XPost: sci.astro.research   
   From: helbig@asclothestro.multivax.de   
      
   In article , Gary Harnagel   
    writes   
      
   > > [[Mod. note --   
   > > It is very likely that 50 years from now our understanding of cosmology   
   > > will be different than it is today. It is also very likely that 50 years   
   > > from now our understanding of cosmology will in the main *refine* (as   
   > > opposed to overthrow) our understanding today.   
   >   
   > Although "refinement" has been THE process for the past half-century,   
   > "overthrow happened in 1905-1916 with the advent of relativity.  It   
   > happened again with quantum physics in the 1930's and the 1960's with   
   > QFT.  The discovery of "island universes" seems to have been more than a   
   > "refinement" and the application of GR to cosmology was, too.   
      
   While I agree on the quantum stuff, I'm not so sure about the others.   
   There had been a debate on whether nebulae, as they were then known,   
   were within the Milky Way or extragalactic systems at least since the   
   time of William Herschel.  The question was decided about a hundred   
   years ago, but since it was a decision between two alternatives which   
   had both been around for a while, I don't see it as a revolution in the   
   same sense as the quantum developments you mention.  As to relativity,   
   with regard to special relativity I think that Rovelli makes a   
   convincing case that Einstein's formulation of it grew out of his   
   conservative attitude to physics (which, famously, led him to part   
   company with the quantum theory he helped to create).  One could argue   
   that the results of SR were revolutionary, but the theory itself, so   
   Rovelli, really wasn't.  He discusses this in his book about Anaximander   
   which, partly because we don't know much about Anaximander, is also   
   about many other things.  With GR, I think that all agree that it was   
   revolutionary.   
      
   In general, though, the fact that there were some revolutions in the   
   past doesn't necessarily mean that there will be any in the future.   
      
   > The problem with GR applied to cosmology is that theories such as the   
   > FLRW metric aren't the only possibilities.   
      
   A Friedmann-Lemaître-Robertson-Walker (FLRW) model is a model a) based   
   on GR and b) homogeneous and isotropic.  It can contain an arbitrary   
   number of components with various equations of state.  Historically, and   
   probably physically as well, the most important have been   
   non-relativistic matter (known as "dust" in cosmology, where a dust   
   particle is a supercluster of galaxies), radiation, and the cosmological   
   constant.  Due to the different equations of state, the respective   
   energy densities evolve differently with time, hence radiation is most   
   important at the beginning, then matter, then the cosmological constant.   
   But as you mention there are many other possibilities for cosmological   
   models based on GR.   
      
   50--100 years ago, homogeneity and isotropy were essentially simplifying   
   assumptions.  Today, they are observational facts.  (To be more exact,   
   we observe a high degree of isotropy which implies homogeneity unless we   
   are in a special location for which there is no evidence.)   
      
   > If a "big bang" could happen   
   > in our brane (taking a concept from M-theory), it may not have been   
   > unique.  Suppose a "big bang" happened before our present one and it's   
   > way, way out there and expanding faster than we are.  What would that   
   > look like?  It happened long, long ago so star formation has stopped.   
   > All it would consist of would be red dwarfs, whose spectra might look   
   > something like the CMBR.   
      
   Regardless of the other points, the idea that the CMB is reprocessed   
   starlight is an old one, going back at least to Fred Hoyle who wanted to   
   have an explanation for the CMB in the steady-state model.  The presence   
   of the CMB doesn't directly contradict the steady-state model (though in   
   that model its temperature would be constant while in conventional   
   cosmology it decreases with time, something which one could at least in   
   principle measure), but it is not something which arises naturally.   
   However, today so many details about the CMB are known that reprocessed   
   starlight is not a valid explanation.  (By coincidence, the energy in   
   the CMB is about the same as that in starlight.)   
      
   > THAT would overthrow the FLRW model since all   
   > of our big bang is not all that there is.  There's other stuff out there   
   > that has a gravitational effect on us, as well as spacetime itself.   
      
   Yes and no.  Conceptually, yes.  However, it is known that an FLRW model   
   is a good description of our Universe, and that would still be the case   
   even if the big picture were different.   
      
   > It might also do away with FLRW's need for dark energy.  GR predicts   
   > that, since we are closer to this expanding side of the previous "big   
   > bang," we would be dragged (accelerated) along its line of motion by the   
   > Lense-Thirring effect, thus explaining dark energy.   
      
   Does that explanation work quantitatively?   
      
   I always wonder why people think that they have to explain dark energy.   
   GR has two physical constants, G and Lambda.  Hardly anyone wants to   
   explain G in the same manner, i.e. explain why it is non-zero, calculate   
   its value from something else, and so on.  That does not rule out that   
   dark energy might be something which behaves like the cosmological   
   constant (note that there is no evidence that dark energy---a more   
   general term---is anything more complicated that the cosmological   
   constant), but in that case one would have to explain why Lambda, the   
   "bare" cosmological constant, is zero.  Usually if Nature has a degree   
   of freedom it is used and the absence of something like that, i.e. a   
   parameter being zero, implies a new symmetry, quantum number,   
   conservation law, etc., and the burden of proof is on those who make   
   that claim.   
      
   > The ekpyrotic theory of Steinhardt, Khoury, Turok and Ovrut, suitably   
   > modified and verified, could do a GREAT deal of damage to our present   
   > cosmological model.   
      
   Suffice it to say that it has not (yet?) convinced the community.   
   Again, a modified version would have to make concrete, testable   
   predictions for it to be verified (in the sense that none of those   
   predictions rule it out).   
      
   > Whether this flight of fancy has any semblance of   
   > reality, I think FLRW is in trouble.   
      
   Why do you think that FLRW is in trouble?   
      
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