The Cosmological Parameters (2025)

TL;DR

This review synthesizes cosmological parameter knowledge as of late 2025, emphasizing the core ${\Lambda}$CDM framework while outlining plausible extensions motivated by data and theory. It outlines how inflationary perturbations, dark matter properties, relativistic species, and dark energy are described within a parameter-led cosmology, and how a diverse set of probes—$CMB$, SN Ia, BAO, galaxy clustering, IGM, and weak lensing—constrain the parameters and break degeneracies. It highlights robust results such as flat geometry, adiabatic Gaussian initial conditions, and a nearly scale-invariant spectrum with $n_s<1$, while noting tensions in $H_0$ and hints of evolving dark energy that warrant further investigation. The outlook surveys upcoming experiments (CMB-S4 alternatives, Simons Observatory, Rubin-LSST, Euclid, Roman, SKA, 21-cm) that will sharpen parameter measurements, probe neutrino physics, and test beyond-${\Lambda}$CDM physics.

Abstract

This is a review article for The Review of Particle Physics 2026 (aka the Particle Data Book), appearing as Chapter 25. It forms a compact review of knowledge of the cosmological parameters near the end of 2025. Topics included are Parametrizing the Universe; Extensions to the standard model; Probes; Bringing observations together; Outlook for the future.