# I Future Cosmology knowledge

1. Feb 15, 2017

### maroubrabeach

OK, let's imagine that in a few billion years time, when our local group of galaxies have merged, our Sun has reached and surpassed it's giant red phase stage and has become a White Dwarf, and we as a species are extinct.
At the same time the distant galaxies will have moved beyond our observable universe.
Let's say that around this time another species has evolved and reached a stage of intelligence compared to our own.
They will not have any evidence of any recessional velocity of these distant galaxies, as they have moved beyond the observable horizon. Also the CMBR will be much lower, possibly undetectable.....So how would such a species reach a conclusion that we were living in a dynamical expanding universe?
Would cosmology knowledge be curtailed for them by these distant events?

2. Feb 15, 2017

### Staff: Mentor

You placed a lot of big "IF" in your thought experiment. What is the goal of it? I don't think that this is a debatable framework, as it has not the least implication to us or our understanding of the universe. Basically you ask: What will a species like ours (1st assumption) can find out about the universe (who cares?) in the middle of the big chill (2nd although likely assumption)?

3. Feb 15, 2017

### Chalnoth

The time scales for these two things are vastly, vastly different. The first will take place in a few billion years. The second will take closer to a couple trillion years.

Yes. Learning about cosmology will become virtually impossible after a couple trillion years, provided the expansion continues as expected from the $\Lambda$CDM model.

4. Feb 15, 2017

### Staff: Mentor

There's another knock out argument (I think): In the middle of the big chill, there won't be any (young) stars left, around which life could evolve or survive. Even if some solar system would remain, it will probably we a very cold one.

5. Feb 15, 2017

### Chalnoth

Star formation doesn't stop until roughly 100 trillion years, so there's still plenty of time for stars to form where cosmology likely won't be possible at all. For a breakdown of the timeline, see here:
https://en.wikipedia.org/wiki/Future_of_an_expanding_universe

6. Feb 15, 2017

### Staff: Mentor

Wow. I couldn't (can't) imagine that there will still be enough material around to build new stars.

7. Feb 15, 2017

### maroubrabeach

Yep, accepted on the time scale difference argument, but on that subject, the Sun will reach White Dwarf stage in around 5 billion years, while the distant galaxies should be shifted beyond view in a few hundred billion years I thought.
The trillions of years would certainly apply to the age when no more stars could form, BH's would evaporate? At least that was the impression I was under.....

8. Feb 15, 2017

### newjerseyrunner

I think it'd still be possible to determine the universe was expanding by using logic.

Lets make the assumption that your species has come to understand gravity and the electromagnetic and nuclear forces.
You're species would likely be living in an island universe that's one big galaxy and not be able to see anything else beyond it.

You'd have to come up with the math of general relativity. I'd expect any species capable of doing that will see the same problem with the math that Einstein saw: the universe must be expanding, contracting, or perfectly balanced forever. Seeing nothing beyond your own galaxy would be strong evidence against a collapsing universe so you're left with expanding or eternal. By calculating that stars burn themselves out and would eventually remove all raw materials from the galaxy should allow the species to realize that the universe has not always been, and therefore much be expanding.

9. Feb 15, 2017

### Chalnoth

Read the link I posted above. It goes through the whole timeline, and I think it's reasonably accurate.

10. Feb 15, 2017

### Chalnoth

True, but they wouldn't be able to measure much of anything about said expansion.

Maybe they could, with extremely detailed measurements of the gravitational potentials of the remaining visible galaxies, measure the cosmological constant. But they likely couldn't get much about the history of the universe.

11. Feb 15, 2017

### maroubrabeach

OK, I have learnt something with regards to that point...thanks.
[I would have presumed earlier, see my last post]

12. Feb 15, 2017

### newjerseyrunner

Agreed, they would find it very difficult to measure any expansion. They could simply come to the logical conclusion that it is.

13. Feb 15, 2017

### Chalnoth

And that it was an accelerated expansion, likely dominated by a cosmological constant at late times. If there were no dark energy or cosmological constant, then it would take far, far longer for the objects not bound gravitationally to us to be redshifted to undetectability. Without doing the calculations, my bet is there's a good chance it wouldn't happen until after star formation ceased.

14. Feb 15, 2017

### newjerseyrunner

Agreed, I think this thread is more a thought experiment about the logic than actual physics.

So assuming the criteria that an intelligence evolves in a galaxy so deep in time that it can't see anything else.

I don't think that you would develop the concept of a cosmological constant. You could assume the cosmological constant to be zero and still have an expanding universe if you lower the energy density. The logical conclusion given the available data would be that there simply wasn't enough mass in the universe to gravitationally collapse.

15. Feb 15, 2017

### Staff: Mentor

Now it's getting interesting: Taking this argument and turning it on us, the question is: Could there have been a state of the universe, in which we would have been able to conclude more, than we are now capable of? Are there truths thinkable, which we cannot reveal due to the fact that our universe is already 13.8 Gy old?

16. Feb 15, 2017

### Chalnoth

The issue is that the rate of expansion in a universe without a cosmological constant slows significantly in the far future. Right now, the rate of expansion is roughly four times what it would be if the value of the cosmological constant were zero (assuming all other densities were kept the same, and the early-universe expansion rate was the same).

Eventually, I'm sure that a universe without a cosmological constant would still end up in a state where only gravitationally-bound objects were detectable, but it would take much longer. Quite possibly longer than it takes for star formation to cease.

17. Feb 15, 2017

Staff Emeritus
Why do you think the CMBR is undetectable if the universe is 100x older?

18. Feb 15, 2017

### Chalnoth

It gets redshifted to the point that the wavelengths of the photons are larger than the Hubble scale.

19. Feb 15, 2017

### Staff: Mentor

I don't think this is correct. If there is zero cosmological constant, the expansion decelerates forever (we'll assume that it doesn't stop so the universe is either just at critical density or below it, i.e., spatially flat or open, not closed). If the expansion decelerates forever, then I think there is no cosmological event horizon: given enough time, any pair of comoving observers will eventually be in each other's past light cones.

20. Feb 15, 2017