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?

  2. Actually, the acceleration is not constant; it is increasing. And the force behind it is, perhaps somewhat surprisingly, gravity.

  3. Let me explain.

  4. 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.)

  5. However, there was gravity. And gravity of course tries to pull things together. So it acts against the expansion, slowing it down. 

  6. And indeed, in the first 8–9 billion years of its existence, the expansion of the universe has been slowing.

  7. Enter dark energy. Let me not try to tell you what dark energy is, since nobody knows. 

  8. There are many conjectures, but none have so far received any experimental support. 

  9. 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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  11. Dark energy exhibits a huge negative pressure, causing the universe to accelerate in its expansion

  12. This negative pressure is associated with the energy density of dark energy and is responsible for the repulsive force that propels galaxies further apart. 

  13. 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. 
  14. source
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  16. Why is that relevant? Well, think about what happens to a “normal” cloud of gas under self-gravity. 

  17. Gravity does work (work in the physics sense) when it pulls that gas together. 

  18. 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.

  19. 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. 

  20. 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.

  21. 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. 

  22. As the universe expanded, these components were diluted as you would normally expect: when the volume doubled, the density halved.

  23. 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. 

  24. About 4–5 billion years ago, we reached the point when dark energy became dominant. 

  25. So now the expansion of the universe is governed primarily by how dark energy reacts to its own self-gravity. 

  26. And it reacts by expanding at an accelerated rate.

  27. These “fairy tales”, by the way, are not fairy tales at all. Rather, 

  28. I just put into words what Einstein’s field equations tell us about an expanding cosmos. 

  29. In the specific case of a homogeneous and isotropic universe, these are called the 

  30. 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.

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