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The Danger of Inflation

The theory of cosmic inflation has been very successful in explaining the flatness of the universe and the large-scale distribution of matter. This is a part of the beautiful success of the current Standard Models of particle physics and cosmology. Still, we have many reasons to believe that the Standard Model of particle physics is incomplete and to expect that new particles and forces will soon be discovered. Earlier articles on these pages have described models of particle physics beyond the Standard Model based on supersymmetry, extra dimensions, and other marvelous principles. But in all of these theories, we have to worry, like the Federal Reserve, about the danger of inflation. Unlike the economy, though, the danger here is not that the universe will overheat but that it will cool down in the wrong way.

I have been particularly worried about theories with supersymmetry, which I find a particularly attractive route to new models of particle physics. Supersymmetry generalizes the Standard Model by adding a new particle with the opposite statistics for every known particle. We add, then, a fermionic photon (which could be the cosmic dark matter) and bosonic electrons and quarks, which could help to explain the origin of the W and Z boson masses. The theory has many other successes, including a connection to the idea of Grand Unification.

But there is a problem. At high temperature, in the very early universe after the Big Bang, the universe starts in a very symmetrical state. Broken symmetry is needed to give mass to the W and Z and to the quark and leptons. But a supersymmetric model often has many different possible broken symmetry states. During inflation, the universe cools rapidly, and it can easily become stuck in the wrong configuration. This would cause irreparable damage.

Recently, in collaboration with Yuuki Shimbara and Tsutomu Yanagida at Tokyo University, I showed that this problem is particularly severe in so-called "gravity-mediation" models of supersymmetry, a class of models that includes most of those studied in literature. In particular, if the expansion rate of the universe during inflation is as high as is usually expected, a condition that corresponds to a very high potential energy during inflation, the theory typically falls into a bad configuration.

It is still possible that the energy during inflation was never so high. But a high value has a definite signal in the cosmic microwave background radiation, corresponding to a contribution induced by gravitational waves. The Planck satellite, which will be launched in 2008, might see this effect. Ground based experiments, for example, the QUIET experiment at the South Pole in which Sarah Church of KIPAC is a collaborator, are also searching for this signal.

In the meantime, it is important to devise new models of supersymmetry that are resistant to the dangers of inflation. Here at SLAC, Ryuichiro Kitano and I have been developing a new model that addresses this question. During inflation, the theory is in a state that is far from the correct one, but it is natural that the theory falls down after the end of inflation and settles into the correct configuration. As a result of this transition, the theory produces a cosmic density of fermionic gravitons, which can naturally make up the dark matter of the universe. The other unique feature of this model is a prediction of the relatively light bosonic tau lepton, which leads to a distinctive signal in collider experiments.

Maybe the next generation of cosmic microwave background experiments—and the special properties of supersymmetric particles discovered at the LHC—will tell us that this is the right direction.

óMasahiro Ibe, May 3, 2007