Quote:
Originally Posted by CraigS
Ok;
However, if there was originally a point, a singularity of infinite density, mass, etc, etc, which I think is the main message of the BB, then I think by definition, this is the point of origin (and hence there is a trajectory from thereon).
|
No. Yes, there was a point of infinite density and zero size in the traditional theory....a singularity if you will. However, there is still no trajectory because that point of infinite density and zero size occurred everywhere.
All of
space and
time expanded out of that condition, so in effect
everywhere was experiencing that condition. There was nothing outside of that "everywhere" which could be defined as a destination point and no vector to define a trajectory, so that renders your idea of a trajectory moot.
Quote:
Originally Posted by CraigS
Not trusting myself on this I just checked one of my favourite books (although, probably just as dodgy as my DVD sources - Brian Greene's 'Fabric of the Cosmos'). He states that one alternative possibility of the BB "is that if the universe is spatially infinite, there was already an infinite spatial expanse at the moment of the big bang. At this moment the energy density soared, large temperatures (etc) ... but these extreme conditions existed everywhere, not just at one point ... instead the BB eruption took place everywhere on the infinite spatial expanse. After the bang, space swelled but its overall size didn't increase since something already infinite can't get any bigger. What did increase are the separations between objects like galaxies (once they formed)."
So, from this perspective (which seems to be an alternative spin on the traditional BB), the term 'trajectory' would have no meaning since there is no single point of origin - my problem seems to disappear (as per renormalised's comments).
|
That scenario would only work if the universe was defined as an M-brane and the BB was caused by the collision between two adjacent branes. Though, even in this case, the two branes would have to collide perfectly perpendicular to one another, otherwise you would have a ripple effect in the resultant energy distribution in the universe as the BB occurred in different parts of the branes at different times. That would show up in the CMB much more clearly and unambiguously in the temp fluctuations than what they find. For instance, it would appear to be colder in the direction away from the observer in the direction of travel of the BB ripple, but warmer when looking towards the point of contact between the two branes. In effect, you would have a temp gradient across the universe. This is not supported by the present or past observations that have been made.
Brian is also having two bites a the cherry here. He is saying, in his hypothesis, that space swelled but didn't increase in size because it was already infinite. That's contradictory. Yes, the space between the galaxies increased. However, if you have an infinite sized container with galaxies spread throughout it, the only way to increase the distance between the galaxies is to move them through space, not with space, as space can't get any bigger. In this case, it's not space which is expanding, it's the galaxies themselves which are moving apart. Now, if you look at that, there's only two ways of explaining this. The universe is not infinite but is expanding into an infinite space of something, or, the universe is infinite, but it's contents came from a finite sized source. In that case, there was a pre-existing space into which the contents are moving out into. If it's contents are finite in dimension, then the BB can't have occurred everywhere in spacetime, which by definition, is infinite in this case. The BB would be occurring at a point in spacetime and would not be an expansion of spacetime itself. That would induce a vector to the movement of the galaxies, but in order to produce the CMB results we see, that movement would have to be incredibly smooth in all directions. Which would be far more difficult to achieve in an actual "explosive" event of just matter into a pre-existing space than of an explosively rapid expansion of spacetime and matter itself.
Quote:
Originally Posted by CraigS
It seems that my original question, perhaps, was born from my naivity of understanding of the rubbery state of the BB Theory (already known by Alex). It seems that its not as clear-cut as I thought - ie: that it didn't necessarily all start from a single point.
Which is OK. I can live with a "we're not really sure" answer. Actually, that's quite cool - and an honest answer.
Cheers
|
All theories are rubbery, to some extent. They're only ever an approximation of what is observable and in most cases you can never really know everything there is to observe. Something will always turn up to upset the apple cart
