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Pressure/energy of the universe

  1. Jan 4, 2012 #1
    If "dark energy" is causing the Hubble expansion of the universe to increase due to its repulsive force does this mean the pressure and energy of the universe are increasing per unit volume (and as a whole). How far can this go before it explodes? That is how high can the energy per unit volume go before something(?) happens? It is hard to see where an explosion would go to.
  2. jcsd
  3. Jan 4, 2012 #2


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    I think the Big Rip scenario describes what you are asking about. See here for more: http://en.wikipedia.org/wiki/Big_rip

    I believe you are correct in thinking that it would be hard for an explosion to "go anywhere". I don't think the universe would "explode" in any sort of conventional way.
  4. Jan 4, 2012 #3


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    If there's a nonzero cosmological constant, then the energy density is decreasing toward a final value equivalent to the energy of about 30 hydrogen atoms/m3. This will not cause any 'instability' in the typical sense of the word.
    Gotta be careful here though. The nonzero cosmological constant only acts equivalent to a final, constant density of hypothetical dark energy.
    Last edited: Jan 4, 2012
  5. Jan 5, 2012 #4
    Relevant (well, sort of - different question, sort of similar topic) inquiry I have...

    Since the universe is by all observational data thought to be expanding, and that expansion accelerating, does that mean that the amount of dark energy is increasing to allow for this expansion? And if so, why is that not considered creation of energy? Energy/matter isn't thought to be able to be created or destroyed. I'm sure there is a very simple answer to this, but it still stumped me when I considered it earlier. Thanks
  6. Jan 5, 2012 #5
    Tech, I am also interested in the question you ask. I think I have heard astrophysicists say yes more is created and yes it violates conservation of energy. Why they accept that I do not know.
  7. Jan 5, 2012 #6


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    There is no rule of global conservation of energy, which is only a local rule. That being said, the idea that 'dark energy density is constant over time' almost seems like energy conservation. It's not actually conservation because objects are getting farther apart. If there were global conservation we would expect the energy density to scale as the inverse cube of the expansion.
  8. Jan 5, 2012 #7


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    Consider the special case of constant dark energy density, i.e. the cosmological constant. Here we find that since the energy density is constant in time, that the amount of energy in a comoving volume is increasing in time. A local observer must then conclude that energy is not conserved within this volume. But that's OK, because there's no such requirement that energy be conserved in general relativity. Rather, it is the stress-energy tensor that satisfies a conservation requirement. For a nice description of this, see Baez:http://johanw.home.xs4all.nl/PhysFAQ/Relativity/GR/energy_gr.html
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