Justin Khoury - UPENN (Friday seminar)

Fri, 04/26/2019 - 2:30pm
Pupin Hall Theory Center, 8th Floor


"Search Optimization, Natural Selection, and Higgs Metastability"

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. I will 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. I will 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 recurrent random walks. 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 Standard Model metastability estimates; 2. The SUSY breaking scale should be nearly Planckian; and 3. 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. The present framework suggests a correspondence between the near-criticality of our universe and non-equilibrium critical phenomena on the landscape.

About the speaker

Justin's research interests lie at the interface of particle physics and cosmology. A central theme of his research program is the possibility that the dark sector includes new light degrees of freedom that couple not only to dark matter but also to baryonic matter. Part of Justin's research efforts over the last few years have focused on the development of screening mechanisms, such as chameleon and symmetron, to explain why such scalars, if light, have escaped detection from laboratory/solar system tests of gravity. The manifestation of these scalar fields therefore depends sensitively on their environment, which in turn leads to striking experimental signatures. Another central theme of Justin's research program is the development of novel theories of the very early universe that can address the traditional problems of standard big bang cosmology and generate density perturbations consistent with observations. Part of his research efforts focus on alternatives to the inflationary paradigm, such as the Ekpyrotic Universe, in which the seeds of structure formation are generated in a long phase of slow contraction before the big bang.

More details on Justin's research can be found here.