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I know in order for something to expand or grow it has to be fueled by something? So, if the universe is expanding their must be something that's keeping the universe at a temperature high enough for it to expand. For something to expand it has to be heated to a certain amount, so I was thinking maybe the massive heat surge, blast and radiation from the big bang is heating the universe enough to expand. Then another thought occurred to me what happens when its not hot enough to expand will it just stop.

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    $\begingroup$ @HDE226868 Temperature DOES affect cosmological expansion and dark energy is only one other factor that affects expansion. Dark energy is currently the dominant factor, but only became the dominant factor several billion years into the life of the Universe and is not a necessary factor for expansion. $\endgroup$ – John Davis Oct 4 '14 at 19:59
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    $\begingroup$ Dark energy explains why the expansion is accelerating. It is quite clear that the FLRW metric does not need a dark energy-like term for expansion. Indeed dark energy is a recent discovery. $\endgroup$ – John Davis Oct 4 '14 at 20:06
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    $\begingroup$ @HDE 226868, this is getting a bit silly. In the FLRW metric expansion still occurs even when the cosmological constant is set to zero. Until the discovery of accelerated expansion about 15 years ago, the cosmological constant was generally believed to be zero. $\endgroup$ – John Davis Oct 4 '14 at 20:16
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    $\begingroup$ @HDE226868: the universe's expansion would have slowed down over time until it was negligible - without dark energy Yes, it would have slowed down, but it wouldn't have stopped by now, for realistic cosmological models. $\endgroup$ – Ben Crowell Oct 4 '14 at 20:36
  • $\begingroup$ @BenCrowell My reasoning was completely wrong, as this (physics.stackexchange.com/q/48573) shows. Thank you for correcting me. The last sentence of your answer pointed me in the right direction. $\endgroup$ – HDE 226868 Oct 4 '14 at 20:52
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The basic reason for cosmological expansion is simply inertia. Because the universe was in an expanding state soon after the big bang, it kept expanding. This is roughly analogous to Newton's first law of motion.

In addition to this, dark energy is currently causing a significant acceleration of the expansion. (Its effect was not dominant in the past, and was negligible in the early universe.)

I know in order for something to expand or grow it has to be fueled by something?

Not true. In an ordinary explosion, there is some initial expenditure of energy, but once the matter is spreading apart, there is no further need for a source of energy.

Cosmological expansion is also different from an ordinary explosion because in current cosmological models, there was no time before the big bang. Therefore there is no need to posit a cause for the big bang at an earlier time, or a preexisting supply of energy.

As a side issue, energy is not globally conserved in general relativity in cosmological spacetimes.

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  • $\begingroup$ Ben: "energy is not globally conserved in general relativity in cosmological spacetimes". Could you develop this point or give some other reference to read? I'm surprised by this Energy disappearance or creation. $\endgroup$ – dan Jan 2 '15 at 13:32
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Ulitimately the Universe's expansion is due to the initial conditions, unfortunately explaining why these initial conditions exist is beyond the scope of classical big bang theory as they exist as parameters than can be adjusted.

However the expansion of the Universe is not independent of the matter it contains and the Friedmann equations link the rate of expansion to the pressure, density, curvature of the Universe and the comsological constant. The temperature is dependent on pressure and density, so it is fair to say it affects expansion, but it is wrong to say that it is the driving cause of expansion.

In a Universe with a zero cosmological constant the rate of expansion always slows over time leaving 3 possible outcomes:

  1. The expansion rate can become negative causing the Universe to collapse to a big crunch
  2. The expansion rate can asymptotically go to zero (i.e. expansion becomes negligible at late times)
  3. The expansion rate can asymptotically approach a positive value (i.e. it is not true to say that without a cosmological constant the expansion will necessarily become negligible at late times).

Which of these 3 fates greets the cosmological-constantless Universe depends on its curvature.

Recently though it has been discovered that the expansion of the Universe is accelerating and the simplest way to explain this is by setting the cosmological constant to a non-zero positive value. In this scenario the rate of expansion asymptotically becomes an exponential function of cosmological time, meaning that in late times the Universe resembles something called De Sitter space.

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