Results 1 to 8 of 8

Thread: --->DIFFICULT<--- question

  1. #1
    Join Date
    Feb 2006
    Posts
    431

    Exclamation --->DIFFICULT<--- question

    This is actually a few related questions.

    What is the estimated mass of the universe? Assuming its not infinitely big. Or at least the visible universe.

    How big of a "ball" could you make with all of this stuff. The diameter. Assuming it doesnt completely collase into nothing (but similar to a black hole).

  2. #2
    Join Date
    Oct 2005
    Posts
    27,229
    Interesting questions. The way to get a rough estimate of the mass is to assume the critical density over a volume with about 10 million LY radius. The critical density is then found by using the constant G and the age of the universe until you get units of mass density, which is 1/(G age^2), or 10 to the minus 27 grams per cc. Then multiply that by the volume, which is about 10 to the 84 cc, giving about 10 to the 57 grams in the observable universe, very roughly. That's about 10 to the 24 solar masses, but it counts dark matter and dark energy as mass energy. A black hole with that mass would have a radius of about G M/ c^2, which comes out to 10 to the 28 cm, or just about the visible horizon of the universe. This is not a coincidence-- the critical density is of order the density inside the event horizon of a universe-sized black hole. Inside the event horizon, the black hole singularity might have infinite density, so you can't really answer that part. If we made a "ball" made of that much water, and ignored the spectacular pressure, it would be 10 to the 19 cm in radius, or only about the space between each star in our galaxy.

  3. #3
    Join Date
    May 2003
    Posts
    6,174
    Quote Originally Posted by Ken G
    Interesting questions. The way to get a rough estimate of the mass is to assume the critical density over a volume with about 10 million LY radius.
    You mean billion light years of course, though looking at your actual calculations, the numbers look right there.
    Conserve energy. Commute with the Hamiltonian.

  4. #4
    Join Date
    Dec 2004
    Posts
    14,782
    Quote Originally Posted by Ken G
    If we made a "ball" made of that much water, and ignored the
    spectacular pressure, it would be 10 to the 19 cm in radius,
    or only about the space between each star in our galaxy.
    I haven't tried to analyze how you arrived at this figure,
    but I get 10 to the 19 cm = just over 100 light-years.
    The average distance between nearest star systems in our
    neighborhood is a bit less than ten light-years.

    -- Jeff, in Minneapolis

  5. #5
    Join Date
    Nov 2001
    Posts
    4,101
    10^19 cm = 10^17 m = 10^14 km

    1 light-year = 9.6 * 10^12 km

    Yes, 10^19 cm ~ 10 ly

  6. #6
    Join Date
    Dec 2004
    Posts
    14,782
    I see what I did wrong. Ken said the ball of water would be
    "10 to the 19 cm in radius". But I repeatedly put "10 Exp 19"
    into my calculator, when I should have put in "1 Exp 19".

    -- Jeff, in Minneapolis

  7. #7
    Join Date
    Oct 2005
    Posts
    27,229
    I was just trying to put the number in perspective, not be too accurate. It seemed a little surprising to me that we could fit all the matter in the universe into that small of a space at a density of 1 g/cc, but so it is with volumes. And yes, I did mean billion LY, that was a typo. Oops.

  8. #8
    Join Date
    Sep 2005
    Posts
    3,066
    Quote Originally Posted by Ken G
    Interesting questions. The way to get a rough estimate of the mass is to assume the critical density over a volume with about 10 million LY radius. The critical density is then found by using the constant G and the age of the universe until you get units of mass density, which is 1/(G age^2), or 10 to the minus 27 grams per cc. Then multiply that by the volume, which is about 10 to the 84 cc, giving about 10 to the 57 grams in the observable universe, very roughly. That's about 10 to the 24 solar masses, but it counts dark matter and dark energy as mass energy. A black hole with that mass would have a radius of about G M/ c^2, which comes out to 10 to the 28 cm, or just about the visible horizon of the universe. This is not a coincidence-- the critical density is of order the density inside the event horizon of a universe-sized black hole. Inside the event horizon, the black hole singularity might have infinite density, so you can't really answer that part. If we made a "ball" made of that much water, and ignored the spectacular pressure, it would be 10 to the 19 cm in radius, or only about the space between each star in our galaxy.
    [That's about 10 to the 24 solar masses, but it counts dark matter and dark energy as mass energy.]

    This is the 70% Dark Energy
    26% Dark Matter Non-Baryonic
    and 4% Baryonic Matter

    Isn't Critical Density all about figuring out Omega and what that means for the universes expansion or contraction?

    So, if Omega =1 exactly, and could stay that way, balanced forever, we could live in a static universe...highly unlikely.

    So before 1998, what was the critical density?
    How does the missing Baryonic Matter (not for the rotation curves for the galaxies or the clusters) factor into this?

    [which comes out to 10 to the 28 cm,] to the visible horizon
    [would be 10 to the 19 cm in radius,] space between stars in our galaxy

    These figures seem off to me, the top one = 13 billion light years
    The bottom one = about 4 light years

Similar Threads

  1. difficult
    By browolf in forum Discuss Ice Investigators Images
    Replies: 0
    Last Post: 2012-Apr-23, 03:01 PM
  2. OK, pretty simple question here, but surprisingly difficult to Google
    By parallaxicality in forum Space/Astronomy Questions and Answers
    Replies: 6
    Last Post: 2011-May-18, 02:10 PM
  3. Replies: 5
    Last Post: 2011-Feb-22, 04:11 AM
  4. Is it that difficult? A question about trying.
    By LotusExcelle in forum Off-Topic Babbling
    Replies: 44
    Last Post: 2010-Feb-26, 09:33 AM
  5. A difficult question about black hole
    By sorrsuki in forum Astronomy
    Replies: 4
    Last Post: 2005-Oct-03, 04:44 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •