Yes, if we extrapolate the standard Big Bang model backward in time, the density increases everywhere without bound as T approaches zero. In an infinite universe, this means every region, even in the unobservable part, reaches arbitrarily high density simultaneously. This is why the Big Bang is often described as a singularity in time, not in space—it’s not a localized point, but rather a state where all of space was at infinitely high density at the same time.
However, in modern cosmology, the singularity at T=0 is generally considered an indication that our current physical theories break down rather than an actual point of "infinite density everywhere." Quantum gravity effects (which we don’t yet fully understand) would likely smooth out this singularity, preventing true infinite density. Inflationary models also suggest that what we call the "Big Bang" may not be a singular beginning but instead a transition from a pre-existing state (such as a quantum fluctuation, an eternal inflation scenario, or a bounce from a prior contracting phase). So, while classical general relativity predicts a singularity of infinite density everywhere as T -> 0, most physicists suspect this is a limitation of the theory, and quantum gravity will provide a more complete picture. Quentin All those moments will be lost in time, like tears in rain. (Roy Batty/Rutger Hauer) Le mer. 19 mars 2025, 15:13, Alan Grayson <agrayson2...@gmail.com> a écrit : > > > On Wednesday, March 19, 2025 at 6:44:47 AM UTC-6 Quentin Anciaux wrote: > > Well guess I have to use my fingers for you... you know any decent email > client has a search function: > > > AG, your statement "density can't diverge unless volume goes to zero" > assumes a finite volume, which doesn’t apply in an infinite universe. In an > infinite universe, density can increase indefinitely everywhere without > requiring a total volume to shrink. > > > Brent is correct that the observable universe (the region we can see) > shrinks as we go back in time, but that doesn’t mean the entire universe > (including the unobservable part) does the same. The observable universe is > just a region within an infinite space, and as we go back in time, the > light cone that defines what we can observe gets smaller. > > If the entire universe is infinite, its total volume remains infinite at > all times—but its density can still increase without bound. There’s no > contradiction. > > > If average galactic distances decrease as we go back in time, the density > increases locally everywhere without limit in the unobservable region. > Won't this be a singularity of infinite density everywhere as T decreases > to zero? AG > > > Quentin > > This is *one* of numerous answers given. > > All those moments will be lost in time, like tears in rain. (Roy > Batty/Rutger Hauer) > Le mer. 19 mars 2025, 12:56, Alan Grayson <agrays...@gmail.com> a écrit : > > > > On Wednesday, March 19, 2025 at 5:40:48 AM UTC-6 John Clark wrote: > > On Wed, Mar 19, 2025 at 4:30 AM Alan Grayson <agrays...@gmail.com> wrote: > > *> If the universe is infinite in spatial extent, and we run the clock > backward, is all the mass/energy of the observable region confined to a > tiny or zero volume?* > > > *The short answer is nobody knows what will happen if you run the clock > back to zero, and the mystery remains regardless of if the universe is > finite or infinite. Nobody knows what will happen when things get super > small because our two best physical theories, Quantum Mechanics and General > Relativity, disagree with each other. Most believe that something will > prevent a zero volume from ever occurring, but nobody knows what that > "something" is. * > > *John K Clark See what's on my new list at Extropolis > <https://groups.google.com/g/extropolis>* > > > Maybe it's a 5th force. What I'd like to know is this; assuming an > infinite spatial universe and that it gets very very small as we run the > clock backward, the observable regions shrinks, but what happens to the > unobservable region? Quentin claimed to have an answer, but I can't recall > what it was. 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