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Was the universe already expanding before inflation occured or did inflation cause the universe to start off expanding?

By “cause it to start off expanding” , I mean the cause of the initial expansion.

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Was the universe already expanding before inflation occured or did inflation cause the universe to start off expanding?

It's not possible for the universe to be in a non-expanding phase and then enter an inflationary phase.

The idea of cosmological expansion is based on two pieces of evidence, one theoretical and one observational. Observationally, Hubble et al. observed the Hubble law. Theoretically, it's not possible to construct a static cosmology in general relativity without fine-tuning.

For the very early universe, we have little clear observational evidence, but the theoretical argument still holds.

All of this is on much more solid ground than anything having to do with the inflation hypothesis. We actually don't know whether inflation is right or not, although it is a very popular idea among theorists.

It's a popular misconception that inflation caused expansion, is needed in order to explain expansion, or is the same thing as expansion. It's not.

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    $\begingroup$ You answered "yes" without explaining exactly why the inflation could not have started exactly at the moment of the Big Bang. The answer is that (assuming the universe started from a singular state) mathematically it is impossible to transition from a singular state to the inflationary state. There must have been a brief moment of expansion between the Big Bang and inflation. Or else the uninverse did not start from a singular state with arbitrary high energies (which would open a new can of worms). $\endgroup$ – safesphere Aug 12 '18 at 4:16
  • $\begingroup$ I’ve read that most models of inflation supported by current data are eternal inflation models. In eternal inflation, the whole spacetime is inflating then inflation ends in some regions which give rise to hot Big Bangs. Is inflation the cause of expansion in this picture? $\endgroup$ – parker Aug 12 '18 at 18:55
  • $\begingroup$ @safesphere: Good point, I've edited my answer accordingly. $\endgroup$ – Ben Crowell Aug 12 '18 at 23:27
  • $\begingroup$ @parker: In eternal inflation, the whole spacetime is inflating then inflation ends in some regions which give rise to hot Big Bangs. No, I think you've got this wrong. In standard cosmological models, there is only one big bang. $\endgroup$ – Ben Crowell Aug 12 '18 at 23:28
  • $\begingroup$ @Ben Crowell: there are separate reheating events in each region that drops out of the inflating background; phenomenologically these are “hot big bangs” in each region. $\endgroup$ – bapowell Aug 12 '18 at 23:40
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Here is a link discussing cosmological models, as one has to keep in mind that the models develop in order to fit/explain observations using current physics theories, classical and quantum mechanical.

The current mainstream model can be seen here, and your question is about what happens at the quantum era, before $10^{-32}$ seconds.

Here is an image of the current mainstream chronology that includes the current models for that time:

history of universe

Between the fuzzy ball that has replaced the classical singularity and to the end of inflation at $10^{-32}$ seconds, quantum gravity reigns, and quantum gravity is still an open problem.

The above current model assumes a period before inflation starts, where the inflation is very small. The inflation period starts later than the fuzzy beginning of universe, and is

Triggered by the symmetry breaking that separates off the strong force, models suggest an extraordinary inflationary phase in the era $10^{-36}$ seconds to $10^{-32}$ seconds. More expansion is presumed to have occurred in this instant than in the entire period ( 14 billion years?) since.

infla

So the answer is that in the mainstream cosmological model the universe is expanding after the fuzzy beginning at a very low rate, and suddenly inflation sets in because the temperature has dropped to the symmetry breaking between strong and electroweak forces.

Keep in mind that cosmology is an active field of research, and we are discussing here the current mainstream model.

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  • $\begingroup$ I guess that without some pre-inflation expansion to cool the universe then the symmetry-breaking phase transition couldn't have occurred. But I guess Quantum Gravity could provide an alternative mechanism. $\endgroup$ – PM 2Ring Aug 12 '18 at 9:02
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    $\begingroup$ @PM2Ring It is all conjecture at that level, if the mathematics works, it is allowed ! It has to be quantum gravity at that time. I have the impression that it is the original classcal bb rate of expansion, at least the image I quote above shows the same slope before and after inflation, but could not find any references. $\endgroup$ – anna v Aug 12 '18 at 10:15

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