The death of universe

[Nernico]

How will universe die? Energy cant be created or destroyed so is that possible that the energy of this dead universe could create a new big bang?

[Ryan_m_b]

Hi there Nernico. You are right in saying that energy cannot be created or destroyed however thanks to entropy (http://en.wikipedia.org/wiki/Entropy) the universe is heading towards its maximum state of disorder. This essentially means eventually the energy will be spread out evenly, it is currently thought that this will lead to heat death (http://en.wikipedia.org/wiki/Heat_death_of_the_universe) in about 10100 years.

There are other ideas regarding the ultimate fate of the universe (http://en.wikipedia.org/wiki/Fate_of_the_universe) including the idea of a Big Crunch (http://en.wikipedia.org/wiki/Big_Crunch) resulting in a Big Bounce (http://en.wikipedia.org/wiki/Big_Bounce) (creating a new universe) but as I understand it the heat death is the only idea that has evidence to support it.

[Tanelorn]

Hi Nernico, my understanding of the current theory is that all matter and energy eventually leaves the universe the same way as it entered, through a singularity or singularities (as in black holes).

[bcrowell]

Hi Nernico, my understanding of the current theory is that all matter and energy eventually leaves the universe the same way as it entered, through a singularity or singularities (as in black holes).

I don't think that's right. The WP article that ryan linked to is actually pretty good.

[Tanelorn]

Ben last time I looked at this was quite a while ago so maybe I am out of date. The last few sections here is how I recall the end:

http://www.universetoday.com/11430/the-end-of-everything/


much regards,
Chris

[Chalnoth]

How will universe die?
Most likely heat death. Wikipedia has a good breakdown:
http://en.wikipedia.org/wiki/Future_of_an_expanding_universe

Energy cant be created or destroyed so is that possible that the energy of this dead universe could create a new big bang?
This isn't entirely true. There is no concept of global energy conservation in General Relativity. This is a good read on the subject:
http://www.desy.de/user/projects/Physics/Relativity/GR/energy_gr.html

In the end, all matter energy will eventually become radiation energy. And radiation energy tends to dilute as the fourth power of the scale factor, so that in an expanding universe, the total energy in a set comoving volume decreases with time (a comoving volume is one that expands along with the expansion of the universe). In these coordinates, the total energy of a comoving volume of the universe tends towards zero.

[bcrowell]

Ben last time I looked at this was quite a while ago so maybe I am out of date. The last few sections here is how I recall the end:

http://www.universetoday.com/11430/the-end-of-everything/



The article doesn't support what you said in #3, that "all matter and energy eventually leaves the universe the same way as it entered, through a singularity or singularities (as in black holes)." It talks about converting baryonic matter into photons through hawking radiation.

[bcrowell]

In the end, all matter energy will eventually become radiation energy.

I don't think this is quite right. There is no reason to believe that every single atom in the universe will get consumed by a black hole and then recycled into Hawking radiation. This is actually one of the problems with Penrose's CCC. To make CCC work, he needs 100% conversion of baryonic matter into photons, but theory doesn't offer any reason to believe that it will be 100%.

[Ryan_m_b]

I don't think this is quite right. There is no reason to believe that every single atom in the universe will get consumed by a black hole and then recycled into Hawking radiation. This is actually one of the problems with Penrose's CCC. To make CCC work, he needs 100% conversion of baryonic matter into photons, but theory doesn't offer any reason to believe that it will be 100%.

Would an individual atom not eventually decay into something else or are they perpetually stable? I'm not proposing that it will decay I genuinely don't know.

[bcrowell]

Would an individual atom not eventually decay into something else or is it perpetually stable? I'm not proposing that it will decay I genuinely don't know.

There is speculation about proton decay, http://en.wikipedia.org/wiki/Proton_decay , but there's no actual evidence for it.

Even if proton decay exists, there are other forms of matter such as neutrinos that are stable as far as we know, and in the far future you can also have individual electrons that end up isolated inside their own cosmological event horizons forever.

