and the movements of the galaxies all traced back to a singularity (curvature singularity, not black hole singularity. Important difference) some 380000 years before the CMB was emitted... – Jim Jul 05 '16 at 12:49
  • It was the CMB that provided one of the problems to the old big bang theory. And to resolve that, we now involve inflation in the theory. Before inflation, we do not say the universe was necessarily a singularity. We admit the possibility of such, but also that it is possible the universe was simply very very small. There is also the possibility that the universe as a whole is infinite in extent and undergoing inflation forever and the part we see is only a "bubble" of space where inflation stopped. Truthfully, there are many competing theories for what the universe was like at its origin. – Jim Jul 05 '16 at 12:54
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    While I've not yet found the words to answer your question directly, it is worth noting that modern physics does not simply say that the universe started out as a singularity. We admit the possibility of this along with the possibility of many other things. Because, as you indirectly pointed out, we don't have a very clear picture of the extremely earliest times. – Jim Jul 05 '16 at 12:56
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    @Overmind What you consider is irrelevant to what is. Observation supports the big bang. That is what science says. If you choose not to accept that, science doesn't care. – Jim Jul 05 '16 at 12:58
  • Observations can be valid with multiple theories in the same time. Does not make any of them better then the other. Additionally, the observations may be incorrect from an universal point of view. – Overmind Jul 05 '16 at 12:59
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    @Overmind Observations are never incorrect. What you observe is what you observe. Only the interpretations and explanations of them can be incorrect. However, when a theory logically and mathematically predicts that you should observe a specific something and that specific something is observed, it supports the theory. Potentially multiple theories, yes. But even should a theory be proven wrong eventually, it does not make the observations that supported it wrong. That said, if you would like to suggest a mathematically and observationally valid alternative theory to the Big Bang, be my guest. – Jim Jul 05 '16 at 13:05
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    The Big Bang didn't happen at a point – John Rennie Jul 05 '16 at 14:30
  • Suggesting one here would kind of be offtopic. And there are instances when the observation may be only valid locally and/or irrelevant from an universal perspective. – Overmind Jul 06 '16 at 05:12

2 Answers2

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This seems to be a common misconception about the big bang.

At present our theories can only suggest what happened AFTER the "bang". We cannot formulate what occurred AT the singularity with our current knowledge of physics.

At a small neighborhood around a spacetime singularity quantum gravity becomes important and we simply have no clue at present how to deal with the general relativistic singularities (e.g. black holes as well!)

When you hear of physicists talking about the big bang it is almost always a discussion of the dynamics AFTER the singularity.

When physicists discuss what happened during the planck era or at a singularity it is entirely (very) educated guesswork within the bounds of current theory (much like how theoretical physicists like hawking have come up with convincing theories about some black hole properties).

Hope that helps!

user122066
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    This is heavily dependent on how you define "The Big Bang". – Jim Jul 05 '16 at 13:00
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    You keep referring to "the singularity". What singularity?! – lemon Jul 05 '16 at 13:33
  • @lemon im not sure if youre being antagonistic or serious? If youre serious then a singularity is an area of spacetime (which does not depend on the coordinates chosen) where where the gravitational field becomes infinite. If youre being antagonistic then please kindly offer construction rather than agression. Im sorry but tone is difficult to discern through type sometimes. I hope you understand :D – user122066 Jul 05 '16 at 16:52
  • @WillihamTotland thank you for pointing this out and being constructive. :D – user122066 Jul 05 '16 at 17:00
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    Also, I think what @lemon means is that singularities probably don't actually exist, they're just times and places where our math breaks down. There's not some spot inside a black hole that's fundamentally distinct from spacetime, and there's not some timespot at the beginning of the universe that's fundamentally distinct; it's just that we don't have the math to explain exactly what's going on at these timeplaces, because our math throws its hands up in disgust and storms out in a huff. – Williham Totland Jul 05 '16 at 17:03
  • @WillihamTotland thank you for your kind and constructive help. I always thought that was the implied idea behind singularities? – user122066 Jul 05 '16 at 17:14
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    It is, but when answering fundamental questions it's good to be explicit. :) – Williham Totland Jul 05 '16 at 17:21
  • @WillihamTotland according to at least some models, there is actually a point with the least proper time, in the same sense there is a lowest point in a parabola. – John Dvorak Jul 05 '16 at 20:40
  • @JanDvorak That is interesting, to be sure. But it seems as iff it implies that some quantities or qualities go to infinity, those models kind of have to be wrong? Intuitively, anyway, but I do realise that intuition is a dangerous road to go down. – Williham Totland Jul 05 '16 at 21:25
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We know some physics that occurred with the early universe up to periods of time close to the big bang. The actual moment of the big bang is not known at least empirically. That moment did not occur at point, but was a process where the spacetime of the observable universe emerged. This was probably a bubble nucleation event similar to that proposed by Coleman and de Luccia. A quantum field at a high energy vacuum tunnels through a barrier and emerges as a "bubble of spacetime." This and related variants are thought to be involved with the earliest moments in the observable cosmology.

This tunneling process converts a spacetime region, which is an de Sitter spacetime or anti-de Sitter spacetime, from one with a very large cosmological constant to one with a very small one. This is similar to radioactive decay and the energy gap between the two vacua generates particles and radiation. The de Sitter spacetime is rapidly inflating due to a large cosmological constant and is associated with the inflationary period. Data from WMAP and Planck spacecrafts have found anisotropic distributions in CMB radiation. The early anisotropy of the universe left imprints on the much later period where the radiation dominated phase ended $380,000$ years into the evolution of the universe

There are other more recent forms of inflationary cosmology with chaotic inflation, where the bubble emerges from an "eternally inflating" de Sitter spacetime and is but one of an infinite number of cosmologies. It is curiously similar in a way to the old Hoyle-Bondi theory of continuous creation in a steady state cosmology. What we observe as a big bang is just one of an infinite number of "creation events."

After this time period predictions with the number of quarks, where more than three families would increase the effective heat capacity of the early quark plasma, conform to just three families. Other predictions such as deuteron and helium production in the first three minutes of the universe are also supported by data.

There are then ways in which we can observe moments prior to the CMB event or the end of the radiation dominated period. The radiation in the CMB is itself thought to be a detector of its own, such as holding imprints of gravity waves produced during inflation. B-modes are finger prints of gravity waves, and in 2014 the BICEP-2 announced their discovery. However, subsequent concern with EM polarization from galactic dust has reduced the experimental certainty below 5-sigma, so the search is in effect still on.

Lawrence B. Crowell
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