The Cosmic Microwave Background (CMB), the afterglow of the Big Bang, was first measured in 1964. By revealing a snapshot of the Universe as it existed almost 14 billion years ago, when the hot and dense early Universe first became transparent, the CMB has provided cosmologists a wealth of information about the history, evolution, and contents of the Universe. The photons that make up the CMB have travelled freely since that early time, and their wavelengths have been stretched with the expansion of the Universe to a few millimeters length, making them microwaves today.
Subtle variations in the CMB temperature and polarization across the sky encode information about the ingredients of the plasma that filled the early Universe, the spectrum of primordial fluctuations that serve as the seeds of cosmological structure, and the evolution of the Universe over the past 14 billion years of cosmic history. As the oldest and furthest light we observe in the universe, the CMB backlights all other structures. Telescopes that measure the CMB can also detect any cosmic object that emits microwave radiation.
CMB-S4 will provide measurements of the CMB at unprecedented precision and permit fundamental new insights into a broad range of physics. These measurements will enable the search for signatures of primordial gravitational waves, probe the nature of dark matter and dark energy, map the matter throughout the Universe, and capture transient phenomena in the microwave sky.