  We propose noncanonical domain walls as a new dark energy model inspired by
grand unified theories (GUTs). We investigate the cosmic dynamics and discover
that the domain walls act as either dark energy or dark matter at different
times, depending on the velocity v in the observer's comoving frame. We find a
single stable solution to the dynamics, i.e., only freezing (v = 0)
noncanonical domain walls can enter the phantom zone without having to
experience ghost field instability. This means that the solution has an
equation of state (EoS) w_dw < -1 without having to possess negative kinetic
energy. These domain walls give rise to a late-time cosmic acceleration
starting from z = 0.8, resulting in w_dw = -1.5 and w_eff = -1.03 today. We
learn that the EoS of the noncanonical domain walls is independent of the
potential form. We also investigate the perturbation dynamics following the
model. Our simulations show that compared to LCDM, the amplitude of the dark
matter power spectrum in the noncanonical domain wall model is lower, while the
CMB power spectrum is shifted slighly to lower l multipoles. The proposed model
gives a smaller sigma8 compared to that of LCDM.
