Celestial nuisance |
The cosmos has become a lab for the Big Questions in physics. Along the way, we’ve lost sight of its glorious messiness
Journey to a satellite galaxy roughly 200,000 light-years from Earth, featuring the young star cluster NGC 602. With data from Webb’s NIRCam (Near-InfraRed Camera) and MIRI (Mid-InfraRed Instrument). Courtesy HUBBLE/WEBB/ESA
is a fellow at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford Universityin California, US. His work focuses on radio astronomy and cosmology.
Edited byRichard Fisher
In the story of cosmic history, one of the most important supporting characters is dust. As clouds of interstellar gas cooled and condensed in the early Universe, dust grains absorbed and re-emitted radiation, eventually forming stars. Later, dust provided the raw material from which planets could form and grow. Dust grains may even have been the surfaces on which the first complex organic molecules formed. Yet, if you ask most cosmologists, they’ll tell you that dust is a sheer nuisance.
In astrophysics, ‘nuisance’ is a quasi-technical term. Astrophysicists call any phenomenon that gets in the way of the thing they actually want to study a ‘nuisance’. Dust grains emit polarised light that can mimic subtle signals from the afterglow of the Big Bang (the cosmic microwave background) that cosmologists have spent decades and billions of dollars trying to detect. A cosmologist studies dust not to uncover its role in shaping the cosmos, but to subtract it – the term ‘sweeping under the rug’ couldn’t be more apt.
This dichotomy between ‘signal’ and ‘nuisance’ is found across many sub-fields in astronomy. Astronomers who study a phenomenon called ‘weak lensing’ use the subtle distortions of light caused by gravity to map invisible dark matter. For them, the ordinary matter that makes up everything we can see is a nuisance. Pulsar timing astronomers use rapidly spinning neutron stars called pulsars as extraordinarily precise clocks to detect the passage of gravitational waves through the galaxy. For them, the nuisance is ionised interstellar gas, which scatters and disperses the light from pulsars.
Yet one astronomer’s nuisance is another’s treasure. Just as a laser beam reveals the intricate swirls of smoke in a dark room, pulsars illuminate the fine structure of the interstellar medium, the unlit space between galaxies. If you want to study the interstellar medium and its role in shaping our galaxy’s evolution, the light-scattering gas is an invaluable resource.
The ESO-operated Very Large Telescope based at the Cerro Paranal site in the Atacama Desert of northern Chile. Photo by Miguel Claro/ESO
Whether a phenomenon is a nuisance or something worth studying in its own right clearly depends on the investigator and their interests. But in a discipline as collaborative as astronomy, shared priorities must be developed. Gone are the days of Galileo and other lone scientists in the tower, pointing telescopes built by their own hands wherever curiosity dictates. Modern astronomy is a highly specialised discipline, marked by large collaborations. The tools of the trade – telescopes – are designed, built and maintained by hundreds of scientists and engineers across many institutions. But you can’t just stroll in and ask to use one. Investigators must fiercely compete for time with these advanced instruments. To guide how that access is allocated, the research community has developed a set of shared priorities. Some questions are elevated to the status of ‘Big Questions’ to which all others inevitably become subordinate.
But are these Big Questions necessarily the right ones? Who decides where to draw the boundaries between signal and nuisance, or big and small? Fields of enquiry are defined by the questions they seek to answer. If we want to understand modern astrophysics, it is not enough to understand the scientific and mathematical content of its theories. We also need to understand the human forces that shape the questions themselves and cause some questions to be prioritised, and other questions to be viewed as irrelevant or merely means to those bigger ends.
To identify the Big Questions that dominate the field, you need look no further than the flagship projects set to shape the discipline for decades. These include the Square Kilometre Array Observatory, the James Webb Space Telescope, the Vera C Rubin Observatory, the Nancy Grace Roman Space Telescope, and the Laser Interferometer Space Antenna. Each of these projects will cost several billion dollars over its lifetime.
When we look at the stated science goals of these projects, consistent themes emerge. For example, the Square Kilometre Array Observatory at Jodrell Bank in the UK is a massive radio array that will define the field of radio astronomy for the foreseeable future. Its website contains a list of science goals. On that list, the early Universe, gravitational waves, dark energy and cosmology are all listed before stars, the interstellar medium and the origins of life. The official page for the much-covered James Webb Space Telescope similarly lists the early Universe – the ‘cosmic dawn’ – before more prosaic matters such as stars, planets and dust. Such priorities are often reflected in media coverage of these projects, which further shape perceptions among young researchers, politicians, funders and the general public about which questions matter most.
The boundaries between the big and small questions often fall along the lines between ‘fundamental’ physics and ‘messy’ astrophysics. This is reflected in the language of many cosmologists who study dark matter, for example. For decades, the study of dark matter has focused on the largest physical scales – giant clusters of galaxies – where the gravitational effects of dark matter can be directly inferred. On small scales, within and around galaxies, dark matter’s effects become entangled with the complex dynamics of interstellar gas. To use the terminology that is........