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.

The culmination of Puget’s work, building on his technical and scientific knowledge of far-infrared/ submillimetre/ millimetre astronomy, has been his leadership of the development and scientific exploitation of the High Frequency Instrument (HFI) on the European Space Agency’s Planck satellite. Between 2009 and 2013, Puget and his international team used HFI’s novel cryogenic sensors to measure the CMB plus the foreground emission due to the Milky Way’s dust and gas with superb sensitivity between 350 μm and 3 mm. By working at these short wavelengths, HFI has studied the CMB with an angular resolution three times better than any alternative all-sky CMB map. HFI is also uniquely sensitive to foreground dust emission, which dominates at short wavelengths. The ability to separate foregrounds was critical to the Planck results, which measure the cosmological parameters to exquisite precision − for example, the total density of dark matter is measured to 2% accuracy.

These precise results potentially allow us to probe within a small fraction of one second after the Big Bang, in the hypothesized inflationary era, when amplified quantum fluctuations may have created both the initial density fluctuations and relic gravitational waves. The gravitational waves induce very weak distortions of the CMB, but their detection is challenging due to possible confusion with dust foreground emission. Planck’s precise foreground separation showed that the true level of primordial gravitational waves must lie below the predictions of the simplest inflationary models. It also showed that the density fluctuations are purely Gaussian, with no detectable phase correlations. At a stroke, these results removed large classes of inflation theories from consideration. We thus still lack definitive evidence that inflation happened, and what exactly caused it − but we now know a great deal about what it is not.

Today, the expansion of the Universe expands is accelerating. If Einstein's relativistic theory of gravity is correct, this requires a non-zero vacuum energy density. Alternatively the acceleration may indicate a modification of the strength of gravity on large scales, in which case density fluctuations in the Universe would develop at a non-standard rate. Planck can measure this effect, because the CMB radiation is deflected by intervening mass fluctuations. This “gravitational lensing” effect has been mapped comprehensively by Planck, and matches the expectations of standard gravity. This work nicely closes the loop on Puget’s career, since the inferred foreground mass concentrations correlate in position with fluctuations in the far-infrared background: the dusty star forming galaxies whose integrated effect was first found by Puget more than two decades ago.

Finally, the CMB detects scattering due to ionized gas that is created by the first stars and quasars. The latest Planck data indicate that the onset of this “reionization era” was more recent than previously supposed: at a redshift less than 10, in other words, when the Universe has already reached 1/10 of its present size. This result complements the far-infrared background's measurement of total energy release, closing in on a complete picture of the history of cosmic star formation.

Jean-Loup Puget’s wide-ranging contributions and leadership in infrared and submillimetre astronomy make him a fitting recipient for the 2018 Shaw Prize.

26 September 2018   Hong Kong