Blow-up criteria for the 3D cubic nonlinear Schrödinger equation

We consider solutions u to the 3D nonlinear Schrödinger equation i∂_tu + Δu + |u|²u = 0. In particular, we are interested in finding criteria on the initial data u₀ that predict the asymptotic behaviour of u(t), e.g., whether u(t) blows up in finite time, exists globally in time but behaves like a linear solution for large times (scatters), or exists globally in time but does not scatter. This question has been resolved (at least for H¹ data) (Duyckaerts-Holmer-Roudenko) if M[u]E[u] ≤ M[Q]E[Q], where M[u] and E[u] denote the mass and energy of u and Q denotes the ground state solution to -Q + ΔQ + |Q|²Q = 0. Here we consider the complementary case M[u]E[u] > M[Q]E[Q]. In the first (analytical) part of the paper, we present a result due to Lushnikov, based on the virial identity and the generalized uncertainty principle, giving a sufficient condition for blow-up. By replacing the uncertainty principle in his argument with an interpolation-type inequality, we obtain a new blow-up condition that in some cases improves upon Lushnikov's condition. Our approach also allows for an adaptation to radial infinite-variance initial data that has a conceptual interpretation: for real-valued initial data, if a certain fraction of the mass is contained within the ball of radius M[u], then blow up occurs. We also show analytically (if one takes the numerically computed value of ∥Q∥Ḣ1/2) that there exist Gaussian initial data u₀ with negative quadratic phase such that ∥u₀∥ Ḣ1/2 < ∥Q∥_Ḣ1/2 but the solution u(t) blows up. In the second (numerical) part of the paper, we examine several different classes of initial data - Gaussian, super Gaussian, off-centred Gaussian, and oscillatory Gaussian - and for each class give the theoretical predictions for scattering or blow-up provided by the above theorems as well as the results of numerical simulation. We find that depending upon the form of the initial conditions, any of the three analytical criteria for blow-up can be optimal. We formulate several conjectures, among them that for real initial data, the quantity ∥Q∥_Ḣ1/2 provides the threshold for scattering.