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, then deceleration, then acceleration?

  1. Feb 8, 2008 #1
    My first post here.
    My level of expertise in this field = 0, although my interest is very high!

    If I understand well what i've been reading:

    after the BB the universe starting expanding
    then the expansion slowed down
    then for the last 5 billion years the expansion reversed its course and started to
    accelerate.

    Why?
    AND
    Could the expansion slow again and even result in "the big crunch"?
     
  2. jcsd
  3. Feb 8, 2008 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    there is a standard explanation for this in terms of the prevailing LCDM model.
    one can think up other explanations, but I personally don't pay them much heed. The standard LCDM makes good enough sense.

    the explanation is simple. dark energy is assumed to have a constant density which starts off very small compared with matter density (measured as it's energy equivalent to have everything in the same terms)
    BUT as space expands the matter density becomes less and less!
    Finally dark energy becomes the dominant form, just by staying constant as matter thins out.

    So the dark energy accelerative effect prevails.

    While matter dominates, expansion slows, then when dark energy dominates, percentagewise, expansion speeds up.
    =================================
    if you want more detail, look up Friedmann equation in wikipedia

    the equations that all cosmology is based on are the two Friedmann equations which control the evolution of the scale factor a(t) in the Friedmann Robertson Walker metric.

    the first Friedmann equation says that the the sign of the second time derivative a" (t) is determined by the expression
    -(rho(t) + 3p(t)) where rho is the average energy density and p is the pressure

    matter typically has zero pressure and DE has negative pressure, so while matter is dominant this term is mostly just minus rho(t).
    So the expression is negative and a'(t) decreases

    but after a while the important part is the negative p(t) and minusminus is plus, so the expression becomes positive and a'(t) increases.
    ======================

    According to the standard LCDM model, collapse is not in the cards. Dark energy is constant, the pressure it exerts is constant. expansion continues gradually accelerating indefinitely.
     
    Last edited: Feb 8, 2008
  4. Feb 8, 2008 #3
    Thank you!!
     
  5. Feb 8, 2008 #4
    Acceleration of the universe is expanding space. How then does gravitation and dark energy play a role? Gravitation attracts matter while dark energy repulses it. Space is not matter (in the non-quantum sense). It should not be effected by either (other than curviture). Gravitationaly bound objects would appear to be static in expanding space but would be moving though space thus have velocity. Dark energy would give the look of expantion to an observer but would also create velocity. Objects in expanding space should be comoving but have no velocity (if you take away their peculiar motion). Space is expanding and we are along for the ride i.e. the rubber sheet universe model. The way I figure it, if dark energy has anything to do with acceleration of expanding space it is creating more of it. How do you see it?
     
    Last edited: Feb 9, 2008
  6. Mar 1, 2010 #5
    Now the universe is accelerating, how can matter dominate again?
    If the accelerated expansion occurs, then how the density of matter can increase again?
     
  7. Mar 1, 2010 #6

    bapowell

    User Avatar
    Science Advisor
    2015 Award

    It might not, Atran. The universe could go right on accelerating forever. However, if the dark energy decays away, then matter will once again rule the cosmos.
     
  8. Mar 1, 2010 #7

    Ich

    User Avatar
    Science Advisor

    Yes, you just misread marcus' post. Put his last sentence in your quote on top.
     
  9. Mar 1, 2010 #8

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I'm not sure I understand your problem. For a long time, for billions of years, matter dominated, and the expansion was gradually slowing down.
    Then the matter density got low enough that dark energy began to dominate, and the expansion started to speed up.

    It seems straightforward. I guess it's possible you misunderstood something and Ich may have correctly identified your confusion. But I'm not sure.

