The Radio Sky Is Brighter Than Astronomers Thought, SKA Prototype Reveals
Featured image: An SKALA4.1 log-periodic dipole antenna at the Murchison Radio-astronomy Observatory in Western Australia. [Credit: CSIRO / SKA Observatory]
Astronomers have been underestimating the brightness of the low-frequency radio sky by a significant margin, according to new measurements using a prototype antenna for the Square Kilometre Array published in Nature Astronomy. The findings reveal that the diffuse radio background between 60 and 350 megahertz is 20 percent brighter than previous models at the lower end of that range and 50 percent brighter at 350 megahertz.
The discovery has immediate implications for the calibration of the upcoming SKA-Low telescope, the interpretation of cosmic dawn experiments seeking the 21-centimeter hydrogen signal, and our understanding of the high-energy electron population in the Milky Way.
“We have determined the absolute background brightness at low radio frequencies for a very large part of the sky, with unprecedented precision,” said Michiel Brentjens of ASTRON, the Netherlands Institute for Radio Astronomy, who was not involved in the study. “This has only been possible thanks to recent developments in electronics and increasing computer power.”
The measurements were led by Luke McKay of CSIRO, Australia’s national science agency, using an SKALA4.1 log-periodic dipole antenna, the same design used in SKA-Low stations, placed above a 40-meter diameter SKA-Low station ground mesh at the Inyarrimanha Ilgari Bundara Observatory in Western Australia.
The calibration problem
Measuring the absolute brightness of the low-frequency radio sky is notoriously difficult. At higher frequencies, astronomers can calibrate their instruments by pointing at the Moon or a planet, whose temperature is known. But at frequencies below 350 MHz, the sky itself is the brightest thing in every direction. There is no empty patch to serve as a zero-level reference.
Previous sky models, notably the 2016 Global Sky Model (GSM2016) built from 20th-century observations, carry systematic uncertainties of approximately 20 percent. The new measurements cut that uncertainty dramatically: below 2 percent between 60 and 150 MHz, rising to just under 8 percent at 350 MHz.
The breakthrough came from the GINAN receiver, a new receiver architecture developed by CSIRO that dynamically self-calibrates for receiver noise, bandpass, and impedance mismatch in real time while connected to the antenna. This eliminates the calibration uncertainties that plagued all earlier measurements over this frequency range.
What is making the sky brighter?
The low-frequency radio sky is dominated by synchrotron radiation, emissions from high-energy cosmic-ray electrons spiraling along the Milky Way’s magnetic field lines. The new measurements suggest there are more of these high-energy electrons than current models account for.
But the Galactic electron population is not the only possible source. Unresolved populations of distant, very faint radio sources, at the nanojansky level, far below the detection threshold of existing surveys, could also contribute to the excess. A combination of both is considered the most likely explanation.
The paper also speculates on a more exotic possibility: that some fraction of the excess could come from the decay of dark matter particles. Brentjens described the dark matter evidence as “very thin,” but noted that the precise measurement provides better upper limits on possible dark matter decay and annihilation signals.
Implications for SKA-Low and beyond
The most immediate practical impact is on the calibration of SKA-Low, which will operate from 50 to 350 MHz, exactly the frequency range covered by these measurements. The new data provide an accurate absolute flux-density scale that can serve as a stable primary reference for calibrating all SKA-Low observations.
The implications extend to epoch of reionization (EoR) experiments, which seek to detect the 21-centimeter hydrogen signal from the cosmic dawn. These observations must subtract the diffuse radio foreground, which is orders of magnitude brighter than the signal they are trying to detect. An incorrect sky model produces systematic errors that could either swamp or mimic the EoR signal.
Other low-frequency radio telescopes, including LOFAR in Europe, the Murchison Widefield Array in Australia, and OVRO-LWA in California, will need to revisit their flux calibration scales. The existing standard, the Baars flux-density scale, has systematic uncertainties of approximately 20 percent that can now be corrected.
Co-author Ron Ekers, a legendary figure in Australian radio astronomy, noted that the result is a reminder of how much foundational astrophysics remains to be done even as astronomy moves into the era of billion-dollar facilities. “We built a better radiometer and found the sky is not what we thought it was,” he said. “Sometimes the most important discoveries come from simply measuring things more carefully.”