Astronomers unveil the largest-ever map of the Universe’s hidden magnetic fields

Astronomers have produced the most extensive and detailed map yet of the Universe’s magnetic fields, revealing the vast, invisible structures that help shape how galaxies form and evolve. Using Australia’s most powerful radio telescope, the new survey dramatically expands previous datasets and offers an unprecedented view of magnetism on cosmic scales.

The map, created with the Australian Square Kilometre Array Pathfinder (ASKAP), traces how radio waves from hundreds of thousands of distant galaxies are twisted as they travel through magnetic fields. This twisting, known as polarisation, allows scientists to reconstruct where magnetic fields lie in the space between galaxies and within our own Milky Way.

Magnetic fields: invisible engines of the cosmos

Magnetic fields are woven throughout the Universe. They influence how charged particles move, affect the flow of gas, and can regulate the birth of new stars. Yet, despite their importance, their origin and evolution remain major open questions in astrophysics.

In extreme environments such as neutron stars and black holes, magnetic fields can be trillions of times stronger than Earth’s. In the vast regions between stars and galaxies, they can be millions of times weaker than Earth’s magnetic field but still play a key role in how galaxies develop over billions of years. These large-scale fields can store enormous amounts of energy, potentially slowing or even suppressing star formation in some regions.

Because magnetic fields themselves are invisible, astronomers rely on their impact on light to detect them. Light is an electromagnetic wave, which means it has both electric and magnetic components. As these waves move through space threaded with magnetic fields, their orientation can be altered in measurable ways.

How twisted light reveals hidden magnetism

The new map is based on the phenomenon of polarisation – the direction in which a light wave oscillates. Light can be polarised in different directions, such as up-and-down or side-to-side. When radio waves travel through regions with magnetic fields and charged particles, their polarisation can be rotated. This process is often referred to as Faraday rotation.

By observing the polarisation of radio waves from distant galaxies, astronomers can infer how much the light has been twisted along its journey. Each object in the map acts like a backlight shining through the Universe, with its polarisation pattern carrying information about the magnetic fields between the source and Earth.

Australian radio telescopes have a long history in this kind of work. In 1962, Murriyang (CSIRO’s Parkes radio telescope) was the first to detect twisted polarisation from cosmic magnetic fields beyond Earth. Since then, researchers have gradually expanded the number of known polarised sources, building ever more detailed maps of cosmic magnetism.

A leap forward from earlier magnetic maps

Before this new survey, the last major all-sky map of cosmic magnetic fields dated back to 2009. Although it was groundbreaking at the time, progress since then has been limited by the number of suitable radio sources and the sensitivity of available instruments.

The new map, known as SPICE-RACS, marks a substantial advance. Where earlier efforts relied on relatively small samples of objects, this survey uses data from hundreds of thousands of galaxies, greatly improving both the resolution and coverage of the magnetic sky.

The map uses colour to indicate the direction of the inferred magnetic fields along our line of sight: regions where the fields appear to point towards Earth are shown in red, while those pointing away are shown in blue, similar to the north–south orientation of a compass. Much of the intricate, swirling structure in the image arises from magnetic fields in our own Milky Way, superimposed on signatures from more distant regions of the Universe.

ASKAP: a radio telescope built for fast sky surveys

The breakthrough comes from ASKAP, located at Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory on Wajarri Yamaji Country in Western Australia. ASKAP consists of 36 dishes, each 12 metres across, designed to capture very wide areas of the sky in a single observation.

ASKAP is one of the key precursor instruments for the Square Kilometre Array (SKA), a next-generation global radio observatory being constructed in Australia and South Africa. These precursors are testing technologies and survey strategies that will later be scaled up for the SKA itself.

To prepare for detailed magnetic studies, ASKAP’s team conducted the Rapid ASKAP Continuum Surveys (RACS), effectively building a radio atlas of the sky. Recent RACS releases have catalogued nearly 4 million distant galaxies, around half of which had never been detected before at radio wavelengths.

SPICE-RACS: combining surveys to map cosmic magnetism

The SPICE-RACS map is the result of a collaboration between the teams behind RACS and the Polarisation Sky Survey of the Universe’s Magnetism (POSSUM), ASKAP’s flagship project for studying cosmic magnetic fields.

Researchers examined every galaxy detected in RACS, searching for clear polarisation signals that could be used to probe intervening magnetic fields. From the roughly 4 million galaxies in the RACS catalogue, about 350,000 provided sufficiently strong and reliable polarisation measurements for inclusion.

This dataset is nearly ten times larger than the sample used in the previous largest polarisation-based magnetic map and about five times larger than all earlier observations combined. The result is currently the most comprehensive and detailed map of the Universe’s large-scale magnetic fields.

What the new map can tell us

The SPICE-RACS map is already enabling a wide range of studies. With its much denser coverage of the sky, astronomers can begin to:

• Trace the strength and structure of magnetic fields within the Milky Way in finer detail.
• Investigate how magnetic fields behave in different parts of the cosmic web, including galaxy clusters and intergalactic space.
• Test theories for how cosmic magnetic fields emerged and evolved since the Big Bang.
• Improve models of how magnetism affects gas flows, star formation, and galaxy evolution.

Because the underlying data are publicly available, research groups around the world can use the map to explore their own questions about magnetism and large-scale structure in the Universe.

Looking ahead to next-generation magnetic maps

The current SPICE-RACS release is not the end point. Astronomers plan to combine all versions of the RACS surveys to build an even more detailed and extensive polarisation map in the coming years.

Meanwhile, the full POSSUM survey is expected to complete its observations by around 2030. With deeper and more precise measurements, POSSUM will sharpen the view provided by SPICE-RACS, allowing scientists to probe magnetic fields in more distant galaxies and further back in cosmic time.

As the SKA Observatory comes online later this decade, it will push these techniques even further, potentially transforming our understanding of how magnetic fields emerged in the early Universe and how they have guided the formation of cosmic structures ever since.