80. Watching worlds collide: bubbles, bubbles, bubbles
Save This Post as PDFGetting tired of malicious attacks by anti-landscapists, Spencer Chang, Matt Kleban and Thomas Levi released a paper about one observable effect on string theory landscape.
As we remember, when people talk about dynamics of eternal inflation on the string theory landscape (see for example my own paper and references therein), they keep in mind the picture of Coleman-de Luccia tunnelings between different vacua on the landscape. These tunnelings happen through bubble nucleation; you can live in the bubble with one vacuum, and then suddenly another bubble near you starts to grow, then another – inside the bubble N2 etc., etc.
Each bubble expands exponentially, and the constant reproduction of bubbles makes inflation eternal. The overall causal structure of the spacetime is rather complicated. In a first very naive approximation one can visualize it as a tea pot full of boiling water. There is a certain rate of bubble nucleation inside the pot (boiling is first order phase transition), but the size of each bubble is governed by the equilibrium between its surface tension and the pressure of the gas inside the bubble. In eternal inflation, there is no such equilibrium, so all bubbles continue to expand forever 
For an observer living inside the bubble 1, bubble walls are expanding exponentially due to infation, while for an observer outside it bubble walls of the bubble 1 are moving with the speed of light. So, poor observer doesn’t have any chance to notice how bubble wall hits him – he just dies innocent as infant. Right?? Wrong.
Actually, one has to recall that trivial fact that inflation has came to the end in our bubble, and the matter dominated Universe is now expanding according to the power law. It means that poor observer now will be able to notice the act of collision – in particular, one can expect strong CMB anisotropy in the direction where bubble walls collided. Who knows – maybe, famous black spot is the actual result of one of these bubble collisions?
To qualitatively approach the problem, one has to take reheating surface in one of the colliding bubbles and analyze behavior of geodesics inside the bubble. This will allow constructing the redshift as a function of angle of the observer’s sky.
I think, the analysis in the paper can be significantly improved in at least two ways: a) by taking into account the fact the the inflaton potential is not flat, and b) by taking details of reheating into account (both the position and the width of reheating surface actually depend on the coupling between the inflaton and matter fields).
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