Future of Universe: Big Freeze, Big Rip, Big Crunch, Big Bounce?

[wolram]

Some thing i have been wondering, this from wiki.

is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch, or a Big Bounce? Or is it part of an infinitely recurring cyclic model?

 

[phinds]

The current consensus is that it is headed for the big freeze. The big rip seems to have been pretty much ruled out, although I don't know that that's conclusive. All the others have been promoted by various folks at various times but don't have any empirical evidence.

 

[bapowell]

I think we can reasonably propose that the big crunch is unlikely, because it requires a closed universe that is not vacuum-dominated. The others, bounce (as perhaps part of a cyclic model), freeze, or rip at this time are all perfectly plausible.

 

[Doug Huffman]

The Value of the Cosmological Constant http://arxiv.org/abs/1105.3105
John D. Barrow, Douglas J. Shaw
Abstract (Submitted on 16 May 2011)
We make the cosmological constant, {\Lambda}, into a field and restrict the variations of the action with respect to it by causality. This creates an additional Einstein constraint equation. It restricts the solutions of the standard Einstein equations and is the requirement that the cosmological wave function possesses a classical limit. When applied to the Friedmann metric it requires that the cosmological constant measured today, t_{U}, be {\Lambda} ~ t_{U}^(-2) ~ 10^(-122), as observed. This is the classical value of {\Lambda} that dominates the wave function of the universe. Our new field equation determines {\Lambda} in terms of other astronomically measurable quantities. Specifically, it predicts that the spatial curvature parameter of the universe is {\Omega}_{k0} \equiv -k/a_(0)^(2)H^2= -0.0055, which will be tested by Planck Satellite data. Our theory also creates a new picture of self-consistent quantum cosmological history.

Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant, which is measured to be on the order of 10^−52 m^−2, in metric units. Multiplied by other constants that appear in the equations, it is often expressed as 10^−35 s^−2, 10^−47GeV, 10^−29 g/cm^3. In terms of Planck units, and as a natural dimensionless value, the cosmological constant, λ, is on the order of 10^−122.
As was only recently seen, by works of 't Hooft, Susskind and others, a positive cosmological constant has surprising consequences, such as a finite maximum entropy of the observable universe (see the holographic principle). https://en.wikipedia.org/wiki/Cosmological_constant#Positive_value

I believe Leonard Susskind derives it during one of his lectures, certainly to demonstrate its small positive magnitude.

 

[Mr.CROWLER]

Yes most scientists agree that it will most likely be a big freeze where galaxies are so far apart, but are still intact. Eventually stars stop being made the last ones left eventually burn out and then the only thing left to govern the universe will be black holes. The universe won't grow big enough to produce enough dark energy for the big rip. Another possibility for the end of the universe as we know it is a phase change which I'm trying to learn more about right now.

 

[phinds]

Yes most scientists agree that it will most likely be a big freeze where galaxies are so far apart, but are still intact. Eventually stars stop being made the last ones left eventually burn out and then the only thing left to govern the universe will be black holes. The universe won't grow big enough to produce enough dark energy for the big rip. Another possibility for the end of the universe as we know it is a phase change which I'm trying to learn more about right now.

I have to disagree with a couple of your points. First, the size of the universe is not, I think, relevant to the big rip scenario, and second, the universe will continue to grow and the amount of dark energy will continue to increase forever (that's in the big freeze scenario that you describe, which as I have already said, is, I believe, the consensus opinion on what will happen.

The big rip in not contingent so much on the amount of dark energy but whether or not it ever takes hold inside gravitationally bound systems. Since the amount of dark energy in the neighborhood of gravitationally bound systems is not going to increase (the amount per unit volume of space is roughly constant over time), it seems unlikely that this could ever happen, since it hasn't already, but some people do believe that it will, or at least could (but most seem to think it unlikely).

 

[Chalnoth]

The current consensus is that it is headed for the big freeze. The big rip seems to have been pretty much ruled out, although I don't know that that's conclusive. All the others have been promoted by various folks at various times but don't have any empirical evidence.

It was never ruled out. It's just that it was never taken very seriously. The big rip scenario has all sorts of theoretical problems that make it very implausible.

