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Gold Barz
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I need to get up to date on the current consensus on the likely fate of the universe? Is it heat death, big crunch, big freeze, big bounce (didn't this get disproved a couple of years ago?)
Chalnoth said:Heat death is currently the most likely result.
The 'big rip' scenario has tremendous theoretical difficulties, and is highly unlikely. Bounce/crunch models would require very special behavior for dark energy, as well as slightly positive spatial curvature, and are also unlikely.
Since we're talking about the ultimate fate of the universe, they always will be. The current evidence, as it stands, still points towards heat death. Perhaps that will change in the future, but that's the way things stand now.bapowell said:Sure, but all that is certainly debatable. From a purely observational perspective, all are still in the running.
It would require violation of the weak energy condition (that matter energy is always non-negative), which most consider to be impossible, especially on cosmological scales where whatever microscopic physics produce gravity are likely to be less relevant.bapowell said:I would not equate "tremendous theoretical difficulties" with "highly unlikely" since that implies that you are happy with the current state of gravity theory, which is widely perceived to be incomplete.
Dark energy, so far, has been decaying very slowly or not at all. A bounce/crunch would require that at some point in the future, this behavior changes so that it starts decaying more rapidly than [itex]1/a^2[/itex]. I'm not aware that any models do this.bapowell said:With regards to bounce/crunch, what very special behavior is required?
The difference is that we have strong observational reasons to believe inflation occurred.bapowell said:For example, the inflaton potential is terribly fine-tuned and the initial conditions perhaps even more so. That's very special behavior but few people argue that inflation is unlikely to have occurred.
OK, I guess I just disagree with this statement. As far as evidence is concerned, baring any arguments regarding the theoretical viability of phantom energy, a Big Rip is still in agreement with data ([tex] w < -1.10 \pm 0.14 [/tex] at 68% CL from WMAP7). Also, since we have no constraints on [tex]\dot{w}[/tex], I would suggest that, observationally, big crunch models are also still OK. I am certainly not an advocate for either of these models, but strictly speaking, observational evidence does not point to heat death over other alternatives.Chalnoth said:Since we're talking about the ultimate fate of the universe, they always will be. The current evidence, as it stands, still points towards heat death. Perhaps that will change in the future, but that's the way things stand now.
It would require violation of the weak energy condition (that matter energy is always non-negative), which most consider to be impossible, especially on cosmological scales where whatever microscopic physics produce gravity are likely to be less relevant.
Why can't you? Is there a theoretical reason? I'm no expert on dark energy, but I would think it would be simply a matter of 'designing' the right potential that decays sufficiently fast at some point in the future.Dark energy, so far, has been decaying very slowly or not at all. A bounce/crunch would require that at some point in the future, this behavior changes so that it starts decaying more rapidly than [itex]1/a^2[/itex]. I'm not aware that any models do this.
OK, but that seems kind of like an a posteriori qualification for what makes a theory 'likely'.The difference is that we have strong observational reasons to believe inflation occurred.
Interacting dark energy models can provide apparent [itex]w < -1[/itex], but do not lead to a big rip scenario. They are the only somewhat realistic physical models which have been shown to produce a measured [itex]w < -1[/itex]. So if anything, this might be evidence for interacting dark energy. But since a one sigma deviation from -1 is the expected deviation, and since that deviation could, by random chance, fall on either side of [itex]w = -1[/itex], there really isn't any evidence of [itex]w < -1[/itex].bapowell said:OK, I guess I just disagree with this statement. As far as evidence is concerned, baring any arguments regarding the theoretical viability of phantom energy, a Big Rip is still in agreement with data ([tex] w < -1.10 \pm 0.14 [/tex] at 68% CL from WMAP7).
Well, what would cause the decay? The longer that the dark energy acts like a cosmological constant, the less the universe changes going into the future.bapowell said:Why can't you? Is there a theoretical reason? I'm no expert on dark energy, but I would think it would be simply a matter of 'designing' the right potential that decays sufficiently fast at some point in the future.
