Quora expansion

Viktor T. Toth

1. If the universe is expanding with a constant acceleration it means that there is a force working on it. Which force is it? Where does the universe take energy to grow up?
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3. Actually, the acceleration is not constant; it is increasing. And the force behind it is, perhaps somewhat surprisingly, gravity.
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5. Let me explain.
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7. The universe was born in a state of flying apart. Everything was flying apart from everything else in every direction everywhere. (This is what “homogeneous and isotropic” means.)
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9. However, there was gravity. And gravity of course tries to pull things together. So it acts against the expansion, slowing it down. 
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11. And indeed, in the first 8–9 billion years of its existence, the expansion of the universe has been slowing.
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13. Enter dark energy. Let me not try to tell you what dark energy is, since nobody knows. 
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15. There are many conjectures, but none have so far received any experimental support. 
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17. So let me focus instead on the one property of dark energy that we do know (indeed, this is what defines dark energy): negative pressure. Huge negative pressure.
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21. Dark energy exhibits a huge negative pressure, causing the universe to accelerate in its expansion. 
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23. This negative pressure is associated with the energy density of dark energy and is responsible for the repulsive force that propels galaxies further apart. 
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25. The effect of this negative pressure is so strong that it overcomes the usual attractive pull of gravity, leading to the observed acceleration of the universe. 
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29. Why is that relevant? Well, think about what happens to a “normal” cloud of gas under self-gravity. 
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31. Gravity does work (work in the physics sense) when it pulls that gas together. 
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33. The work shows up ultimately as heat: the gas that is being pulled together heats up and its pressure increases, in accordance with its so-called equation of state.
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35. Dark energy has negative pressure. The denser it is, the more negative pressure it has. So gravity cannot do work on dark energy by making it denser. 
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37. Rather, gravity works on dark energy by making it expand. And what happens to the work done by gravity? Well, it actually becomes more dark energy.
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39. So here is the picture. Most of the early universe was either “normal” matter or “dark matter”, both of which have very small positive pressure. 
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41. As the universe expanded, these components were diluted as you would normally expect: when the volume doubled, the density halved.
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43. Meanwhile, the density of the small amount of dark energy remained constant. So eventually, the density of all the other stuff fell below that of dark energy. 
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45. About 4–5 billion years ago, we reached the point when dark energy became dominant. 
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47. So now the expansion of the universe is governed primarily by how dark energy reacts to its own self-gravity. 
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49. And it reacts by expanding at an accelerated rate.
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51. These “fairy tales”, by the way, are not fairy tales at all. Rather, 
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53. I just put into words what Einstein’s field equations tell us about an expanding cosmos. 
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55. In the specific case of a homogeneous and isotropic universe, these are called the 
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57. Friedmann equations, and they give precise predictions of how the universe evolves, in the presence of matter with (positive or negative) pressure, under its self gravity, and no other forces.