 

[Mr.CROWLER]

I have to disagree with a couple of your points. First, the size of the universe is not, I think, relevant to the big rip scenario, and second, the universe will continue to grow and the amount of dark energy will continue to increase forever (that's in the big freeze scenario that you describe, which as I have already said, is, I believe, the consensus opinion on what will happen.

The big rip in not contingent so much on the amount of dark energy but whether or not it ever takes hold inside gravitationally bound systems. Since the amount of dark energy in the neighborhood of gravitationally bound systems is not going to increase (the amount per unit volume of space is roughly constant over time), it seems unlikely that this could ever happen, since it hasn't already, but some people do believe that it will, or at least could (but most seem to think it unlikely).

Well from what I read my understanding is when the universe is twice as big there will be twice as much dark energy and the universe will expand faster when the amount of dark energy increases.

 

[phinds]

Well from what I read my understanding is when the universe is twice as big there will be twice as much dark energy and the universe will expand faster when the amount of dark energy increases.

Yes, but that is, as I said, irrelevant to local considerations. The amount of dark energy will increase because the size of the universe will increase. The amount of dark energy near galactic clusters will NOT increase because of that. You need to reread my post.

 

[Chalnoth]

Well from what I read my understanding is when the universe is twice as big there will be twice as much dark energy and the universe will expand faster when the amount of dark energy increases.

No.

The rate of expansion is proportional to the energy density. As the universe expands, the density of normal matter and dark matter go down, while the density of dark energy stays at least close to the same (perhaps exactly the same). So overall, the rate of expansion drops slowly, approaching a constant (or nearly constant) value set by the density of dark energy.

The reason why this is called an "accelerated expansion" is because with a constant expansion rate, the distances between objects accelerates.

 

[Mr.CROWLER]

No.

The rate of expansion is proportional to the energy density. As the universe expands, the density of normal matter and dark matter go down, while the density of dark energy stays at least close to the same (perhaps exactly the same). So overall, the rate of expansion drops slowly, approaching a constant (or nearly constant) value set by the density of dark energy.

The reason why this is called an "accelerated expansion" is because with a constant expansion rate, the distances between objects accelerates.

So technically normal matter and dark matter have been losing density since the creation of dark energy? If that's the case then thanks for clearing that up so basically it's almost like a star where gravity wants to crush it and fusion is pushing out. The density of normal matter and DM essentially is trying to pull things closer, but without the density dark energy could push things away from each other pretty much without any resistance.

 

[Chalnoth]

So technically normal matter and dark matter have been losing density since the creation of dark energy?

No. Normal matter and dark matter have been losing density as long as our universe has been expanding. They lose density because in an expanding universe, galaxies get further apart. Same amount of matter, bigger volume = lower density.

If that's the case then thanks for clearing that up so basically it's almost like a star where gravity wants to crush it and fusion is pushing out. The density of normal matter and DM essentially is trying to pull things closer, but without the density dark energy could push things away from each other pretty much without any resistance.

This part is more or less accurate. Normal matter and dark matter act to slow the rate of expansion.

 

[Mr.CROWLER]

What is the less part, so I can fully understand...I appreciate your help.

 

[Chalnoth]

What is the less part, so I can fully understand...I appreciate your help.

I'm not sure I can get much more accurate without also being a lot more confusing. The super-short version is that fusion doesn't actually push out (fusion keeps the star hot: in order to collapse, the star has to cool, but fusion prevents the star cooling), and the fact that the universe is expanding while the star isn't leads to other things that cause the analogy to break down.

But in super-simplistic terms, matter pulling inward while dark energy is pushing outward is good enough to go along with.

 

[Mr.CROWLER]

Fair enough thanks buddy

 

[phinds]

Dark energy is believed to have been around since the earliest times in the universe, it just wasn't able to overcome gravity until about 5 or 6 billion years ago. That is, it has always had an effect, just not one that was as noticeable as it is now that it has become dominant.

 

[julcab12]

Which one is more data friendly? Are there any oddities on the metric parameter -- density of matter and the cosmological constant.

 

[bapowell]

Which one is more data friendly? Are there any oddities on the metric parameter -- density of matter and the cosmological constant.

