Death of Universe: Is a New Big Bang Possible?

[Chalnoth]

You may have read about http://en.wikipedia.org/wiki/Virtual_particle" , even if a virtual particle were to become real the process would still obey conservation of energy.

While potentially true, new universes produced out of quantum fluctuations don't need to have any energy: in the Hamiltonian formalism, a closed FRW universe has energy identically equal to zero.

I should have clarified that the sum total of energy in the universe remains the same no matter what.

This is patently false. Just consider the case of a radiation-dominated universe. In a radiation-dominated universe, if the universe expands by a factor of two, then the volume increases by a factor of eight, but the energy density of the photons drops by a factor of 16. So the total energy in the volume is cut in half.

 

[Ryan_m_b]

This is patently false. Just consider the case of a radiation-dominated universe. In a radiation-dominated universe, if the universe expands by a factor of two, then the volume increases by a factor of eight, but the energy density of the photons drops by a factor of 16. So the total energy in the volume is cut in half.

My apologies, I have not heard of this before. Am I not right in saying that because can be neither created nor destroyed the universe as an isolated system has a constant amount of energy inside of it?

 

[Tanelorn]

Ryan, by spontaneous I meant an instantaneous random quantum fluctuation at the Planck level which created everything. I may be well out of date though, I am pretty retro (1980s) :)

 

[Chalnoth]

My apologies, I have not heard of this before. Am I not right in saying that because can be neither created nor destroyed the universe as an isolated system has a constant amount of energy inside of it?

The problem is that there is no unique definition of total energy in a volume in General Relativity. Since there is no unique definition of total energy in a volume, it can't be a conserved quantity.

Now, you can come up with definitions of energy within General Relativity that are conserved under certain conditions, but there can't be any general conservation law.

Another way of pointing out this fact is to look at where conservation laws come from in the first place: by Noether's theorem, we know that conservation laws reflect symmetries of the underlying laws of physics. Energy conservation, for example, stems from certain properties of the system being invariant with respect to time. In particular, if the metric changes over time, then energy probably isn't going to be conserved. And in the case of an expanding universe, the metric changes over time.

Anyway, you may like reading this essay on the subject:
http://www.desy.de/user/projects/Physics/Relativity/GR/energy_gr.html

One other comment I'd like to make is that there is an absolute conservation law in General Relativity: the stress-energy tensor is absolutely conserved. This includes energy density, but also other things as well. In flat space-time, conservation of the stress-energy tensor reduces to the continuity equation: that the change in energy at a point is equal to the energy flowing into/out of that point. So in the right limit, you do get energy conservation. It just can't be taken to happen globally in a curved space-time.

 

[Tanelorn]

From post 25:

Ben, one other point in that link you provided. I am havng a hard time visualising how most of the stars will eventually be ejected from old eliptical galaxies.

I am still unable to figure out why this happens. Can anyone offer a reason?

 

[Chalnoth]

From post 25:

Ben, one other point in that link you provided. I am havng a hard time visualising how most of the stars will eventually be ejected from old eliptical galaxies.

I am still unable to figure out why this happens. Can anyone offer a reason?

As I understand it, gravitational interactions between the different stars in the galaxy continually push the kinetic energy distribution of the stars towards thermal equilibrium. But in thermal equilibrium, there is always a high-energy tail that has a velocity above the escape velocity for the galaxy.

 

[Tanelorn]

If there are forces that eject nearly all the stars out of an eliptical galaxy you kind of wonder how they ever got gravitationally bound in the first place! :)

 

[Chalnoth]

If there are forces that eject nearly all the stars out of an eliptical galaxy you kind of wonder how they ever got gravitationally bound in the first place! :)

Well, two points here. First, it is only a minuscule fraction that are ejected at any given time. The rate of ejection due to this effect is vastly, vastly slower than the rate of formation, so it just isn't an impediment. Secondly, a major factor in forming galaxies is the friction of the gas that is later formed into stars. This friction tends to cause the gas to collapse inward quite rapidly as it loses energy. Once the stars form and the friction basically goes away, this collapse is halted. It is only then that some tiny fraction of the stars start getting ejected from the galaxy.

One way to think about it is that as long as you have friction, the high-kinetic energy tail of the distribution will be slowed down much more than the objects with less kinetic energy, so that until the friction goes away, you just won't get a whole lot of ejection.

 

[Chronos]

What suggests decaying orbits result in gravitational collapse of an orbital system?

 

[Chalnoth]

What suggests decaying orbits result in gravitational collapse of an orbital system?

I don't know what you're asking. To me, those are different words for the exact same thing.

