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B Expansion of the universe

  1. Apr 16, 2016 #1
    I have a doubt on the expansion of the universe. In the recent years, it has been observed that the universe is expanding in an increasing speed. So the existence of dark energy was suggested. Actually is the speculation that the universe is expanding in an increasing speed because of dark matter necessary of true?
    When we throw a ball upwards, first its velocity increases from zero to a certain level and then it decelerates into zero and again increases and finally comes to zero. Then couldn't the current accelerating expansion be considered as the initial increase in velocity as in the case of the ball? Why is it believed that dark energy drives the expansion. I would like to know there reason behind it.
    If this is far fetched and such questions aren't allowed in PF then please forgive me.
     
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  3. Apr 16, 2016 #2
    Be careful not to confuse dark matter with dark energy, they are completely different phenomena.
    We know almost nothing about whatever force it is that is causing accelerated expansion, but it is observed to be happening.
    People have tried tinkering with relativity theory and other mathematical approaches to explaining it, but as far as I know, there isn't a hypothesis so far which looks convincing.
     
  4. Apr 16, 2016 #3

    phinds

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    This misunderstanding (that the acceleration of the universe isn't real) is regularly debunked here. I suggest a forum search.
     
  5. Apr 16, 2016 #4

    PeterDonis

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    Ned Wright's cosmology FAQ (which is worth reading in its entirety, along with his tutorial, in any case) has a good brief explanation of the data that leads us to believe the expansion is accelerating (more precisely, that it has been accelerating for the past few billion years--before that, it was decelerating).

    http://www.astro.ucla.edu/~wright/cosmology_faq.html#CC
     
  6. Apr 19, 2016 #5
    I just read the article.
    But sir it is given that the expansion would have been slower in the past. It is just like in the case of the ball which i have mentioned. Until the ball reached certain point, it had been accelerating. So on looking to the past from that point one could see that the acceleration was happening slower.
    And i do not question the expansion. It is only why the existence of dark matter which is added to the theory that i'm doubted on.
    I'm sorry if I misunderstood something or failed to grasp something from your honorable posts
     
  7. Apr 19, 2016 #6

    PeterDonis

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    For a few billion years into the past, yes. But farther back than that, the expansion would have been faster. In other words, if we look at the behavior of expansion from the Big Bang on, it first slowed down for billions of years, then started speeding up.

    No, it isn't. The case of the ball does not reproduce the entire behavior I described above.

    Do you mean dark matter or dark energy? They're not the same thing, as rootone has pointed out. Dark energy is our current term for whatever it is that is causing the expansion to accelerate. Dark matter is our current term for whatever it is that makes large bound systems like galaxies behave as if their total mass is larger than what can be accounted for by adding up all the mass we can see.
     
  8. Apr 19, 2016 #7
    Sir I meant dark energy
     
  9. Apr 19, 2016 #8

    PeterDonis

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    Ok, so what do you "doubt" with regard to dark energy? We've already shown that your "ball" model can't account for what we actually observe; or, in more general terms, the accelerating expansion we currently see can't just be the product of the initial impulse given by the Big Bang. Something else must be at work, and "dark energy" is what we call that something else.
     
  10. Apr 19, 2016 #9
    Thank you for your reply sir.
    And my doubt is cleared.
     
  11. Apr 20, 2016 #10

    timmdeeg

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    Just to understand you correctly, the data are still consistent with the assumption, that the accelerated expansion is caused by the Cosmological Constant, right?
     
  12. Apr 20, 2016 #11

    PeterDonis

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    The Cosmological Constant is one particular subtype of dark energy, so yes. But that still doesn't tell us why there is a Cosmological Constant and why it has the value it has. So calling it a Cosmological Constant doesn't really say anything more than calling it dark energy, except that it commits you to the belief that its value is the same everywhere.
     
  13. Apr 20, 2016 #12
    My ball doesn't do that. It starts decelerating right away after I finished pushing it.
     
  14. Apr 20, 2016 #13
    Your ball still goes from 0 to a specific velocity. It doesn't go from 0 to 0.
     
  15. Apr 20, 2016 #14
    Well, it's not a mystery why it's doing that. I'm pushing on it.

    Allen_Wolf seems to think that Universe and thrown balls accelerate by themselves.
     
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