Papers › On the Generality and Persistence of Cosmological Stasis
On the Generality and Persistence of Cosmological Stasis
James Halverson, Sneh Pandya
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Hierarchical decays of N matter species to radiation may balance against Hubble expansion to yield stasis, a new phase of cosmological evolution with constant matter and radiation abundances. We analyze stasis with various machine learning techniques on the full 2N-dimensional space of decay rates and abundances, which serve as inputs to the system of Boltzmann equations that governs the dynamics. We construct a differentiable Boltzmann solver to maximize the number of stasis e-folds 𝒩. High-stasis configurations obtained by gradient ascent motivate log-uniform distributions on rates and abundances to accompany power-law distributions of previous works. We demonstrate that random configurations drawn from these families of distributions regularly exhibit many e-folds of stasis. We additionally use them as priors in a Bayesian analysis conditioned on stasis, using stochastic variational inference with normalizing flows to model the posterior. All three numerical analyses demonstrate the generality of stasis and point to a new model in which the rates and abundances are exponential in the species index. We show that the exponential model solves the exact stasis equations, is an attractor, and satisfies 𝒩∝N, exhibiting inflation-level e-folding with a relatively low number of species. This is contrasted with the 𝒩∝log(N) scaling of power-law models. Finally, we discuss implications for the emergent string conjecture and string axiverse.
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