The infrared-to-millimetre spectral range (1 to 10,000 μm) offers a unique and little explored window on the Universe. Such radiation probes cold, dusty objects such as dense interstellar material, forming stars, and obscured young galaxies. The longest wavelengths, near a few millimetres, also give information on conditions when the Universe was around 400,000 years old, via the Cosmic Microwave Background (CMB). To detect the faint cosmic signals in this waveband in the presence of very large instrumental, atmospheric and astronomical foreground radiation is challenging. It requires special cryogenic sensors, and optimized telescopes above the Earth’s atmosphere and in space. Jean-Loup Puget has made pivotal contributions to all these aspects, scientific as well as technical.
In the 1970s and early 1980s mysterious spectral emission features between 3 and 12 μm were discovered in Galactic reflection nebulae. Léger & Puget (1984) and independently Allamandola, Tielens and Barker (1985) proposed that these puzzling features come from large “polycyclic aromatic hydrocarbon” (PAH) molecules, similar to car exhaust, and composed mainly of carbon-hydrogen rings. The PAHs represent a new form of interstellar “dust”. Dust grains and PAHs are heated when they absorb ultraviolet radiation from massive stars. They re-emit this energy as a grey-body thermal continuum in the infrared-submillimetre band and as PAH features. The total intensity of such emission measures cosmic star formation, integrated over the entire history of the Universe. In 1996 Puget and co-workers discovered in data of the NASA COBE satellite a pervasive 100 μm background radiation plausibly from an active star formation phase about 10 billion years ago. Many infrared measurements have since confirmed this discovery and shown that this was the epoch when most of stars in galaxies were formed.