The observable universe, the actual uinverse, and CMB

[curioushuman]

Chronos,

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

When you say "they" left before they were ignited, and I take it you mean the gas clouds from which those stars and galaxies ignited. If so, those pre-star gas clouds would have been in our cosmic horizon at some point, and we could get a telescope, in principle, and look at them. And if we can see the gas clouds, then we can see the space (albeit at a much older time and in a less-expanded state) in which those stars (now beyond our cosmic horizon) later formed. That's my point: the fact we can see the CMB seems to imply that there are not vast regions of our universe that are inaccessible to us since we can, at least, see those regions when they were younger.

Naty1,
Discussion on topics like this isn't "useless." Just ignore it if it doesn't appeal to you.

 

[George Jones]

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.

 

[Naty1]

Discussion on topics like this isn't "useless."

That's not what I posted.
It's repeatedly asking the same questions that is useless.
If you'd read posts carefully you'd see you have your answers.

 

[marcus]

Marcus, you added the qualification, "NEARER THAN 41 MILLION LY at the time the CMB was produced." That's fine, but wouldn't 41 million ly include everything in the universe at the time the CMB was produced (anyway)?

No. It would not include "everything in the universe". It would include everything in the currently observable portion of the universe.

==quote from same post by Curious==
On a different topic, later on you say, and this kind of shocked me, "Galaxies never leave our observable universe... they never stop in principle being observable." How do you square that with the claim that we all keep making that (my line #3) that there must be regions of space --full of stars and galaxies-- that we will never (in principle) see?
==endquote==

It is true that galaxies never LEAVE our observable universe. This is a simple consequence of the mathematical model of the universe which cosmologists use. In the longterm the light from them can be so red-shifted that no practical-size antenna or telescope can detect it. but we will be getting their light so they will still be (by definition) in our observable.

However the observable portion is NOT THE WHOLE MODEL. It is a slowly growing piece of the whole. The observable universe is always including more and more matter.

The current distance to the most distant matter that we are getting signals from is called the "particle horizon". It is around 45-46 Gly. It is INCREASING. The most distant matter we will ever be able to get signals from is NOW 63 Gly from us. As far as we know there is plenty of space and matter out beyond that.

But according to model, it is our destiny that the farthest matter we will ever see is matter which is now 63 billion lightyears from us. This is if you could stop the expansion process to let you measure.

You can see the 63 Gly (approximate) on this chart. It is the "Figure 1" that I keep in signature at the end of every post.
http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg
Look at the bottom figure, where the distance scale is socalled "comoving distance"
this is a permanent distance number attached to each bit of matter which is its distance from us NOW.

You can see from the Figure 1 that the most distant bits of matter we will ever get light or other signal from are the bits that are NOW at a distance of 63 Gly (if you could stop expansion so as to get a chance to measure).

 

[Chronos]

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.

 

[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.