There's a brief discussion of this kind of thing in the following paper by Penrose on CCC: http://epaper.kek.jp/e06/PAPERS/THESPA01.PDF

[Chalnoth]

I don't think this is quite right. There is no reason to believe that every single atom in the universe will get consumed by a black hole and then recycled into Hawking radiation.
Talking with the high energy physics people, proton decay is generally expected to be inevitable. That is why all matter will eventually become radiation.

This is actually one of the problems with Penrose's CCC. To make CCC work, he needs 100% conversion of baryonic matter into photons, but theory doesn't offer any reason to believe that it will be 100%.
The main problem is that there is no physical mechanism.

[Tanelorn]

Ben, all matter ending up in black holes is one of the possibilities discussed here and it stuck with me for decades. Perhaps it is no longer considered likely. Dark energy expansion of the universe may help prevent it.

"Chunks of matter and binary black dwarfs will merge together creating new black holes, and these black holes will be consumed by even larger black holes. It might be that in the far future, all matter will exist in a few, truly massive black holes."

http://www.universetoday.com/11430/t...of-everything/

[bcrowell]

Talking with the high energy physics people, proton decay is generally expected to be inevitable. That is why all matter will eventually become radiation.
Sorry, but I'm not buying it based on your secondhand impression that other people think it's inevitable. In any case, it has nothing to do with the arguments about neutrinos, or electrons isolated inside cosmological horizons. There's also the issue of dark matter, which is probably stable as well, although we know almost nothing about it.


The main problem is that there is no physical mechanism.
I wasn't pointing you to the paper to say that I think CCC is right. I was pointing you to the paper because it discusses the reasons why it is incorrect to claim that all matter will become radiation. Penrose would like all matter to become radiation, because it's necessary if he wants CCC to work. Nevertheless he honestly surveys what we know about the question, and concludes that based on current knowledge of particle physics, it won't happen. For this reason, one of the predictions he was originally pushing as part of CCC was nonstandard particle physics that would allow what you're claiming to be true.

[bcrowell]

Ben, all matter ending up in black holes is one of the possibilities discussed here and it stuck with me for decades. Perhaps it is no longer considered likely. Dark energy expansion of the universe may help prevent it.

"Chunks of matter and binary black dwarfs will merge together creating new black holes, and these black holes will be consumed by even larger black holes. It might be that in the far future, all matter will exist in a few, truly massive black holes."

http://www.universetoday.com/11430/t...of-everything/

Note the "it might be." And again, this does not support your statement that "all matter and energy eventually leaves the universe the same way as it entered, through a singularity or singularities (as in black holes)." The mass-energy recycled through Hawking radiation does not "leave the universe." For that matter, nothing "entered" the universe "through" the big-bang singularity.

Tanelorn and Chalnoth, let's try to raise the level of discussion here. I've given you guys a reference to an actual scientific paper. You've given me statements that you think other people think X is true, and to a popular-level article that doesn't support your claim.

[bcrowell]

I turned up a good review article on this topic: Adams and Laughlin, A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects, , Rev.Mod.Phys.69:337-372,1997, http://arxiv.org/abs/astro-ph/9701131

"Thus, unless the dark matter particles themselves
decay into radiation, it seems that enough non-baryonic dark matter should survive to keep the universe
matter dominated at all future epochs; in addition, the leftover electrons and positrons will help prevent
the universe from becoming radiation dominated" (p. 38)

[Tanelorn]

Ben, I have no proof, it was just one of several possible endings that I read in the popular press, possibly even before Hawking radiation was proposed, and to me at least, the one that seems the most elegant. ie. Matter and energy entered the universe via a singularity, and at the end leaves the universe via a singularity. As I said I am probably well out of date, but I will read the paper you linked soon. Just started reading it, it is impressive.

Would you not agree however that the orbit of all the stars in our galaxy over an extremely long time will eventually decay until the central black hole makes its kill?

