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

Dark energy question

  1. Dec 13, 2015 #1
    I'm not qualified to purpose a theory at all, however I want to ask a question to see what the problem is with my reasoning. Could someone explain why this wouldn't produce the same results as we are observing today?

    At the big bang, there was an expansion. At first, the rate of the expansion of the the universe decelerated due to gravity attempting to put everything back into a singularity. As time continued, however, the matter and therefore mass of everything within the confines of the universe spread out. This dispersal made the gravity in the universe significantly weaker, and so the expansion began to accelerate. This would explain why the universe is accelerating in expansion.
  2. jcsd
  3. Dec 13, 2015 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    No it would not. Gravity caused by matter is still attractive and if matter was all there was there would always be deceleration, although it would decrease as the universe expanded. In order to model an accelerated expansion you need something like dark energy.
  4. Dec 13, 2015 #3
    However if the deceleration within the universe started to decrease, wouldn't we observe this as acceleration?
  5. Dec 13, 2015 #4


    User Avatar
    Science Advisor

    No. If your car is decelerating(i.e going slower), and it begins to decelerate at a slower rate (i.e. still losing speed, but losing speed at a slower rate), do you think it is accelerating (i.e going faster)?
  6. Dec 13, 2015 #5
    Well I didn't think the universe was accelerating, at least not in the conventional manner.
    I thought that all this screaming of acceleration was just that the rate of decline in expansion was decreasing.

    If the universe was truly accelerating, how could the Hubble sphere continue to grow?
  7. Dec 13, 2015 #6


    User Avatar
    Science Advisor
    Gold Member

    The Hubble sphere presently still grows due to a significant amount of mass-energy around (~30%), but the 70% dark energy already dominates, causing a mild acceleration in the expansion. When mass-energy eventually gets diluted into insignificance, the Hubble sphere will remain constant in proper distance terms.
    V_gen (red) tracks the recession rate of a galaxy that is presently on the Hubble sphere. The curve for the Hubble radius is the inverse of Ho.
  8. Dec 13, 2015 #7
    Oh. I was told this:
    "So leaving Dark Energy acceleration aside for now, the essential fact of the size of the Hubble Sphere is that it expands when the overall expansion rate of the universe is slowing down.......and the overall expansion has been slowing down ever since the big bang when space and energy first burst outward. So the Hubble Sphere has been expanding in size relative to the rest of the universe over all of the age of the universe too.

    That's the basic picture......and that was the total picture until 1998 when astrophysicists made a hugely important (2011 Nobel Prize winning) discovery that is usually just described in the popular science media as the ACCELERATION of the universe. Something called "Dark Energy" was theoretically postulated to explain it. On a surface level......for someone who knows that the universe has been slowing down and that the Hubble Parameter has been decreasing over all the age of the universe.......this is confusing. How can the universe be slowing down if the popular science headlines say that it's “Accelerating”?

    On one level, you can just say that it's a form of acceleration that has the effect of reducing the rate of decline of the Hubble Parameter over time. But the Hubble Parameter is still declining and the Hubble Sphere is still expanding as before.......except just not as fast as they were before Dark Energy acceleration kicked in about 5 billion years ago."
  9. Dec 14, 2015 #8


    User Avatar
    Science Advisor
    Gold Member

    Yes, it can be confusing, because even in technical writings "expansion rate" is sometimes used interchangeably for the Hubble value H and the time derivative of the scale factor, da/dt. On this Forum (at least), we found consensus by naming H(t) the expansion rate and V_generic = aH(t) the recession rate.
    H(t) has the same properties as an interest rate on investment - the "expansion rate of an investment".
    V_generic, the red curve above, is the generic recession rate of a galaxy presently on the Hubble radius, hence presently receding at c.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook