Unparticle dark energy
  We examine a dark energy model where a scalar unparticle degree of freedom
plays the role of quintessence. In particular, we study a model where the
unparticle degree of freedom has a standard kinetic term and a simple mass
potential, the evolution is slowly rolling and the field value is of the order
of the unparticle energy scale ($\lambda_u$). We study how the evolution of $w$
depends on the parameters $B$ (a function of the unparticle scaling dimension
$d_u$), the initial value of the field $\phi_i$ (or equivalently, $\lambda_u$)
and the present matter density $\Omega_{m0}$. We use observational data from
Type Ia supernovae, BAO and CMB to constrain the model parameters and find that
these models are not ruled out by the observational data. From a theoretical
point of view, an unparticle dark energy model is very attractive, since
unparticles (being bound states of fundamental fermions) are protected from
radiative corrections. Further, coupling of unparticles to the standard model
fields can be arbitrarily suppressed by raising the fundamental energy scale
$M_F$, making the unparticle dark energy model free of most of the problems
that plague conventional scalar field quintessence models.
