Universe Expanding?

[SkepticalScience]

I was wondering how scientists know that the universe is expanding.

I understand red-shifting, but doesn't it still take time for light to reach us?

So in other words, if scientists are looking at starts right now - that are 15 billion light years away and they are red shifted - how do we know that the universe hasn't stopped expanding?

We SEE the red shift, but since it would take that light 15 billion years to reach us, perhaps the universe is not currently expanding, but has in the past?

Can someone help me out with this?
Thanks!
SS

 

[CurtC]

Well, we can always look at the galaxies that are a "mere" one billion light years away, and they're also red-shifted.

 

[patnray]

To expand on what CurtC said, at every distance we find that red shift (expansion rate) is proportional to distance. Light from nearby objects is more recent, but still fits the pattern...

 

[SkepticalScience]

Hmmm. . . interesting.

But. . . you know how on a beach, before a wave comes, all the sand and water go AWAY from you first?

Why couldn't something like that be at play in the universe?

That, locally, we see stars going away from us, but everything after 10 billion light years from us are caving in. . . a super universe tsunami!

 

[drkitten]

Hmmm. . . interesting.

But. . . you know how on a beach, before a wave comes, all the sand and water go AWAY from you first?

Why couldn't something like that be at play in the universe?

That, locally, we see stars going away from us, but everything after 10 billion light years from us are caving in. . . a super universe tsunami!

Then we wouldn't see the nice proportional relationship of distance to red-shift that we see.

 

[SkepticalScience]

Why wouldn't we "see the nice proportional relationship of distance to red-shift" even if the universe was standing still or even collapsing?

It still takes time for the light to reach us . . . .

For the sake of argument, say we were the center of the known universe, and that the universe has been expanding for 100 billion years.

Then say, this year, the universe starts closing in on itself, or even slows to a halt.

So, say the star on the outer edge of the universe has stopped expanding.

We wouldn't really know that for a 100 billion years right? Cause the light from that star would take a 100 Billion Light years to reach us. . . .

So, if we were to observe the star even 1000 years from now, it would still appear red shifted.

Am I right?

 

[KingMerv00]

Here is a link:

http://www.sciam.com/article.cfm?ch...23-1213-852383414B7F0147&pageNumber=5&catID=2

I don't know if it will answer any of your questions but it sure is cool.

 

[CurtC]

For the sake of argument, say we were the center of the known universe, and that the universe has been expanding for 100 billion years.

By the way, we are, really and truly, at the center of the known universe.

The red-shift of galaxies seems to hold pretty true as you get closer to us, up to some point. The very closest galaxies don't really go by this rule because our relative motion towards the Andromeda galaxy (due to our mutual gravity) overwhelms the expansion of space in that small (2Mly) distance. I'm not sure how far you have to go to be able to reliably measure it - let's say it's 100 million light-years. So we can observe that the rule holds pretty well from the beginning 13.7 billion years ago, to as recently as .1 billion years ago. I'm not an expert, but it seems that we can pretty safely assume that the universe probably won't hit some kind of wall at that point and start going the other way.

 

[Ziggurat]

Hmmm. . . interesting.

But. . . you know how on a beach, before a wave comes, all the sand and water go AWAY from you first?

Why couldn't something like that be at play in the universe?

That, locally, we see stars going away from us, but everything after 10 billion light years from us are caving in. . . a super universe tsunami!

Basically it comes down to the idea of homogeneity. We assume that we are not special, that the universe is, globally speaking, pretty uniform. *IF* that is the case, then there are only a few possible cosmologies (topological structures for the universe), and regardless of which one we live in, the universe must still be expanding. In order for it to only be expanding locally but contracting globally, our location in the universe would need to be special: we would need to just happen to be near the center of this local expansion, and not, say, next to it. So it comes down to the idea that our location in the universe is not special, in which case we can determine to some extent what the universe as a whole is doing, versus the idea that we just happen to be right in the center of this one spot in the universe where everything around us is spherically symmetric but not uniform, in which case we cannot determine what the rest of the universe is really doing, with no way of distinguishing between the limitless possibilities. The first assumption is easy and simple, but the second, while we can't exactly disprove it, requires a rather larger degree of faith.

 

[thatguywhojuggles]

Get a balloon, and a marker or sharpie. Use your marker to put dots all over the balloon. Those dots are galaxies.

Blow up the balloon.

Oh, and then there was something about the universe is three dimensional, and it is expanding into the 4th dimension. (The blowing up of the balloon is an example of a two dimensional universe expanding into the third dimension.)

I think that's all I remember from my Cosmos class.

 

[Iconoclast]

By the way, we are, really and truly, at the center of the known universe.

I didn't think the "known" qualifier is required, we're at the centre of the universe by any definition.

 

[PixyMisa]

I didn't think the "known" qualifier is required, we're at the centre of the universe by any definition.

Well, "observable" would be correct. I saw a recent paper trying to figure out the actual size of the universe, and coming up with a likely figure about six times the radius of the observable universe. (Based on various assumptions which may or may not be correct.)