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The galaxies in the universe appear to be accelerating away from each other according to the Doppler shift that is present in the light we receive from other galaxies.

According to Newtonian mechanics for something to accelerate, a force must be applied to it, and if the accleration is constant, then the force must be constantly applied.

If this is the case, then what force or forces is (or are) causing galaxies to accelerate?

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    $\begingroup$ Essentially answered in How does dark energy allow the universes expansion to accelerate? $\endgroup$ – ACuriousMind Mar 19 '16 at 19:34
  • $\begingroup$ We do not know for sure what causes the universe to expand. Dark energy is a hypothesis, created and named to explain the expansion. Science is not even close to understanding it. But the expansion is caused by expansion of space itself, and not by speeding up the galaxies through space. So whatever does it, acts on the space, not on galaxies. Everything moves with space. $\endgroup$ – kpv Mar 20 '16 at 3:01
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Newtonian mechanics does not explain the dynamics of the universe.

The expansion of space is apparently "caused" by an innate property of the vacuum.

The name we give to this property is dark energy.

In the absence of any gravitating matter, the dark energy content of the universe causes space to expand as a natural consequence of solving the dynamics of the whole universe according to the theory of General Relativity. In GR, mass, energy and pressure all act to gravitationally decelerate any expansion. Dark energy acts like a negative pressure that causes accelerating expansion, and there is no evidence at the moment that its energy density (i.e. energy per unit volume) changes with time - so it is sometimes referred to as the cosmological constant.

It is still a matter of theoretical speculation at present as to what dark energy is.

However, before observations of the cosmic microwave background and distant type Ia supernovae revealed the need for dark energy, it was still known that the universe expanded and the equations of GR still permit an expanding (although decelerating) universe without dark energy. One idea is that the universal expansion was kicked off by cosmic inflation - a souped-up version of dark energy that caused the universe to expand exponentially for a short period of time in the first tiny fraction of a second after the big bang.

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  • $\begingroup$ Hi Rob, in regards to your statement, "there is no evidence at the moment that its energy density (i.e. energy per unit volume) changes with time" actually the observations indicate that we are living in a very special time with this value being a very slowly varying function of time. In fact Dark Energy had slightly less contribution to the mass and energy budget of early Universe than it has today. It is not exactly a constant as it is called. And the still open question is that why we are living in such a special time where it seems it is not changing much compared to early times. $\endgroup$ – Benjamin Mar 19 '16 at 23:50
  • $\begingroup$ @Benjamin, that is not correct. Where did you get that from? I think it is wrong to make such statements without even providing a reference. $\endgroup$ – Virgo Mar 20 '16 at 0:49
  • $\begingroup$ Hi Virgo, thanks for the critic. I am not an expert. We can view it as 'cosmological' perspectives or as 'field' theoretical perspectives as argued by this wonderful paper.physics.rutgers.edu/~saurabh/690/Padmanabhan-2003.pdf I should have been more specific. What I truly meant is that it is not exactly true to treat this 'constant' only based on cosmological observations as they are incomplete as of now. But instead I really meant its 'field' treatment where it could be varying just fine. I have to admit though that I am biased towards this perspective until enough observations. $\endgroup$ – Benjamin Mar 20 '16 at 3:01
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    $\begingroup$ @Benjamin There is no evidence for a time-varying dark energy density. It is treated as a constant in all elementary work. There are of course theories as to why it might vary, and it might vary, but there is no evidence for that. $\endgroup$ – Rob Jeffries Mar 20 '16 at 8:21
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Dark Energy is thought to be responsible for an ever expanding Universe. And we cannot understand this phenomenon based on Newtonian Mechanics as it is not applicable in cosmological scales. You can think of this energy as a negative pressure due to vacuum. But we truly don't know anything else about its nature and characteristics. However, DES survey is under way to understand more of it in the coming decade.

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    $\begingroup$ Interesting. Dark Energy. Are the Sith involved? $\endgroup$ – Ambrose Swasey Mar 19 '16 at 19:07
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    $\begingroup$ Sorry what do you mean by Sith? $\endgroup$ – Benjamin Mar 19 '16 at 19:28

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