# How can we 'see' back to the early universe?

1. Aug 8, 2013

### Graeme M

This has probably been covered here before, and it's a pretty basic question but one I've often wondered at.

I read somewhere recently (and have heard it said) that we can now 'see' very distant galaxies. I forget the exact numbers but things like we are seeing galaxies that are maybe 80% as distant as the age of the universe.

This is basically saying we are seeing the light that was emitted from a galaxy at some time very close to the origin of the universe. Let's say the universe is 14 billion years old and we can detect a galaxy that is 10 billion years old.

If though the universe is expanding, it must mean that such a galaxy was not actually 10 billion light years away from us 10 billion years ago. Again I have no idea of numbers or the rate of expansion of the universe, but one could imagine that 10 billion years ago the point where we are and where that galaxy is were very much closer together.

How come the light didn't get to us say 2 billion, 3 billion, 6 billion years ago?

2. Aug 8, 2013

### phinds

Yes, that is correct.

Because the universe is expanding and the space through which the light had to travel is getting bigger, so farther to travel than just the old distance from it to us.

3. Aug 8, 2013

### Staff: Mentor

The distance between us and the light is also expanding, so light emitted X years ago from an object X light-years distant at the time of emission has to cover more than X light-years to reach us.

Imagine a snail crawling along an elastic band that is being uniformly stretched. As long as the snail is moving more quickly than the far end of the elastic band is stretching away, it will eventually get to the far end. But the distance it has to cover may be much greater than the length at the moment when it started its journey.

4. Aug 8, 2013

### Graeme M

I see. Thanks for that. Does that mean that the rate of expansion then is effectively a significant percentage of the speed of light (I guess we could work it out from the facts already presented)? Or is that not really a question that makes sense in this context?

And here I am probably getting into territory beyond my pay-grade, but if the space is expanding at such a speed, then doesn't that mean that the galaxy in question is (was) not actually 10 billion light years away and therefore did not emit light 10 billion years ago but at a time much closer to the present?

5. Aug 8, 2013

### davenn

hi Graeme

the expansion rate is proportional to distance .... that is the further away an object is from us the faster its motion
from wiki.....

there's tons more info on the www just do a search on "universe expansion rate" or similar phrases

cheers
Dave

6. Aug 8, 2013

### Staff: Mentor

Expansion isn't measured with a speed, but with a rate. For example, if I'm stretching a rubber band and it takes me 5 seconds to make that rubber band twice its original length, then I can use that to calculate how fast each point on the rubber band moves away from every other point. But here's the thing. That speed that I'm calculating will be different depending on how far apart the two points were to begin with. Two points initially close together move away at a slower speed than two points that are initially far apart.

So, applying that to space, we say that the rate of expansion causes objects to recede from us at a velocity of about 73 km/s for every megaparsec between us. So an object at 1 megaparsec moves away at 73 km/s while an object at 10 megaparsecs moves away at 730 km/s.

As we look at objects that are further and further away, we eventually reach a point where they are indeed expanding at, and faster than, the speed of light. This is allowed because it is a result of the geometry of spacetime, not because anything is traveling "through space" faster than light.

7. Aug 8, 2013

### Graeme M

Thanks again. That rate aspect - does that explain why we don't see say the moon receding from us - at that sort of scale it IS receding but at a very small rate? And at the molecular level it is to all intents and purposes not detectable at all?

8. Aug 8, 2013

### Staff: Mentor

That's... a little complicated.

The short version is that the model developed that calculates all this had to use some 'shortcuts' in order to even do the math. One of the shortcuts is that it only deals with a homogenous universe. That is, a universe with no clumping or anything. Obviously our universe isn't like this. Clumping abounds in the form of stars, galaxies, planets, etc.

This clumping has the effect of completely overcoming the expansion on galactic and smaller scales. Gravity and the other forces simply swamp it, not allowing expansion to separate anything bound together at all.

I'm not sure if space is still expanding within galaxies, just with no effect, or if it isn't happening at all. Like I said the model that we use to calculate all this just doesn't say anything about things on our scale. But either way it isn't having any noticeable effect on objects until they are separated by intergalactic distances at least.

