# How fast is a true vacuum bubble going to expand?

If a true vacuum bubble will occur in our universe, how fast is it going to expand?

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The speed of light –  Michael Brown Sep 2 '13 at 8:00
@MichaelBrown is going to expand at our value of speed of light (300.000 km/s) because is the top speed for information to travel? –  user27494 Sep 2 '13 at 8:09
@MichaelBrown: It's generally said that a phase change would propagate at the speed of light, but is this just a general principle that it would propagate as fast as it can or is there a calculation to back up the claim? –  John Rennie Sep 2 '13 at 8:46
@JohnRennie It's general for the false vacuum case. The calculations are found in the old papers by Coleman. Basically you have an $O(4)$ invariant bubble solution in Euclidean space which you Wick rotate back to Minkowski spacetime to get a hyperboloid which asymptotes to the light cone. So not really $c$, but exponentially close to it for macroscopic times. In the finite temperature case (like a first order phase transition in the early universe) the plasma causes friction which slows down the bubble wall, possibly even to subsonic velocities (see the electroweak phase transition literature) –  Michael Brown Sep 2 '13 at 10:45