h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

In astronomy, and physics in general, it is accepted that the further away you peer into the universe, the further back in time you are looking.
There then usually follows some comment on the hubble ultra deep field and something about how we are seeing further and further back.

Last night, on The Story of Science (BBC proggy taking a historical and contextual approach) they did a cgi zoom into the universe going back in time and overlayed that at the end with a 'big bang' type graphic.

Now, it strikes me that it is impossible for us to 'see' back to the big bang itself, unless a couple of things hold true.
This is, obviously, purely a thought experiment - there were no stars - heck, no photons, back at the absolute start. But anyway...

If we take two points in the expanding universe which are, relative to each other, moving apart and then consider what happens to the photons traveling from them, unless the expansion is greater than the speed of light then the early on photons will make the journey pretty quickly. It is only as the distance between the points becomes greater that light takes a significant time to travel between them and thus you have the 'looking into the past' effect.

So, assuming a constant light speed, this would mean that there is a practical limit to how far back in time we can see because beyond that time any two points would be close enough together for the light from one to have already arrived at the other.

So what am I forgetting and where am I going wrong?

Interesting thought, Ictoan

But then - you say >>... unless the expansion is greater than the speed of light ...<<
Haven't I read somewhere that at a pretty early stage, the puff of smoke actually was expanding faster?
I'm sure I have [I remember thinking "well, it's happened once, so..."], but is that still accepted?

yes, inflation it is termed. Seems to fairly well accepted, several of its predictions have been confirmed.
Although not sure what velocity is involved, if that even makes sense.

In an early, small universe we might imagine a photon travelling around the universe several times. That wouldn't stop it from teaching us about the universe it was produced in.

However, there is a practical problem: The early universe was very hot and busy, so photons wouldn't have survived that long. What we get is the Cosmic Microwave Background radiation produced as the universe was starting to settle down, after a few hundred thousand years.

Something else to note is that according to inflationary theory the universe did indeed expand faster than the speed of light for its first fraction of a second.

Excellent question!

Sadly one I'm in no position to begin to answer.


But I await expert physicist's replies with interest

OK, let's see. I do know a bit about this.

The expansion of the universe was (and probably still is) faster than light. That's not a violation of Einstein's theories, because it was space itself that was expanding, not the things in it moving apart.

And if we look back far enough in any direction, we do in fact see the big bang, although the stretching of space has reduced that intense flash to a mere 3 degree microwave glow.

Something else which might be relevant: The big bang did not happen at a single point and expand from there - it happened at every point in the universe simultaneously, as they were all the one point.

"And if we look back far enough in any direction, we do in fact see the big bang, although the stretching of space has reduced that intense flash to a mere 3 degree microwave glow."

But the CMB at 3degs is what we see of the big bang *now*. If we looked back in time, it should be hotter should it not?

"The big bang did not happen at a single point and expand from there - it happened at every point in the universe simultaneously, as they were all the one point"
Exactly. So when every point was a lot closer to every other point, it will have taken light a lot less time to get from A to B and thus we won't be able to see that light, no matter how closely we look because it has already arrived (and perhaps, passed) our point in space?

One thing I am now confusing myself on. If the universe is (or has) expanded faster than light, then light from point A could not reach point B (during that period of expansion) - correct?
I.e. if spacetime expands, does that mean that what was once 1km is now 2km? Or is it, to us inside the system, still 1km?

Still 1km.

so then the expansion of the universe in terms of spacetime is irrelevant to the consideration, and it is only the expansion of the universe in terms of the material within it that needs to be considered?

But the expansion of the Universe IS the expansion of space/time and is the reason that the light from some parts of the universe have not reached us yet and some never will.

The speed of light is an absolute limit in the Universe but the fabric of the Universe itself is expanding and one part can be moving away from another distant part at greater than the speed of light.

In the early Universe light could not move far without being absorbed and re-emitted until around 400,000 year. Some pages to look at are here: -

http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang#Early_universe
http://en.wikipedia.org/wiki/Graphical_timeline_of_the_Big_Bang

This has nothing to do with expansion but I thought it was great - http://www.atlasoftheuniverse.com/index.html

It might also be helpful to listen to this Astronomy Cast podcast from Fraser Cain and Pamela Gay - http://www.astronomycast.com/astronomy/cosmology/ep-178-mysteries-of-the-universe-part-1/ . They discuss the expansion of the Universe, recession velocities and so on.

Documentary makers love to depict the Big Bang as a huge explosion but there is no way we can say (at the moment) what it might have looked like because there is no way we will ever see it as we can't look back before about 400,000 after the event.

t.

thanks for that, shall go read your links. Interesting on the 400k year limit as well

Gnomon: "The expansion of the universe was (and probably still is) faster than light."

I thought it was only during inflation that that could be said.