Evolution of the Observable Universe

**hellfire** · May17-06, 04:40 AM

Originally Posted by marcus

What I am saying is that the farthest crud that we will EVER see is crud that at the present time is 62 Gly from us.

Yes, you and Lineweaver are right. I was wrong when I claimed in my last post that always new objects will be visible. I will explain the technical reason for this. First, let's summarize the result from the calculations above: In a de-Sitter model, that can be regarded as the asymptotic limit of our universe, the particle horizon (the current location of the most distant objects or crud whose light we are receiving today) reaches a constant comoving distance when t $LaTeX Code: \\rightarrow \\infty$ , but increases always its proper distance. So far we have not said anything about new crud that is located farther away.

Speaking about comoving or proper distances is a matter of definitions and conventions. The question to understand Lineweaver's claim is the following: If a particle horizon is at constant comoving distance of a comoving observer, does the crud that is located farther away decrease its comoving distance and enter the particle horizon? Equivalently, if the proper distance to the particle horizon increases at a given rate, does the crud that is farther away increase its proper distance to us slower so that it can enter the particle horizon?

The easy question to approach is the first one. The point is that, due to the definition of comoving distance, comoving objects and crud located at a comoving distance Dc will be always located at the comoving distance Dc during the expansion of space. They will be located at a larger proper distance in new spatial hypersurfaces, but their comoving distance will be the same. This means that no new crud can enter the particle horizon if the particle horizon it is located at constant comoving distance of us.

**marcus** · May17-06, 01:09 PM

Originally Posted by hellfire

Yes, you and Lineweaver are right. I was wrong when I claimed in my last post that always new objects will be visible. I will explain the technical reason for this. First, let's summarize the result from the calculations above: In a de-Sitter model, that can be regarded as the asymptotic limit of our universe, the particle horizon (the current location of the most distant objects or crud whose light we are receiving today) reaches a constant comoving distance when t $LaTeX Code: \\rightarrow \\infty$ , but increases always its proper distance. So far we have not said anything about new crud that is located farther away.

Speaking about comoving or proper distances is a matter of definitions and conventions. The question to understand Lineweaver's claim is the following: If a particle horizon is at constant comoving distance of a comoving observer, does the crud that is located farther away decrease its comoving distance and enter the particle horizon? Equivalently, if the proper distance to the particle horizon increases at a given rate, does the crud that is farther away increase its proper distance to us slower so that it can enter the particle horizon?

The easy question to approach is the first one. The point is that, due to the definition of comoving distance, comoving objects and crud located at a comoving distance Dc will be always located at the comoving distance Dc during the expansion of space. They will be located at a larger proper distance in new spatial hypersurfaces, but their comoving distance will be the same. This means that no new crud can enter the particle horizon if the particle horizon it is located at constant comoving distance of us.

I didnt follow everything but I think you are probably right. I think it is a very interesting topic and wish more people would discuss what is the ultimate horizon of the ultimately observable universe. And I would say you hellfire are in charge of the thread from this point (I dont think I have any more thoughts about it).

more people should be asking about this. the cosmological event horizon for example might have a defined entropy. what happens when a galaxy or black hole passes across it by its own proper motion? what happens to the entropy. am I confused? yes very much? can that even happen? Tamara Davis wrote her PhD thesis for charles lineweaver about this very topic of horizons in cosmology. there are various kinds. her thesis had too much in it for me, but parts of it were quite interesting. Somebody with a similar name, but different, like Davies, was also writing about this. whatever else, have fun

**hellfire** · May18-06, 04:01 AM

Originally Posted by marcus

the cosmological event horizon for example might have a defined entropy. what happens when a galaxy or black hole passes across it by its own proper motion? what happens to the entropy. am I confused? yes very much? can that even happen?

Who is not confused? Horizons are weird.

For black holes, matter can only cross the horizon from the outside to the inside. Without considering quantum effects, the horizon area can only increase or stay constant according to the generalized second law of thermodynamics for asymptotic observers outside. Considering a quantum scalar field, as usual, the horizon area may decrease due to the thermal Hawking radiation that transports entropy out of the black hole.

In case of a de-Sitter horizon, matter can only cross the horizon from the inside to the outside. If we assume that something similar to the generalized second law applies for comoving observers inside, and without considering quantum effects, the horizon area on a hypersurface of constant time can only increase or stay constant.

However, considering a quantum scalar field, the de-Sitter horizon does also radiate towards the comoving observer inside. Does this mean that it may also decrease its area?

What does this entropy measure?

**Chronos** · May19-06, 06:52 AM

A fairly confusing discussion. How/why did 'horizons' creep into this conversation?

selfAdjoint · May19-06, 08:08 AM

Originally Posted by Chronos

A fairly confusing discussion. How/why did 'horizons' creep into this conversation?

Via "observable universe". We have an observation horizon and it constrains what we can see (know?) about evolution.