Dynamical dark energy in light of the latest observations

Abstract: A flat Friedman-Roberson-Walker universe dominated by a cosmological constant ($\Lambda$) and cold dark matter (CDM) has been the working model preferred by cosmologists since the discovery of cosmic acceleration. However, tensions of various degrees of significance are known to be present among existing datasets within the $\Lambda$CDM framework. In particular, the Lyman-$\alpha$ forest measurement of the Baryon Acoustic Oscillations (BAO) by the Baryon Oscillation Spectroscopic Survey (BOSS) prefers a smaller value of the matter density fraction $\Omega_{\rm M}$ compared to the value preferred by cosmic microwave background (CMB). Also, the recently measured value of the Hubble constant, $H_0=73.24\pm1.74 \ {\rm km}\ {\rm s}^{-1} \ {\rm Mpc}^{-1}$, is $3.4\sigma$ higher than $66.93\pm0.62 \ {\rm km}\ {\rm s}^{-1} \ {\rm Mpc}^{-1}$ inferred from the Planck CMB data. In this work, we investigate if these tensions can be interpreted as evidence for a non-constant dynamical dark energy (DE). Using the Kullback-Leibler (KL) divergence to quantify the tension between datasets, we find that the tensions are relieved by an evolving DE, with the dynamical DE model preferred at a $3.5\sigma$ significance level based on the improvement in the fit alone. While, at present, the Bayesian evidence for the dynamical DE is insufficient to favour it over $\Lambda$CDM, we show that, if the current best fit DE happened to be the true model, it would be decisively detected by the upcoming DESI survey.

• The paper introduces a dynamical dark energy model using Bayesian non-parametric reconstruction to address tensions in key cosmological measurements.
• It employs a correlated prior method and KL divergence to reveal a 3.5σ significance for an evolving dark energy equation of state.
• The study forecasts that future surveys like DESI could decisively distinguish between evolving dark energy and the standard ΛCDM model.

Dynamical Dark Energy in Light of the Latest Observations

The paper explores the possibility of a dynamical dark energy (DE) model as an alternative to the canonical flat $\Lambda$CDM framework, motivated by persistent tensions observed between various cosmological datasets. These tensions, notably in the measurements of the Hubble constant $H_0$, the Baryon Acoustic Oscillations (BAO), and the Cosmic Microwave Background (CMB), suggest inconsistencies that could indicate the presence of a non-constant DE component.

Analysis and Methodology

The authors employ the Kullback-Leibler (KL) divergence to quantify the level of tension between different datasets within the $\Lambda$CDM paradigm. By adopting a Bayesian framework that allows for a non-parametric reconstruction of the DE equation of state $w(z)$, they analyze its evolution over time. The framework incorporates a correlated prior method that facilitates exploration of the parameter space without significant bias, despite the increased freedom allowed by the model.

The datasets include recent measurements of the CMB, SN Ia luminosity distances, BAO from multiple sources, and direct measurements of the Hubble constant. The resulting reconstructed $w(z)$ model exhibits an evolving DE equation of state, preferred over the constant $\Lambda\cdm$ model by a significance of $3.5\sigma$.

Results and Implications

Key results include the alleviation of dataset tensions in the dynamical DE model compared to $\Lambda\cdm$, with a significant reduction in tension scores for $H_0$, Ly$\alpha\FB$, and SN Ia data. The reconstructed $w(z)$ indicates a potential crossing of the $w = -1$ boundary, a behavior incompatible with single-field quintessence models, yet possible within frameworks involving multiple scalar fields such as Quintom models or in scenarios involving Modified Gravity.

The principal component analysis reveals that the effective degrees of freedom added by the $w(z)\cdm$ model are constrained such that only four additional components are effectively required, ensuring the increased complexity does not excessively depend on the prior. By comparing the posterior eigenvalues to those of the prior, the analysis shows data predominantly constrain the principal components.

Future Prospects

The authors forecast that future measurements, particularly from the DESI survey, will offer significantly improved constraints on dynamical DE, potentially achieving decisive Bayesian evidence if the $w(z)$cdm\ model reflects the true nature of DE. This projection highlights the potential for upcoming observational campaigns to distinguish between $\Lambda\cdm$ and dynamical DE models confidently.

Conclusion

This work provides a robust framework for analyzing and interpreting tensions in cosmological observations through the lens of dynamical DE models. While not conclusively favoring dynamical DE over $\Lambda\cdm$ based on current Bayesian evidence, the analysis demonstrates statistically significant potential departures from $\Lambda\cdm$, positing a framework through which future data can offer further insights into the DE dynamics shaping our universe.