The dark degeneracy: On the number and nature of dark components

Abstract

We use that gravity probes only the total energy momentum tensor to show how this leads to a degeneracy for generalised dark energy models. Because of this degeneracy, Omega_m cannot be measured. We demonstrate this explicitely by showing that the CMB and supernova data is compatible with very large and very small values of Omega_m for a specific family of dark energy models. We also show that for the same reason interacting dark energy is always equivalent to a family of non-interacting models. We argue that it is better to face this degeneracy and to parametrise the actual observables.

Figures (2)

• Figure 1: We try to determine $\Omega_m$ with the SNLS 1yr data and the $R_{0.35}$ constraint from the baryon acoustic oscillations measured by SDSS LRG, but using an equation of state that exhibits a degeneracy between dark matter and dark energy. We find that the background data cannot determine $\Omega_m$.
• Figure 2: This figures shows that supernova and CMB data together cannot measure $\Omega_m$ for generalised dark energy models. The filled contours show $1$ and $2\sigma$ limits for a model with $c_s^2 =0$, while the open contours show the limits for an effective scalar field model. The lower limit on $\Omega_m$ is due to the baryons which we know to exist.