Inflation is still in trouble: Probing the early universe with numerical relativity

Joshua Shterenberg
Ph.D Candidate
Department of Physics, Princeton University, Princeton, NJ

Abstract: The standard picture of modern cosmology posits inflation as the dynamical, classical process that smooths generic fluctuations in the early universe. To analyze the effectiveness of inflation in the regime of large, non-perturbative fluctuations, we perform numerical relativity simulations using a custom tetrad-based formulation of the Einstein-scalar field equations with a plateau-shaped potential. Based on gauge-invariant diagnostic variables, we find that inflation can only smooth nearly-Friedmann-Robertson-Walker initial conditions beginning at the Planck energy scale. In these simulations, we observe that the unbounded growth of fluctuations—particularly those in gravitational shear density and curvature—is a generic effect in these models that tends to dominate over the potential energy and prevent inflation from occurring. As a result, the parameter space of initial conditions at the Planck density that inflation can smooth must be heavily restricted, rendering inflation a rare phenomenon.