Couple of cosmic questions

Discussion in 'Physics' started by boriz, Aug 17, 2009.

  1. boriz

    Thread Starter New Member

    Jul 16, 2009
    Was space always expanding at its current rate? If the rate of expansion is slowing, in the past it was expanding faster. What implication does this have for the speed of light? Was the speed of light proportionally faster also? Would that mean our estimates of distance based upon redshift are too small?


    When the universe was smaller, was it actually denser? Or were all the cosmological constants equally compressed making all measurements the same then as now? (IE. A 1m ruler would, in the past, still read 1m even though it was smaller.)? If we say it would (now) take n years for a photon to circumnavigate the universe, would it also take n years when the universe was half its current size, or would it take n/2 years?

    Is the time it takes a photon to circumnavigate the universe a constant? Regardless of the size of the expanding universe? Does that mean that (by today’s standards) light used to travel slower? Would that mean our estimates of distance based upon redshift are too large?


    About 300000 years after the big bang, the universes fell below a critical temperature allowing atoms to form, making space transparent and releasing the cosmic microwave background radiation still detectable today. So why were only hydrogen and helium formed. Why are heavy elements only made in stars and novas? Wasn’t the big bang fireball hot enough?
  2. Wendy


    Mar 24, 2008
    My impression is the big bang fireball (if it holds up with new theories) is it was too intense for any complexity to form. That and a lot of laws we take for granted hadn't had a chance to present themselves, everything was still merged in one universal law. I'm not qualified to have a good opinion though, but I like reading science.
  3. steveb

    Senior Member

    Jul 3, 2008
    These are all greate questions, but you would probably get better answers on a Physics forum. Professional cosmologists inhabit those areas and are usually eager to give good answers.

    I'm tempted to point out some answers I've read about, but as this is not my field, I'm likely to misrepresent some critical information, or not include all the debatable points.
  4. rspuzio

    Active Member

    Jan 19, 2009
    > So why were only hydrogen and helium formed.
    > Why are heavy elements only made in stars and novas?

    The crucial first step in forming elements heavier than helium is forming
    carbon. For that to happen, you basically have to have three helium atoms
    collide into each other within a time frame of a femtosecond. This is only
    likely if there is a high density of helium around. Such is the case in the
    core of stars, where the gas is compressed due to the weight of the star and
    there is extra helium produced by the reactions which make the star shine.
    Hence, heavy nuclei are routinely produced in the cores of stars.

    With the Big Bang, the situation was different. Early on, the density was there,
    but there was only hydrogen around. Later on, there were significant amounts
    of helium produced from the hydrogen but, by then, the universe had expanded
    so the density was likely two low for there to be a reasonable chance of finding
    three helium at the same place at the same time and, concomitantly, the temperature
    had gone down so that, even when that might happen, there might not be enough
    energy for them to react. Hence, I would say that there was no opportunity for carbon
    to form in any appreciable amount in the early universe. Since I haven't taken
    the time to work through the numbers, I won't say this too surely, but I feel
    pretty confident this is the answer to your question.