This is the first part of a two-paper series that establishes the uniqueness and
regularity of a threshold energy wave map that does not scatter in both time
directions.
Consider the
-valued
equivariant energy critical wave maps equation on
, with equivariance
class
. It is
known that every topologically trivial wave map with energy less than twice that of the unique
-equivariant
harmonic map
scatters in both time directions. We study maps with precisely the threshold energy
.
In this paper, we give a refined construction of a wave map with threshold
energy that converges to a superposition of two harmonic maps (bubbles),
asymptotically decoupling in scale. We show that this two-bubble solution possesses
regularity. We give a precise dynamical description of the modulation parameters as
well as an expansion of the map into profiles. In the next paper in the series, we show
that this solution is unique (up to the natural invariances of the equation) relying
crucially on the detailed properties of the solution constructed here. Combined with
our earlier work (in 2018), we can now give an exact description of every threshold
wave map.