The relevant data is about the dynamics of the current dark energy-fueled expansion. It is not yet known whether the energy density causing the accelerated expansion is decreasing (potentially Big Crunch, but unlikely; most likely Big Freeze), increasing (Big Rip), or staying constant (Big Freeze). The empirical quantity of interest is $\dot{w}$, the rate of change of the equation of state parameter, $w = p/\rho$, giving the ratio of pressure to density of the fluid driving the expansion.

Cyclic models are harder to address empirically, because we are only observing one cycle. These tend to involve looking for evidence in the early universe that might resemble a bounce (typically based on assumptions of the theory underlying the bouncing/cyclic cosmology).

 

[julcab12]

The relevant data is about the dynamics of the current dark energy-fueled expansion. .

Ah, Thanks. I did a little skimming and found out that the current best measurements so far put w at −1 but with an uncertainty of 5%. ΛCDM still the simplest and favors freeze for now. What ever happens to quintessence?

 

[bapowell]

The best fit value of $w$ is only part of the story. We need to know how it is changing in order to project what the future holds. Quintessence may still be in the running -- importantly, it requires a changing $w$. Hence the importance of this measurement!

 

[Torbjorn_L]

is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch, or a Big Bounce?

I think the current consensus is, as reflected in this thread, that unless new physics or new constraints on LCDM appear, the universe is headed towards a Big Freeze. Which era is a nice symmetry to the initial cold inflation era, in my opinion.

 

[Chalnoth]

I think the current consensus is, as reflected in this thread, that unless new physics or new constraints on LCDM appear, the universe is headed towards a Big Freeze. Which era is a nice symmetry to the initial cold inflation era, in my opinion.

The energy scale is different by tens of orders of magnitude, however.

 

[Torbjorn_L]

Agreed. symmetry by similarity only.

 

[bahamagreen]

Isn't the present thinking that the universe will continue to expand? The long term implication for that would be the increasing separation of structures not gravitationally bound beyond some threshold... unbound structures recede until they move out of the visible universe, leaving the Milky Way and Andromeda galaxies, and a few little local ones, as all that remain within the horizon of the visible universe.

Assuming that present intelligent life continues or new intelligence continues to emerge, it looks like there would come a time when the entire known universe (for us or our replacements) would comprise entirely these two galaxies... nothing beyond.

The ancient myths and writings might mention a time when the universe was observed to be full of super clusters of galaxies like dust, etc. but would those of the future believe it? All they would have to observe would be a universe with just a pair of surviving galaxies... or maybe since Andromeda is now moving toward us, by then there might just be one galaxy alone in the visible universe...kind of like was thought about a hundred years ago...

 

[bapowell]

Isn't the present thinking that the universe will continue to expand?

Yes, that is one possibility sometimes called the Big Freeze. But if you read through this thread, other fates are possible.

 

[Chronos]

I believe the local group is gravitationally bound, so, even in the remote future these galaxies should remain within our observable horizon. The universe would be oddly empty beyond the local group. Even the CMB would eventually redshift beyond detectability. Assuming historical scientific data also retreated into obscurity, it is easy to see how cosmology could revert from a science to mythology.

 

[PeterDonis]

I believe the local group is gravitationally bound

Actually, the cluster of which the local group is part is gravitationally bound, at least according to our best current estimate, correct?

 

[Chalnoth]

I believe the local group is gravitationally bound, so, even in the remote future these galaxies should remain within our observable horizon. The universe would be oddly empty beyond the local group. Even the CMB would eventually redshift beyond detectability. Assuming historical scientific data also retreated into obscurity, it is easy to see how cosmology could revert from a science to mythology.

I honestly wonder if any life will be around by the time all galaxies that aren't gravitationally-bound become redshifted to undetectability. Certainly the Earth will be long-gone.

If we assume the limit of detectability is the current CMB temperature, then our universe would have to expand by a factor of over 1000 before all galaxies not gravitationally-bound to us are redshifted to undetectability. A super-rough back-of-the-envelope calculation leads me to think that this will take on the order of a hundred billion years.

