GunnerJ

My dad recently sent an e-mail to me asking:

In answered with something about how spacetime itself is moving along with the galaxies, creating a warp effect. Is this remotely correct? If so, does it solve the problem? If not to either, what's the real deal?

Shadowy Man

There are a couple of problems with his viewpoint. One is the idea that the material in the universe was "centrally located". He seems to be implying that there was a lot of matter in one point in space and things are expanding out from there. This is a common misperception of the Big Bang theory.

The universe could be very much larger than what we see - we refer to the "visible" universe; i.e. we have a horizon problem generated by the finite "age" of the universe. Since light travels at a finite speed we can only see as far back as the available time for light to travel allows. So, if the universe is 12 billion years old, we can't see farther than 12 billion light years away. That doesn't mean there isn't material farther away than that. The other part of the horizon problem is that we are looking backward in time as well.

Another problem that arose was that the cosmic microwave background, which emanates from the time of last scattering, the furthest back in time that we can actually "see" (earlier than that the universe was opaque), is incredibly smooth, indicating that virtually the entire universe was in thermodynamic equilibrium at that time. It seems like that should not be possible because there wouldn't have been enough time for the universe to thermalize equally. This is one thing that led to the generation of the inflationary theory.

Stephen T-B

Shadowy Man, be very kind and please explain this: “He seems to be implying that there was a lot of matter in one point in space and things are expanding out from there. This is a common misperception of the Big Bang theory.”
Thanks

Wyz_sub10

Rimstalker,

Let me just say that your dad is cool. I wish my folks had these curiosities.

Jesse

Stephen T-B:
Shadowy Man, be very kind and please explain this: “He seems to be implying that there was a lot of matter in one point in space and things are expanding out from there. This is a common misperception of the Big Bang theory.”
Thanks

A common analogy is that of a balloon with a bunch of dots painted on it representing galaxies--as you blow up the balloon, all of them get farther apart from each other, but there is no point on the 2-D surface that is the "center" of the expansion. Similarly, the idea of the big bang theory is that our 3-D space is expanding without any "center" in the space itself--you could imagine a center in a 4-D hyperspace (just like the center of a 2-D balloon's surface would be in the 3-D space it sits in), but for mathematical reasons you don't actually need a higher-dimensional space for a curved lower-dimensional space to "sit in." I had a long discussion about these ideas a while ago on this ARN thread, you might want to take a look at it...there are also some useful links to other pages which talk about this issue there.

Shadowy Man

Jesse:

Another analogy used is that of raisin bread in an oven.

When you first put the bread in, the raisins are close together, but as the bread rises and expands, the raisins get farther apart from each other. However, it's not like they started close together in a bread of fixed sized and moved through the bread while in the oven. It was the bread that expanded.

Make sense Stephen T-B?


Now all you have to do is imagine an infinitely large loaf.

GunnerJ

Thanks, shadowyman and jesse. Jesse, that analogy is the exact same one I used.

Tim Thompson

Unfortunately, general relativity is such a beast that "distance" just doesn't cut it. You have to be more specific about which kind of distance you want to talk about. With that in mind, I recommend Ned Wright's ABC's of Distance (which really doesn't quite get across what I am thinking of), and perhaps even more interesting, his javascript Cosmology Calculator. On the calculator, you can input a small cosmological model, and it will calculate several "distances" for you. the one you will most commonly see in print is the one he calls "light travel time"

So, "12 billion light years away" means it took light 12 billion years to get here, but that is not the distance you would get if you laid out a bunch of rulers along the path the light took.

The recession of the galaxies is caused by the expansion of space-time. It is not that the galaxies are speeding away from each other by moving through space, but that space carries the galaxies along for the ride as it expands. Edit that the read "space-time" where I wrote "space" (which is easier for most people to understand), and you've got the basic picture of the expanding universe.

Cheers.

Nickle

Jesse, you have some great posts there. Too bad Quicksilver
couldn't grasp any of it.

Stephen T-B

I don’t mean to try your patience, good people, but I’m still confused.
From http://www.damtp.cam.ac.uk/user/gr/public/cs_phase.html
“As we move backwards in time towards the moment of creation, prior to one hundredth of a second, the universe becomes hotter and denser...”
So how dense?
Does this mean a concentration of matter in a small area, or a spread of dense matter in a vast one? Or is the concept of “matter” occupying any sort of “space” in the first moments of the Big Bang erroneous?