Fitting Type Ia supernovae with coupled dark energy

Abstract

We discuss the possible consistency of the recently discovered Type Ia supernovae at z>1 with models in which dark energy is strongly coupled to a significant fraction of dark matter, and in which an (asymptotic) accelerated phase exists where dark matter and dark energy scale in the same way. Such a coupling has been suggested for a possible solution of the coincidence problem, and is also motivated by string cosmology models of "late time" dilaton interactions. Our analysis shows that, for coupled dark energy models, the recent data are still consistent with acceleration starting as early as at $z=3$ (to within 90% c.l.), although at the price of a large "non-universality" of the dark energy coupling to different matter fields. Also, as opposed to uncoupled models which seem to prefer a ``phantom'' dark energy, we find that a large amount of coupled dark matter is compatible with present data only if the dark energy field has a conventional equation of state w>-1.

Figures (6)

• Figure 1: Approximations to $H^{2}$ (assuming $\beta =4,\mu =3,\Omega _{b}=0.05,\Omega _{c}=0.3$). The dotted curve is the numerical result, the full curve the linear fit. In the inset, the relative error.
• Figure 2: Contour plot obtained from the SNe Ia of 9, cutting to $w>-1$. Likelihood contours, from inside to ouside, are at the 68%, 90%, 95%, 99% confidence level. $\Omega _{R}$ is the effective fraction of uncoupled matter evolving as $a^{-3}$ (see Eq. (\ref{['eq:ha']})). The line labelled by $\Omega _{b}$ marks the present baryon fraction.
• Figure 3: The SNe Ia results projected on the plane $\beta ,\Omega _{u}$, at fixed $\Omega _{m}=0.27$. Likelihood contours, from inside to outside, are at the 68%, 90%, 95%, 99% confidence level. The dotted curves mark the parameter values for which $z_{\rm{acc}}$ equals $1$, $2$ and $3$, respectively. Values of $z_{\rm{acc}}$ above $1$ are within one $\sigma$, and even $z_{\rm{acc}}=3$ is not excluded at more than 90% c.l..
• Figure 4: Best fits of the distance modulus $\Delta (m-M)$ for three different models: $\Lambda$CDM ($\Omega _{m}=0.27,\Omega _{\Lambda }=0.73$), coupled dark energy $(w=-0.6,~$$\Omega _{R}=0.05$ ) and a varying equation of state $w=w_{0}+w_{1}z$, with $w_{0}=-1.31$, $w_{1}=-1.48$, fixing $\Omega _{m}=0.27$. The data represent the gold sample of 9.
• Figure 5: Contour plot obtained from the SNe Ia of 9, including $w<-1$. Likelihood contours, from inside to ouside, are at the 95% and 99% confidence level. The variables $w,\Omega _{R},\Omega _{b}$ are the same as in Fig. 2.