# About the Big Bang Thing

1. Jul 13, 2008

### GTrax

Starting with all the universe mass squeezed into the same little start place (however intuitively improbable) , it went bang and started all those real masses moving apart from each other, at speeds quite high, but not not so high that they could exceed light-speed relative to each other.

Any information about the future fate of any part could be observed, at some time later, from any other part, (though possibly with a red shift).

When the latest and greatest telescopes are set to doing what they do, we are told that they are looking at light that emanates from the very edges of the Universe, back to a time near to when the "event" happened. Eh? I am thinking that the light associated with that cataclysm overtook the moving masses and put itself into places unavailable to us quite quickly.

So how does what we see now so readily point to a Big Bang past ?

2. Jul 13, 2008

### Fredrik

Staff Emeritus
It's not quite correct to say that the masses were squeezed into a small region of space because that suggests that there existed "more space" outside of the region with matter. In all of the solutions of Einstein's equation that describe a big bang (FLRW solutions), the entire universe is filled with matter at all times, but there's a scale factor (a function that must be included in the calculation of the spatial distance between two pieces of matter) that depends on the FLRW time coordinate and goes to zero as time goes to zero. Two of the three FLRW solutions actually describe a universe that's infinite at all times, but the distance between two arbitrary points still goes to zero as the FLRW time coordinate goes to zero.

It's definitely possible for two pieces of matter to move away from each other at speeds faster than c (because they aren't really "moving", it's just the space between them that grows). See my answers in this thread for more details.

You seem to be thinking that the big bang was an explosion at some point in space. It wasn't. It's just a name for the limit $t\rightarrow 0$. The earliest light we can see comes from the time of decoupling, i.e. from the time when the universe had expanded enough and cooled enough to allow atoms to form without immediately being smashed to pieces by a collision with something else. At that time (about 380000 years after the big bang), the universe became "transparent". It wasn't before, because light interacted too much with all those electrons what were flying around without being attached to an atom.

Everything we see agrees with general relativity. The universe is roughly homogeneous and isotropic on large scales, and solutions of Einstein's equation (the main equation of GR) that describe homogeneous and isotropic universes all predict an expansion. The galaxies we see are redshifted, indicating that they are moving away from us. The more distant they are, the more redshifted they are. We can also see the light from the time of decoupling.

3. Jul 13, 2008

### GTrax

Thanks much for providing more insight Fredrik
Yes indeed! Given the name we have for it, maybe I can be forgiven for that.

More - the red shift is called so because the shift is always toward the red. Everything is expanding, moving away from everything else. Its not to hard to see why one would (maybe simplistically) conclude that in the past (a fragile concept??) maybe the stuff was somewhat closer together.

4. Jul 13, 2008

### Fredrik

Staff Emeritus
No.

The stuff was a lot closer together. As I said, the distance between any two points goes to zero as (FLRW) time goes to zero, so the density goes to infinity in that limit. The part that's non-intuitive is that this doesn't mean that any of the "stuff" has really moved. Matter isn't moving in space (in FLRW solutions). What happens is that space itself is growing.

You are right that "the past" is a fragile concept in GR, but one of the features of the FLRW solutions is that they suggest a preferred global coordinate system that makes concepts such as "the past" well-defined. "The past" would be all events with a lesser value of the time coordinate in that coordinate system.