Science & Technology

Polarized Shockwaves Shake the Universe’s Cosmic Web

A composite image of 3 different observations of the cosmic web (gas, radio, and magnetic) accompanied by a composite image. Credit: F. Vazza, D. Wittor and J. West, Composition by K. Brown

The cosmic web, a network of interconnecting filaments and clusters containing gases, galaxies, and winding around cosmic voids that span millions of light-years, represents the largest scale of the universe. Predicted by astrophysicists in the 1960s, computer modeling in the 1980s provided a visual representation of this vast network.

In recent decades, astronomers have made significant progress in mapping the Cosmic Web, which has opened up new avenues for answering some of the most significant questions in the field. One area of particular interest is the study of magnetic fields at a cosmic scale and their role in shaping both galactic and cosmic structures.

New research published in Science Advances and led by the International Centre for Radio Astronomy Research (ICRAR) in partnership with CSIRO, Australia’s national science agency, is helping us to further understand these cosmic magnetic fields.

Dr. Tessa Vernstrom from The University of Western Australia’s (UWA) node of ICRAR, is the lead author of the research and describes magnetism as a fundamental force in nature.

A composite image showing the magnetic fields of the cosmic web, featuring a pull out of how radio data was stacked. Credit: Vernstrom et al. 2023

“Magnetic fields pervade the universe – from planets and stars to the largest spaces in-between galaxies. However, many aspects of cosmic magnetism are not yet fully understood, especially at the scales seen in the cosmic web.” When matter merges in the universe, it produces a shockwave which accelerates particles, amplifying these intergalactic magnetic fields,” said Dr. Vernstrom.

Her research has recorded radio emissions coming from the cosmic web – the first observational evidence of strong shockwaves.

This phenomenon had previously only been observed in the universe’s largest galaxy clusters and was predicted to be the ‘signature’ of matter collisions throughout the cosmic web.

“These shockwaves give off radio emissions which should result in the cosmic web ‘glowing’ in the radio spectrum, but it had never really been conclusively detected due to how faint the signals are.”

Data from the Murchison Widefield Array (MWA) radio telescope in Western Australia provided all-sky radio maps for the research. Credit: Dragonfly Media

Dr. Vernstrom’s team began searching for the cosmic web’s ‘radio glow’ in 2020 and initially found signals which could be attributed to these cosmic waves.

However, as these initial signals could have included emissions from galaxies and celestial objects other than the shockwaves, Vernstrom opted for a different signal type with less background ‘noise’ – polarised radio light.

“As very few sources emit polarised radio light, our search was less prone to contamination and we have been able to provide much stronger evidence that we are seeing emissions from the shockwaves in the largest structures in the universe, which helps to confirm our models for the growth of this large-scale structure.”

An extracted still from Simulation of the Cosmic Web’s magnetic field video. The blue and green colors give the (growing) strength of magnetic fields in the simulation, while the red color marks the gas temperature. Credit: Vazza F; ENZO; Piz-Daint CSCS (Lugano)

The research utilized data and all-sky radio maps from the Global Magneto-Ionic Medium Survey, the Planck Legacy Archive, the Owens Valley Long Wavelength Array, and the Murchison Widefield Array, stacking the data over the known clusters and filaments in the cosmic web.

The stacking method helps to strengthen the faint signal above the image noise, which was then compared to state-of-the-art cosmological simulations generated through the Enzo Project.

These simulations are the first of their kind to include predictions of the polarised radio light from the cosmic shockwaves observed as part of this research.

Our understanding of these magnetic fields could be used to expand and refine our theories on how the universe grows and has the potential to help us solve the mystery of the origins of cosmic magnetism.

Reference: “Polarized accretion shocks from the cosmic web” by Tessa Vernstrom, Jennifer West, Franco Vazza, Denis Wittor, Christopher John Riseley and George Heald, 15 February 2023, Science Advances.
DOI: 10.1126/sciadv.ade7233

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