Emre Özülker

Recent observational analyses have revealed a significant tension in the growth index γ , which characterizes the growth rate of cosmic structures. When γ is treated as a free parameter within lambda cold dark matter ( Λ CDM ), the PL 18 + f σ 8 combination yields γ ≈ 0.64 , in ∼ 4 σ tension with the theoretically expected value γ ≈ 0.55 (assuming general relativity), suggesting a preference for suppressed late-time growth relative to the standard expectation. In this work, we investigate whether the Λ s CDM framework—featuring a rapid sign-switching cosmological constant (mirror anti–de Sitter-to-de Sitter (AdS-to-dS) transition) in the late universe at redshift z † ∼ 2 —can alleviate the γ tension together with the H 0 and S 8 tensions. We find that γ Λ s CDM is able to reduce the γ and H 0 tensions relative to γ Λ CDM while maintaining low S 8 values. For PL 18 + f σ 8 , the combination underlying the γ tension, γ Λ s CDM yields z † > 1.58 (95% CL) and reduces the γ and H 0 discrepancies from 3.7 σ and 4.2 σ in γ Λ CDM to 2.1 σ and 1.1 σ , respectively, while leaving χ min 2 essentially unchanged. The largest improvement among the free- z † analyses is obtained for PL 18 + f σ 8 + PP & SH 0 ES , for which the marginalized posterior constraint is z † = 1.8 2 − 0.28 + 0.32 (95% CL); the γ and H 0 tensions are reduced to 1.8 σ and 0.6 σ , respectively, and the model also provides an improved fit to the data. The efficacy of the mechanism is, however, dataset dependent: the standard 3D BAO measurements used in the present analysis—from SDSS-IV/eBOSS and the first year of DESI observations—tend to push the transition to higher redshifts, z † ≳ 2.3 – 2.4 , and weaken the simultaneous alleviation, particularly of the H 0 tension. We also examine the benchmark case with fixed z † = 1.7 , previously identified in studies of the standard ( γ ≃ 0.55 ) Λ s CDM framework as a redshift of interest for alleviating multiple major cosmological tensions—including those in H 0 , M B , and S 8 . For the restricted combinations in which it is considered, this benchmark yields low residual tensions (e.g., a 1.6 σ γ tension and an effectively vanishing H 0 tension for PL 18 + f σ 8 ), albeit with somewhat worse fits than γ Λ CDM . It is, however, excluded at more than 2 σ by the free- z † analyses once either the uncalibrated PantheonPlus SN Ia sample or the 3D BAO data are included. Our results indicate that Λ s CDM can contribute to the joint alleviation of multiple cosmological tensions, although its impact is dataset dependent.

In this work, we present updated observational constraints on the parameter space of the DMS20 dark-energy model, a member of the (ODE) class. Our analysis combines multiple cosmic microwave background datasets—including measurements from the Planck satellite (PL18), the South Pole Telescope (SPT), and the Wilkinson Microwave Anisotropy Probe (WMAP)—with Type Ia supernova data from the Pantheon + catalog (PP), and baryon acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument and Sloan Digital Sky Survey. We find that certain data combinations, such as SPT + WMAP + BAO and P L 18 + BAO , can reduce the significance of the H 0 tension below 1 σ , but with considerably large uncertainties. However, the inclusion of PP data restores the tension in H 0 . To provide a comprehensive view of the ODE phenomenology, we also investigate the evolution of its energy density, emphasizing its dynamical behavior at low redshifts. Our results generically exhibit multiple phantom divide line crossings in a single expansion history; if confirmed, this points beyond the simplest minimally coupled canonical single-field quintessence/phantom descriptions and motivates more general dark-sector realizations.

Context. The direct empirical determination of the local value of the Hubble constant ( H ₀ ) has markedly advanced thanks to improved instrumentation, measurement techniques, and distance estimators. However, combining determinations from different estimators is nontrivial due to their correlated calibrations and different analysis methodologies. Aims. Using covariance weighting and leveraging community expertise, we have constructed a rigorous and transparent “Distance Network” to find a consensus value and uncertainty for the locally measured Hubble constant. Methods. Experts across all relevant distance measurement domains were invited to critically review the available datasets spanning parallaxes, detached eclipsing binaries, masers, Cepheids, the tip of the red giant branch, Miras, carbon-rich asymptotic giant branch stars, Type Ia (SNe Ia) and Type II supernovae, surface brightness fluctuations, the fundamental plane, and Tully–Fisher relations. Before any calculations, the group voted for first-rank indicators to define a “baseline” Distance Network. Other indicators were included to assess the robustness and sensitivity of the results. We provide open-source software and data products to support full transparency and future extensions of this effort. Results. Our key findings are as follows: (1) The local H ₀ is robustly determined, with first-rank indicators internally consistent within their uncertainties. (2) A covariance-weighted combination yields a relative uncertainty of 1.1% (baseline) or 0.9% (all estimators). (3) The contribution from SNe Ia is consistent across compilations of optical or NIR magnitudes. (4) Removing either Cepheids or the tip of the red giant branch has a minimal effect on the central value of H ₀ . (5) Replacing SNe Ia with galaxy-based indicators changes H ₀ by less than 0.1 km s ⁻¹ Mpc ⁻¹ while doubling its uncertainty. (6) The baseline result is H ₀ = 73.50 ± 0.81 km s ⁻¹ Mpc ⁻¹ , 7.1 σ from the early Universe plus ΛCDM result 67.24 ± 0.35 km s ⁻¹ Mpc ⁻¹ and 5.0 σ from BBN+BAO within a flat ΛCDM DESI DR2 (68.51 ± 0.58 km s ⁻¹ Mpc ⁻¹ ). Conclusions. A networked approach, such as the one presented here, is invaluable for enabling further progress in Hubble constant measurements, as it provides the much needed advances in accuracy and precision without overreliance on any single method, sample, or group.