# Since the Universe is expand we rule out galaxies whose light will never reach us?

1. Apr 3, 2007

### ensabah6

We see galaxies as their light reaches us after billions of years of travel.

But if the Universe itself is expanding, there should be an indefinite number of galaxies whose light not only did not have enough time to reach us, but will never reach us due to expansion.

Perhaps the mass of these galaxies are attracting the nearby galaxies pulling them outward.

2. Apr 3, 2007

### Wallace

This is true, there are no doubt galaxies whose 'future light cone' (the region of space that light from them can reach) will never intersect our position.

However the mass of these galaxies is included in our model of the universe. On large enough scales we see that the universe is homogeneous, that is to same, the same density on average everywhere. The scale on which we see the homogeneity is much smaller than the size of the observable universe. This means that the total gravitational acceleration on each part of the universe is zero (so we are moving towards Andromeda, while the whole local groups moves towards the Great Attractor but this is because the universe is not homogenous on these smaller scales). Hence the mass of these galaxies won't be pulling nearby galaxies away, since there is as much mass pulling them the opposite way such that the total acceleration is zero.

Interestingly though, there are theories that suggest that even though the universe appears homogeneous, it may be possible that on scales larger than the observable universe the universe is not homogeneous and perhaps we are in some over or (more commonly suggested) an underdense region. This has been put forward as an explanation for the accelerated expansion of the universe. The theory is problematic since it is hard to explain why the universe would be lumpy on small scales, very smooth on large scales but then get lumpy again on even larger scales. It also isn't supported well by any observational evidence (which would be hard, since by definition we can't see beyond the observable universe!) but I believe it hasn't been ruled out, in that I don't think we can point to something and say 'it would look like this if the theory was true but it doesn't'.

If anyone is interested in these theories, the technical name they go by is "super horizon sized perturbations".

Last edited: Apr 3, 2007
3. Apr 3, 2007

4. Apr 3, 2007

### Wallace

.ultimate several people pointed out the misconceptions you based you idea in the other thread upon. I would suggest if you still disagree with them you should continue the discussion in that thread rather than trolling it in others.

5. Apr 4, 2007

### hellfire

This does not follow. The current nearest location of a photon that is traveling towards us but that will never reach us in future is called cosmological event horizon. Not all cosmological models in which space expands contain cosmological event horizons. This depends on the dynamics of the expansion that is driven by the content of the universe. However, it is assumed that our universe actually contains cosmological event horizons and ours should be currently located 15 Gly away from us.

Last edited: Apr 4, 2007
6. Apr 4, 2007

### marcus

Yes! I agree! And the figure of 15 seems about right although I did not know an exact figure.

what you say agrees with the explanation and pictures in Lineweaver's easy to understand "Inflation and the CMB" paper of around 2003. Do you have any particularly good clear sources?

My understanding is the reason we have a cosmo event horizon is because Lambda is positive.

Before 1998 or whenever, when Lambda was thought to be zero, people didn't think we had a cosmo event horizon and so at least in the simple flat case, all the photons coming our way would eventually reach us!

Lineweaver has a picture that shows the Hubble radius 13.7 Gly
and somewhat out beyond that the Cosmo Event Horizon. It could be 15 as I recall it visually, but he doesnt give a figure. at least as I recall.

7. Apr 4, 2007

### hellfire

I did the calculation and I implemented this in my cosmological calculator. However, the "Proper distance to the event horizon then" output of the calculator does not work in general. I believe the formula I used is correct and the output seams to be correct for the standard cosmological model that is loaded with the page, but it seams to be wrong for some others. For this special output I think I still have to learn about some basic numerical methods and their implementation, but I did not yet find any time for this. You are free to read or get the code for own use in the cc_e.js file.

Last edited: Apr 4, 2007
8. Apr 4, 2007

### marcus

Thanks! I am not going to try to do the calculation. BTW you are outstanding in that sector!
But I will keep an eye out for some published paper that has a figure for the CEH.

9. Apr 4, 2007

### wolram

10. Apr 5, 2007

### hellfire

With cosmic light horizon it is meant there the particle horizon or radius of the observable universe. This is different to the event horizon. The particle horizon is the current position of a photon emited at t = 0 from our comoving position. The event horizon is the position at t = $\infty$ of a photon sent from our current comoving position now. This is equivalent to the other definition I have given above.