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

The expansion of the universe

  1. Apr 19, 2005 #1
    Alright, my first post here, and I feel like a questionmark if it is justifiable to post this here, but I'll give it a shot.

    Recently I've pondered on some stuff regarding the expansion of the universe. Since I'm only in grade 12, attending the International Baccalaurate programme, I cannot state that I have Real knowledge about anything, which makes my questions probably rediculous, but what do I know?

    Here is the deal, We have learned that the universe is thought to be expanding, and as the universe expands the entropy increases and the temperature decreases, and so does the density and so on. What I have thought of, firsly is that the universe didn't start by expanding with the speed of light since, as the Big Bang started, and eventually as photons were released, they got different displacements and thus some were further away than others. This means that the expansion is uneven. Also, as the uneven expansion is due to maybe photons colliding or something like that. Then as the the universe expands, the expansion after a while should not expand uniformly, cause after a while the photon's angle from its original position decreases, and there are parts that do should be left untouched by the photons, hence uneven expansion.

    So far the model of the universe as I have been taught is that, that the universe is trying to reach equilibrium. A problem now is, let's assume that the universe is now expanding with the speed of light, which means that the photons explore with the speed of light. As these photons are namely light, they should have that speed, and as light does not have a mass, it can have infinite energy? - that is, go on forever?

    Let's assume it can, if it does, then the following thing would happen: The mass in the universe is moving with a speed of (c-k), as it has a mass and is not able to move with the speed of light. The first question that arrise here for me is, why would it even want to create an equilibrium? I mean, the mass of the universe does not behave like a liquid in a container, or does it? And even if it did, eventually it would not be able to fill the bottom of the container, so that there would be empty space between the photons that keep on expanding and the mass that is left behind.

    Also, if we assume that it is trying to reach an equilibrium, and that it is affected by a force or a motion in the direction of the expansions, then another problem would arise, namely, light is able to keep on traveling, but the mass's velocoty would decrease all the time. As the mass has the velocity of (c-k) this value of k is likely to increase since it attracts more and the more mass it is, the less will the velocity become, so wouldn't the mass eventually come to rest? That is, wouldn't the universe stop reaching for equilibrium?

    Then in all this mess, what would then really happen to the pressure, density, temperature and entropy of the universe?

    *sigh* This must be pretty rediculous to hear, and I'm sorry if it is, but I just wanted some clarity for my ponderings. And, no, we have not studied Einsteins relativity theories, but I have a feeling that he is likely to pop up in explaining this, but feel free to use all things available, cause I'm willing to learn :)

    Thank you for your time,
  2. jcsd
  3. Apr 19, 2005 #2

    First, welcome to PF! :smile: We are glad to have you here. Now, what you have posted does not sound ridiculous. In fact, we were just discussing the topic of universal expansion a few days ago. You might want to search for the threads entitled: Universal Expansion and Heat Death. There is much imformation there. You might find it useful in answering the questions you posted.

    The only thing we did not discuss was the affects on light the expansion might have caused. I do believe light has an infinite amount of energy. Perhaps if we could recruit one of the relativity gurus or SpaceTiger, they might be able to help.

  4. Apr 20, 2005 #3
    Thank you, I'll go through them :)
  5. Apr 20, 2005 #4


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    someone in 12th grade of the Int. Bacc. is probably going to know some differential calculus, and may have learned some differential equations.

    Caroly, if you are familiar with some simple differential equations that would make it easier for you to understand, and for anyone here who knows some cosmology it would make it easier for them to explain.

    do you know some differential equations?

    If so, one of us can find a link to the Friedmann equation (actually a pair of very simple ones) which is the main model used in cosmology
  6. Apr 20, 2005 #5


    User Avatar
    Science Advisor
    Gold Member
    2015 Award

  7. Apr 20, 2005 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's not a bad thing to think about at all. In fact, you've basically just restated the "flatness problem". That is, why does the topology of the universe appear to be so trivial in the observable universe? The most popular explanation at the moment is "inflation", an extremely rapid expansion early in the history of the universe. In a sense, it "smoothed" things out.

    Photons of light have a finite energy (planck's constant times the frequency), but their lorentz factor diverges, meaning they effectively experience no time. Perhaps that's what you're thinking of.

    Equilibrium is a fairly general concept, actually. You can think of it very simply by just imagining that a system will change until it reaches a point of "stability". Until it reaches the stability point, it will be out of equilibrium and, by definition, changing. This change will allow it to explore its parameter space, so to speak, and look for these stability points.

    Now it's a fair question to ask whether these stability points even exist in the universe and I think the answer to that is that it depends on the model you're using. There's one model of the universe in which it undergoes a "big rip" and continues to expand faster and faster. I'm not sure such a universe could self-interact enough to reach equilibrium.

    What you're describing sounds like the "heat death" and I would say that it corresponds to a sort of equilibrium. There was a discussion on this recently, so I suggest a search.
  8. Apr 22, 2005 #7
    Thanks guys, I'm going to read them all through this weekend :)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: The expansion of the universe
  1. Universal Expansion? (Replies: 30)