[bcrowell]

Ben, I have no proof, it was just one of several possible endings and to me at least the one that seems the most elegant.

Would you not agree however that the orbit of all the stars in our galaxy over an extremely long time will eventually decay until the central black hole makes its kill?

You don't have to reinvent the wheel. Read the paper by Adams and Laughlin.

The Adams/Laughlin review dates back to before we knew about the nonzero cosmological constant. Here is a discussion that's more up to date but less detailed:

Baez, J., 2004, "The End of the Universe.", http://math.ucr.edu/home/baez/end.html

[Tanelorn]

Thanks Ben, I read that entirely. It seems that matter and energy never leaves the universe at all then. Perhaps it always was here then before the BB.

He does also mention that Cosmology is a fast moving science at the moment. In my design / integration phase in work we say that a product is still no where near ready to ship whilst it still rapidly undergoing mods due to bugs. If we consider how many cosmological theories have been proposed since the ancients first thought the earth was the center of the universe, then what is the chance that the latest theories are correct? However, I agree that the latest is probably the best we have.

[Chalnoth]

Sorry, but I'm not buying it based on your secondhand impression that other people think it's inevitable.
The way I understand it, the argument goes that in order to have baryogensis, you need baryon number to not be exactly conserved. And if baryon number isn't exactly conserved, protons will, at some point, decay.

There's also the issue of dark matter, which is probably stable as well, although we know almost nothing about it.
Yes, but the dark matter is expected to have been produced in equal parts matter and anti-matter, meaning it will eventually annihilate.

[bcrowell]

Yes, but the dark matter is expected to have been produced in equal parts matter and anti-matter, meaning it will eventually annihilate.

This would contradict the statement by Adams and Laughlin that I quoted in #15. Even if dark matter is an equal mix of particles and antiparticles (which I doubt can be stated with any certainty right now), there is the fact that some individual particles find themselves isolated inside their own cosmological horizons. This is discussed in both the Penrose paper and the Baez page.

[Haelfix]

Even if there was no Beyond the standard model theory out there (where dimension 6 terms are often generated naturally) proton decay would occur, given long enough timeframes, by Sphaeleron decay and other nonpertubative effects.

The timeframes involved are astronomical though, albeit small relative to the recurrence time of a DeSitter end state.

[Chalnoth]

This would contradict the statement by Adams and Laughlin that I quoted in #15. Even if dark matter is an equal mix of particles and antiparticles (which I doubt can be stated with any certainty right now), there is the fact that some individual particles find themselves isolated inside their own cosmological horizons. This is discussed in both the Penrose paper and the Baez page.
Yes, well, one particle in a sphere roughly 16 billion light years in radius is close enough.

[bcrowell]

Yes, well, one particle in a sphere roughly 16 billion light years in radius is close enough.

Close enough to...? Close enough so that...?

The point is that the universe is predicted to be matter-dominated, not radiation-dominated. It's the opposite of what you claimed, which was that all matter would be converted to radiation.

[Chalnoth]

Close enough to...? Close enough so that...?

The point is that the universe is predicted to be matter-dominated, not radiation-dominated. It's the opposite of what you claimed, which was that all matter would be converted to radiation.
Well, dark energy-dominated. What I was basically saying is that matter gets converted to radiation, which then redshifts away to irrelevance.

[Tanelorn]

So what we are saying then is that nothing in this universe is eternal, all matter and energy just disperses and fades away into insignificance, perhaps like wiping the board clean as it was before the last BB, ready and waiting for something to create another BB?

It is also interesting that this stage in the life cycle of our universe is by no means the main stage but is very temporary in comparison. Any life that still exists later on had better be pretty frugal with its energy usage.


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.

[Ryan_m_b]

So what we are saying then is that the nothing in this universe is eternal, all matter and energy just disperses and fades away into insignificance, perhaps like wiping the board clean as before the last BB, ready and waiting for something to create another BB?

