# B The cosmic horizon and light

1. Oct 26, 2016

### jacobassett

I have a question about the cosmic horizon. If theoretically I go out 20 billion light years to the cosmic horizon as there a point in space where there is a huge build up of light where space is moving away at exactly the speed of light? Is this this the 2d representation of reality they talk about in the holographic principle?

2. Oct 26, 2016

### phinds

I don't know where you got 20 billion light years but no matter how far out in space you go, there will be an observable universe horizon about 47 billion light years away from you. There IS no "point in space where there is a huge build up of light".

3. Oct 26, 2016

### jacobassett

The observable universe may be 47 billion ly across but I was saying to the nearest horizon. I know you can't actually go there because the horizon depends on the observers position in space. But theoretically is there a sphere of intense light surrounding us at the point of the horizon?

4. Oct 26, 2016

### phinds

I say again, There IS no "point in space where there is a huge build up of light".

5. Oct 26, 2016

### jacobassett

Alright so what is at the point where the universe is expanding at the rate equal to the speed of light?

6. Oct 26, 2016

### phinds

Nothing in particular. It's just a mathematical surface. It has no physical presence.

7. Oct 26, 2016

### jacobassett

How does it have no physical presence? There is a literal point in space where space is expanding at the speed of light away from us...

8. Oct 26, 2016

### phinds

I think you are confusing yourself by your use of incorrect (though common in pop-sci) that "the universe is expanding at the speed of light".

There IS a mathematical surface on which objects are receding from us at c but that is true for every single point in the universe. If there WERE such a physical presence as what you are thinking of then all of space, every single point, would be on such a surface. Do you see how this doesn' t make sense? Google "metric expansion"

9. Oct 26, 2016

### jacobassett

1. I know that....every surface would be on that point but this is based on the observer...if the observer moves than the cosmic horizon moves. I'm not saying you can see this space I'm just saying that it must exist.

10. Oct 26, 2016

### phinds

Yeah, I know. You KEEP saying that. Saying it doesn't make it true. It "exists" only as a mathematical surface with no physical presence. I am tired of saying that. I get that you don't believe it, but it's true whether you believe it or not.

11. Oct 26, 2016

### jacobassett

Uh no you're wrong. We wouldn't be talking about it if it didn't exist, or at least theoretically exist. It may or may not but you can't just say it's a mathematical representation and that's it.

12. Oct 26, 2016

### Staff: Mentor

No.

No, you are wrong, and phinds is correct. The idea of "space expanding" is common in pop science presentations, but it is just a heuristic idea with limited usefulness, and you are trying to push it beyond its useful range. Thread closed.