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

Universe expansion is slowing down

  1. May 6, 2012 #1
    Standard point of view: When we look at distant galaxies, we see that redshift is higher then expected so we conclude that the universe is expanding at accelerating rate.
    I don't see the logic here.

    When we talk about acceleration, we mean how velocity changes over time. If velocity over time increases - it means object is accelerating.
    If velocity was higher long time ago and becomes slower over time we have to conclude that velocity decreases over time (decelerating) instead.
  2. jcsd
  3. May 6, 2012 #2
    dont think about velocity.
    acceleration was much higher while inflating.
    current expansion acceleration is lower but it still accelerates.
  4. May 6, 2012 #3
    You are mixing something. If universe expansion speed is accelerating from the big bang till now it means that current expansion speed is higher then it was in the inflation. Which doesn't make sense.
    Last edited: May 6, 2012
  5. May 6, 2012 #4
    Myslius, the universe has been accelerating since approximately 7 billion years after the big bang. NOT since the big bang itself. In fact, it was decelerating until roughly the 7 billion year mark. At that time, the density of dark energy (the cosmological constant, so it probably doesn't ever lose density) overcame the density of matter/dark matter, which falls very fast.
  6. May 6, 2012 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    No, that is not what we mean by acceleration in cosmology. There is no definite velocity that characterizes the expansion. We are not talking about the change over time of some velocity.

    I'll try to explain how the term is used.

    First learn the idea of the Hubble rate, which is a percentage growth rate of distance (measured in the universal rest frame, in which distances grow but galaxies mostly don't move save for small random motions).

    The Hubble rate has been decreasing since very early days and is expected to continue to decline, but less rapidly. It is currently about 1/140 of one percent every million years. That is on average largescale distances (between objects at rest) increase by about 1/140 of a percent every million years.

    There are online calculators which can tell you, for a given redshift, what the Hubble rate was back then when the distant galaxy emitted the light that now has that redshift, that we see it with.

    Then learn about the scale factor a(t) a function of time. By convention a(present)=1.
    It turns out that the Hubble rate is equal to a'(t)/a(t). That is the ratio of the derivative or slope of the scalefactor to the scalefactor itself. A fractional increase per unit time. You can express a'(t)/a(t) as a percentage growth of distance per unit time.

    a(t) is not a velocity. a'(t) is not a velocity. the Hubble rate is not a velocity (it is a percentage growth rate of distances between stationary objects.)

    a(t) and the Hubble rate are governed by an equation or more exactly by a pair of equations. They are simple equations but out of laziness I nearly always use the readymade online calculators that solve them for you automatically.

    What is meant in cosmology by the acceleration or "speeding up" of expansion is that the second derivative a"(t) of the scalefactor a(t) is positive.

    Not only is the a(t) increasing with time but the slope of the a(t) curve is increasing.

    Do you understand how a'(t) can be increasing and yet the Hubble rate a'(t)/a(t) can be decreasing?
    It sometimes helps to think of money in the bank---say in a savings account where the bank is very gradually decreasing the percentage of interest that they pay to savers. But the total dollars in your account nevertheless keeps going up by a greater and greater amount each year. Because the bank is easing off the interest rate only very slowly.

    I don't believe that thinking in terms of conventional motion or velocity will help you understand this. Distance change is convenient to think about as a change in *geometry*, not motion as we know it. Everybody's distances increase and nobody gets anywhere by it. Relative positions don't change (except for small individual random motions whose effect on the big picture is negligible).
    Last edited: May 6, 2012
  7. May 6, 2012 #6
    Hubble "constant" decreases over time. And it describes the expansion rate (speed) and that universe expands at the speed of c (flat)

    H(t) = (Mpc/c*t)*c per Mpc
    H(now) = 3.08 * 10^22 m / (3*10^9 km/s * 13.7by) * 3*10^9 km/s per Mpc = 72km/s/Mpc
    (Mpc/c*t)*c per Mpc = 1/t or simply scale factor
    and H decreases over time because t increases. So the expansion rate is slowing down. Of course derivative is positive because the universe is expanding.

    Hubble constant says nothing about accelerating expansion. It says that total expansion is flat and is c, and that local expansion decreases over time. So where is the acceleration again?
    Last edited: May 6, 2012
  8. May 6, 2012 #7


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Can't help. Thanks for responding.
  9. May 6, 2012 #8
    We can measure the redshift of distant Type Ia Supernovae to determine that the universe is accelerating. This chart here plots distance against redshift:


    Hubble's Law for an expanding universe predicts a linear relationship - as distance increases, redshift increases. Without any increase in slope. However, if we look at the chart, we see that since z = 0.5 Hubble's Law is violated, and the slope is increasing. As marcus said, the SECOND derivative is positive. Expansion has been acceleration since z = 0.51, about 7 billion years ago.
  10. May 6, 2012 #9
    Try to look from perspective when time was 0 and plot this graph till now.
  11. May 6, 2012 #10
    What? Did you not read my post? The expansion has been accelerating since 7 billion years after the big bang. NOT since t = 0.
  12. May 6, 2012 #11
    ... We do see distant galaxies as they were few billion years after big bang. It's 7b y old information, faster expansion was 7b years ago, not now. We see no increase when we look 7b years around.
    Last edited: May 6, 2012
  13. May 6, 2012 #12


    User Avatar
    Staff Emeritus
    Science Advisor

    Am I correct in thinking that the increase in the rate of expansion would show up the most when viewing very distance galaxies since they are receding at a much faster velocity than closer ones are, and would show the greatest increase in recession velocity with any increase in expansion rate?
  14. May 6, 2012 #13
    Yes we do! Did you not look at the chart? After z = 0.51, Hubble's law is violated.
  15. May 6, 2012 #14
    Correct. Just when you see the increase in the rate of expansion that increase was billions of years ago, not now. If you want to know how the rate of expansion changed (or will change) over time you have to reverse the timescale.
  16. May 6, 2012 #15
    Mark M, yes, i noticed.
  17. May 6, 2012 #16
    Increase of rate should merge with inflation period at some point.
  18. May 6, 2012 #17
    Why do you keep getting caught up on inflation? We can't see what the universe looked like when inflation occurred. We are limited by the cosmic microwave background, the first light emitted at the recombination. This occurred 380,000 years after inflation. Nothing before this is visible to us.
  19. May 6, 2012 #18
    Just a though. I would rather have explained and described phenomena than something that has no math.
  20. May 6, 2012 #19
    :confused:I'm confused about the standoff here.
    As an observer so far of this exchange we seem to be talking at cross purposes.
    The point under discussion appears to be is .51 older or younger than .49 in terms of observed phenomena.
    So which is it?
    Have I missed a trick here or should this be obvious?
  21. May 6, 2012 #20
    It's not about age. It's about dark energy. Current theories state that space expansion was accelerating, decelerating and is accelerating again, and that we have this repulsive (i guess, source isn't described) and unknown force called dark energy.

    Instead of this it could be simply explained by deceleration. And known force - gravity? It's a hypothesis, same as dark energy.
    Last edited: May 6, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook