Big Bang

July 2, 2004 8:06 AM

Okay all you astronomers, I just asked this is an older MeFi thread but figured I might get a better response here.

Go back to the big bang and you start off with a single point that exploded and has kept on expanding. So when you look back in time by way of observing distant places would you not end up looking at exactly the same place (the point) no matter which direction you looked?
posted by zeoslap to science & nature (13 comments total)

IANAA, but as I understand it, in the microseconds that followed the "big bang", most of what we know understand as dimensions was much more scrambled and inchoate, without the structured dimensional space we're now used to. By the time there was an "inside" to be in, and an "outer edge" to look at, it really wasn't a single point any more. I think the idea of a "single point" means that it really couldn't be measured on any dimension, not really implying that it was like a teeny-tiny Hoberman sphere.
posted by LairBob at 8:22 AM on July 2, 2004

Not quite. The universe is not expanding into empty space, space itself is expanding. It isn't an entirely correct analogy, but imagine a balloon inflating. There's no centre of expansion or point of origin on the surface of the balloon, but the surface area of the balloon gets larger as it inflates.
posted by bonehead at 8:24 AM on July 2, 2004

would you not end up looking at exactly the same place (the point) no matter which direction you looked?

This is why background microwave radiation seems to be coming from all directions, isn't it?
posted by gimonca at 8:27 AM on July 2, 2004

IANAA, but I am a theoretical physicist who's been known to think about such things from time to time.

In practical terms, no. The early Universe was significantly hotter and denser than the Universe we see now, so much so that it was essentially filled with dissociated electrons & protons (i.e. plasma) instead of neutral atoms. Light doesn't propagate very well in a plasma, so when we look "back in time" we can't see any point in time before this plasma cooled off and turned into hydrogen, helium & trace amounts of heavier elements. This "last scattering" of the photons in the early Universe was what the WMAP satellite was designed to study; you might have heard about its first results being released about this time last year.

Now if this last scattering surface didn't exist, then I think the answer is still no, but I'm going to have to try to think of a good way to explain it without bringing differential geometry into the equation.
posted by Johnny Assay at 8:31 AM on July 2, 2004

It is my understanding that when you look 'back in time' the furthest back you can see is the background radiation. This is not all the way to the 'big bang', it is to this side of the inflation which occured in the first second of the universes existence.
There are several models for the expansion of the universe, which are explained quite well here.
It certainly is a challenge for me to explain these fundamental building blocks of my understanding of the universe. I should do it more often.
on preview; must learn to type more quickly!
posted by asok at 8:34 AM on July 2, 2004

Thanks for the Encarta link asok and the radiation info Johnny. So do we(that's the royal we) know where the center of the Universe is? Are all things expanding equally with regards to a single point? With an ever expanding Universe do you end up with just a big ball of nothingness in the middle and all the 'stuff' at the edges ?
posted by zeoslap at 8:53 AM on July 2, 2004

Zeoslap:

There is no centre.

There is no single point at which you can stand and have everything expand away from you at the same rate.

There are no middle or edges.

Some more technical explanations can be found here and here.

Your mental model is wrong. The universe did not start a a single point in space then get bigger. The universe did not start as an acorn of matter in an existing space. The extent of the universe is part of the universe. Space itself, the distances between stuff, is getting bigger.

According to most theories, at the big bang the distance between any two points was very small. Since that time, space has expanded between those two point enormously, but space has also expanded between any other two points (whether or not space has expanded uniformly is an open question). In three dimentional space, there appears to be no common point that everything has expanded from.
posted by bonehead at 9:30 AM on July 2, 2004

Upon posting, I'm sorry if that came off rudely. I certainly didn't mean it that way. This isn't very intuitive and most people have a hard time getting their head around it the first time.
posted by bonehead at 9:33 AM on July 2, 2004

No worries bonehead, you're right my model was wrong and that second link made more sense. I'd pictured us inside the ball as opposed to being on the surface.
posted by zeoslap at 9:50 AM on July 2, 2004

I was gonna write something up but some quick googling found an answer online that summarizes my understanding of the situation:

Question:
I saw a story on WMAP that featured images of the Universe when it was only 380,000 years old. If the Universe is 13.7 billion years old, why did it take so long for this light to catch up with us? How did our our galaxy outrun the light that was emitted a mere 380,000 years after the Big Bang?

Answer:
You are indeed touching upon a very perplexing situation. At an age of 380,000 years (after the Big Bang) the "observable universe", i.e. the entire volume of the Universe that we can see in electromagnetic waves (light, microwaves, radio, etc.), is so incredibly small (with a radius of only 380,000 light years, give or take a few, depending on how much the Universe is accelerating). Yet it took the microwaves of that afterglow originating from its outskirts 13.7 billion years to reach us.

The problem is that the entire Universe has been and still is expanding at an incredible rate. So, what was a miniscule distance of 380,000 light years back then turned into the enormous distance of 13.7 billion light years as of today. The light from the afterglow, on its way, had to cross that ever-expanding space and was literally "stretched" along with that expansion. Its wavelength increased from that of visible light to that of microwaves, which we see today.

To imagine this situation think of the Universe as of a gigantic - maybe even infinitely large - dough of raisin bread. Scale your imagination down to a finite piece of dough, where you are sitting on a raisin in its center. The outer edge of the dough is equivalent to the edge of the "observable universe", from where we see the cosmic microwave background radiation. You have put a lot of yeast into your raisin bread, and you have set up microwave transmitters all around on the outer edge, blasting inward. Imagine that in reaction to this heating the dough rises at such a furious rate that the outer edge recedes from the center at almost the speed of light. Although the microwave transmitters were only at a relatively short distance at the beginning of this "gedankenexperiment" (thought experiment), the microwaves have to cross a much larger (continually growing) distance because of the ongoing expansion.

I would like to add a cautionary note, in case you might be wondering. Don't get confused! This expansion does not violate Einstein's theory of relativity, even though the imaginary dough of the even larger Universe, which we can't see beyond that edge, appears to recede at speeds larger than the speed of light. The dough represents space itself, and in our expanding Universe space itself is expanding, carrying the galaxies (represented by the raisins) along on a ride. Einstein's limit to the speed of light applies only to motion through space, and not to expansion of space itself.

Also, bonehead is right, the universe has no center. (actually, it may turn out to have a center in a more complex topological higher dimensional space, but thats a digression)
posted by vacapinta at 9:52 AM on July 2, 2004

Vacapinta, you're entirely correct, but I didn't want to over complicate things.

To digress, as I mentioned above, the universe is like the surface of a milti-dimensional balloon (how many dimensions depends on which cosmologist you're speaking to and what they had for lunch that day), except instead of a smooth balloon, it's got all kinds of creases and through-holes (maybe). In short, we live in a 12(?)-D parade float that's still being blown up.
posted by bonehead at 10:35 AM on July 2, 2004

IANAA, but I am a theoretical physicist

Only on Metafilter. That's going down as one of my favourite responses ever.
posted by humuhumu at 2:09 PM on July 2, 2004

Getting the wrong notions on this results simply from the term "big bang". We uninitiates then tend to think of it like a giant firework explosion, where all the matter flies out from the center explosion.

What a great thread, cosmology is a favorite subject of mine. A pity I didn't learn of it 'til I was too old to take up the mathematics needed to pursue it with rigor.
posted by Goofyy at 2:47 AM on July 3, 2004