Future Cosmology knowledge

[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?

 

[fresh_42]

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)?

 

[Chalnoth]

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.

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.

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?

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

 

[fresh_42]

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.

 

[Chalnoth]

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.

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

 

[fresh_42]

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

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

 

[maroubrabeach]

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.

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...

 

[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.

 

[Chalnoth]

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...

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

 

[Chalnoth]

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.

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.

 

[maroubrabeach]

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

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

 

[newjerseyrunner]

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.

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

 

[Chalnoth]

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

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.

 

[newjerseyrunner]

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.

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.

 

[fresh_42]

The logical conclusion given the available data would be that there simply wasn't enough mass in the universe to gravitationally collapse.

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?

 

[Chalnoth]

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.

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.

 

[Vanadium 50]

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

 

[Chalnoth]

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

 

[PeterDonis]

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

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.

 

[maroubrabeach]

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

The CMBR was around 3000K when it was first emitted around 400,000 years post BB, now it is 2.73K and will continue to fall as spacetime/universe expands.
Some nice more technical answers here.....
https://www.quora.com/Will-the-cosmic-microwave-background-radiation-eventually-disappear

 

[Vanadium 50]

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

In only a trillion years?

 

[Chronos]

Red dwarfs are extremely long lived, its not inconceivable they might still harbor life.

 

[Chalnoth]

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.

Hmm, okay, yes, definitely then.

I was thinking that because there's still redshift that increases with distance, that redshift might eventually reach a point where non-bound galaxies are undetectable. But that's not right. Without dark energy or a cosmological constant, the recession velocity of any given galaxy will always be decelerating, so that at late times, far-away galaxies will become more detectable, not less.

Edit: except that they will become dimmer, being further away.

 

[Garth]

It does seem that we live in a favourable 'cosmological time zone' in which the universe is old enough for intelligent beings to evolve and discover cosmological history and yet not too old that the neccesary information (CMB etc.) has become undetectable...

Garth

 

[Chalnoth]

It does seem that we live in a favourable 'cosmological time zone' in which the universe is old enough for intelligent beings to evolve and discover cosmological history and yet not too old that the neccesary information (CMB etc.) has become undetectable...

Garth

Without looking into it too closely, my bet would be that a majority of civilizations will arise while cosmology is still possible. Star formation may continue for a long, long time, but it slows dramatically over time as well. For some recent data:
http://www.popsci.com/science/artic...e-old-and-universe-making-hardly-any-new-ones

I'm not completely sure if the Wikipedia claim that stars will continue to form for 100 trillion years has been updated, or whether it's consistent with these numbers.

 

[phinds]

The trillions of years would certainly apply to the age when no more stars could form, BH's would evaporate?

I see no one has addressed this directly although Channoth's link does show it: compared to the time that it takes for black holes to evaporate, trillions of years is not significantly different from zero.

 

[Chalnoth]

I see no one has addressed this directly although Channoth's link does show it: compared to the time that it takes for black holes to evaporate, trillions of years is not significantly different from zero.

Yup. A solar mass black hole takes around $10^{67}$ years to evaporate, more than $10^{50}$ times as long as a trillion years.

 

[maroubrabeach]

It does seem that we live in a favourable 'cosmological time zone' in which the universe is old enough for intelligent beings to evolve and discover cosmological history and yet not too old that the neccesary information (CMB etc.) has become undetectable...

Garth

Yes, in essence that's about the message I was trying to deliver.
Sort of reminds me of the awesome nature of total eclipses...I have been lucky enough to see one near Cairns a few years ago...There will be a distant future time when they will be impossible to see, and if we are still around, only Annular eclipses will be viewable.

 

[bahamagreen]

It wasn't so long ago that WE thought our galaxy was the whole universe...

 

[Chalnoth]

It wasn't so long ago that WE thought our galaxy was the whole universe...

What is the point of this post?

 

[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.