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

Question on Universal Expansion

  1. Feb 28, 2014 #1
    I was reading an article about the recent type Ia supernova in the big dipper when the question popped into my head that I've forgotten about until now.

    Since we use the Ia supernova as our candles to measure the expansion of the universe, how do we know that everything is accelerating?

    It's easy to look at objects that are further away and see them moving away from us faster because that's how universal expansion works. What I'm wondering about is from seeing the universe expanding ever faster as we look at further objects, how do we know that it is ACCELERATING rather than expanding at a constant rate?

    What I'm thinking in my head is the universe might be expanding at a constant rate, but objects further away may just appear to be receding faster just because of the expansion process (With all the red shifting and such). Not sure how we're extrapolating an acceleration because we're already seeing something similar.

    I'm hoping I'm just misconstruing things here and that I don't have any misconceptions of how the universe works.
  2. jcsd
  3. Feb 28, 2014 #2


    User Avatar
    Gold Member

    If there were no acceleration there would be no differences in red shift but differences are observed that make it obvious that there has to be acceleration.

    This topic has been discussed here many dozens of times, if not hundreds. Try a forum search.
  4. Feb 28, 2014 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The choice of words has been unfortunate. There isn't any one speed that the U is expanding. So "acceleration" in this case doesn't mean that "the speed the U is expanding is increasing". Since there isn't one definite speed for the whole, that would be a confusingly vague way to put it.

    You realize that the speed any given distance increases is proportional to its size. So at any given era there is a PERCENTAGE RATE that distances are increasing. That's the best thing to focus on.

    You understand exponential growth, like money in a savings account. IF THE PERCENTAGE RATE WERE CONSTANT then the growth of cosmic distances between clusters of galaxies etc would be EXPONENTIAL. So obviously any one given distance's growth speed would be accelerating.

    But until 1998 astronomers thought that the percentage rate of distance growth was DECLINING rapidly enough that you would NOT be getting that kind of exponential increase in the size of any one given distance. The universe's own gravity was gradually bogging down the percentage growth rate enough to prevent acceleration.

    Then in 1998 they discovered that you get a better fit to the data with a model where YES THE PERCENTAGE RATE IS STILL DECLINING BUT THE DECLINE is not as rapid as we thought and IS LEVELING OFF TO A CONSTANT percentage RATE AND WE ARE ALREADY SEEING something approximating exponential growth.

    To put numbers on it, today's growth rate is 1/144 % per million years. We used to think that was on track to gradually tail off to ZERO % per million years. But in 1998 they realized you get a significantly better fit if you say the decline is on track to level off at 1/173% per million years.
    A small, but nevertheless positive percentage rate.

    And we are already getting so near the eventual target now that expansion is beginning to act as if the percentage rate is nearly constant,which means NEARLY (but still very gradual) exponential growth, which means the curves bend very slightly up, the slope gently increases---the kind of thing we mentally associate with "acceleration". So in 1998 they trumpeted the news in great excitement.

    But you could think of it more calmly (and boringly:biggrin:) as simply observing that the percentage growth rate is still declining but is tending to level out so the decline is more gradual than we thought before.

    I don't know if that is a good way or a bad way to explain it. Let me know if this doesn't work for you, and ask more questions.
    Last edited: Feb 28, 2014
  5. Mar 3, 2014 #4
    Thank you so much Marcus, that definitely clears it up just about perfectly for me.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook