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Critical mass

  1. Jun 2, 2003 #1
    now, i'm a little confused on an important fact: how close are we now in our measurment to the critical mass for the expansion? the stuff i've been studying is from the 80's or something so i'm wondering what our current estimate is.
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  3. Jun 3, 2003 #2


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    In the 80s and 90s it was a big issue with cosmologists
    whether rho was equal to rho crit-----the so-called "spatially flat" case where the universe would continue expanding (just barely) foreever.

    or whether rho was greater than rho crit-----the "closed" case where the expansion would eventually end and things would fall back together in a crunch

    or whether rho was less than rho crit----the "open" case where expansion would continue forever but in a brisk manner rather than the borderline "just barely not stopping" way

    The critical energy density rhocrit is easy to calculate from Hubble's parameter H0 by a formula

    (3/8pi) c4 /G H02

    but the joke was, in the 80s and 90s they didnt know H0 at all well. there were widely differing measurments of it.

    Around 1998 it all came together and there was a terrific change, people call it a revolution, in cosmology.

    They got an accurate H0 value of 1/13.8 billion years.

    And they discovered at last that the universe was spatially flat. That is the real rho is equal to the critical rho needed for flatness.

    this would have been an enormous solace and relief to cosmologists except that at the same time (around 1998)
    they discoverd that 73 percent of the energy density (the nice rho they had determined so carefully) was nothing anybody had ever seen so far called "dark energy". The name means nothing, they could as well call it X energy because nobody has a clue what it is. This means that cosmology is probably the greatest standup comedy act in the whole history of science---but also
    in a certain way the most interesting that it could possibly be.
    a contant source of headline news and entertainment.

    So rho turned out to equal rho crit, after all, but 73 percent of rho is dark energy.

    AFTERTHOUGHT your books may talk about Ω which is
    the rieal rho divided by rho crit.

    Another way to desribe the three cases is to ask if
    Ω = 1 (flat case)
    Ω > 1 (closed case)
    &Omega; < 1 (open case)

    it is just the same spiel but with trivially different notation
    Last edited: Jun 3, 2003
  4. Jun 3, 2003 #3
    Isn't there also the factor of &lambda; coming into play?
    &lambda; is the cosmological constant, invented by Einstein, but also negated by him (calling it his "biggest" blunder), and then re-invented later on?
  5. Jun 3, 2003 #4


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    That is right. Lamda is how many people refer to the dark energy density.

    this is estimated to be 73 percent of the actual energy density.

    No one knows what it is, they just call it various things:


    dark energy density


    rhoX ----- the unknown energy X

    vacuum energy density

    there are promising possible avenues of explanation but no one has yet succeeded in saying what it is-----maybe this is one of the things that will eventually drive people to move on past the
    Standard Model in particle physics-----and there may be experimental searches for the dark energy particle (if there is one) in the next decade. I dont know if they will find anything but there is interest in looking.

    Ordinary matter (leptons, quarks, ordinary proton-electron matter) accounts for 4 percent of the total density. What we dont understand (dark matter, dark energy) is around 96 percent. It is a very strange situation for science to be in.

    Also it is ironical and odd that Einstein should have had this Lambda in his 1916 equation and then disavowed it and rejected it and then around 1998 it comes back into the picture with a lot of fanfare. Still without any explanation of what it might be.
  6. Jun 3, 2003 #5
    Re: Re: critical mass

    i don't quite understand what you mean by "the universe is spacially flat. and quite frankly, i don't know if you've answered me straight. what are you saying?
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