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Thread: Question on Dark Energy

  1. #1 Question on Dark Energy 
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    if the expansion of the universe seems to be accelerating the farther away in distance(and time) we look,
    then since we're looking into the past because light has a finite speed,
    and the further you go in the past, the faster the universe's expansion was.
    why should there be any dark energy at all? shouldn't that be what is expected?
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  2. #2  
    Senior Member AlexG's Avatar
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    why should there be any dark energy at all? shouldn't that be what is expected?
    While was not expected is that rather than being constant or slowing down, the RATE of expansion is accelerating.
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  3. #3  
    Senior Member Janus's Avatar
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    You seem to be confusing the higher recession velocities we see with greater distances with the acceleration of the expansion.

    The first is a consequence of the expansion. It is something you would see even if the expansion rate was constant over time or even slowing down. What tells us whether or the expansion rate has slowed down, stayed the same or sped up with time is the exact relationship between the distance and recession velocity. (for instance if we measured that there was an exact 1 to 1 ratio between distance and recession speed, this would tell us that the expansion rate has remained constant over time.)

    The fact the recession speeds increased with distance has been known for a long time, but it wasn't until the 1990's that a study was able to measure the ratio accurately enough to determine the change in the expansion rate over time. the expectation was that it was slowing, and they were trying to nail down just how quickly it was slowing. Finding out that it was speeding up came as quite a surprise.
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  4. #4  
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    but then you would only be observing the accelerated expansion of the universe from the present to the past, yes?

    edit: nvm, I understand what you mean, but couldn't this perceived growth be the simple fact that the universe is decelerating, and any light we receive at all will be from a time when things were moving faster apart from one another?
    Last edited by exDark; 08-21-2014 at 04:38 AM.
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  5. #5  
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    What people expected to see as they looked into the past was that the rate of expansion got faster and faster the farther we looked back. This is what you expect if the mass of the universe is dragging the expansion down to a stop (and maybe a reversal).

    But that isn't what you see when you look into the past. The expected increase does not happen until a long while ago. This means that there must be some force counter-acting the effect of gravity. There are various ways to use cosmological observations to measure the relative strengths of the force of matter pulling expansion down and the force accelerating it.
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  6. #6  
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    maybe there is a flaw in that dark energy argument, and maybe the CMB is a natural background temperature of space.
    if you'd like to see how Monty Python might explain this, visit a show video I posted at
    Monty Python and the CMB Argument - YouTube
    Last edited by KJW; 09-12-2014 at 12:55 PM. Reason: Adjusted Link URL
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