# Homework Help: Causal contact from opposite sides of Universe?

1. Nov 18, 2013

### bowlbase

1. The problem statement, all variables and given/known data
No math required, mostly just a conceptual question. Consider two points at direct opposite sides of the expanding visible universe. Could these two points ever been in causal contact?

2. Relevant equations

3. The attempt at a solution
I take causal contact to mean that a photon, without encountering any medium that could cause it to take a less than perfect path, could have been exchanged by the two points.
To me, all points (including non-visible) should have been in causal contact at some point directly after the big bang. Unless expansion has always been greater than the speed of light?

But, I know that right now that these two points, according to us, are moving away from each other at a speed greater than the speed of light due to expansion. Meaning they could not possibly have had any causal contact.

So, I guess it comes down to how quickly the universe was expanding at it's lowest expansion velocity. We haven't discussed expansion in that much detail so perhaps I'm thinking of this wrong.

Thanks for the help!

2. Nov 18, 2013

### phinds

Then how do you explain the fact that the edge of the observable universe IS observable and yet is receding from us at about 3c ? How do we and it have any casual connection?

3. Nov 18, 2013

### bowlbase

Well, we can't since it would take faster than light (FTL) travel to get there. It's observable only because it hasn't redshifted out of the spectrums we use with our telescopes yet. But, at some point we were a lot closer together and FTL wouldn't have been required since the expansion, as I understand it, was not always FTL.

4. Nov 18, 2013

### phinds

But if a photon is emitted from the edge of the observable universe "now", it would STILL reach us eventually. Is that not a casual connection? That is, it is NOT just the photons that were emitted 13billion years ago that are reaching us now, it is that the "now" photons will also reach us (a hell of a long time in the future, to be sure, but they WILL reach us).

In fact, I seem to recall that matter very close outside the observable universe is going to move into the observable universe in the future, by which it is meant that the light from that matter will reach us eventually.

5. Nov 18, 2013

### bowlbase

I always understood it that the space between objects in the universe is expanding, not just that we are moving away from each other. As the expansion increases in speed it eventually expands faster than the speed of light. So even though a star emits a photon 14 billion light years away it will never reach us because the space between us is increasing faster than it can traverse it (this is assuming the expansion accelerates to v>c within 14billion years of emission).

Also, if an object is moving at 3c away from me, how could I ever hope to send information to it? I guess I can't explain how its possible for us to see galaxies at the edge of the visible universe if they are moving at 3c.

Last edited: Nov 18, 2013
6. Nov 18, 2013

### phinds

You are missing the simple point that it doesn't MATTER how fast an emitting object is moving away from us, it only matters how FAR away it is. If something is 1 light year away from us and moving at 10c (impossible, of course, but read on ...), the light will reach us in 1 year. This gets more complicated when something is far enough away that the recession velocity matters because that figures into where the edge of the observable universe is.

7. Nov 18, 2013

### bowlbase

It just seems like you're assuming that as soon as the light is emitted then the whole universe comes to a stand still. If, from our perspective, the galaxy that emits the light is moving away at 3c then we, from their perspective are moving away at 3c.

It's not like a bullet fired from a moving car towards an unmoving target. Since it is the space between the car and target that is causing the apparent relative velocity.

Clearly, I must be wrong since we can see galaxies moving away from us but I'm not seeing the reasoning.

8. Nov 19, 2013

### phinds

Perhaps it will help if you realize that your point of view IS valid after a certain distance, otherwise light from any arbitrary large distance away would arrive here eventually given enough time, but that is not the case. In the future, the observable universe will contain a LITTLE more than it does now but not a huge increase beyond the matter that is in it now. It just gets bigger and bigger due to expansion. Do a forum search for maybe "size of the universe" and look for some of Marcus's posts. He explains all this very well, and shows a table that helps you see how the observable universe works over time.

9. Nov 19, 2013

### bowlbase

Another point I have trouble with is if these 2 points are at the very edge of my visible universe then wouldn't that put them outside of each other's visible universe? They should not be able to see each other at all. Still, my basic question remains as to whether they every had the ability to interact. I say yes because they were close together during early expansion. But, I do not know if they moving apart so fast as that they were unable to interact. If they moved away faster than c (always) then they couldn't have exchanged information.

I'm trying to find the specific posts you spoke about.

10. Nov 20, 2013

### phinds

I keep hoping Marcus will jump in here and give you a more lucid explanation that I can, but I do have one comment on the above which is that I agree w/ you that objects at opposite points on the edge of our OU are definitely way outside each other's OU and should not interact w/ each other in the future --- but I understand your main question is whether or not they ever could have interacted and I can't answer that one.

By the way, just FYI my comment about the OU eventually increasing to include a small amount that is currently just outside it is based on the fact that the Hubble constant isn't actually quite constant over time. It is the fact that this increase in the radius of the OU is small that makes me confident that the two points are not ever going to be in each others OU.