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Could the Big Bang have been a Vacuum decay?
Suppose the universe expands to a state where the vacuum energy went under whatever threshold it has and had a quantum event caused vacuum decay at some point in the universe, causing some localized region to decay to a state with a lower vacuum energy.

We've all heard this scenario: Vacuum decay would spread at the speed of light, profoundly changing the physics of the decayed region, (overwhelmingly) likely making it incompatible with life that evolved before the decay and (most) likely incompatible with life in general. And we'd never see it coming because the horizon of decayed space would spread at light speed.  I have heard physicists speculating that if our universe underwent vacuum decay, then among other profound changes in physics, photons would acquire mass and the speed of light would no longer be a constant, which is interesting but not material to this discussion. 

But what if a universe at a higher-energy state than ours experienced a vacuum decay giving it a region with the kind of physics that would be familiar to us?  And that the bubble then spread at the speed of light (whatever that would mean) in that universe, eventually merging with other spreading bubbles from other vacuum decay events elsewhere in that universe?

My question is, as seen from the inside thirteen-and-a-half billion years later, would that event look substantially different from the universe we see around us today?  Cosmic inflation might map to joining other spreading shells of decay, which would be reasonable to expect as the vacuum energy would be declining at more or less the same rate all across that universe.  The Hubble expansion might map to a change in the spacetime metric from the progenitor universe.  And the expansion of the universe might map to the expansion of that vacuum decay horizon.

I'm no cosmologist; but I got sucked into that trap of wondering about "before" the Big Bang and why the question makes no sense because our own notion of time doesn't apply.  But if there was a previous version of the universe, with an enormous energy density and whatever analogue for time would allow a "quantum event" to definitely happen, could it be the remains of a vacuum decay?
I don't have the background to say one way or the other, I'm afraid. I'm not sure that anyone in RL does, including practicing physicists.

That said, I have seen an article or book from time to time that talks in terms of an open universe aging/expanding to the point where eventually there is nothing but a void filled with vacuum fluctuations and that eventually (some incredibly long time into the future) one of those vacuum fluctuations would essentially act as/trigger/become a Big Bang event. Whether that would be equivalent to a vacuum decay event, I have no idea.

If anyone knows a physicist they could ask and get back to us that would be kind of cool. Smile

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