Adding my--- Whoops, has Anyone Seen My Drachma? (Lk15:8)
kalle437 said:
If all matter in this universe was together, wouldn't that create big enough gravity to never let that matter go? Or would it create some kind of implotion? Or how could Big Bang be?
I think your question is: if the gravitational self-attraction of a configuration of matter is (all other things being equal) stronger when this configuration has a larger density, and if the Big Bang model says that the average density of matter in the universe was once much denser than it is today, how could the Hubble expansion "get started"?
I think the best short answer is that modern cosmology has yielded, by combining observation with theory, a picture of the gross features of the history of our universe, back to a certain point in time when indeed the average density of matter in the universe was much larger than it is today. In particular, fairly simple theoretical models constructed in the context of general relativity (exact perfect fluid solutions with nonzero Lambda) reproduce very nicely the most large scale features as determined by astronomical observation (in the context of our other physical theories). In these models, contrary to your expectation, the Einstein field equation does not in fact prevent the expansion, as you expected, even though the density and curvature increases without bound as "time goes to zero". To discuss why not, with all the mathematical detail, I think you would need to read a good textbook such as D'Inverno,
Understanding Einstein's Relativity, and then post a more sophisticated question. Otherwise you should probably just take my word for it
Seele appears to be referring to some other issues involving inflation, fluctuations, and perhaps the "horizon problem", one of the problems with the original Big Bang scenario which was "solved" by the introduction of the "inflationary scenario". It is now widely acknowledged that inflation has raised more questions than it answered, but this is not cause for dismay since that is how things usually work in science! Be this as it may, I wish it were possible to easily convey to students and to the general public a better sense of what claims about modern cosmology they may see in the popular science literature are better established and better understood than others.
Perhaps the most difficult thing of all to convey is the fact that our understanding of cosmology is based upon an intricate network of reasoning which combines a vast number of observations; in a sense, it is this network of observations and the theoretical glue holding it all together which forms the substance of cosmology. New observations can add new items of information to this network, and advances in theory can establish new theoretical connections between different observations or change our interpretation of observations, but these changes, even "revolutionary" ones, would be better said to
enrich rather than
invalidate previous work!
So when almost overnight observations from two groups resulted in the acceptance of the suggestion that a nonzero Lambda term must be added to our crudest cosmological models (the FRW models), which radically changes the long term behavior of those models and some other aspects of our understanding, this change was in another sense not as "radical" as some members of the public appear to believe. That is, some amateur critics have been heard to complain, "if cosmologists got Lambda wrong, why should we take their word that Hubble expansion is not in doubt?" But this type of criticism reflects a serious misunderstanding of the nature of structure of modern cosmology, even of science itself.
