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

Infinite density to finite density

  1. Feb 15, 2009 #1
    I'm guessing there has been an attempt to address this, and I've looked through some threads but I remain unsatisfied.

    I'm not getting it. If relativity predicts that the universe was a singularity (I get that the universe wasn't necessarily a tiny point) with infinite density; then how could that singularity ever not be a singularity? If it was infinitely dense then how could it ever not be infinitely dense?

    If you increase the distance between all infinitely close points within the singularity by some set value (say an increase in distance between all points of 10^10000000000%), wont they still be infinitely close together?

    Or is this just what is meant by very little being known about the singularity?

    Was there some sort of a jump between the infinite density to a finite though very large density where the equations start to work? I'm imagining something of an asymptote and a jump to another part of the graph where it is no longer infinite. I don't know how valid this analogy would be though.

    A link that would address these specific questions would satisfy me. Thanks.

    Edit- Ok, I now see that it is possible there was no singularity. Though answers to my questions might still be helpful.
     
    Last edited: Feb 15, 2009
  2. jcsd
  3. Feb 15, 2009 #2

    Nabeshin

    User Avatar
    Science Advisor

    First off, GR doesn't exactly predict a singularity, per se. All GR says is that it doesn't work at that point.

    As far as the infinite density goes, any amount of energy crammed into an infinitesimal (zero) volume results in infinite density. Classically speaking, when the universe began to expand, the volume was no longer zero, so the density of the universe was no longer infinite.
     
  4. Feb 15, 2009 #3

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    You are getting the idea without anyone helping.

    As far as is known, there is no such thing as a singularity in nature.
    When a mathematical model predicts a singularity (like infinite density, infinite temperature, infinite curvature) it doesn't mean it's real. It simply means the model has broken, and fails to apply outside a certain range.

    Physics models in the past (of the atom, of thermal radiation, etc...) have had singularities, that is breakdowns, and it simply meant you had to come up with a better model that would apply where the old model gave meaningless answers.

    GR was invented in 1915. It has a limited range of applicability. It blows up in certain situations and fails to compute meaningful numbers.

    All that means is you shouldn't apply it in those situations, and you should try to find a replacement that doesn't blow up. This is called "resolving the singularity". It has been done with other theories about other phenomena in the past. GR is just one case.

    Now a lot of people are actually working on resolving the BB singularity.

    Here is a database search for research publications since 2005 in the field called "quantum cosmology" which mainly concerns conditions around the t=0 where GR blows up. You can sample it if you want to get a rough impression of the current work. I see this particular search comes up with 282 papers published starting 2006, essentially in the last 3 years. The papers are ranked by how much they are cited for reference in other research, a rough measure of how innovative or useful they have proven to be. The most highly cited papers are listed first:
    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+and+DATE+%3E+2005&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]
    If you want to glance at a brief summary of any paper, called its abstract, just click on where it says "abstract", a couple of lines below the paper's title and authors. Besides a brief description, the abstract page will normally have a link to a PDF file of the whole paper which is free for download.
    I'm not urging detailed study, just offering a way to get a taste of what is going on in this area of research in case anyone is curious.
     
    Last edited by a moderator: May 4, 2017
  5. Feb 16, 2009 #4

    Chalnoth

    User Avatar
    Science Advisor

    A good way of seeing why we can't take the prediction of singularities in General Relativity seriously is the fact that there's a built-in energy scale to the theory: the Planck scale. We typically expect that the theory of gravity should start to break down when energy densities get close to the Planck scale. So obviously a singularity, which goes far beyond the Planck scale in energy density, is not something we can take seriously.
     
  6. Feb 16, 2009 #5
    Thanks guys. I'll check out some of those papers Marcus, thanks for posting the link.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Infinite density to finite density
  1. Infinite? finite? (Replies: 68)

  2. The density parameter (Replies: 9)

  3. LCDM Densities? (Replies: 10)

  4. Critical Density (Replies: 10)

  5. Density Perturbations (Replies: 1)

Loading...