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

Acceleration of expanding space

  1. May 20, 2003 #1
    What's the reason for it? Is there a mechanism already existing in physics to explain it, such as virtual particle energy density, to account for the anomalous acceleration? Will such an explanation solve the mystery of the acceleration of the Pioneer spacecraft?
    To know we've got to solve http://math.ucr.edu/home/baez/physics/Relativity/GR/cosConstant.html [Broken] .
    because, according to this web site, the vacuum density predicted by quantum mechanics is 120 orders of magnitude too high (a density of 1091g/cm3.
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. May 20, 2003 #2
    Yup, the vacuum thing is a problem. There are contributions of opposite sign from a bunch of sources, so clearly there must be some miraculous cancellations going on. That -- vacuum energy, or virtual particle energy density if you prefer -- is prob the preferred solution for the dark energy. There are other ones, like quintessence, which do things like introduce new particles. It's still a big open problem, though.
  4. May 20, 2003 #3
    Do you know why the quantum fluctuations dont cancel each other out completely? It seems like there would have to be some asymmetry, like the Cassimir effect.
  5. May 22, 2003 #4
    May be the problem is in matter pressure? Matter has enormous energy density while practically zero pressure. But pressure and energy density are one and the same thing (same units, etc).

    So there is something which "holds" energy of matter preventing it from exerting pressure (say, e/m and nuclear forces). Without knowing exact structure of these forces (and why they hold energy from spreading) we probably can't solve this misfit.
  6. May 23, 2003 #5
    Wait, it sounds like you're talking about the reason spacetime is bent as result of mass, and that's not quite what I'm asking (or is it? I dont know how expanding space and curved space are specifically related, other than energy conservation between them.)
    I've heard that the energy holding the nuclear particles together makes up nearly 1/2 the mass of the whole particle. The e/m and nuclear forces have positive energy definitions, while gravity has negative energy. Ostensibly photons make up the binding energy of particles. Seems like if we want to know how space expands we need to know what makes it curve, better.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook