How big is our Galaxy.

Unless I've forgotten something, but the observable universe is roughly 78 billion light years across.I know you could shrink it in terms of radius and not have cataclysmic stuff happen, but I'm just thinking that that level concentration would effect filament formation.I could be missing something here.This message has been edited by Son Goku, 01-14-2006 08:28 AM

Since the furthest objects we can see are around 13 billion light years away, the diameter of the observable universe is around 26 billion light years. It gets at least 2 light years larger every year, more if we improve our observational capabilities.--Percy

The furthest objects are emitting light thats 13 billion years old, but has been carried by the expansion, so that it has covered ~78 billion light years.I'll get a paper on it, if anybody is sceptical.This message has been edited by Son Goku, 01-14-2006 08:53 AM

Oh, I see what you're getting at. The size of the observable universe *is* about 26 billion light years. The 78 billion light year figure is for the current size of what can now see of the universe, which is much larger since it has continued to expand through the past 13.7 billion years (the estimated age of the universe). Of course, the current universe isn't observable.--PercyThis message has been edited by Percy, 01-14-2006 09:26 AM

Again no, the expansion of the universe does not accelerate the propagation of light, it increases the distance between points in space. The light we see from 13 billion years away comes from a point that was only 2 billion years away when the light left. It took the light 13 billion years to cover the distance because the distance kept increasing as it went. The location the light was from also kept receding, at what is from our point of view an ever-increasing speed. But no more light will ever reach us from that location, it is now 65 billion light-years beyond the visible light horizon. Again, from our point of view anything beyond 13.7 billion light years away is receding from us at a speed faster than light. Absolutely nothing can be observed as moving towards anything else at a speed faster than light.And that's all assuming expansion, inflation, and the red-shift and cosmic 4k signal that prove them are reliable. Percy is right that the observable universe is 26-28 billion light years in diameter, with the observer as the center.

I'm not so sure about this. I think our observable horizon is larger than 13.7 billion light years, and that the reason we won't be able to see anything further away, no matter how good our observational equipment becomes, is because that would predate the big bang. It would be a remarkable coincidence if right now just happened to be the point in the universe's lifetime where the expansion constant happened to place the limit of observability precisely at the big bang.I'd have to look up how far away the furthest observed object actually is, but I know we're approaching that period a couple hundred million years after the big bang when the universe first became transparent to electromagnetic radiation.--Percy

No sir, a billion years ago the limit of observability would have been a little more than a billion years closer. Furthermore, light would have stopped being able to reach us from those points between then and now, it would have red-shifted out of existence.Saying that it is improbable that the observable universe would be the same size as its age is really still begging the whole spacetime question.

This is one of those issues that hurts my head, there is a nice pop article of it hereI'm sure herr Goku will have a better paper to read though!

No, pretty much spot on. Though the 2 billion lyrs figure is rather inflation dependent. If we could see beyond last scattering right back to pre-inflation, the initial distance would be zero.

We couldn't observe that light though, because it would have already traveled past us in 0 seconds. Just a smidgin more than 0 seconds later though, those same points would have been say 2 billion light years away. The scream they emitted at that point is the cmb, which has been propagated at us from the visible light horizon from that moment since, that one-signal red-shifted across the entire observable universe.

Iblis is right... the cosmological horizon is precisely that which represents infinite red shift (recession at c) and the big bang. It's not a coincidence.

Well, what about photons (not that there were any photons) facing away from us That's right, though as I said, that is an inflationary based figure, and is certainly not known with any kind of certainty, especially as we now have lambda screwing up all of the old calculations. No, the cmb came much later... around 270,000 yrs later. It is the image of the surface of last scattering... the point where the universe first became transparent.This message has been edited by cavediver, 01-14-2006 10:46 AM

Hi SG. Don't confuse the obs universe with "now", the comoving hypersurface we ride upon. The 78 billion lyrs is the size of the universe on that surface, though is obviously highly lambda dependent, so we don't really have a clue. Yes, I'm sure you're right. Though of course we could be "small" and multiply connected...

Hi Iblis,I think you may have misinterpreted what I was saying. I agree. My previous reply to Son Goku said pretty much the same thing when I said that the diameter of the observable universe increases annually by 2 light years. I don't agree with this if you're saying that objects at the limit of observability a billion years ago are no longer visible to us today. It's easy to see why this couldn't be so. A billion years ago we would have been able to see objects that were 12.5 billion years old. Now it's a billion years later, and we can still see these objects that are now 13.5 billion years old. Of course. I think when I said "limit of observability" it was confusing. I should have said "theoretical limit of observability". What I meant was that the expansion rate of the universe has not carried objects older than 13.7 billion years out of our theoretical observability horizon. The reason we can't see anything older than 13.7 billion years isn't because of the observability horizon, but simply because nothing existed before that time.--Percy

I'm not understanding that statement properly I don't think. I thought the scattering was 13.4 billion light years out and we could tell where it comes from, it represents distinct bodies of density. The cmb is an almost perfectly homogenous signal at 4 degrees kelvin from nowhere in particular, but theoretically 13.7 billion light years away as that is when inflation would have ended and normal expansion begun.I am pretty sure the homogenity of the cmb is supposed to prove inflation, you can't prove something that you happened more than 200 million years after as result of decrease in dust.