Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Vacuum solution with nonzero cosmological constant

  1. Oct 22, 2008 #1
    Consider the vacuum solution to Einstein's equations with non-zero cosmological constant. Following Carroll, we can find the equation for the radial geodesic with the aid of killing vectors. It takes the standard form: E = T + V. But, with non-zero cosmological constant V(r) now has a term proportional to r^2. This obviously doesn't reduce to ordinary Newtonian gravity. If there were actually a term proportional to r^2 we would have experimentally detected it. What's going on?
  2. jcsd
  3. Oct 22, 2008 #2
    General Relativity with a non zero cosmological constant is not compatible with special relativity in flat spacetimes or Newtonian gravity in the limit.
  4. Oct 22, 2008 #3
    Ok. But if a nonzero cosmological constant has an r^2 potential it can't be compatible with our universe either. How does one reconcile this with the fact that people think the cosmological constant isn't zero?
  5. Oct 22, 2008 #4
    How do you conclude as such?
  6. Oct 22, 2008 #5
    Wouldn't we have experimentally detected a potential that grows as r^2?
  7. Oct 23, 2008 #6


    User Avatar
    Science Advisor
    Gold Member

    Not if the constant of proportionality is exceedingly tiny!

    The effect of a tiny cosmological constant is only noticeable on huge (cosmological!) scales, and it's only in the last decade or so that we've found astronomical evidence to suggest it isn't zero.
  8. Oct 23, 2008 #7
    I don't see how that makes sense. No matter how small the cosmological constant is, I can find an r much much greater than the cosmological constant. Unless you're saying the cosmological constant is small even compared to the size of the universe.
  9. Oct 23, 2008 #8
    I take back what I just said. The condition would be that Lambda R^2 would have to be much less than 1, where R is the maximum distance over which Newtonian gravity has been observed to hold. Is this consistent with current estimates of the cosmological constant? In particular, the current upper bound would have to be less than or equal to the upper bound implied by this criterion. Anyone know if this is true?
  10. Oct 23, 2008 #9
    I found a paper that gives the estimate Lambda ~ 10^-52. That is indeed pretty small.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook