Chiclayo guy said:
The link listed by Marcus in post #4 has this to say in part:
“At this moment, all the galaxies that we see around us today were compressed into a region of zero volume - to a single point in space.”
I checked and it does NOT imply that actually was the case. You missed the qualification. What he is explaining is what is wrong with GR and why people are working on improving it.
If we simply follow the predictions of Einstein's theory of general relativity for the evolution of a simple expanding, homogeneous universe filled with matter and radiation, then our journey into the past will eventually come to an end - a point in time where we cannot go back any further. At this moment, all the galaxies that we see around us today were compressed into a region of zero volume - to a single point in space. Since density is defined as mass divided by volume, the density was infinite. In Einstein's theory, matter influences the way that the geometry of space and time is distorted, and at this moment of infinite matter density, the curvature of spacetime was infinite, as well. Within the simple cosmological models based on general relativity, there is no possibility to go to any earlier times than this. Such a boundary of time (or, more generally, of spacetime) is called a singularity...
You missed the "If we simply follow..." Do you see Chiclayo?
Farther down the page, he says:
Most cosmologists would be very surprised if it turned out that our universe really did have an infinitely dense, infinitely hot, infinitely curved beginning. Commonly, the fact that a model predicts infinite values for some physical quantity indicates that the model is too simple and fails to include some crucial aspect of the real world. In fact, we already know what the usual cosmological models fail to include: At ultra-high densities, with the whole of the observable universe squeezed into a volume much smaller than that of an atom, we would expect quantum effects to become crucially important...
In other words Einstein's original 1915 predicts infinite density, so it is probably too simple and fails because it overlooks something, namely the quantum effects that kick in at very high density. Indeed when you move to a quantum version of GR it seems likely that at very high density quantum effects cause gravity to repel. A collapsing region would be unable to get down to a volumeless point, and would rebound. Infinite density does not happen IOW.
If the OP does not mind, I have my own stupid question. Often on this forum various posters say the big bang “happened everywhere.” At other times it is described as in the above quote…happening at a single point. Is this a contradiction or am I missing something?
The universe may be spatially finite or infinite (we don't yet know which and may perhaps never) but in either case
what we can see occupies a finite volume which, if you could stop expansion to give yourself a chance to measure, would have radius 45 to 46 billion LY. The expansion ratio is estimated to have been such that this volume would have begun very small---nearly zero volume.
According to the standard cosmic model space is boundaryless, edgeless, there is no space outside of space. And this has always been the case.
This leads to the simplest best fit math model that we have so far. Any geometric properties (like curvature) and any geometric processes can only be experienced from INSIDE. By noticing that triangles don't add to 180 degrees or that distances between stationary objects increase according to some largescale average pattern. There is no "outside" space that space "expands into", and no "center" of expansion that we can point to. It turns out to be mathematically clean and convenient to model that way, the fewest extra assumptions. After all we have no evidence of a boundary or edge or center, so including them would just mess things up needlessly.
As far as we know whenever and however expansion began it began at every point in space.
There is no evidence that it began at different points at different times. (Although some people IMAGINE that may have happened, just too far away for us to know about.) So the simplest cleanest assumption is that expansion began at all points at once. That's built into the model, which has a kind of uniformity assumption. ("homogeneity")
the proof of the pudding is in the eating. The simple model (called LCDM) gives a remarkably good fit to the data! So there is a strong motive to not mess things up by adding extra features.
