Undergrad How Would Future Species Understand a Dynamic Universe?

[bahamagreen]

What is the point of this post?

The point of this thread was consideration of the far future, when local groups had merged and all else had slipped past the horizon, the remaining galaxies comprising the apparent contents of the whole universe to any observers.

The point of my post was that we have been in that same position, when it was thought that the Milky Way constituted the entire contents of the universe.
The famous Shapley-Curtis debate of 1920 was about this; not resolved until the 1930s...

 

[Chalnoth]

The point of this thread was consideration of the far future, when local groups had merged and all else had slipped past the horizon, the remaining galaxies comprising the apparent contents of the whole universe to any observers.

The point of my post was that we have been in that same position, when it was thought that the Milky Way constituted the entire contents of the universe.
The famous Shapley-Curtis debate of 1920 was about this; not resolved until the 1930s...

I don't see any similarity there at all. Back before the 20's, we saw many objects that were outside the Milky Way. We didn't have conclusive evidence at that time that they were outside our galaxy. That debate was settled, if I recall, by observations of Cepheid variable stars in some of these galaxies.

Today, we have an actual causal horizon which prevents all observations of our universe beyond a certain point. No longer do we just have a situation of not looking carefully enough at existing data to demonstrate how far away other galaxies are, but of a fundamental physical limitation on the potential extent of any observations we could ever make.

 

[Chronos]

Causal connection is the gift that never stops giving. We cannot escape causal connection from events that occurred between the time of initial causal contact with a body until it recedes beyond our our horizon. This time, however, gets stretched out to infinity in our reference frame because of cosmological time dilation. Distant galaxies will ultimately redshift beyond detectability, but, never just vanish. It's sort of like watching Bob fall into a black hole. You know he's going to be eaten, but, never see it happen. In the interest of clarity, I'll try quoting from 'Expanding Confusion', p4 "... all galaxies become increasingly redshifted as we watch them approach the cosmological event horizon (z → ∞ as t → ∞)."

 

[bahamagreen]

If "...galaxies will ultimately redshift beyond detectability..." yet some "causal connection" must necessarily remain, is this thinking of something like entanglment?

 

[Chalnoth]

In practical terms, yes, once the far-away galaxy has redshifted beyond detectability, there is no causal connection of any consequence remaining.

At least in terms of classical General Relativity, this causal connection still exists, but it's just so weak as to be virtually meaningless. I would tend to expect that quantum gravity might allow the causal connection to drop to become actually zero eventually, but that isn't completely clear.