The observable universe, the actual uinverse, and CMB

[marcus]

There are 'tons' of stars and galaxies we will never see because they left our cosmological horizon before they ignited.

It is impossible to leave the cosmological horizon.

We are in disagreement, George. I contend objects routinely exit our cosmological event horizon. That doesn't mean they 'vanish', merely that they redshift into obscurity before any photons they emit 'now' can reach us.

Chronos, I find what you said about "tons" of stars and galaxies confusing. You clearly are talking about the cosmological event horizon (CEH). The CEH distance is now something like 15.6 Gly. But most of the galaxies we observe with the Hubble Space Telescope are farther than that.
So a galaxy being beyond the CEH does not imply it cannot be seen!
I can't make sense of your statement.

My hunch is there is some semantic inconsistency here. We need to sort out what is meant by the words.

One way to think about it is that the CEH is a limit of *reachabiity*, not a limit of *detectability*. If a galaxy is beyond 15.6 Gly then if you started off for it today you would never ever get there even traveling at the speed of light. But I suppose by "cosmological horizon" one might also mean the edge of the observable---now something like 46 Gly. That is very different from the CEH (a mere 15.6 Gly). It is usually called the particle horizon and at first i thought that might be what you meant when you said cosmological horizon. But then what you said would be wrong since I think nothing ever exits over the particle horizon. In comoving distance terms it keeps on growing and including more and more matter.

Figure 1 from Lineweaver Davis "Expanding Confusion" illustrates the particle horizon growth in comoving distance terms. Readers new to this can check page 3 of http://arxiv.org/pdf/astro-ph/0310808.pdf
or go directly to http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg
The vertical lines are the paths of matter particles or galaxies, in the two diagrams using comoving distance (because matter's comoving distance essentially does not change.)

So we may have a bit of a terminology vagueness here that could be sorted out

 

[curioushuman]

Go away Naty1!

 

[Chronos]

Agreed, of course we see stars and galaxies beyond our cosmological event horizon. Its just that we will never receive any photons they emitted after they crossed our CEH. In the paper Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe, http://arxiv.org/abs/astro-ph/0310808, on page 4 it is stated "Most observationally viable cosmological models have event horizons and in the CDM model of Fig. 1, galaxies with redshift z ~ 1.8 are currently crossing our event horizon. These are the most distant objects from which we will ever be able to receive information about the present day." I take this to mean galaxies at z~1.8 will cross our CEH at some point in the finite future and any photons emitted thereafter will never reach us. If you extend this to a star birthing region at z~1.8, the light emitted by any stars birthed after it crosses our CEH will never be observable by us. Hence, I deem it logical to assert there are vast numbers of stars that 'currently' exist in the universe whose light will never reach us.

 

[Tanelorn]

Chronos, I find what you said about "tons" of stars and galaxies confusing. You clearly are talking about the cosmological event horizon (CEH). The CEH distance is now something like 15.6 Gly. But most of the galaxies we observe with the Hubble Space Telescope are farther than that.
So a galaxy being beyond the CEH does not imply it cannot be seen!
I can't make sense of your statement.

I think it depends on whether we are talking about light being emitted today that we may view billions of years in our future or light being emitted in the distant past that we are just viewing now.