Is the $Λ$CDM Model in Crisis?

TL;DR

The paper investigates whether the cosmological constant is in crisis by testing multiple dynamical dark energy models against DESI DR2 BAO, Type Ia SN compilations, and compressed CMB priors. It adopts a Bayesian framework with MCMC sampling (SimpleMC) to compare ΛCDM against $ω(z)$ parameterizations (e.g., $ω_0ω_a$CDM, Logarithmic, Exponential, JBP, BA, GEDE) and nonflat extensions, evaluating the impact of neutrino physics via $\sum m_ν$ and $N_{\rm eff}$. The results show strong evidence for $ω ≠ -1$ and Quintom-B-type behavior in several models, with the Universe remaining spatially flat ($Ω_k \approx 0$) and tight upper bounds on $\sum m_ν$ that depend on the model and dataset; however, no model reaches $5σ$ versus ΛCDM, and the $H_0$ tension persists. Overall, the work identifies emerging cracks in the cosmological constant paradigm while highlighting the need for further data to pin down the dark sector physics.

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.

Figures (11)

• Figure 1: The figure shows the summary of inferred values of $h$ for different cosmological models, with and without varying $\sum m_\nu$ and $N_\mathrm{eff}$
• Figure 2: The figure shows the posterior distributions of different planes of the o$\Lambda$CDM, $\omega$CDM, o$\omega$CDM, $\omega_a \omega_0$CDM, Logarithmic, Exponential, JBP, BA, and GEDE models using DESI DR2 measurements in combination with the CMB and SNe Ia measurements, at the 68% (1$\sigma$) and 95% (2$\sigma$) confidence intervals.
• Figure 3: The figure shows the posterior distributions of different planes of the o$\Lambda$CDM + $\sum m_\nu$, $\omega$CDM + $\sum m_\nu$, o$\omega$CDM + $\sum m_\nu$, $\omega_a \omega_0$CDM + $\sum m_\nu$, Logarithmic + $\sum m_\nu$, Exponential + $\sum m_\nu$, JBP + $\sum m_\nu$, BA + $\sum m_\nu$, and GEDE + $\sum m_\nu$ models using DESI DR2 measurements in combination with the CMB and SNe Ia measurements, at the 68% (1$\sigma$) and 95% (2$\sigma$) confidence intervals.
• Figure 4: The figure shows the contour plot in the $\Omega_m$–$\sum m_\nu$ plane for the $\Lambda$CDM model, using the CMB alone and combined with the DESI DR2 dataset, showing the 68% and 95% confidence level contours.
• Figure 5: The figure shows the posterior distributions different planes of the o$\Lambda$CDM + $\sum m_\nu$, $\omega$CDM + $\sum m_\nu$, o$\omega$CDM + $\sum m_\nu$, $\omega_a \omega_0$CDM + $\sum m_\nu$, Logarithmic + $\sum m_\nu$, Exponential + $\sum m_\nu$, JBP + $\sum m_\nu$, BA + $\sum m_\nu$, and GEDE + $\sum m_\nu$ models using DESI DR2 measurements in combination with the CMB and SNe Ia measurements, at the 68% (1$\sigma$) and 95% (2$\sigma$) confidence intervals.