
#19
Feb2312, 04:39 AM

P: 2,889

Anyway a static universe doesn't imply there is no "local" change, it is just the global cosmological parameters as a whole that don't change wrt time. Otherwise it would have been plain silly from Einstein and the physicist of their time to even consider static spacetimes as models for our universe knowing as it is obvious that locally things change. 



#20
Feb2312, 08:31 AM

PF Gold
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#21
Feb2312, 09:39 AM

P: 2,889

Could you address the thought experiment? 



#22
Feb2312, 09:55 AM

P: 2,068

The answer is, anything you want to happen. Since you have thrown the laws of physics out the window and made up your own laws of physics, we can't meaningfully speculate unless you tell us what laws of physics do apply. 



#23
Feb2312, 10:18 AM

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#24
Feb2312, 02:18 PM

Sci Advisor
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While physical laws are generally time reversible, but, we have no meaningful evidence of time reversed processes. This is why we observe and do experiments. Just because something is mathematically possible does not mean it is physically meaningful  e.g., quadratic equations have two solutions, but, both are not necessarily meaningful.




#25
Feb2312, 09:48 PM

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#26
Feb2412, 02:53 AM

P: 2,889





#27
Feb2412, 03:53 AM

P: 2,889

I don't quite understand the resistance to even consider this thought experiment, when actually all the classical tests of relativity are computed using a similarly unrealistic model (even more because it is supposed to be an empty universe): the static exterior of a nonrotating star. I see no nitpicking in this case because everyone understands it is an exact solution of the EFE that allows a valid local approximation however unphysical the model looks.
Well, the OP imaginary universe is certainly no EFE solution, and of course I didn't expect anything valid for our universe to come out of it, but all thought experiments allow certain divergence from physical reality. That is why they are thought experiments. Mathal was right that in such universe, without constraining it in any way, every distribution would be valid, that is why I asked if it was possible to apply the known stellar nuclear reactions and core conditions to single out some more probable equilibrium distribution. Maybe the problem is not wellposed to single out a certain distribution but so far nobody has said so. 



#28
Feb2412, 04:37 AM

P: 6,863

Nuclear statistical equilibrium is pretty independent of temperature, density, and mass for normal temperatures. The equilibrium distributions will change only once you reach "nuclear" temperatures and densities. Anything less than that, it's 100% iron. So now that question one has been set up, I'd like to make a universe that's slightly more interesting. 



#29
Feb2412, 04:45 AM

P: 6,863

http://user.numazuct.ac.jp/~sumi/eos/ It will change for high temperatures and densities (i.e. T>1 million kelvin rho > 10^7 g/cm^2) but for anything under that, it's 100% iron nuclei. For high densities it start going to neutronium and maybe quark soup. For high temperatures, it starts turning into nucleon gas if it gets really hot. Next question? 



#30
Feb2412, 04:46 AM

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#31
Feb2412, 06:16 AM

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#32
Feb2412, 03:53 PM

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#33
Feb2412, 04:18 PM

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#34
Feb2812, 02:18 AM

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#35
Feb2812, 03:24 AM

P: 2,889

A a spacetime is said to be static if it admits a global, nonvanishing, timelike Killing vector field K which is irrotational, this is the standard definition and the one I'm following in my thought experiment as scenario for a putative plausible imaginary equilibrium distribution of chemical elements abundance. Now, as was pointed out before, in abstract terms every distribution is compatible with such a universe. My question is, is there a way to constrain this with the known nuclear reactions (in reversible form) and the physical conditions of stellar's cores? I thought this was an interesting exercise, I'm not so sure now. 



#36
Feb2812, 08:33 AM

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