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10 Aug 2017 01:46

Watsisname wrote:
We're more than 99.7% sure that w is between -1.2 and -0.8.  And something like 99.9 and about 90 more 9's percent sure that it is less than zero.  Basically, a negative w means the energy density is diluted more slowly than what you would expect for the increase in volume with the expanding universe.  w = -1 means the energy density is constant as the universe expands, which is the simplest model for dark energy and most consistent with observations.


Heat death is just what happens if the expansion goes on forever, which applies to any open or flat universe model.  So I think the cyclic universe model with dark energy must have some more complicated requirement than just w being negative -- i.e. some more exotic form of dynamical dark energy.

Something called phantom energy I think.  I will read up again and repost. edit- I just did, note the part where it says that W can be close to but must be less than -1.  Imagine if it was something like -1.1 or -1.2 !
https://en.wikipedia.org/wiki/Cyclic_model
The Baum–Frampton model[edit]
This more recent cyclic model of 2007 makes a different technical assumption concerning the equation of state of the dark energy which relates pressure and density through a parameter w.[7][10] It assumes w < −1 (a condition called phantom energy) throughout a cycle, including at present. (By contrast, Steinhardt–Turok assume w is never less than −1.) In the Baum–Frampton model, a septillionth (or less) of a second (i.e. 10−24 seconds or less) before the would-be Big Rip, a turnaround occurs and only one causal patch is retained as our universe. The generic patch contains no quark, lepton or force carrier; only dark energy – and its entropy thereby vanishes. The adiabatic process of contraction of this much smaller universe takes place with constant vanishing entropy and with no matter including no black holes which disintegrated before turnaround.

The idea that the universe "comes back empty" is a central new idea of this cyclic model, and avoids many difficulties confronting matter in a contracting phase such as excessive structure formation, proliferation and expansion of black holes, as well as going through phase transitions such as those of QCD and electroweak symmetry restoration. Any of these would tend strongly to produce an unwanted premature bounce, simply to avoid violation of the second law of thermodynamics. The surprising w < −1 condition may be logically inevitable in a truly infinitely cyclic cosmology because of the entropy problem. Nevertheless, many technical back up calculations are necessary to confirm consistency of the approach. Although the model borrows ideas from string theory, it is not necessarily committed to strings, or to higher dimensions, yet such speculative devices may provide the most expeditious methods to investigate the internal consistency. The value of w in the Baum–Frampton model can be made arbitrarily close to, but must be less than, −1.

Cyclic steady-state model[edit]
Astrophysicist Geoffrey Burbidge proposes a realistic cyclic steady-state model (2008) in which the universe continuously goes through stages of expansion and contraction, each cycle on a length far greater than 1012 years, with very hot and massive active galaxies and black holes playing roles in the formation of elements in each cycle. The model attempts to address issues with standard Lambda-CDM cosmology, including the lack of viable or confirmed non-baryonic dark matter sources, uncertainty regarding dark energy, early galaxy formation and constraints on inflation, while also reconciling and taking account with recent observations and element abundances without the need of additional new physics beyond the Standard Model.[11]

Other cyclic models[edit]
Conformal cyclic cosmology—a general relativity based theory due to Roger Penrose in which the universe expands until all the matter decays and is turned to light—so there is nothing in the universe that has any time or distance scale associated with it. This permits it to become identical with the Big Bang, so starting the next cycle.
Loop quantum cosmology which predicts a "quantum bridge" between contracting and expanding cosmological branches.
 
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10 Aug 2017 01:54

A-L-E-X wrote:
Source of the post Something called phantom energy I think.  I will read up again and repost. edit- I just did, note the part where it says that W can be close to but must be less than -1.  Imagine if it was something like -1.1 or -1.2 !

Being less than -1 does sound like a requirement, but I think it still must have more requirements than that.  The most simple model where w is less than -1 just leads to a Big Rip.  Or in other words, it seems like the Big Rip is a required starting point, and then it becomes a rebound model after making some further assumptions about the physics in that scenario.
 
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10 Aug 2017 02:02

Watsisname wrote:
A-L-E-X wrote:
Source of the post Something called phantom energy I think.  I will read up again and repost. edit- I just did, note the part where it says that W can be close to but must be less than -1.  Imagine if it was something like -1.1 or -1.2 !

Being less than -1 does sound like a requirement, but I think it still must have more requirements than that.  The most simple model where w is less than -1 just leads to a Big Rip.  Or in other words, it seems like the Big Rip is a required starting point, and then it becomes a rebound model after making some further assumptions about the physics in that scenario.

Is the Big Rip more or less likely than simple heat death?
 
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10 Aug 2017 03:33

A-L-E-X wrote:
Source of the post Is the Big Rip more or less likely than simple heat death?

I'll quote a segment from the Panel Reports - New Worlds, New Horizons in astronomy and astrophysics:

The are divergent opinions in this panel on the a priori likelihood of w = -1.

In other words "we don't know."  :P

To elaborate, the simplest model we have for dark energy is as a cosmological constant, where w is identically -1 and we get no Big Rip. To current observational constraints, w = -1 is pretty much in the middle of the uncertainty range.  If we disregard the a priori likelihood of it being -1 and allow the possibility of dynamical dark energy, then it could be a bit larger (Heat Death but no Big Rip), or a bit smaller (Big Rip at some sufficiently great time in the future).