 

[Cosmo Novice]

Assuming final entropic disorder and a heat death, does this mean at the exact "time" of final heat death (all matter/energy existing independently in the smallest quantities possible within its own cosmological event horizon) that the second law of thermodynamics ceases to be a valid law?

I am not promoting this as true in anyway but I always percieve final heat death to be t=0 and think that we should be counting backwards to 0 instead of forwards :) Maybe my brain ticks the wrong way.

 

[Chalnoth]

Assuming final entropic disorder and a heat death, does this mean at the exact "time" of final heat death (all matter/energy existing independently in the smallest quantities possible within its own cosmological event horizon) that the second law of thermodynamics ceases to be a valid law?

Why would it cease to be valid? The second law of thermodynamics states that entropy never decreases. When the universe reaches equilibrium, entropy will be constant, so not decreasing.

 

[bcrowell]

Assuming final entropic disorder and a heat death, does this mean at the exact "time" of final heat death (all matter/energy existing independently in the smallest quantities possible within its own cosmological event horizon) that the second law of thermodynamics ceases to be a valid law?

I am not promoting this as true in anyway but I always percieve final heat death to be t=0 and think that we should be counting backwards to 0 instead of forwards :) Maybe my brain ticks the wrong way.

Usually when a system approaches equilibrium, it does it asymptotically, like an exponential function. So I don't think there would be a well-defined moment when it would max out.

I also don't think it's really known whether entropy and equilibrium can be defined properly in this context. There's an argument that the true equilibrium would be one in which gravitational degrees of freedom were equilibrated with the others, which would give you a scenario with a jumble of random gravitational waves, maybe somewhat like Misner's mixmaster universe. I don't know of any mechanism that would allow our universe to relax into such a state.

One way of seeing that this is not presently well understood is that if heat death occurs, then computation must cease. But it seems to be controversial whether infinite computations are physically possible in realistic cosmologies:

Krauss and Starkman, 1999, Life, The Universe, and Nothing: Life and Death in an Ever-Expanding Universe, http://arxiv.org/abs/astro-ph/9902189

Katherine Freese and William Kinney, 2002, The ultimate fate of life in an accelerating universe, http://www.arxiv.org/abs/astro-ph/0205279

Some possibly relevant speculation near the end of this:
http://math.ucr.edu/home/baez/end.html

 

[Tanelorn]

Some possibly relevant speculation near the end of this:
http://math.ucr.edu/home/baez/end.html

Again a very sad conclusion and end to the universe really. No hope of a repeat cycle, just a long slow fade away, like the fading heat of a bonfire from the night before. Darn.. makes you wonder what is the point sometimes..

 

[Chalnoth]

Again a very sad conclusion and end to the universe really. No hope of a repeat cycle, just a long slow fade away, like the fading heat of a bonfire from the night before. Darn.. makes you wonder what is the point sometimes..

I don't think so. The fact that our universe started is, to me, proof positive that another universe can start again.

 

[Tanelorn]

Chalnoth, I think you are just trying to make me feel better, well... you succeeded! Thanks Chris

 

[Chalnoth]

Chalnoth, I think you are just trying to make me feel better, well... you succeeded! Thanks Chris

I should mention, however, that the problem that Baez pointed out is a very real problem. It is very clear that there is a significant issue with respect to how a new universe can begin. These days, I'm leaning towards the de Sitter Equilibrium idea. Here is a paper describing how inflation is likely in this picture:
http://arxiv.org/abs/hep-th/0405270

 

[Tanelorn]

Perhaps we can view the gradual winding down of the universe as rather like a great cuckoo clock. When it stops it just needs someone or something to come along and wind it back up again.

All that would be required would be for dark energy to slow down and switch sign. Perhaps this way it could yet be the driving force for a cyclical universe?

 

[Chalnoth]

Perhaps we can view the gradual winding down of the universe as rather like a great cuckoo clock. When it stops it just needs someone or something to come along and wind it back up again.

Well, not quite. That view runs into the precise problems that Baez mentioned in that essay you linked. However, it is conceivable that new regions of space-time are periodically born that spawn new universes. Ours, however, is most likely to undergo heat death and remain that way forever, even if it does spawn new universes.

All that would be required would be for dark energy to slow down and switch sign. Perhaps this way it could yet be the driving force for a cyclical universe?

That seems to me a pretty tall order.

 

[bcrowell]

Perhaps we can view the gradual winding down of the universe as rather like a great cuckoo clock. When it stops it just needs someone or something to come along and wind it back up again.