Pretty sure that doesn't work. Inflation depends upon "slow roll" which requires that the expansion of the universe put sufficient friction on the scalar field so that it takes a while to reach its potential minimum. If I remember correctly, the parameters are all wrong for the same sort of thing to be occurring now.Chronos said:Keep in mind the universe experienced an extreme inflationary epoch immediately following the big event. It died out rather quickly. It appears we are now experiencing an 'aftershock'. My hunch is it too will die out.
Why? Slow-roll mimics de Sitter expansion. What is needed for late-time acceleration is a potential that is also slowly rolling, but even more so than during primordial inflation (since the epoch of late-time acceleration has lasted so much longer than the epoch of primordial inflation.) I'm not sure what parameters you're referring to.Chalnoth said:Pretty sure that doesn't work. Inflation depends upon "slow roll" which requires that the expansion of the universe put sufficient friction on the scalar field so that it takes a while to reach its potential minimum. If I remember correctly, the parameters are all wrong for the same sort of thing to be occurring now.
Well, I suppose you could do it, but it would require a completely different potential energy than the one that drove inflation. The expansion rate has just been too slow to retard the decay of an inflaton. So it might be similar to inflation, but it wouldn't appear to be directly related to it.bapowell said:Why? Slow-roll mimics de Sitter expansion. What is needed for late-time acceleration is a potential that is also slowly rolling, but even more so than during primordial inflation (since the epoch of late-time acceleration has lasted so much longer than the epoch of primordial inflation.) I'm not sure what parameters you're referring to.
Well, first you're making the assumption that a "true vacuum" has zero energy, which we certainly don't know. There is no known symmetry that sets the vacuum energy to zero.bapowell said:With regards to your earlier question about what would make such a scalar field decay nowadays -- why not the same thing that made the inflaton decay? Perhaps 'decay' is a poor choice of words -- all we really need is for the late-time scalar to lose potential energy. This will occur for any suitable potential that is monotonically decreasing with the expansion of the universe. In other words, to get late time acceleration that is asymptotically approaching a matter dominated universe, I should be able to pick my favorite inflationary potential with a true vacuum minimum, adjust its magnitude and flatten it out sufficiently so that it came to dominate the energy density of the universe 5 billion years ago, and press go.
Chalnoth said:Well, I suppose you could do it, but it would require a completely different potential energy than the one that drove inflation. The expansion rate has just been too slow to retard the decay of an inflaton. So it might be similar to inflation, but it wouldn't appear to be directly related to it.
Well, first you're making the assumption that a "true vacuum" has zero energy, which we certainly don't know. There is no known symmetry that sets the vacuum energy to zero.
But it really has to lose potential energy quickly, not simply continue its slow roll trajectory. This means that it would have to, for instance, oscillate about the minimum of its potential (as inflaton did), producing radiation and other stuff.
To get the accelerated expansion to slow, you need the energy density to decay more rapidly than [itex]1/a^2[/itex]. Fail to do this, and the expansion will continue to accelerate.bapowell said:This is where I think we are missing each other. Why does the potential energy need to decay quickly?
Or you just need a potential that goes to zero at finite time. Am I missing something?Chalnoth said:To get the accelerated expansion to slow, you need the energy density to decay more rapidly than [itex]1/a^2[/itex]. Fail to do this, and the expansion will continue to accelerate.
If the total energy density doesn't decay more rapidly than [itex]1/a^2[/itex], then it will never reach zero.bapowell said:Or you just need a potential that goes to zero at finite time. Am I missing something?
The theory of heat death suggests that the universe will eventually reach a state of maximum entropy, where all energy is evenly distributed and no work can be done. This would mean that the universe would essentially become cold, dark, and dead.
Yes, the theory of heat death is widely accepted by scientists as a possible fate of the universe. However, it is still just a theory and there are other theories about the fate of the universe that are also being explored.
The Big Crunch theory suggests that the expansion of the universe will eventually slow down and reverse, causing all matter and energy to collapse back into a single point. This is essentially the opposite of the Big Bang theory.
Currently, the evidence suggests that the universe is expanding at an accelerating rate, which makes the Big Crunch theory less likely. However, further research and observations are needed to fully understand the fate of the universe.
As of now, there is no known way to prevent the eventual fate of the universe. However, some scientists are exploring the concept of a "big rip" where the universe would expand at an increasing rate until everything is torn apart. This is still just a theory and would also result in the end of the universe.