The prior dependence of the DESI results

Understand

In this short contribution, we explore the prior dependence of the recent DESI results, according to which a Bayesian comparison of $\Lambda$CDM with $w_0w_a$CDM cosmologies favors dynamical dark energy.

• It is a simple exercise to show that extending the prior lower bound of the dark energy parameters tilts the preference in favor of $\Lambda$CDM.
• Adopting the PantheonPlus supernovae catalog, for instance, a shift of the lower bound for $w_0,w_a$ from $-3$ to beyond $-4.6$ and $-5$, respectively, is sufficient to favor $\Lambda$CDM.
• This calls for caution when interpreting DESI results in the Bayesian context.