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If according to Newton’s 3rd law (action = reaction) the pressure to expand the universe, to accelerate galaxy clusters away from each other never can exceed nor precede in time their opposition to it powered by gravity, then wouldn’t its expansion be unable to accelerate -or decelerate, for that matter?

Newton’s third law states the obvious fact that the force between objects only can be as strong as the opposition they offer to it, as the counterforce it is able to evoke and cannot precede in time that counterforce and clearly also holds in general relativity. If energy is a source of gravity, then shouldn’t the pressure (Friedmann equation) act as an equally strong counterforce to it? If that energy is as strong a source of repulsion as of gravity, then it cannot drive the expansion of space not would massive particles then be able to contract at places. If the same goes for Λ -the cosmological constant / vacuum energy / dark energy, then there shouldn’t be any expansion or curvature of space.

Put differently, how can energy be both -simultaneously- a source of gravity and antigravity?

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    $\begingroup$ Applying Newtonian mechanics to the metric expansion of space? $\endgroup$ – Alfred Centauri Jun 30 '17 at 2:43
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    $\begingroup$ Newtonian mechanics is no longer valid on such scales. You must use general relativity. $\endgroup$ – probably_someone Jun 30 '17 at 3:33
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    $\begingroup$ I he comments are right, you need GR. But classically, a cloud of gas will expand with repulsive pressure from the kinetic energy of the molecules. And if you heat it it will accelerate. You just need the right combination of energy and pressure. $\endgroup$ – Bob Bee Jun 30 '17 at 4:45
  • $\begingroup$ Also, a common misconception of the expansion of the Universe is that it is expanding "into something", like a balloon being blown up. The thing is that space itself is expanding, as opposed to "the Universe is expanding in space". $\endgroup$ – lemdan Jul 1 '17 at 0:12

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