Noncanonical Domain Wall as a Unified Model of Dark Energy and Dark Matter: I. Cosmic Dynamics

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 nu in the observer's comoving frame. We find a single stable solution to the dynamics, i.e., only freezing (nu=0) noncanonical domain walls can enter the phantom zone without having to experience ghost field instability. This means that the solution has the equation of state (EoS) omega(dw)<-1 without having to possess negative kinetic energy. These domain walls give rise to a late-time cosmic acceleration starting from z approximate to 0.24, resulting in omega(dw)=-1.5 and omega(eff)=-1.03 today. We learn that the EoS of the noncanonical domain walls is independent from the potential form. We also investigate the perturbation dynamics following the model. Our simulations show that, compared to Lambda CDM, the amplitude of the dark matter power spectrum in the noncanonical domain wall model is lower, while the CMB power spectrum is shifted slightly to lower l multipoles. The proposed model gives a smaller sigma(8) as compared to that of Lambda CDM.