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

Horizon of view

  1. Jul 27, 2006 #1
    Let me preface this by saying that I know very little about astronomy, but this is a question that has been bugging me for quite a while. I've tried looking it up online and in books, but I can't find it directly addressed anwhere.

    My question is that if the universe began with a single point, and everything expanded outward from there with a big bang, then am I right in assuming that the expanding matter could not travel faster then light? However, if this was the case then why can't we see then entire universe? If nothing did travel faster then light, then the objects never would get beyond the distance that its light traveled in the opposite direction. I'm wondering, because I was under the belief that we can only observe a certain portion of the universe, beyond that horizon the light has not yet reached us, so every year we can see one light year farther. If the matter in the big bang didn't go faster then light, what explanation is there for this?
  2. jcsd
  3. Jul 27, 2006 #2


    User Avatar
    Science Advisor
    Gold Member

    Hi Dawguard!

    It is a consequence of the principles of SR that objects with inertial mass cannot travel through space at a speed equal to or greater than the speed of light, as measured locally. Massless entities such as photons travel at the speed of light (obviously!) but not greater than that.

    However in the cosmological solution of Einstein's GR space-time itself is found to expand or contract. Because we observe Hubble red shift we conclude that our universe is in the expanding phase, that is, every point within it is moving apart from every other point.

    Because of the universe's size objects on the far side of the universe are actually expanding away from us at speeds greater than the speed of light. But note it is space-time itself that is expanding, everything is being carried along within it. They all have their own sub-luminal proper motions.

    Indeed, depending on how you define distance and time at these cosmological distances, it is possible to observe objects that are moving away from us at super-luminal speeds.

    However, as you rightly said, there are portions of the universe that we cannot see, these lie behind our Particle horizon, there are also objects further away that we will never be able to see, no matter how long we wait. These lie behind our Event horizon.

    These horizons are determined by Hubble's constant, the expansion rate and the curvature of space.

  4. Jul 27, 2006 #3
    Thanks Garth, that's great answer, I apreciate it!
  5. Jul 27, 2006 #4


    User Avatar
    Science Advisor
    Gold Member

    Garth answered your question most competently, but just a small point that he did not address (although he has done so may times before!) We have no reason to believe that the total universe began as a single point. We think it possibly began as a singularity, meaning of infinite density, but not of infinitesimal size. It may have been infinitely large at the time of the BB! We may think of our observable piece of universe starting as a single point, but not the universe at large.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook