A New Theory to Create Bubble Universes

Imagine each universe is a bubble. Physicists recently found that a classical collision of two bubble universes results in a new, long-lived one. There is a chance our universe was created by this collision process, and cosmological observations are one way to test that theory. Professor Richard Easther of Yale University and collaborators submitted the theory to arXiv (an open e-print archive for mathematics and physical sciences) on July 19, 2009.

In the string theory landscape, each universe is described by a function of a scalar field and its potential energy. Therefore, each universe behaves like a soap bubble or a pocket of air in boiling water. It is formed within a potential well, a region of low potential energy (labeled by V in Figure 1) surrounded by walls of high energy. A stable bubble stays where it is and has a long life time, whereas unstable bubbles either fall to another well with lower energy, or evaporate away. Quantum mechanically, the bubbles can tunnel, or go through the wall, from one potential well (A) to another well (B), given that B has lower energy than A (see Figure 1). As the bubble tunnels, it expands and the energy difference between A and B is stored in the bubble wall. Until yesterday, this quantum tunneling process was the best theory to produce a stable, expanding universe.

Richard Easther and his collaborators proposed that the bubbles can collide while tunneling from A to B, merge, and form a new bubble, like small bubbles in boiling water can merge while rising up to the surface. Such a collision is classical, and can release the energy stored in the original bubbles' walls. Loosely speaking, it is similar to a collision between two cars, which releases enough energy to heat up the gas tank and give an explosion. The energy dissipation from bubbles' collision can be big enough for the daughter bubble to roll over a potential barrier and into yet another potential well (C), whose energy is lower than that of both A and B. Therefore, the daughter bubble is stable. This classical collision process now adds a number of universes into the already populated landscape of string theory (which contains roughly 10500 possible universes!).

The question remains which process, classical collision or quantum tunneling, is more likely to create universes like ours. Can this collision process explain the small value of the cosmological constant in general relativity? Easther and his group promise more experimental prediction to come.

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