THE COSMIC MICROWAVE BACKGROUND

to appear in PNAS The cosmic microwave background originated some 400,000 years after the Big Bang. It provides a detailed representation of the physical state of the Universe in its early youth. Recent balloon-borne experiments have accurately measured the tiny fluctuations in the intensity of the microwave background radiation. The results show a remarkable consistency with other cosmological measurements and allow the conclusion that the Universe is spatially flat, has low matter density of which only a small fraction is baryonic (made of atoms), and is dominated today by some form of dark energy, perhaps a cosmological constant. Moreover, the measurements confirm the theory of the hot Big Bang and argue in favor of cosmological inflation. According to the cosmological standard model (see e.g. [6, 4]), the Universe originated in an extremely hot and dense state, from which it expanded and cooled. As the temperature fell, the elementary particles formed, among them protons, neutrons and electrons. The charged particles were in thermal equilib-rium with a sea of photons. About three minutes after the Bi