What does "the big bang happened everywhere at once" mean?

[bahamagreen]

"the comoving frame coordinate system"... an expanding system of coordinates, very interesting, I'm looking into this... I think many questions will be answered. Thanks, Ken G!

 

[Buzz Bloom]

If the Universe in now flat, it must also be infinite. If it is infinite now, it must have always been infinite, even at that singularity at time zero.

Please see the thread: Big Bang Singularity.

 

[PeterDonis]

If it is infinite now, it must have always been infinite, even at that singularity at time zero.

No, this is not correct. The singularity at "time zero" is not part of spacetime, so it is meaningless to ask whether or not the universe was spatially infinite there. (Also, the model in which this singularity appears is not the actual Big Bang model used in cosmology; that model leaves it open what came before the inflationary era, and does not make any claims about an initial singularity.)

 

[Buzz Bloom]

No, this is not correct. The singularity at "time zero" is not part of spacetime, so it is meaningless to ask whether or not the universe was spatially infinite there.

Hi PeterDoris::

Thank you for you comment.

I am OK with the concept that the singularity is not part of spacetime, but that would also imply it is not a point either, wouldn't it?

I also understand that the current thinking about the singularity is that in first Planck time (5.4 x 10^-44 secs) of the universe, a General Relativity (GR) model would have to be replaced by some currently unspecified quantum model. However, my comment is about extraolating the geometry of the spacetime of an infinite GR model to time zero. This extrapolation would not not result in a point; the spacetime would remain infinite.

I started a thread on this point and my impression of the consensus of the responses there seem to agree with the above point of view. Please see
Big Bang Singularity.

 

[phinds]

...that would also imply it is not a point either, wouldn't it?

Correct, and this is exactly what was already explained to you in the other thread. "Singulaity" does not mean "point" except in the phrase "this is the point where the math model breaks down" in which case it mean "place" not a dimensionless point.