So is a Big Rip more or less likely than simple heat death?  It depends on your view of the likelihood of the reality following the simplest model vs. a more complex model.  Disregarding that likelihood, current constraints indicate it is about as likely as not.  

A big goal of current research in cosmology is to try to place better constraints on the value of w, by perhaps a factor of 10.  If those constraints end up ruling out -1 that would be very interesting.  But the funny thing about the case if w is equal to -1 is that no matter how well you constrain it, it is still equally probable to be less than or greater than -1.  What happens by constraining w to be closer to -1 is that the potential for a Big Rip must be even further in the future, if it happens at all.  Equivalently, it would make "dark energy as a cosmological constant" a more precise model of reality.
 
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10 Aug 2017 03:36

Watsisname wrote:
A-L-E-X wrote:
Source of the post Is the Big Rip more or less likely than simple heat death?

I'll quote a segment from the Panel Reports - New Worlds, New Horizons in astronomy and astrophysics:

The are divergent opinions in this panel on the a priori likelihood of w = -1.

In other words "we don't know."  :P

To elaborate, the simplest model we have for dark energy is as a cosmological constant, where w is identically -1 and we get no Big Rip. To current observational constraints, w = -1 is pretty much in the middle of the uncertainty range.  If we disregard the a priori likelihood of it being -1 and allow the possibility of dynamical dark energy, then it could be a bit larger (Heat Death but no Big Rip), or a bit smaller (Big Rip at some sufficiently great time in the future).

So is a Big Rip more or less likely than simple heat death?  It depends on your view of the likelihood of the reality following the simplest model vs. a more complex model.  Disregarding that likelihood, current constraints indicate it is about as likely as not.  

A big goal of current research in cosmology is to try to place better constraints on the value of w, by perhaps a factor of 10.  If those constraints end up ruling out -1 that would be very interesting.  But the funny thing about the case if w is equal to -1 is that no matter how well you constrain it, it is still equally probable to be less than or greater than -1.  What happens by constraining w to be closer to -1 is that the potential for a Big Rip must be even further in the future, if it happens at all.  Equivalently, it would make "dark energy as a cosmological constant" a more precise model of reality.

I think it would be really strange if w was exactly -1.  The fine-tuned people would have a field day with that!  Even that it is really close to -1 makes me feel weird about it.  Not in a good or a bad way, just that there is probably something deeper behind the value than just the number itself.
 
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10 Aug 2017 03:45

The equation of state for a universe filled with nonrelativistic matter is identical to 0.  And for radiation it is identical to 1/3.  Is that strange fine tuning?

Remember w = 0 just means the "stuff" dilutes exactly in proportion to the increasing volume.  Exactly what you expect for particles of matter.  w = -1 means it stays constant -- exactly what you expect if the "dark energy" is a property of space itself (like vacuum energy).

w = 1/3 might seem strange for radiation.  What it means is that the photons get diluted just like matter does, but there is an extra dilution factor due to the photons being stretched out (redshifted) as well.  Or expressed another way, the density of photons in expanding space decreases with the scale factor (size of universe) cubed, but the photons also get stretched in proportion to the scale factor, and the energy of a photon is inversely proportional to its wavelength, so the energy density of radiation actually decreases as the scale factor to the fourth power (diluted faster than matter).

Aside:  This is actually why the early universe was "radiation dominated" and then became "matter dominated". :)
 
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10 Aug 2017 04:09

Watsisname wrote:
The equation of state for a universe filled with nonrelativistic matter is identical to 0.  And for radiation it is identical to 1/3.  Is that strange fine tuning?

Remember w = 0 just means the "stuff" dilutes exactly in proportion to the increasing volume.  Exactly what you expect for particles of matter.  w = -1 means it stays constant -- exactly what you expect if the "dark energy" is a property of space itself (like vacuum energy).

w = 1/3 might seem strange for radiation.  What it means is that the photons get diluted just like matter does, but there is an extra dilution factor due to the photons being stretched out (redshifted) as well.  Or expressed another way, the density of photons in expanding space decreases with the scale factor (size of universe) cubed, but the photons also get stretched in proportion to the scale factor, and the energy of a photon is inversely proportional to its wavelength, so the energy density of radiation actually decreases as the scale factor to the fourth power (diluted faster than matter).

Aside:  This is actually why the early universe was "radiation dominated" and then became "matter dominated". :)

But the "strangeness" comes from the fact is if it were truly an exact value like -1.00000 or 0.000000, etc.  How much of a difference would there be for something like -1.0 vs -1.1 (rounded)?  Not in terms of mathematical equations but in terms of the destiny (and density!) of the universe?  What about -1.00 vs -1.05?  Or 0.90 vs .0.95 vs 1.00 vs 1.05 vs 1.10 (rounded)?

Wow that's fascinating about radiation, so it actually dilutes at the rate of "square squared" compared to matter!
 
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10 Aug 2017 08:58

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17 Aug 2017 17:22

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24 Aug 2017 08:04

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24 Aug 2017 08:59

DoctorOfSpace, was just about to post it when i saw you did first.
great series!
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31 Aug 2017 11:59

"Exploration is in our nature. We began as wanderers, and we are wanderers still"
-carl sagan
 
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07 Sep 2017 08:22

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18 Sep 2017 11:17

When will we be able to travel faster than light? I say something like wormholes if you can control
 
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18 Sep 2017 11:32

gamadh wrote:
Source of the post When will we be able to travel faster than light? I say something like wormholes if you can control

99.99% sure the answer is never
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