Abstract: |
The most pressing fine-tuning puzzles of the Standard Model --- the cosmological constant and weak hierarchy problems, as well as the Higgs metastability --- can all be understood as problems of near criticality. We present a natural selection mechanism based on search optimization on the string landscape. The working assumption is that cosmological evolution on the multiverse has occurred for a finite time, much shorter than the exponentially-long global mixing time for the landscape. We argue this imposes a strong selection pressure among hospitable vacua, favoring those that lie in optimal regions where the search algorithm is efficient. This satisfies the basic requirements for natural selection: a diverse gene pool, offered ab initio by the landscape; vacuum replication through cosmological expansion; and competition for a finite resource, namely the fraction of comoving volume. Optimality is defined by two competing requirements: search efficiency, which requires minimizing the mean-first passage time, and sweeping exploration, which requires recurrence. Optimal landscape regions reach a compromise by lying at the critical boundary between recurrence and transience, thereby realizing the idea of self-organized criticality. The framework makes concrete phenomenological predictions: 1. The expected lifetime of our universe is ~10^{130} years, consistent with current electroweak metastability constraints; 2. The SUSY breaking scale should be nearly Planckian; and the predicted cosmological constant is M_Pl^4/N, which can account for the inferred vacuum energy if our optimal region contains N ~10^{120} vacua. Importantly, these predictions do not rely on anthropic reasoning and instead follow readily from optimality. |