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Well, no, as well all know. However this misapprehension comes up time and time again, and surprisingly there doesn't seem to be a single question addressing precisely this issue (though it's covered by lots of related questions).

So do we need a Danuesque canonical question and answer(s) with the title above so we can point people to it? If the consensus is yes then I hereby volunteer to create a community wiki question and a trial answer.

If I've missed an obvious Q/A that already covers this then shout and I'll delete this question.

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    $\begingroup$ I have my own adjective? Right on!!! Also, I definitely approve of this idea, and don't know of any alternative canonical question/answer :) $\endgroup$
    – Danu
    Commented Sep 23, 2014 at 5:42
  • $\begingroup$ Is voting on this Q like a poll? (That's the impression I get from the word consensus.) If yes, then +1 :) $\endgroup$
    – 299792458
    Commented Sep 23, 2014 at 7:41
  • $\begingroup$ @New_new_newbie: I guess so. Anyone who thinks it's a silly idea should downvote. $\endgroup$
    – John Rennie
    Commented Sep 23, 2014 at 7:57
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    $\begingroup$ I've gone ahead and posted the question and answer. $\endgroup$
    – John Rennie
    Commented Sep 23, 2014 at 10:12
  • $\begingroup$ I feel the added text in this revision is clunky and detracts from the question. Maybe put it in a comment, or encase it in a <sup></sup> pair? The question should really contain mostly just the question. (+2, by the way.) $\endgroup$
    – Emilio Pisanty
    Commented Sep 23, 2014 at 11:17
  • $\begingroup$ @EmilioPisanty: yes. I added the second paragraph as an afterthought because the last time I did a question and answer like this I got people asking why I had answered my own question. Assuming the post meets with approval I'll give it a few days then roll back the edit. $\endgroup$
    – John Rennie
    Commented Sep 23, 2014 at 11:28
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    $\begingroup$ Essentially a duplicate of physics.stackexchange.com/q/25591/2451 and links therein. $\endgroup$
    – Qmechanic Mod
    Commented Sep 23, 2014 at 12:21

2 Answers 2

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I think it may be worth mentioning that if you conformally map the standard cosmological spacetime to Minkowski spacetime you find it doesn't map to the whole of Minkowski spacetime. In the conformal map can be extended so the big bang singularity can be mapped to a standard spatial slice of Minkowski spacetime (i.e. it remains spatially infinite) and only the observable Universe at t=0 shrinks to a point.

Of course it doesn't mean that the Universe was actually infinite at the big bang, the singular nature of the big bang prevent a meaningful answer to the question, but it is useful when understanding why the particle horizon exists even though by rewinding back in time you can make any comoving point arbitrarily close.

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    $\begingroup$ This seems like it belongs as an answer to the posted question here and not on the meta site asking if this was a good idea.... $\endgroup$
    – Kyle Kanos
    Commented Sep 30, 2014 at 2:09
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    $\begingroup$ Hi John. If you talking about the conformal mapping as discussed by Infeld and Schild that's an area I never really understood. If you could turn this into a "for gifted amateurs" answer and post it as an answer I'd be very interested to read it. $\endgroup$
    – John Rennie
    Commented Sep 30, 2014 at 5:23
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Just for completeness I'll post an answer. I've posted my proposed Q/A as a community Wiki at Did the Big Bang happen at a point?.

The reception seems favourable, but that shouldn't stop you if you have any ideas for improving it. For example I'm contemplating adding a second answer for gifted amateurs that goes into a lot more detail on the points I glossed over in the main answer. All such contributions are welcome.

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