$Λ$CDM: The path forward

TL;DR

The paper assesses whether the standard $\Lambda$CDM cosmology can be supplanted by a more fundamental theory by focusing on evolving dark energy as revealed by DESI, CMB, and SNe data. It analyzes the $w_0w_a$ parameterization and scalar-field dark-energy models, finding that DESI data favor a sharply peaked dark-energy density near $z\approx 0.5$ and a dynamic equation of state, though $\Lambda$CDM remains a good fit. Scalar-field models offer only modest improvements over $\Lambda$CDM and generally do not outperform the best $w_0w_a$ fits, while Pantheon+SH0ES data push toward nontrivial dark-energy behavior with $\beta$ in a broad range. Overall, the evidence hints at interesting deviations from a cosmological constant but stops short of a definitive replacement, emphasizing an evolutionary or potentially revolutionary path forward driven by future data, including DESI and persistent Hubble-tension signals.

Abstract

The current cosmological paradigm, $Λ$CDM, is characterized its expansive description of the history of the Universe, its deep connections to particle physics and the large amounts of data that support it. Nonetheless, $Λ$CDM's critics and boosters alike agree on one thing: it is the not the final cosmological theory and they are anxious to see it replaced by something better! After reviewing some of the impactful events in cosmology since the last \Le Workshop, I focus on the role that the recent evidence for evolving dark energy may play in getting cosmology that better theory.

Figures (6)

• Figure 1: CMB temperature anisotropy power spectrum as measured by Planck PlanckLegacy; the curve is the best-fit $\rm\Lambda$CDM model. The concordance of $\Lambda$CDM with the precision CMB measurements is impressive and perhaps the strongest testimony to $\Lambda$CDM.
• Figure 2: The four quadrants of the $\alpha$-$w_a$ plane that define the qualitative behavior of dark energy in the $w_0w_a$ parameterization. $\Lambda$ corresponds to $\alpha = w_a = 0$, a canonical scalar field is described by $\alpha =0$ and $w_a <0$ and a phantom scalar field by $\alpha =0$ and $w_a > 0$. DR2 and DR2+ refer to the best fit models to DESI DR2 with (DR2+) and without (DR2) the SNe and CMB data; both are in the SW quadrant where $\rho_{DE}$ has a peak (at $z\simeq 0.5$) and $w$ crosses the phantom line.
• Figure 3: Evolution of the energy density of dark energy (in units of the critical density today) for the extreme DESI-data only best-fit (blue), the DESI+ best-fit (red), and $\Lambda$. The extreme DESI model has $\Omega_M = 0.4$, $w_0=0.016$, and $w_a = -3.69$. The DESI+ best-fit includes SNe and CMB data, and has $\Omega_M = 0.33$, $w_0=-0.7$, and $w_a = -1$
• Figure 4: Best-fit, DESI-only data $w_0w_a$ model ($\Omega_M = 0.4$, $w_0 = 0.016$ and $w_a = -3.69$): Energy density of dark energy (red), dark energy EOS (dotted blue) and deceleration parameter (blue). Note, in this model the Universe is not accelerating today, $q_0 \simeq 0.5$.
• Figure 5: $\chi^2$ vs. $\beta$ for the massive scalar field model and DR2 with the CMB constraint and the value of $\Omega_M$ that minimizes $\chi^2$.