What have we learned from observational cosmology ?

Understand

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.