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

How far back in time can we see?

  1. Nov 15, 2005 #1
    Everyone knows that the farther away an object is the farther back in time we're seeing it when its light reaches us.

    But, if the universe began with a singularity or similar, and if the universe is expanding at less than the speed of light, then there has to be some limit to how far back in time we can look.

    For us to be able to see the light from the big bang, for example, we'd have to have sped out to our present distance faster than the first photons could have gotten here. (Yes, I know space itself is expanding, but you get my drift.)

    So as the stuff that became our galaxy traveled away from the big bang, up to a certain distance from the original singularity, all photons emitted prior to the time we reached that distance must have passed us by, by now.

    So what is that horizon? How far back can we really see?
  2. jcsd
  3. Nov 15, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    The limit on our ability to see backwards in time is due to the universe becoming opaque to electromagnetic radiation (including visible light) at about t = 1 million years after the big bang.

    It is not possible for a massive particle to outrace a light beam without a head-start. (With a head-start, it is possible only when the massive particle continuously accelerates).

    In the case of the big bang, there is no head start. Both massive particles and light were present at the big bang. Thus the scenario you described won't work.
  4. Nov 15, 2005 #3
    Maybe I was unclear, because that's actually what I'm talking about. Not sure what scenario you thought I was describing.

    But thanks for the 1 million years answer. Where did you get that from?
  5. Nov 15, 2005 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    The figure of a million years came from an article I quoted in another recent thread

    http://cassfos02.ucsd.edu/public/tutorial/BB.html [Broken]

    look for "recombination era". You may see different numbers for when this "era" occured from other sources, for instance the Wikipedia places this at only about 400,000 years after the big bang.

    Last edited by a moderator: May 2, 2017
  6. Nov 15, 2005 #5


    User Avatar
    Science Advisor

    The correct number for recombination is indeed about 400.000 years after big-bang. If you would calculate the temperature according to Wien's law and the energy needed for an hydrogen atom in ground state to ionize, you would get about 30.000 K. This corresponds to a redshift of z ~ 10.000 which are about 10.000 years after big-bang. However, lower energy photons may also excite neutral hydrogen and afterwards ionize it. Due to the high photon to proton ratio (about 109) the contribution of the photons in the tail of the black body distribution cannot be neglected. A more detailed calculation taking this into account would show that the temperature of recombination was about T ~ 3000 K which corresponds to z ~ 1000 and about 300.000 - 400.000 years after big-bang.
  7. Nov 19, 2005 #6
    This isn't an answer to your question, but just a comment on a couple of the things you said in your original post.

    As I understand it, galaxies that are further apart than a certain distance ARE separating at a rate greater than the speed of light: this is possible because the galaxies aren't moving through space relative to each other, but the space between them is expanding. So, in a way, it could be said that the universe IS expanding faster than the speed of light.

    It's misleading to think of our galaxy "travelling away" from the big bang, or or it being "a certain distance from the original singularity". The big bang is still going on (i.e. the universe is still expanding) and it never had a centre to travel away from. A bit like a balloon covered in dots: as the balloon expands the dots get farther apart, although none of them is travelling across the surface of the balloon, and none of the dots is at the centre of the surface.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook