Dark Energy Properties in DBI Theory
  The Dirac-Born-Infeld (DBI) action from string theory provides several new
classes of dark energy behavior beyond quintessence due to its relativistic
kinematics. We constrain parameters of natural potentials and brane tensions
with cosmological observations as well as showing how to design these functions
for a desired expansion history. We enlarge the attractor solutions, including
new ways of obtaining cosmological constant behavior, to the case of
generalized DBI theory with multiple branes. An interesting novel signature of
DBI attractors is that the sound speed is driven to zero, unlike for
quintessence where it is the speed of light.
