Dark radiation as a signature of dark energy

We propose a simple dark energy model with the following properties: the model predicts a late-time dark radiation component that is not ruled out by current observational data, but which produces a distinctive time-dependent equation of state $w(z)$ for $z<3$. The dark energy field can be coupled strongly enough to standard model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

Figure 1

An example for the evolution of the finite temperature effective potential $V(\varphi ,T)$ of the dark energy field, Eq. (3), as the temperature $T$ decreases through the first-order phase transition region $\sim T_1$.

Figure 2

Likelihood contour for the parameters $f$ and ${\Omega }_{m0}$. The yellow (light) region is excluded at the $2\sigma$ level and the orange (darker) region is excluded at the $1\sigma$ level. The red (darkest) region is not excluded at either confidence level.

Figure 3

$w$ vs $z$ for the choice $f=0.01$, which along with, e.g., a conservative ${\Omega }_{m0}=0.28$ is not excluded at the $1\sigma$ level by the SNIa data (see Fig. 2).

Figure 4

Likelihood contour for the parameters ${\Omega }_{m0}$ and $z_{*}$, the redshift at which the Universe has exited the false vacuum and entered the true vacuum, releasing the energy of the cosmological constant into relativistic modes. The yellow (light) region is excluded at the $2\sigma$ level, and the orange (darker) region is excluded at the $1\sigma$ level. The red (darkest) region is not excluded at either confidence level.

Figure 5

Potential energy surface for a gauge theory, where $\varphi$ is an order parameter for confinement (Polyakov loop) and $\Phi$ a colored scalar field. For $N>2$, $SU(N)$ models will have a first-order confinement-deconfinement transition as the temperature is lowered. However, at zero temperature the deconfined phase ($\Phi =\varphi =0$) is not necessarily metastable.