# Cosmic horizon

1. Oct 8, 2009

### xMonty

if i ran towards the cosmic horizon (really fast) will i see it receding?

2. Oct 8, 2009

### xMonty

Did i ask a stupid question? or i didnt explain the question very much?

3. Oct 9, 2009

### Chalnoth

You can't "see" the horizon. The horizon is the limit of what you can see. But the horizon does change with time depending upon how you move (and even if you don't move at all with respect to the CMB).

4. Oct 9, 2009

### xMonty

Thanks! few more questions about the horizon
1) we cant see past the horizon cause exactly that far away things are receding at the speed of light, is this is only reason for horizon or is there something else.

2) we say we will never be able to see past the horizon is that a law or just that we will never be able to move at the speed of light so there is no hope of catching up with the stuff that disappeared into the horizon. So in principle "if" we could teleport instantly to far distances (beyond horizon) then we could explore the rest of the universe.

5. Oct 9, 2009

### Chalnoth

Our vision is not actually limited by a horizon. It's limited by the cosmic microwave background. Before the CMB was emitted, the universe was opaque.

That's basically correct: the horizon is set by the finite speed of light, and cannot be exceeded. Yes, if you could get to some distant place in our universe without actually traversing the distance, you would have a different horizon that would include regions which are beyond our horizon.

6. Oct 9, 2009

### xMonty

Ok but i heard CMB will dilute in the years to come so in distant future when there is negligible CMB what happens then

7. Oct 9, 2009

### Dmitry67

Wait, wait, it depends. Different observers in different places of the universe do not agree where the horizon is. For some observers WE are behind their horizon. So if you run, horizon changes the position FOR YOU.

Even more, if you accelerate, you see another horizon and observe Unruh radiation from it.

8. Oct 9, 2009

### Chalnoth

Just because the temperature of the CMB will continue to decrease with time doesn't change how it was emitted: before it was emitted, the universe was opaque, so that is the limit of our vision, now and forever.

9. Oct 9, 2009

### xMonty

confusing

So CMB is coming from about 14 Billion light years away because the big bang happened 14 b ly so after a billion years it will be 15 billion light years away from us right?

10. Oct 9, 2009

### Chalnoth

Well, actually, due to the curvature of space-time, it isn't coming from that far away. The part of the CMB that we are observing now was emitted from matter that was, at the time, a mere 45 million light years away or so. But because our universe was expanding so rapidly back then, even though it started out rather close, the light had more and more space to travel across as time went on, such that it only finally reached us after some 13.7 billion years.

The matter that once emitted the part of the CMB that we are observing now is currently some 48 billion light years away (likely beyond our future horizon: we can never interact with that stuff again).

As for how far away we'll be looking in a billion years, I'd have to run through the calculations (it's not terribly simple). But suffice it to say that the picture will remain much the same: we'll be seeing light coming from bits of matter that were a bit further away than the stuff we see now. I'd have to run through the calculations, but I'm pretty sure that as time goes on, we won't be able to see all of the matter that emitted the CMB, but will only see out to a finite distance. But, as I said, I'd have to run through the calculations to determine exactly how far this is.