    In the standard model (socalled LambdaCDM) we do not assume matter will ever dominate again, and we do not assume that the dark energy "decays" or changes in any way. It always stays at a constant density of about 0.6 nanojoule per m^3 and a constant negative pressure of -0.6 nanopascal. There is no foreseen "big rip" or "big crunch" or any other of these exciting prospects.:biggrin:
     
  10. Mar 2, 2010 #9
    According to the standard model, matter will not dominate again, so I know what you mean now.

    Another problem I have: The expansion accelerates, so what happens if the speed of the expansion gets near the speed of light? I know that nothing can reach the speed of light.

    ========================================

    Any idea of why dark energy is constant and matter variable?
    What is meant by "dark energy decays"?

    Thanks for the replies.
     
  11. Mar 2, 2010 #10

    bapowell

    User Avatar
    Science Advisor
    2015 Award

    Even in nonaccelerating spacetimes, objects can move apart at speeds surpassing that of light. This is because the speed at which two objects move apart is proportional to their separation: v = Hr, where v is their recession velocity, r is there separation, and H is a constant known as the Hubble constant. You can see that at r = c/H, their recession velocity is c. There is no contradiction with relativity; the objects themselves are locally at rest. It's the expansion of space that causes them to move apart, and special relativity puts no bounds on the rate of spatial expansion.
    In the standard model, LCDM, the dark energy is provided by a cosmological constant -- the energy of the vacuum. As its name indicates, it possesses a constant energy density. Matter has a variable density because it dilutes as the universe expands. The energy of the vacuum is a very different substance indeed.
    Contrary to LCDM, in which the dark energy is constant, there has also been lots of work on models in which the dark energy is a function of time. By 'decays' I really mean the energy density decreases; I don't strictly mean that the dark energy undergoes any decays in the particle physics sense, although this is possible. It's possible to hypothesize an energy density that decreases as the universe expands, however, at present there is no observational evidence for dark energy that does anything else other than remain constant.
     
  12. Mar 3, 2010 #11
    how we know that universe was decelerating before 5 billon years can any body tell me this
     
  13. Mar 3, 2010 #12

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I think it has been accelerating for at least 5 billion years. I don't think anyone said acceleration started then. One cannot say with precision. Acceleration may have started when the expansion process was 7 billion years old (and so then it would have been going on now for 6.7 billion years.)

    How we know? There are various ways that cross check each other. Different ways, if they are consistent, reinforce and corroborate each other.

    I will tell you how I calculate when acceleration started.

    The present universe data is that (approximately) the dark energy density is 73% of total and (dark and ordinary) matter is 27%. For decel to stop and accel to begin it must be the case that dark energy reaches 1/3 of total. It must be at least HALF as abundant as matter.

    So think of a cubic meter of space having 73 units of DE and 27 units of matter. Twice 73 is 146, that is how much matter we can have per cubic meter when accel starts. What is 146/27? It is 5.4. What is the cube root of 5.4? 1.75.

    That means that if we go back to a time when distances were shorter by a factor of 1.75, then volumes will be reduced by a factor of 5.4, and the matter density will be increased by a factor of 5.4 and it will be 146 units per cubic meter. Then DE is 73 and matter is 146, and accel can start.

    Now the redshift that corresponds to distances shorter by factor 1.75 is redshift z = .75. I have calculated that if I look thru a telescope and see a galaxy whose light is redshifted z = 0.75 then the people I would see living on that galaxy (with a better telescope) are living at a time when acceleration is just beginning. The z=0.75 galaxy which you see is existing in the era when decel stopped and accel. began.

    ================
    If you google "wright calculator" you will get the online calculator that tells how old the universe was at the z=.75 time. It says 7.1 billion years.
    Just type in .75 and it will tell you.

    Therefore according to my quick calculation I would say that, as of today, accel has been going on for 6.6 billion years.
     
    Last edited: Mar 3, 2010
  14. Mar 4, 2010 #13
    first i want to thank you for your good reply
    in this we have taken that dark energy is responsible for acceleration of universe and made calculation but how accept that dark energy is responsible for acceleration. decay of present matter and appearance of dark matter how is this dark matter being appearing with time can i know this
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?