 

[Clayjay]

Observation: Dark Matter,
I do not see why DM would not keep everything gravitationally bound. It keeps the galaxies bound along the filaments. It seems that dark matter would counteract the expansion. What am I missing?

 

[Chalnoth]

Observation: Dark Matter,
I do not see why DM would not keep everything gravitationally bound. It keeps the galaxies bound along the filaments. It seems that dark matter would counteract the expansion. What am I missing?

There isn't enough of it, compared to the rate of expansion, for the dark matter to halt the expansion.

 

[TheDemx27]

http://en.wikipedia.org/wiki/Future_of_an_expanding_universe
*Shudders at the size and scale of the universe of things. Existential crises incoming for the umpteenth time.*

 

[bahamagreen]

http://en.wikipedia.org/wiki/Future_of_an_expanding_universe
*Shudders at the size and scale of the universe of things. Existential crises incoming for the umpteenth time.*

Gotta love wikipedia, after 10^100 years, "What happens after this is speculative." :)

 

[Torbjorn_L]

If we assume the limit of detectability is the current CMB temperature, then our universe would have to expand by a factor of over 1000 before all galaxies not gravitationally-bound to us are redshifted to undetectability. A super-rough back-of-the-envelope calculation leads me to think that this will take on the order of a hundred billion years.

And so most life would be gone. Granted, new red/white dwarfs will be produced for quite a while yet. But star production has dropped a factor 100 on average AFAIK (running out of gas, literary), so that is at the tail end of natural emergent life.

As for the rest of cosmology, it will be knowable for a while after that, just more difficult. The galaxy SMBH will throw the random high speed star out of the galaxy, and those suffice to observe dark energy as I understand it. Hence predicting the else curious CMB spectral peak portions.

"What happens after this is speculative."

Ah, another symmetry (by analogy). Cosmology has speculative physics before and after the known parts.

 

[Clayjay]

There isn't enough of it, compared to the rate of expansion, for the dark matter to halt the expansion.

i am just trying to make sense of this so allow me to round numbers - On a cosmic scale the universe is about 70% dark energy and 30% gravitational energy (power). Of all the gravity 85% is dark matter and 15% is light matter (baryons). The local group is gravitationally bound and so are all galaxy groups. All galaxies groups exist within dark matter and dark matter is gravitational bound.

Where could dark energy expand space? It seems to me the perfect environment for expressing dark energy is in the very huge cosmic special voids. The internal distance or diameter of the voids run around between 50 -150 million light years while the distance across filaments is around 5-10 million light years.

I am not saying I am right but I am expressing my simple logic that counters the idea of dark energy acting on any gravitationally bound matter, even dark matter clouds that envelop baryonic matter. As far as dark or baryonic matter is concerned dark energy does not exist.

 

[Chalnoth]

The bit about gravitational energy is entirely incorrect. It's about 30% matter (currently). Gravitational energy is a somewhat nebulous concept which would be extremely misleading to use in this context, because dark energy influences the expansion history through gravity as well. It's just that different kinds of matter have different impact on the expansion rate.

Dark energy also doesn't "expand space". The expansion is already there. Dark energy just prevents the expansion rate from falling too low. It acts, in effect, as a repulsive force. Basically, it changes gravity in the Newtonian limit from:
$$F = -{G m_1 m_2 \over r^2}$$
to:
$$F = -{G m_1 m_2 \over r^2} + {1 \over 3}\Lambda r$$

This has an impact everywhere in the universe. But the value of $\Lambda$ is so small that it's unmeasurable on scales shorter than billions of light years.

Finally, the the density ratios today really don't tell you much of anything about our universe (except for the ratio of normal matter to dark matter: that's pretty stable over time). The problem is that the density ratios are a function of time. A billion years ago, matter would have been a larger portion of the total energy density. 12 billion years ago dark energy was a negligible component of the total energy density.

In order to get a handle on what the makeup of the universe means, you have to look at each component individually. The dark energy remains roughly constant over time. As the square of the expansion rate is proportional to the density, this means that the current ~70km/s/Mpc expansion rate will slowly decrease to ~60km/s/Mpc in the far future. As long as the dark energy is constant, it will remain at that rate indefinitely. The matter density, meanwhile, will decrease over time until nothing but empty space remains.