  Observing the relics of collisions between bubble universes would provide
direct evidence for the existence of an eternally inflating Multiverse; the
non-observation of such events can also provide important constraints on
inflationary physics. Realizing these prospects requires quantitative
predictions for observables from the properties of the possible scalar field
Lagrangians underlying eternal inflation. Building on previous work, we
establish this connection in detail. We perform a fully relativistic numerical
study of the phenomenology of bubble collisions in models with a single scalar
field, computing the comoving curvature perturbation produced in a wide variety
of models. We also construct a set of analytic predictions, allowing us to
identify the phenomenologically relevant properties of the scalar field
Lagrangian. The agreement between the analytic predictions and numerics in the
relevant regions is excellent, and allows us to generalize our results beyond
the models we adopt for the numerical studies. Specifically, the signature is
completely determined by the spatial profile of the colliding bubble just
before the collision, and the de Sitter invariant distance between the bubble
centers. The analytic and numerical results support a power-law fit with an
index $1< \kappa \lesssim 2$. For collisions between identical bubbles, we
establish a lower-bound on the observed amplitude of collisions that is set by
the present energy density in curvature.
