Immense radio survey discloses our universe’s structure at the largest scales

New observations suggest our estimations of the cosmic dipole effect are in fact in line with the large-scale structure of the cosmos.

When we look out into the cosmos with our unassisted eyes, we are really only seeing a very small chunk of what’s actually out there. That’s simply because there are parts of the electromagnetic spectrum that our visual faculties are not at all sensitive to. Radiation is being emitted by all kinds of cosmic phenomena across this spectrum, but we’re not able to physically see wavelengths outside the visible light range without the help of external tools — but fortunately, astrophysicists have access to telescopes that let them to observe the cosmos across this continuum. The South African MeerKAT radio telescope, positioned 90 km outside the small Northern Cape town of Carnarvon, is one such observatory.

MeerKAT radio telescope allows astrophysicists to probe the radio band emissions of stars, blackholes and galaxies in the surrounding universe. Recently, a global team of astronomers from the MeerKAT Absorption Line Survey (MALS) used a vast catalog of radio sources apprehended by the MeerCAT radio telescope to make a measurement of a phenomenon termed the “cosmic radio dipole.” Observing the radio sky can give astrophysicists insights into the large-scale structure of the cosmos, as radio emissions from far off galaxies can cruise through space on relatively uninterrupted trajectories. The MALS survey has produced a very sensitive catalog of roughly a million radio sources in the sky because the team pointed the telescope array in 391 directions. “The depth and the expanse of this continuum catalog holds a unique position among modern radio continuum surveys,” said Neeraj Gupta (an astronomer at the Inter-University Centre for Astronomy and Astrophysics, who leads the MALS project).

The cosmic radio dipole

It is an effect generated by the motion of our solar system through space as it orbits the center of the Milky Way galaxy, and as the Milky Way gravitationally interacts with other galaxies. The effect makes radio sources seem more numerous in the direction the solar system is traveling in, and far less numerous in the opposite direction. The magnitude of this effect should be directly linked to the velocity of the solar system through space — but, the effect has been found to be far higher based on prior measurements of the solar system’s motion through space. This made astrophysicists question whether the dipole might not merely be caused by the motion of the solar system through space, but rather by some other radio sources (and therefore more galaxies) in the direction that the solar system is traveling. But, the new dipole measurement based on the MALS survey is aligned with predictions based on recent measurements of the solar system’s movement through space.

Astronomers reason this discrepancy may be linked to the design of different surveys, while the MALS survey covered small patches of the sky to a very deep level. By contrast, some other radio surveys have measured wider patches of sky but on much shallower scales. “Measuring the dipole is an extremely important test of cosmology, and can tell us whether our fundamental assumptions about the structure of the Universe are correct,” said Jonah Wagenveld (an astronomer at MPIfR and lead author of the paper that reported the findings). As the new findings show, radio astronomy offers researchers novel ways of observing the cosmos at the largest scales, and so, opportunities to test our best cosmological theories against observational data.