9. Aug 8, 2013

### Graeme M

Intriguing… So does that mean that we can only infer expansion from observing the light of distant objects? Are we reasonably confident that we aren’t just observing some hitherto unknown property of the universe, say the attenuation of light over very large distances? I mean, light travelling that far is going to pass through an awful lot of gravitational or electromagnetic fields.

10. Aug 8, 2013

### Staff: Mentor

Very confident - google for "tired light" to find some of the theories along those lines that have been falsified by observation (be cautious though - you'll come across some crackpots that way as well).

You have to keep the distance scales in mind as well. It seems natural to us to think of the earth as a chunk of rock separated from the another chunk of rock, the moon, by 250,000 miles; and both them separated from an enormous ball of hot dense gas (the sun) by 100,000,000 miles of pretty solid vacuum; and all of that separated by several tens of trillions of miles from other enormous balls of hot dense gas. But at cosmological distances, that lumpiness is just completely irrelevant; we don't worry about it for the same reason that when you're setting up a fan to blow air you don't think of the air as tiny dense lumps (molecules) separated by hard vacuum.

Last edited: Aug 8, 2013
11. Aug 8, 2013

### phinds

NO ... "tired light" was debunked a long time ago. Also, there are MANY reasons pointing to the expansion of the universe, not just redshift. Do some reading and you'll get the answers.

By the way, just FYI, the objects at the edge of our observable universe are receding at about 3 times the speed of light. Google "metric expansion".

12. Aug 8, 2013

### Staff: Mentor

The idea of "tired light" has been tossed around for nearly 100 years. It simply doesn't work as well as the expanding universe idea does. Be aware that we look at more than just redshift as evidence for an expanding universe.

The current model of an expanding universe makes extremely accurate predictions on many different things. One example is the ratio of hydrogen, helium, and other elements in the universe. The first link below has a few other things as well. The second link is about the tired light theories that have been proposed.

http://en.wikipedia.org/wiki/Metric_expansion_of_space#Observational_evidence
http://en.wikipedia.org/wiki/Tired_light

Edit: Wow, 3 replies within 3 minutes of each other.

13. Aug 8, 2013

### phinds

Yeah, and as usual, you're a day late and a dollar short

14. Aug 8, 2013

### Staff: Mentor

But I have pizza. That trumps everything.

15. Aug 8, 2013

### Graeme M

Tired light eh? I am guessing I must have read about that at some time and my subconscious threw that up. Thanks for the explanations. I've read a little but never really seen anything that explained how it is that the light took that long to get here when at that time in the past things were closer together. My problem was thinking that the time taken implied a static distance over time even though I generally understand the expanding universe idea (I think I've said that right). I think I sort of imagined everything expanding away at the same rate. I *do* know when I stop to really think it through, it becomes pretty complicated pretty quickly...

16. Aug 8, 2013

### Graeme M

I was just sitting in the coffee shop thinking all of this through, and while no pizza was involved (though a damned good coffee was), I was still left uncertain about my original question. I understand the way that the universe expands and some of the properties of that expansion, but the fundamental question remains. If expansion is slower for points closer together and if the two points concerned were very much closer together 10 billion years ago, is it likely that it would take 10 billion years for that light to reach us, even allowing for the expansion effect?

I guess that’s just a moot point – obviously so because it happened. But that leads me to this question.

If we know the rate of expansion and we have the time taken for that light to travel to us and we know how fast light travels, can we then back calculate how far apart those points must have been 10 billion years ago? And would that jibe with estimates about the universe’s size then? Or would that contribute to ideas about how big the universe was then? Or is that just a dumb idea that has no relevance to anything?

17. Aug 8, 2013

### Staff: Mentor

Absolutely.

I believe that is how we estimate the universe's size. Knowing the expansion rate plus the age of the universe allows us to calculate its size. Someone correct me if I'm wrong.

18. Aug 8, 2013

### Staff: Mentor

Sure. Go back to my example of the snail crawling on an elastic band... By trying various combinations of speed of the snail and speed of the expansion, you can get an arbitrarily long time for the snail to get to the end, no matter how small the initial distance was.

19. Aug 9, 2013

### Graeme M

Nice analogy! Thanks.