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Sorry if this is a naive question, not being even a part qualified physicist in any way shape or form.

I've read that the universe is expanding and the rate of expansion is increasing. The assumption being that it will continue expanding indefinitely.

However isn't there another possibility.

Let me illustrate my question prior to posing it.

If I throw a ball in the air.. it accelerates away from my hand and keeps accelerating, until momentum is lost and it starts to slow down, eventually falling back to my hand.

Is it possible that the expanding universe is also in the throws (excuse the pun) of an initial acceleration phase, prior to that acceleration slowing and eventually the universe compressing?

Again sorry if its a stupid observation!

Brendan X
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    Your idea is called the Big Crunch. – Kyle Kanos Aug 14 '14 at 17:32
  • Technically, the second the ball leaves your hand, the only force being applied to it is gravity, so it accelerates downward, so it actually isn't accelerating away from your hand. And momentum is always conserved. – HDE 226868 Aug 14 '14 at 17:39
  • Related: http://physics.stackexchange.com/q/25806/2451 and links therein. – Qmechanic Aug 14 '14 at 18:01
  • All we know, for now, is that the CURRENT expansion is accelerating. This has absolutely no consequences for our ability to predict the long term future of the universe, because we don't know the dynamic equation that causes this acceleration. So whenever someone says they universe will expand forever, or contract again, all they really mean is that the model they have chosen (arbitrarily) predicts an expansion, rip or crunch event. What nature is really going to do is, as of today, not knowable based on existing observations and theory. – CuriousOne Aug 14 '14 at 18:34
  • Thank you CuriousOne, a very clear answer. I was indeed reacting to a prediction of continued expansion. Thanks again. – Brendan X Aug 15 '14 at 07:12

2 Answers2

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The ball, in fact, is always accelerating downward, even though it is moving up for the first part of its trip. A ball that kept accelerating upward would just fly up out of the atmosphere.

Zo the Relativist
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Carroll (2004) introducing the standard Friedmann-Lemaitre-Robertson-Walker model, shows that this will become clear in a formula that sheds light on the fate of the cosmos:

To determine the dividing line between perpetual expansion and eventual recollapse, note that collapse requires the Hubble parameter to pass through zero as it changes from positive to negative. The scale factor $a_{*}$ at which this turnaround occurs can be found by setting $H=0$ in the Friedmann equation.

Which after rearranging gives

$\Omega_{\Lambda0}a_{*}^3+(1-\Omega_{M0}-\Omega_{\Lambda})a_{*}+\Omega_{M0}=0$

Where $\Omega's$ are the density parameters associated with vacuum energy and matter in the cosmos defined as $\frac {8 \pi G \rho}{3H^2}.$

The cubic equation in terms of $a_{*}$, the scale factor, gives us predictive power.

If there is no real positive solution to it then we have to expect “perpetual” expansion.

And the current experimental data indeed favor such an open ended cosmic future.