A scenario is presented, based on renormalization group (linear perturbation) ideas, which can explain the universality and scaling observed in a numerical study of gravitational collapse of radiation fluid. In particular, it is shown that the critical exponent β and the largest Lyapunov exponent Reκ of the perturbation is related by β=(Reκ)-1. We find the relevant perturbation mode numerically, and obtain a fairly accurate value of the critical exponent β0.355 801 92, also in agreement with that obtained in numerical simulation.
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