Is the $Λ$CDM Model in Crisis?

Abstract

We present strong evidence for dynamical dark energy that challenges the standard $Λ$CDM model. Several dark energy scenarios are explored, including $ω_0ω_a$CDM, logarithmic, exponential, JBP, and BA parameterizations, along with non-flat cosmologies allowing for spatial curvature ($Ω_k \neq 0$). Our analysis supports a flat Universe with $Ω_k \approx 0$. Using MCMC techniques, we constrain these models with observational data from DESI~DR2 baryon acoustic oscillations, Type~Ia supernovae, and compressed CMB likelihoods. The results provide strong statistical evidence that $ω\neq -1$, favoring dynamical dark-energy behavior consistent with a Quintom-B scenario ($ω_0 > -1$, $ω_a < 0$, and $ω_0 + ω_a < -1$). We also derive upper bounds on the total neutrino mass, $\sum m_ν$, using CMB + DESI~DR2 data: $\sum m_ν< 0.066~\mathrm{eV}$ for $Λ$CDM and $\sum m_ν< 0.075~\mathrm{eV}$ for $ω$CDM. In the non-flat extensions, o$Λ$CDM and o$ω$CDM, the limits relax to $\sum m_ν< 0.263~\mathrm{eV}$ and $\sum m_ν< 0.520~\mathrm{eV}$, respectively. For the other models $ω_0ω_a$CDM, logarithmic, exponential, JBP, BA, and GEDE the constraints range between $<0.043$ and $<0.127~\mathrm{eV}$. The effective number of relativistic species remains consistent with the standard value, $N_{\mathrm{eff}} = 3.044$, across all models. Bayesian evidence further shows that combining DES-SN5Y or Union3 supernova samples with CMB + DESI~DR2 produces measurable deviations from $Λ$CDM. Although no model reaches the $5σ$ significance threshold, several exhibit tensions exceeding $3σ$, suggesting emerging cracks in the cosmological constant paradigm.