Impact of theoretical priors in cosmological analyses: The case of single field quintessence

We investigate the impact of general conditions of theoretical stability and cosmological viability on dynamical dark energy models. As a powerful example, we study whether minimally coupled, single field quintessence models that are safe from ghost instabilities, can source the Chevallier-Polarski-Linder (CPL) expansion history recently shown to be mildly favored by a combination of cosmic microwave background (Planck) and weak lensing (KiDS) data. We find that in their most conservative form, the theoretical conditions impact the analysis in such a way that smooth single field quintessence becomes significantly disfavored with respect to the standard $\mathrm{\Lambda }\mathrm{CDM}$ cosmological model. This is due to the fact that these conditions cut a significant portion of the $(w_0,w_a)$ parameter space for CPL, in particular, eliminating the region that would be favored by weak lensing data. Within the scenario of a smooth dynamical dark energy parametrized with CPL, weak lensing data favors a region that would require multiple fields to ensure gravitational stability.

Figure 1

Viable and nonaccessible regions of the CPL parameter space within the realm of standard quintessence. Different colors correspond to different viability and stability conditions, as shown in legend. In the left panel, we show the effects of mathematical conditions (MC), and in the right one, we show the full combination (FC) of mathematical and physical conditions.

Figure 2

The joint marginalized posterior of ${\mathrm{\Omega }}_m$ and ${\sigma }_8$ (panel a) and $w_0$ and $w_a$ (panel b) as obtained with the Planck data in $\mathrm{\Lambda }\mathrm{CDM}$ (red contours), CPL with MC (blue contours), and with FC (gray contours). The darker and lighter shades correspond, respectively, to the 68% C.L. and the 95% C.L. regions. The dashed line indicates the point corresponding to the $\mathrm{\Lambda }\mathrm{CDM}$ model in the $w_0-w_a$ plane.

Figure 3

The joint marginalized posterior of ${\mathrm{\Omega }}_m$ and ${\sigma }_8$ (panel a) and $w_0$ and $w_a$ (panel b) as obtained analyzing KiDS (green contours) and Planck (red contours) data. Filled contours refer to constraints obtained in CPL with MC, while empty contours (left panel only) refer to $\mathrm{\Lambda }\mathrm{CDM}$ constraints. The darker and lighter shades correspond, respectively, to the 68% C.L. and the 95% C.L. regions. The dashed line indicates the point corresponding to the $\mathrm{\Lambda }\mathrm{CDM}$ model in the $w_0-w_a$ plane.

Figure 4

The joint marginalized posterior of ${\mathrm{\Omega }}_m$ and ${\sigma }_8$ (panel a) and $w_0$ and $w_a$ (panel b) as obtained analyzing KiDS (red contours) and Planck (blue contours) data. Filled contours refer to constraints obtained in CPL with FC, while empty contours (left panel only) refer to $\mathrm{\Lambda }\mathrm{CDM}$ constraints. The darker and lighter shades correspond, respectively, to the 68% C.L. and the 95% C.L. regions. The dashed line indicates the point corresponding to the $\mathrm{\Lambda }\mathrm{CDM}$ model in the $w_0-w_a$ plane.