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Flatness problem?

  1. Mar 13, 2005 #1
    I'm having a problem understanding the resolution of the flatness problem in the standard Big Bang model (k = 0 and Omega = unity).

    If k=0 at the present time, then this implies Omega ~ unity (actual mass/energy density ~ critical mass/energy density).

    For k = 0 and H0 (Hubble parameter for present time) = 71km/s/Mpc then rho(critical) = 3.H^2/8.pi.G = 9.4665 x 10^-27 kg/m3.

    BUT ~73% of this critical mass/energy density (at the present time) is supposed to come from the vacuum energy (Dark Energy) which is hypothesised to be scale-invariant (the vacuum energy density scales as a^0, in other words it does not change as the universe expands). This implies a vacuum energy-density of 6.91 x 10^-27 kg/m3.

    Most of the remaining 27% of mass/energy density (at the present time) is made up of matter (visible and cold dark matter), the density of which scales as a^-3, ie the matter density scales as the inverse cube of the size of the universe). This implies a matter-density of 2.56 x 10^-27 kg/m3 at the present time.

    The contribution from radiation energy-density at the present time is less than 0.01% of the total.

    The above implies that when the universe was one tenth of its present size (which, because a scales as t^2/3 during the matter-dominated era, was when the universe was about 3.16% of its present age, or about 400 million years old) then the mass-density was 1,000 times greater than it is now, which implies a matter mass-density ~2.56 x 10^-24 kg/m3. The vacuum energy density contribution would have been the same as now, at 6.91 x 10^-27 kg/m3. The contribution from radiation energy-density at that time would have still been very small (~0.3% of the total). Thus matter contributed ~99.4% of the total energy density, and the total energy density would have been ~2.58 x 10^-24 kg/m3.

    Assuming flatness (k=0), what would have been the critical mass/energy density when the universe was one tenth of it's present size? The same equation applies, rho(critical) = 3.H^2/8.pi.G. But at that time H would have been much higher than it is now. In fact, during the matter-dominated era H scales as t^-1 (this follows from the definition of H as a'/a, where a' is the expansion velocity; during the matter-dominated era a scales as t^2/3 and a' scales as t^-1/3). Therefore H would have been ~H0/0.0316 = 2,247 km/s/Mpc. This gives a value for the critical energy density at that time of ~9.48 x 10^-24 kg/m3.

    But the actual mass/energy density, from above, was only ~2.58 x 10^-24 kg/m3, which is only ~27% of the critical mass/energy density at that time, hence Omega = 0.27 when the universe was only 400 million years old.

    Where am I going wrong?
  2. jcsd
  3. Mar 13, 2005 #2


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    Go back to the beginning. You are mixing reference frames. You're not wrong, just making the same mistake we have all made.
  4. Mar 13, 2005 #3


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    Well for a start there isn't a resolution to the flatness problem in bog-standard big bang cosmology, it's one of those probelms that need inflation to be resolved.

    Though I think your going to have to spell out your problem more clearly as 0.27~1 (and I must admit I'm not really completely sure what the value should be, but Omega = 0.27 seems reasonable to me).
    Last edited: Mar 13, 2005
  5. Mar 13, 2005 #4
    Thanks. I think I know where I'm going wrong, and am working on it.

    However I disagree with you on your comment "there isn't a resolution to the flatness problem in bog-standard big bang cosmology" - if the universe was created with Omega = 1 then (inflation or no inflation) it was flat by definition. (Inflation perhaps explains how Omega got to be equal to 1.)
  6. Mar 13, 2005 #5


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    It wouldn't be much of a problem if it could be easily be explained by normal big bang theory. The flatness problem IS why the universe should have a value of Omega so incredibly close to 1, there's ceratinly no problem with chucking in an initial value of Omega of ~1 in non-inflationary big bang theory (which would be required to re-create the observed flatness of today).

    edited to add: looking at your calculations you have defintely gone wrong soemwhere though 0.27 still seems like a resoanbel value for Omega.
    Last edited: Mar 13, 2005
  7. Mar 13, 2005 #6


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    Throughout you're assuming a matter-dominated era from the present time to when the universe was a factor of 10 smaller. This isn't the case (because of lambda), so some of the above scalings won't apply (for example, H as t^-1).
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