All that would be required would be for dark energy to slow down and switch sign. Perhaps this way it could yet be the driving force for a cyclical universe?

Tanelorn, please take a look at PF's rules https://www.physicsforums.com/showthread.php?t=414380 about overly speculative posts.

 

[Tanelorn]

Ben, point taken, just trying to be humourous. Gallows humour actually after finally accepting the predicted death of the universe.


"Ours, however, is most likely to undergo heat death and remain that way forever, even if it does spawn new universes."

Thanks Chalnoth so the final outcome remains pretty grim.



"That seems to me a pretty tall order."

I understood that dark energy has already changed once during the history of the universe. I believe I read that it started increasing about 7B years ago, unless I misunderstood something?
So I was just asking if there is any reason why, in the very distant future, dark energy could not start changing again?

 

[Chalnoth]

I understood that dark energy has already changed once during the history of the universe. I believe I read that it started increasing about 7B years ago, unless I misunderstood something?
So I was just asking if there is any reason why, in the very distant future, dark energy could not start changing again?

There isn't any reason to believe this is the case. If dark energy really is a cosmological constant, its value was simply irrelevant until a few billion years ago, but there's no reason to believe it changed.

 

[Tanelorn]

Thanks For clearing that up Chalnoth, I must have misinterpreted something I read about a change in dark energy about 7B years ago.

Also I think I may have been guilty of somehow still wanting the universe to end by eventually returning everything to its initial conditions. It seems more elegant for all matter energy space and time to return to inital conditions, perhaps like conservation of energy or an electron positron pair being spontaneously created and destroyed. There is no possibility of an opposite process to inflation to achieve something like this?

 

[Chalnoth]

It seems more elegant for all matter energy space and time to return to inital conditions, perhaps like conservation of energy or an electron positron pair being spontaneously created and destroyed. There is no possibility of an opposite process to inflation to achieve something like this?

Seems unlikely. I mean, if you want to wait for some obscene number of years, it will happen. But counting on simple recurrence doesn't solve the entropy problems. You need to have an alternative pathway to producing new regions of space-time to make the entropy make sense.

 

[MathematicalPhysicist]

This essentially means eventually the energy will be spread out evenly, it is currently thought that this will lead to http://en.wikipedia.org/wiki/Heat_death_of_the_universe" in about 10¹⁰⁰ years.

A long time to falsify a theory.

 

[Chalnoth]

A long time to falsify a theory.

Well, it's not a theory. It's a conclusion based upon our knowledge of the theories of physics. It isn't absolutely necessary to test each end every conceivable conclusion of a theory to be confident in its veracity.

 

[MathematicalPhysicist]

Well, it's not a theory. It's a conclusion based upon our knowledge of the theories of physics. It isn't absolutely necessary to test each end every conceivable conclusion of a theory to be confident in its veracity.

Indeed, upon our current theories, as every empirical science things may change in the years to come in our perception of nature.

In a class I took as an undergrad, our teacher of QM 2, in the last lecture showed us a calculation that even according to QM (before QFT) the universe shouldn't have gone so far, in fact it should have collapsed in a matter of a few seconds.

So to me asserting how many years does the universe have before it enters a heat death is a little bit like believing in fairytales.

I believe I am becoming a pragmatist in my views.

 

[Chalnoth]

Indeed, upon our current theories, as every empirical science things may change in the years to come in our perception of nature.

While this is true, the ways in which theory would have to change to change the conclusion of heat death are rather unlikely. Basically, in order to avoid heat death you would need:
1. The dark energy to be composed of some sort of dynamical field that will, at some point in the future, start to degrade in energy density quite rapidly (so far, it has stayed constant or nearly so).
2. The spatial curvature of our universe to be slightly positive.

The second point is basically 50/50 given current knowledge, but the first requires extremely contrived models of dark energy, and is thus pretty unlikely.

In a class I took as an undergrad, our teacher of QM 2, in the last lecture showed us a calculation that even according to QM (before QFT) the universe shouldn't have gone so far, in fact it should have collapsed in a matter of a few seconds.

I'd like to know what line of reasoning he was using here. But, suffice it to say, this sort of thing depends quite strongly upon high-energy physics far beyond what we have been able to test in the lab so far, so it is somewhat foolish to draw conclusions there. The heat death of the universe, however, does not depend upon anything that isn't well-tested in the lab, except for the nature of dark energy and the spatial curvature.

 

[Tanelorn]

"So to me asserting how many years does the universe have before it enters a heat death is a little bit like believing in fairytales. I believe I am becoming a pragmatist in my views."


So far, so good, as they say.