Doppelganger defects
  We study k-defects - topological defects in theories with more than two
derivatives and second-order equations of motion - and describe some striking
ways in which these defects both resemble and differ from their analogues in
canonical scalar field theories. We show that, for some models, the homotopy
structure of the vacuum manifold is insufficient to establish the existence of
k-defects, in contrast to the canonical case. These results also constrain
certain families of DBI instanton solutions in the 4-dimensional effective
theory. We then describe a class of k-defect solutions, which we dub
doppelgangers, that precisely match the field profile and energy density of
their canonical scalar field theory counterparts. We give a complete
characterization of Lagrangians which admit doppelganger domain walls. By
numerically computing the fluctuation eigenmodes about domain wall solutions,
we find different spectra for doppelgangers and canonical walls, allowing us to
distinguish between k-defects and the canonical walls they mimic. We search for
doppelgangers for cosmic strings by numerically constructing solutions of DBI
and canonical scalar field theories. Despite investigating several examples, we
are unable to find doppelganger cosmic strings, hence the existence of
doppelgangers for defects with codimension >1 remains an open question.
