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

The universe is expanding wait, really?

  1. Jul 21, 2011 #1
    As you might have guessed from my subtle hint, I've been pondering the nature of dark energy and its involvement in the expanding universe and I tripped over a stupidly simple alternative idea that pretty much changes much of what I understand about cosmology since the times of Albert Einstein. But first, I need more info.

    I'm no scientist (altho I like calling myself one in conversation), I have no degrees, don't know all (well, ANY) of the math involved in cosmological calculations, the majority of what I know and understand come from TV documentaries and factional books by guys like Michio Kaku (he's my hero, totally). So here's what I think I know.

    We agree that the universe is expanding, not only because it's still a reasonably logical conclusion from the big bang, but because of a little phenomenon called the Doppler shift. Light emitted by something moving towards you shifts slightly to the blue, light from something moving away from you shifts to red, due to the funny habit that light has of always moving at exactly the same speed, no matter how fast or in which direction the light emitter is moving, and thus stretching or compacting space itself to accomodate this absolute constant, which in turn causes an increase or decrease in wavelength.

    So when smart people with big telescopes look at the galaxies, they see that not only do galaxies that are farther away suffer a stronger red shift (indicating that they are moving faster than close-by galaxies), but over time they actually shift even further to the red (indicating that the rate of expansion increases exponentially in relation to the distance). In short, things that are far away are moving faster than things that are close, and the farther they move, the faster they move away from us.

    Am I correct in these understandings thus far? If not, please enlighten the I? :)
    Last edited: Jul 21, 2011
  2. jcsd
  3. Jul 21, 2011 #2
    This statement is not really correct. You are speaking of tracking individual objects as they move away, and observing that as they move further away their recession velocity increases. This is not what happens.

    When considering tracking of individual objects, the observation times are too short, the recession velocities are too great, and the acceleration too low for us to measure this directly. Rather, what is done is calculating an average velocity of objects observed to be a certain distance from us, and then fitting a curve to the velocity vs. distance data and inferring an acceleration based on this curve.

    While this amounts to roughly the same thing, this is an important distinction to make.
  4. Jul 21, 2011 #3

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    As Aimless said, observation time have been too short to see redshift change. We are close to being able to do this, but, for economic and other reasons, such a project won't start for several decades. Once started, the project would take a couple of decades to start to get good results. See


    Also, redshifts don't necessarily increase with time. Figure 1 from this paper plots redshift versus time. The three red curves are for objects in our universe. As we watch (over many years) a distant, high redshift object, A, we will see the object's redshift decrease, reach a minimum, and then increase. If we watch a much closer, lower redshift object, B, we see the object's redshift only increase.

    Roughly, when light left A, the universe was in a decelerating matter-dominated phase, and when light left B, the universe was in the accelerating dark energy-dominated phase.
  5. Jul 21, 2011 #4
    Important distinction indeed, you pretty much answered the uncertainty that I had and disproved my thought (which is not a bad thing of course). Thanks muchly :)
  6. Jul 21, 2011 #5
    Actually, it's the other way around. We infer the big bang from the fact that the universe is expanding.

    Strictly, the redshifts observed from objects at cosmological distances are not Doppler shifts. Once one is dealing with General Relativity, there are at least three reasonably distinct way that light can be red (or blue) shifted. The Doppler shift, as you've described it, is one of these - depending on relative motion of observer and source (this is actually a little imprecise in a framework where the geometry of spacetime is dynamic; but, it can be made more precise).

    A second kind of redshift is the gravitational redshift. This essentially comes down to the fact that time runs at different rates as proximity to a gravitating object changes. This effect actually is significant enough that it needs to be accounted for in the operation of the GPS system.

    The redshift that matters in understanding the expansion of the universe is called the cosmological redshift; and, it arises from the expansion (or, hypothetically, contraction) of space. Essentially, what happens here is that, as the light travels, the space it's passing through expands, increasing the distance between successive peaks (well, really just stretching out the entire wavetrain).
  7. Jul 22, 2011 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    This is a beautiful paper. (Or so it seems to me on first encountering Figure 1.)
    For some reason I had never seen a plot like Fig. 1 before. So http://arxiv.org/abs/0802.1532 shows us that if we watch an object that has z = 8, the redshift will decrease over time!
    Assuming usual cosmological parameters.
    And that will hold for pretty much any z > 3 but the effect would be too small to measure as you get down closer to z = 3.

    And for z < 3, it is different----in the case of nearer objects with z = 0.5 or 1 we would observe z increasing.
    Last edited: Jul 22, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook