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 unparticle scaling dimension $d_u$), the initial value of the field ${\varphi }_i$ (or equivalently, ${\lambda }_u$) and the present matter density ${\Omega }_{m0}$. We use observational data from type Ia supernovae, baryon acoustic oscillations and the cosmic microwave background to constrain the model parameters and find that these models are not ruled out by the observational data. From a theoretical point of view, 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.

Figure 1

$1+w(a)$ for different choices of $B$. ${\varphi }_i$ is fixed to 1.

Figure 2

$1+w(a)$ for different choices of ${\varphi }_i$ (on a logarithmic scale). $B$ is fixed to 0.25.

Figure 3

Likelihood plot from $\mathrm{SNIa}+\mathrm{BAO}$ data for the parameters $B$ and ${\Omega }_{\Lambda }$ with ${\varphi }_i=0.01$. The yellow (light) region is excluded at the $2\sigma$ level, and the darker (orange) region is excluded at the $1\sigma$ level. The red (darkest) region is not excluded at either confidence level.

Figure 4

As for Fig. 3 with ${\varphi }_i=0.5$.

Figure 5

As for Fig. 3 with ${\varphi }_i=1$.

Figure 6

Likelihood plot from CMB data for the parameters $B$ and ${\Omega }_{\Lambda }$ with ${\varphi }_i=0.01$. The yellow (light) region is excluded at the $2\sigma$ level, and the darker (orange) region is excluded at the $1\sigma$ level. The red (darkest) region is not excluded at either confidence level.

Figure 7

As for Fig. 6 with ${\varphi }_i=0.5$.

Figure 8

As for Fig. 6 with ${\varphi }_i=1$.