What have we learned from observational cosmology ?

TL;DR

This review summarizes the observational evidence supporting the $b5$CDM model, highlighting key probes and measurements that confirm a flat, dark energy-dominated universe, while acknowledging open questions about its fundamental nature.

Contribution

It provides a comprehensive overview of observational foundations and confirms the $b5$CDM model's validity based on recent precise measurements.

Findings

Verification of the Cosmological Principle with increasing accuracy

Multiple independent probes support a hot, dense early universe

Precise measurements favor a flat universe dominated by dark energy

Abstract

We review the observational foundations of the $\Lambda$CDM model, considered by most cosmologists as the standard model of cosmology. The Cosmological Principle, a key assumption of the model is shown to be verified with increasing accuracy. The fact that the Universe seems to have expanded from and hot and dense past is supported by many independent probes (galaxy redshifts, Cosmic Microwave Background, Big-Bang Nucleosynthesis and reionization). The explosion of detailed observations in the last few decades has allowed for precise measurements of the cosmological parameters within Friedman-Lema\^itre-Robertson-Walker cosmologies leading to the $\Lambda$CDM model: an apparently flat Universe, dominated by a cosmological constant, whose matter component is dominantly dark. We describe and discuss the various observational probes that led to this conclusion and conclude that the $\Lambda$CDM model, although leaving a number of open questions concerning the deep nature of the constituents of the Universe, provides the best theoretical framework to explain the observations.