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

How universe gradually becoming flat?

  1. Dec 22, 2008 #1
    I haven't seen any theory that describe the universe how it becoming flat, from what I saw from here is, universe is isotropic at the beginning, then become flat. What is the explanation here?
  2. jcsd
  3. Dec 23, 2008 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It is a problem.
    Look at this page from Wikipedia

    Different solutions have been proposed, to explain how the universe got so spatially flat. The preferred explanation is inflation.
    In that wikipedia article, scroll down to the section on inflation. It explains how inflation can achieve flatness.

    The mathematical basis is the Friedmann eqn. which they manipulate by simple algebra. They show how. There is a simple explanation that to get flatness a certain quantity must increase. It can only increase if a large part of the energy in the universe is represented by something like dark energy.

    The math symbols they use are rho(t) [the energy density at time t] and a(t) the scale factor at time t.

    The quantity that must increase is a2rho

    but if the energy in the universe is mainly ordinary stuff like matter and radiation, then as a increases the density rho will decrease very fast, so expansion will make a2rho decrease!

    However dark energy has a constant density and if something like that is dominant then rho will be approximately constant. Then a2rho will increase!

    It has to increase as the universe expands becaue rho stays the same and a, the scale factor, increases. So that will cause spatial flatness.

    For more details and more explanation, read the wikipedia article.

    Here is some extra background on the Friedmann equations, if you want.
    They are the core equations in cosmology, so ultimately all explanations you ask for go back to them and are based on them.
    Last edited: Dec 23, 2008
  4. Dec 24, 2008 #3


    User Avatar
    Science Advisor
    Gold Member

    An expanding universe with almost any kind of metric appears flat [with a bit of tuning]. That is why expansion is so attractive to theorists. That dont make it right or wrong, merely convenient. There are other solutions that are not attractive because they create more problems than they solve [e.g., conflicts with observational evidence]. Conventional metrics are obviously not totally right, but, closer than existing competitors.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook