Simetra7 said:
... "We" was probably a poor choice of words. I meant the experts in the field. My understanding of cosmology is limited but i am fascinated by what i have read so far and am keen to learn more. I would be very interested in reading the article that you spoke of. Thanks.
"We" is a fine choice---meaning mainstream working astronomers and cosmologists. Let's include ourselves, as long as we are trying to understand the mainstream consensus of experts. I regret having sounded persnickety at that point.
Did you have a look at the Sci Am article---Lineweaver and Davis? If it was not right for you let us know and I or someone can look for other stuff about the current picture in cosmology.
right now in this thread we are getting some trouble because of a very common misconception----a lot of people don't understand the big bang set of models and think that any big bang picture has to be spatially finite. that just isn't so. From the very start----going back I think to the NINETEEN TWENTIES----people were proposing and studying expanding universe models which were spatially infinite and which started off with a singular hypersurface of infinite spatial extent
the singularity in the model has always been considered possible to be either infinite in extent or finite.
Nick (Tiger) who is a gradstudent going on to be a professional can correct me if I am wrong but this is pretty standard.
the popular misconception is that expansion has to start from a point, which it doesnt, mathematically an infinite thing can start expanding----so this is a "FAQ" type of thing we encounter over and over again at messageboards like PF. I think it comes from several reasons
1. the famous physicist John Wheeler CALLED it onset of expansion by the NAME of "big bang". He loves colorful terminology and is very imaginative. I think he also invented the term "black hole".
After a good name crops up it will often take off on its own and develop its own signif. in the public imagination.
2. people find it hard to imagine a "bang" that happens in an infinite region, they picture some comicbook explosion all emanating from one point. It is a verbal thing where the original mathematical idea gets lost and a verbal preconception dominates.
3. another verbal trouble comes from the word "singularity". In mathematics a singularity can occur at an infinite set of points. It means a PLACE WHERE THE MODEL BREAKS DOWN and fails to compute, often because it starts giving infinite or otherwise unreasonable answers, or no answers.
the classical 1915 Einstein model fails to compute right at the beginning of expansion, so that is called a singularity. It also fails to compute right around the center of a black hole.
but singularity does not mean "single point". Nobody who knows what they are talking about ever said that universe expansion has to begin at a single point, or even at a finite set of points. IT COULD but that hasnt been settled yet. It could also very well begin at an infinite set.
even a black hole singularity does not have to be a single point, in some models it is a ring (I find this hard to picture but mention it only because it is just a common truism that singularties do not in general consist of single points. It is only the WORD that gives people that idea.)
instead of meaning "single point" a singularity is an "oddity" or "peculiarity" that happens when you push a model to the limit of where it applies and it breaks down.
in some proposed new cosmologies (e.g. Loop Quantum Cosmology) the big bang singularity is smoothed out and spacetime evolution extends back to an earlier contracting phase. Eventually something like this will probably become mainstream----some improved theory which does NOT fail at the beginning of expansion will check out and be accepted by cosmologists and will replace the classical 1915 Einstein based cosmology.
then the singularity (or failure) in the classical picture will be FIXED. this has happened in other situations---improved theories (like of the atom) eliminate singularities in older classical theories.
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so don't be misled by the words "bang" and "singularity" into thinking that the mathematical model necessarily begins at a point or has to be spatially finite.
this is actually one of the most interesting questions in big bang cosmology!
there is a number Omega, which can be MEASURED, and for which the measured value is around 1.01 or 1.005 plus/minus something.
and the uncertainty is such that it could easily be exactly 1.000, in which case the universe might actually be of infinite spatial extent! and on a very large scale UNCURVED.
but if this number is slightly more than 1, say it is 1.005, then the universe can be very huge (much bigger than what has been observed so far) but still finite.
and we really don't know, this is one of the big questions in big bang cosmology, and why they sent up satellite observatories like COBE and WMAP----to measure Omega more precisely, to shrink down the errorbounds.
a lot of mainstream cosmologists seem to think Omega is exactly one (infinite bigbang, infinite universe) I guess just because the measurement gives an answer that is so close--maybe they think "it is so close to one, why shouldn't it be exactly one?" But in all honesty the issue is still undecided and there is still an errorbar and more accurate measurements need to be made.
Hope this is helpful, and not too long winded.
