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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?

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    $\begingroup$ Can you link to your source for these claims, e.g. that we know that the universe started out as a point? $\endgroup$ – David Z Jul 5 '16 at 10:36
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    $\begingroup$ 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 why-sci.com/big-bang, big-bang-theory.com, this book: play.google.com/store/books/… $\endgroup$ – Annonymus Jul 5 '16 at 10:44
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    $\begingroup$ @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. $\endgroup$ – Jim Jul 5 '16 at 12:34
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    $\begingroup$ 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. $\endgroup$ – Jim Jul 5 '16 at 12:56
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    $\begingroup$ @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. $\endgroup$ – Jim Jul 5 '16 at 13:05
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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!

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    $\begingroup$ This is heavily dependent on how you define "The Big Bang". $\endgroup$ – Jim Jul 5 '16 at 13:00
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    $\begingroup$ You keep referring to "the singularity". What singularity?! $\endgroup$ – lemon Jul 5 '16 at 13:33
  • $\begingroup$ @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 $\endgroup$ – user122066 Jul 5 '16 at 16:52
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    $\begingroup$ 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. $\endgroup$ – Williham Totland Jul 5 '16 at 17:03
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    $\begingroup$ It is, but when answering fundamental questions it's good to be explicit. :) $\endgroup$ – Williham Totland Jul 5 '16 at 17:21
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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.

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