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

Why can we see light emitted near the beginning of the universe?

  1. May 4, 2014 #1
    ok, I can understand that we can see object billion light-year away because it took light a billion year of travelling to come to us.

    But where I don't understand is why we can see object at 14 billion light year (or close) away or maybe why can we see light emitted close to the beginning of the universe ?

    Because when this light was emitted 14 billion year ago, the what so even form of matter/energy that make us today has to be in the proximity of the source of emission since this time correspond to the beginning of the universe and it shouldn't that big at this moment (compare to today).

    Then how come we manage to get where we are today before this light that was emitted 14 billion year ago ?
  2. jcsd
  3. May 4, 2014 #2


    User Avatar
    Science Advisor
    Gold Member

    Hi, xazz. Welcome to PF!

    Everything should become clear when you realise that the BB was not an explosion from a single point, but it 'happened' everywhere in the universe(possibly infinite in extent now as back then) at once. So when looking at the most distant objects, you're not seeing stuff that was in the same spot as you. It was much closer(expansion, remember?), but still not in one point.

    I say the big bag 'happened' using inverted commas, because it's still not exactly what the theory says. It merely states that distances in the universe are expanding, and at one time in the past all of it was much hotter and denser.

    No need to feel bad about all this stuff, though. The Big Bang is an unfortunate name, that stuck and keeps on misleading people. The popular science programmes that picture it as an explosion do not help in the slightest. If you stick around, you'll notice that this question in one form or another pops up all the time on the forums.

    We even have a humongous sticky thread in the Cosmology section aiming at "getting us all on the same page". You might find some good information in there, or if you do a forum search.
  4. May 4, 2014 #3


    User Avatar
    Gold Member

    xazz, you might find the link in my signature helpful in understanding some of this fundamental cosmology.
  5. May 5, 2014 #4


    User Avatar
    Science Advisor
    Gold Member

    The big bang occured in time, not space.
  6. May 5, 2014 #5
    Picture a solid ball. Instead of being made up of atoms, it is made up of energy - - all of the particles you can think of, electrons, quarks, gravitons, anti-protons, all of them. The ball's density is so BIG that it is incomprehensible. This ball contains all the energy in the Universe, all the mass, all the matter, everything. The size of the ball in not very important, but its small, very small. It has lots of photons in it, crowded by the other particles, and changing into them, and back. Particle-antiparticle annihilations are making more photons, which then make more particles. A seething mass of everything. We will pretend that this ball isn't "the Universe", but instead is "in" the Universe. You don't need to understand, in order to understand why we can see back to about 380,000 years after the BigBang, what the difference is between the Universe as a ball, and the energy of the Universe in a ball embedded in Space-time. OK. So at a specific instant, there are a very, very, very large number of photons in the ball. They are each moving in a random direction; in fact, in all directions. Now, at the very next "instant" the size of the ball "inflates" by 2X. Or by 2,000X or by 2 000 000 000X. Suddenly, the photons are not shoulder to shoulder with the other particles, but might take a femtosecond to travel far enough to collide with something. But what happens if the inflation continues in the subsequent instants? Each instant, the distance between particles increases by a HUGE factor. Lets pick out a photon and call it "Bob". Bob was not right next to you, but wasn't that far away, either. Not only that, but Bob's momentum (direction of travel) was pointed RIGHT AT YOU. Suddenly, there is distance, real distance, between Bob and you. How much distance? Oh, say about 14 billion light-years. So, how long will it take Bob the photon to reach you?
    Now, the inflation didn't happen in "an instant", but you get the idea, I hope. Also, before about 380,000 years after the Big Bang, the Universe was so hot, that it was opaque to light; light couldn't travel (almost) at all before that. But 13,800,000,000 years - 380,000 years = 13,800,000,000 years (rounded).
  7. May 5, 2014 #6


    User Avatar
    Gold Member

    Your explanation to the OP is a good one, but I do find it unfortunate that you use the misleading description in the paragraph above (and speaking of paragraphs, you should learn to limit the size of yours) which gives the impression that the universe was both finite sized and spherical right after the singularity, both of which are wrong (well, the finite MIGHT be right but should not be stated as fact). This propagates misinformation.
  8. May 6, 2014 #7
    abitslow (curious, it sound like me), If suddenly, not instantaneously but in less than 14 billion years, there is 14 billion light-year between me and Bob, it would means that the universe inflate faster than light speed ?
  9. May 6, 2014 #8


    User Avatar
    Gold Member

    The universe went through a period called "inflation" in which things got farther apart at MUCH faster than the speed of light, and currently the expansion is causing things that are very far away from each other to move away from each other faster than c.

    For example, the objects at the edge of our observable universe are receding from us at about 3 times the speed of light.

    No speeding tickets are issued, however, because this is the expansion of space, NOT movement in the way you normally think of it.

    Google "metric expansion" for more information
  10. Jul 14, 2014 #9
    Big bang theory

    Many people believe that the big bang may or may not have started the universe but the light stays there if not drawn back in with gravity allowing us to see the aftermath. The remains are very week so it takes very high powered machines to read the uv and other light wave emissions that came about during that time but about the big bang no one really knows 100% if it really happened or if it just was a big explosion it is taught as happening but i personally think that the big bang was a small explosion that caused the area that we preside in to be created the space we see now is limited most likely there is more than just empty space past our telescopes so it would be very presumptuous of us to say this and think that everything came from a single small event in comparison to the universe we find new things every day we didn't see yesterday that leads me to believe it did not happen in the big bang we are seeing to day.
  11. Jul 14, 2014 #10


    User Avatar

    Staff: Mentor

    Jonson, it would be advantageous to read up on the big bang theory, as you have some misconceptions about it. First and foremost, the big bang was not an explosion within space. It wasn't an "explosion" at all in the regular use of the word. All we know is that the very early universe was very hot and very dense and expanded from there. Note that this dense state existed EVERYWHERE, not just within a small section of the universe.

    See this article for more: http://en.wikipedia.org/wiki/Big_bang
  12. Jul 14, 2014 #11


    User Avatar
    Gold Member

    To add to what Drakkith said, the light remnants are NOT from the early days of the universe (the "explosion" as you incorrectly describe it) but 400,000 years afterwards and it is NOT "uv" it is microwaves. See, there's a hint in the name. It's called the "Cosmic Microwave Background", not the "Cosmic Ultraviolet Background".

    Also, personal theories are welcome at a lot of forums, but not this one.

    Welcome to the forum, by the way, and please be aware we're not trying to give you a hard time here. The point of this forum is to clear up misconceptions about actual science and that's what we're trying to help you with.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook