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

Question regarding the expanding universe

  1. May 19, 2005 #1
    When Big Bang occured everything was thrown in different directions at high speeds.
    Is this speed constant or are the speed at which galaxies travel slowing down?
    Since the general theory seems to be that Big Bang is a neverending procedure, galaxies should be slowed down by the gravity of the galaxies going the other way until everything gets pulled back to where the big bang occurs.
    Is there a flaw in my reasoning?
    If not, would it not be possible to measure how much galaxies are slowed down, and in that case predict where the site of big bang is located and even calculate the time when this will occur?

    Thankfull for answers.

  2. jcsd
  3. May 19, 2005 #2
    Neither. Speeding up.

    No more so than Einstein's. There are various theories as to what force is overcoming gravity. When it's done and dusted, it'll hit the major news stations.

    The big bang did not occur in any particular space that exists now. From the big bang came space and time, so all of the space in the universe now comes from the big bang. It kind of happened everywhere.
  4. May 19, 2005 #3
    What causes galaxies to speed up?
  5. May 19, 2005 #4
    As I said, there are various theories and I know little about most of them, and nothing about some of them no doubt. Dark energy is, I think, still the most prevalent theory but I generally turn off when people talk about something that hasn't been detected that explains it all away... Google it and see what you find.
  6. May 21, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    As El Hombre said, there is no "where" for the Big Bang, but what you're describing is very similar to what we've been trying to do in observational cosmology. Basically, if we can measure how fast the galaxies are moving from one another and how quickly this motion is changing, we can get an estimate of the age of the universe.
  7. May 21, 2005 #6
    If the universe had 4 Hydrogen atoms or over per cubic meter, it would reach a crucial value where inevitably all of the matter would start collapsing back on itself (Like if you reverse Time of the expansion of the universe). Though this sounds like a small value, the known universe is nowhere near this value. The earth and the solar system have a lot of mass involved, as moreso with galaxies. But most of the universe is empty area with galaxies seperating, so it all equals itself out.

    However, there is a theory of Dark Matter which for some reason doesn't interact with photons as we would normally expect matter to. If the theory is true, the universe has enough mass to go way beyond this crucial value, which in this case would cause the universe to stop expanding, and to reverse into the "big crunch" after so much time.

    I think this is what I've heard anyway.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook