I was reading about the history of the early universe in Carroll & Ostlie and specifically looking at Inflation, which, as I understand, occurred because our [observable] part of the universe entered a false vacuum state containing negative pressures, thus fueling expansion. (Carroll & Ostlie state the opposite, but as per the discussion below, that appears to be incorrect).

Does that mean that there are other parts of the Universe (outside of our horizon) that experienced deflation, as inflationary parts pressed against them? Would there be boundaries between separate parts of the larger universe where inflationary parts encounter one another? Any thoughts on what those parts of the Universe are like, such as containing their own CMB that's NOT close to uniform, or where there is a much higher density of matter?

  • $\begingroup$ Irreversibility is a statistical thing. Even if the equilibrium constant is 10^9999 in favor of true vacuum over false vacuum, you're going to have some slight trickle back the other way. en.wikipedia.org/wiki/Equilibrium_constant $\endgroup$ Commented Sep 2, 2015 at 13:47

1 Answer 1


That sounds... confused, or else just wrong. Negative pressure is what causes inflation: something (inflaton field, cosmological constant, or "dark energy" in general) with an energy density that stays approximately constant as space expands has negative pressure and will drive the exponential expansion of space.

The transition to "true" vacuum is what shuts off inflation, after which you get the standard Big Bang, during which the expansion continues but slows down.

(The cosmological constant -- or "dark energy" -- also has negative pressure, and so the universe is gradually moving towards a state where the space between galaxy groups and galaxy clusters will again be expanding exponentially.)

The "eternal inflation" model has our visible universe -- almost certainly a tiny part of a much larger region of true vacuum -- as one of many such pockets of true vacuum embedded within a vastly larger false vacuum that's still expanding exponentially.

  • $\begingroup$ As I understand it, that is correct: negative pressure is the cause of inflation, but not because it is "on the inside pushing out." Rather, areas of greater (closer to zero or even positive) pressure expand, or are pulled, into areas of negative pressure. That appears to be the point being made in the Carroll & Ostlie passage above, as well as in other places, such as this piece by Caltech's Alan Guth link. The shutting off of inflation is what occurs when all space reaches true vacuum. $\endgroup$ Commented Sep 2, 2015 at 21:30
  • $\begingroup$ Apologies, the comment posted before as was ready to submit it, I'm still reading the Alan Guth piece, which appears to be at odds with the relevant section from Carroll & Ostlie quoted in the original question. $\endgroup$ Commented Sep 2, 2015 at 21:39
  • $\begingroup$ Falling deeper into the rabbit hole, I've tentatively concluded: 1) The relevant section in C&O does assert our part of the universe left false vacuum for a true vacuum and this caused Inflation; 2) The arguments and equations they give are not wholly convincing; 3) Other sources, such as the Guth article assert the opposite: negative pressure causes inflation; 4) There is a LOT of debate about of this, such as this lecture by Stephen Hawking and this paper by well-established universities. $\endgroup$ Commented Sep 2, 2015 at 22:29
  • $\begingroup$ (Comment continued) So I am guessing that the mechanism for inflation is still somewhat poorly established? I supposed I should have known better... That may also make my original question un-answerable because "it depends." I'd still be curious to know what a hypothetically "deflating" section of the universe might have evolved into today, or, if different parts of the Universe experienced inflation, what would occur at boundaries of different inflating parts? Still might be nonsensical, and even if not, well beyond the scope of this forum :/ $\endgroup$ Commented Sep 2, 2015 at 22:35
  • $\begingroup$ The equations of General Relativity are pretty clear: a more-or-less uniformly distributed form of energy with negative pressure (e.g., inflaton field, cosmological constant, dark energy) will cause an exponential expansion of space. (Which can, of course, be opposed by the contractive "force" from a more-or-less uniformly distributed source of energy with zero or positive pressure -- like light or ordinary matter.) If you're correct about what C&O say, then they're just wrong. $\endgroup$ Commented Sep 3, 2015 at 18:04

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