[mfulcher (OP)]

?why do we see the early universe in every direction we look in the night sky, if it was much smaller then and we have been moving away from it ever since?

[imatfaal]

This is horrible counter-intuitive, but we have been moving away from it,  and it from us, in every single direction. 

Analogy time...
Imagine a single cherry (that's us) amongst loads of raisins (they're the other galaxies) in some well-blended cake mix.  When you bake your cake the mixture rises (that's the universes expansion/inflation) - and every single raisin ends up further away from the cherry than it started.   That's exactly what we experience - it is a mistake to envisage an explosion (they have centres) - think of the cake mix, or of the dots on the surface of a balloon; everything gets further away from everything else.

We see the same picture in every direction, because the big bang happened everywhere.  The light we see is the last remnants of the hot universe (the era of last scattering) from about 400,000 years after the big bang.  It has been travelling from that time for the last 13ish billion years to reach us.  Why it has taken so long is a question that requires the continuous "dark energy" universal expansion to explain.

[Bill S]

Good answer, imatfaal, but there is another facet to this as well, if the light from some object has taken about 13 billion years to reach us, that object must have been 13 billion light years away from us 13 billion years ago.  Surely, this must mean that the Universe was, at the very least, 26 billion L Y across 13 billion years ago.

I think this was mentioned elsewhere, but it may have been on a different forum.

[yor_on]

Every raisin on imatfaal's cake will see the same stretch of space in every direction, and as there is nowhere in space that is the 'center' you are free to define every raisin as that center. Space is isotropic, meaning that it have no differentiating aspects in any direction, and there is no certain 'spot' from where we can say that it 'came'. The BB came not inside a fixed space, it became the space.

[Geezer]

Every raisin on imatfaal's cake will see the same stretch of space in every direction, and as there is nowhere in space that is the 'center' you are free to define every raisin as that center. Space is isotropic, meaning that it have no differentiating aspects in any direction, and there is no certain 'spot' from where we can say that it 'came'. The BB came not inside a fixed space, it became the space.

Maybe a piccy would help  []

[mfulcher (OP)]

so are you saying it's the space itself which has expanded, not that the galaxies were fired apart in a kinetic way?!?  if so would it follow from this that galaxies themselves are being stretched by the expansion of the universe? - always more questions!

[Bill S]

if so would it follow from this that galaxies themselves are being stretched by the expansion of the universe? - always more questions

No, because on a galactic scale gravity overrides expansion and holds galaxies and groups together.  At least, I think that's how it works.  []

[syhprum]

|f two galaxies are held together by their own gravity but the space between them expands does this not lead to an increase in the potential energy due to the attraction between them contrary to the energy conservation law ?

[yor_on]

The universe have a balance, at least it seems so. Assume it to have a finite size. Then the question is relevant if we by that mean that, even though still balanced the same way, it now will contain more of whatever 'energy' is seen as. If the universe don't have a 'size' the question will be meaningless though as the only thing that defines a before and a after will be the balance inside it, and if that doesn't change then the universe will be 'as always'.

And thinking of Lorentz contraction?
Is distance really 'there'?

It is for us, I agree, but so is the expansion.

[imatfaal]

Good answer, imatfaal, but there is another facet to this as well, if the light from some object has taken about 13 billion years to reach us, that object must have been 13 billion light years away from us 13 billion years ago.  Surely, this must mean that the Universe was, at the very least, 26 billion L Y across 13 billion years ago.

I think this was mentioned elsewhere, but it may have been on a different forum.

No - I think you asked it here, and I thought I answered it - probably just deja vu on my part.  Your assertion is only correct if the universe is static.  It is not static; it is expanding.  It is like running up a down-escaltor; eventually you will make it to the top, but it will take you a lot longer than if you ran up the stairs.  That analogy is bad, I still like it, but it has a flaw - ie the reason that progress is slow is incorrect.  It isn't that the light is impeded or slowed; but that the space in between us and where the light came from has, and is still getting bigger. 

Make 10 marks on an elastic band say 5mm apart when the band is slack - how many seconds at 10 mm/s does it take to cross the number of marks you made.  Now stretch the elastic band and recalculate BUT you must still use 10mm/s not so many marks/sec.  When the light that is now the CMBR set off it might only of had ten marks to cross - but the time to cross those marks might be grossly more than if space had remained static.

The "sphere" that makes up the CMBR that we now can detect was smaller when it emitted the radiation than it is "now."  The numbers are mind-blowing and will probably make you even more sure that I am wrong

The microwave light that we now detect was emitted by hot matter 40 million LY away from us. The light has travelled for c.13.7 billion years to reach us, and that emitting matter is now about 46 billion LY from the earth.

Don't say I didn't warn you - cosmology is too big for the human mind.  As a comfort - remember this was predicted and then observation matched the theory; so it isn't people massaging uncomfortable data to fit a pet theorem.

[imatfaal]

|f two galaxies are held together by their own gravity but the space between them expands does this not lead to an increase in the potential energy due to the attraction between them contrary to the energy conservation law ?

Energy conservation in cosmology is tough - you need to balance over entire system and include the energy of expansion and vacuum energy.  In the end it comes to zero! It's late on a friday and I had a liquid lunch so I can't explain now.  But if you are still interested I will give it a stab when a bit less fuzzy headed

[Bill S]

No - I think you asked it here, and I thought I answered it - probably just deja vu on my part.

I think it's just that it was in another thread, if we both remember it, it must have been somewhere.  []

Anyway, thanks for the answer, I think I'm slowly getting my head round this. 

[Bill S]

But if you are still interested I will give it a stab when a bit less fuzzy headed

Some of us get fuzzy headed just thinking about this stuff.  []