Nick Kaiser obituary
Between 1984 and 1992 the cosmologist Nick Kaiser, who has died of heart failure aged 68, created many of the ideas now used by astronomers to map the large-scale distribution of dark matter in the universe. His analysis of the clustering of galaxies and the distortion of galaxy shapes by gravitational light deflection are at the heart of the leading modern cosmological experiments, particularly the recently launched Euclid satellite.
Kaiser’s research concentrated on the large-scale structure of the universe. Galaxies such as our own, the Milky Way, are congregated in a vast pattern of density fluctuations – superclusters that extend for at least 100m light years. All this structure probably represents the relic of primordial fluctuations that collapsed under their own gravity, generating galaxies and the stars and planets within them. We see these patterns in the clustering of the galaxy distribution, but much evidence tells us that the galaxies are embedded in invisible underlying dark matter, which dominates the gravity – dragging gas with it, which in turn forms into the stars of the visible galaxies.
Kaiser’s first important contribution to this field came in 1984, when he solved the puzzle of biased clustering. In the early 80s there was great confusion because different classes of galaxy clumped differently, each apparently indicating different levels of cosmic inhomogeneity, indicating different degrees of clumping of the overall mass distribution. Kaiser explained this by appealing to classic statistical tools created for the analysis of noise in telephone systems. He argued that peaks in a random noise field will always have a stronger degree of correlation than the field as a whole, with the highest peaks – clumps of dark matter in this case – tending to be found close to each other. In this way, we now understand how the clustering of different galaxies can be related to a single underlying set of clumps in the dark matter.
Once the idea of the biased relation between light and mass was understood, there remained a strong desire to study the underlying distribution of dark matter directly. In 1987 Kaiser developed a novel way to achieve this, by exploiting the so-called “redshiftspace distortions”. These arise in a 3D galaxy survey because the cosmological redshift from the expansion of the universe is changed by Doppler shifts due to the “peculiar velocities” that arise as clumps of matter fall together. The apparent redshift, which is what is used to infer the distance to a given galaxy, is then altered – stretching the map of superclusters along the line of sight. Kaiser showed that this distortion could be used to measure the peculiar velocities and hence determine the total density of the universe.
An alternative means of detecting dark matter directly comes from gravitational lensing: the distortion of background galaxies as their light passes massive foreground galaxies. In 1992, Kaiser published an analysis of “weak lensing”, considering the statistics of small image distortions and how they can reveal the effect of dark matter.
Gravitational lensing can be seen at work in images from the Hubble telescope of the rich galaxy cluster A370.
The bright foreground galaxies deflect light from more distant galaxies, distorting their shapes into striking elongated arcs. In more typical directions, this distortion is much smaller: only stretching images by around 1%. But Kaiser showed how such distortions could be averaged over many galaxies to detect and map the dark matter causing the lensing.
This work was far ahead of its time, and is now an entire field of cosmology in its own right. Many groundbased surveys and most recently the Euclid satellite observatory have been devoted to lensing and redshift-space distortions; more than a billion dollars have been invested in order to turn Kaiser’s 1987 and 1992 visions into a working reality.
Kaiser also innovated in the technical arena. Between 2000 and 2008 he led the development of PanSTARRS, a survey telescope in Hawaii with an unprecedentedly wide field of view. Although designed for cosmology, it has also made important contributions to the study of the solar system, and is now used as part of an early warning system for “killer asteroids” that are at risk of making a catastrophic impact on the Earth.
Born and brought up in Sheffield, Nick was the son of Pamela (nee Pound)and Tom Kaiser, a physics professor at Sheffield University. After gaining a BSc (1978) at Leeds University, Nick went on to a PhD (1982) at the Institute of Astronomy, Cambridge, under the supervision of Martin Rees.
Following postdoctoral research in California and the UK, Kaiser’s career took him to professorships at the Canadian Institute for Theoretical Astrophysics, Toronto (1988); the University of Hawaii (1998); and the École Normale Supérieure, Paris (2017). He was made a Fellow of the Royal Society (2008); was awarded the gold medal of the Royal Astronomical Society (2017), like his father; and received the Gruber prize in cosmology (2019).
A keen athlete, he became an ultra-runner, participating in innumerable marathons, triathlons and extreme competitions such as Iron Man. Characteristically he made a mathematical analysis of the most effective strategy for energy expenditure in his races.
His marriage to Penelope Corbett ended in divorce. He is survived by his partner, Maureen Miller, and by two sons from his marriage, Alex and Louis.
• Nicholas Kaiser, cosmologist, born 15 September 1954; died 13 June 2023