Cosmic microwave background
Guillaume is also known for the "earliest" estimation of the "radiation of the stars” in his 1896 paper "La Température de L'Espace (The Temperature of Space)". This publication made him a pioneer in plasma cosmology, the study of conditions far from any particular star. The concept would later be known as the Cosmic microwave background. He was one of the first people in history to estimate the temperature of space, as 5-6K.