I don't think anyone here is saying that. The notion of "before" the Big Bang does not make sense in contemporary physics, we can only probe back to the Planck epoch (http://en.wikipedia.org/wiki/Planck_epoch). Whether or not time existed 'before' then is unknown.

As for creating another Big Bang I would advise you to read some of the sites linked in this thread. Currently we do not have a comprehensive understanding of the mechanism underlying the Big Bang, at the moment the ultimate fate of the universe looks to be heat death (http://en.wikipedia.org/wiki/Heat_death_of_the_universe) however there are other suggestions (http://en.wikipedia.org/wiki/Fate_of_the_universe).

[Tanelorn]

Ryan I think I was also trying to describe heat death, just couldnt think of the term.

Would I be correct in saying that a fresh supply of Hydrogen would keep up star formation and thus a reasonably steady state universe? Is there any scenario where a small but endless supply of fresh hydrogen could continuouslly be produced from (just brainstorming here) galactic voids, dark energy, dark matter, spontaneous matter creation or some other method? I dont know if such a thing is so outragaous when we consder that the entire universe was created spontaneously.

[Ryan_m_b]

Ryan I think I was also trying to describe heat death, just could think of the term.

I got that part of your statement, it was the latter part "perhaps like wiping the board clean as it was before the last BB, ready and waiting for something to create another BB?" that I was objecting to.

Would I be correct in saying that a fresh supply of Hydrogen would keep up star formation and thus a reasonably steady state universe?

Eventually the mass that this hydrogen is adding to the universe may eventually result in a Big Crunch (http://en.wikipedia.org/wiki/Big_Crunch). I'm sure there are other, bigger objections here too

Is there any scenario where a small but endless supply of fresh hydrogen could continuouslly be produced from (just brainstorming here) galactic voids, dark energy, dark matter, spontaneous matter creation or some other method? I dont know if such a thing is so outragaous when we consder that the entire universe was created spontaneously.

Current understanding is not that the universe was "created" nor was it "spontaneous". As I have already explained we do not have a comprehensive understanding for why the Big Bang occurred but there is no reason to suggest that everything popped into existence, it is my understanding that the universe most likely began as a singularity with all the matter and energy that we have now residing in one place. No extra mass or energy was created. As the the matter creation you are asking about such a thing would violate conservation of energy (http://en.wikipedia.org/wiki/Conservation_of_energy). To specifically address your question'

Galactic Voids - are empty space, there is no reason to think that this would allow matter to pop into existence.
Dark energy - the proposed, homogeneous energy in the universe that is facilitating it's expansion, whilst no comprehensive understanding of this exists there is no reason to think that this would allow matter to pop into existence.
Dark matter - the proposed exotic matter that interacts with normal matter in a limited manner. It accounts for all the "missing mass" in the universe. Again there is no reason to think that this would allow matter to pop into existence

[Tanelorn]

Ryan I do recall reading that some matter is spontaneously produced in free space. Was it electron positron pairs?

PS I like your signature, Asimov's Foundation series is excellent, Galaxia and so on. Of course the earth is flat from my perspective and the earth is round from the moons perspective. This makes me wonder if our cosmological pespective is good.

[Ryan_m_b]

Ryan I do recall reading that some matter is spontaneously produced in free space. Was it electron positron pairs?

You may have read about virtual particles (http://en.wikipedia.org/wiki/Virtual_particle) and quantum fluctuations (http://en.wikipedia.org/wiki/Quantum_fluctuations), even if a virtual particle were to become real the process would still obey conservation of energy. I should have clarified that the sum total of energy in the universe remains the same no matter what.

[Chalnoth]

You may have read about virtual particles (http://en.wikipedia.org/wiki/Virtual_particle) and quantum fluctuations (http://en.wikipedia.org/wiki/Quantum_fluctuations), 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 http://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 acheive 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 acheive 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 heat death (http://en.wikipedia.org/wiki/Heat_death_of_the_universe) in about 10100 years.



A long time to falsify a theory.
:tongue:

[Chalnoth]

A long time to falsify a theory.
:tongue:
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.