Rescue mission for the cosmological principle
New all-sky survey by the MeerKAT radio telescope shows the universe as expected at large distances - unlike previous observations by other telescopes
The cosmological principle is the foundation of modern cosmology and has been confirmed many times by observations and computer models. It states that the universe looks the same from every location and in all directions on a large scale. The latter criterion is called isotropy. However, observations with radio telescopes have in the past discovered a curious deviation from the cosmological principle: in the direction of motion of the solar system through the Milky Way, they found more galaxies than in the opposite direction. The latest data from the MeerKAT radio telescope, however, attempts to solve this mystery
One way to measure the largest structures in the Universe is to use radio telescopes to observe the light of distant galaxies. The brightest galaxies are usually also strong radio emitters and are therefore particularly suitable for looking as far into space as possible and measuring how galaxies are distributed in the part of the Universe that can be seen from Earth. However, a key effect that distorts the data must be taken into account in these measurements: the radio dipole. This is a cosmological effect caused by the proper motion of the Earth and the Solar System through our own galaxy. Radio sources appear brighter in the direction of motion than in the opposite direction. This also means that researchers find more galaxies in this direction than in the opposite direction. This is because every telescope has a limited sensitivity and the further away a light source is, the fainter it appears and the more it eludes a telescope.
More galaxies in the direction of travel?
Although radio observatories have taken this effect into account so far, the observed anisotropy lasts. The latest data from the MeerKAT radio telescope now show no more signs of such anisotropy in the large-scale universe. Researchers led by the Max Planck Institute for Radio Astronomy are trying to clarify why this is the case, i.e. whether the previously observed uneven distribution of galaxies in the sky was purely an observational effect or whether the structures of the universe actually contradict the cosmological principle.
As part of the MeerKAT Absorption Line Survey (MALS) sky survey, astronomers, including researchers from the Max Planck Institute for Radio Astronomy in Bonn, have now compiled the largest catalog to date of radio sources distributed in all directions and at different distances from Earth using the MeerKAT radio telescope. “The depth and the expanse of this continuum catalog are unique among modern radio surveys”, says Neeraj Gupta, an astronomer at the Inter-University Centre for Astronomy and Astrophysics in India and head of the Mals project. Unlike previous surveys, the team found no anisotropy in the distribution of galaxies after correcting for the expected effect of the radio dipole.
The Cosmic microwave background as a reference
The effect of the radio dipole can be traced back to the Doppler effect and also distorts the radio light of the cosmic background radiation received on Earth, which is actually a very homogeneous source of radiation that emerged during the early days of the universe. The Doppler effect ensures that the background radiation appears brighter in the direction of movement than in the opposite direction. There are three reasons for this: Firstly, the movement of the Earth relative to the rest of the universe shifts the spectrum of the cosmic background radiation, which resembles that of a black body. Depending on which part of the spectrum is measured, this can cause the background radiation of the universe to appear brighter. In addition, the Doppler effect increases the intensity of the light through the so-called Doppler boost and the aberration of the light. Both are actually relativistic effects, but they can already be measured at the Earth's own speed as part of the solar system. In the case of aberration, the oncoming light appears to focus.
This distortion is very well understood and is considered a reference for other observations, such as sky surveys, in which this type of distortion must be subtracted out in order to measure the actual distribution of radio-emitting galaxies. This is because the Doppler effect also changes the brightness of individual so-called radio galaxies, which can determine whether a telescope sees the galaxy in a certain direction or not.
However, previous surveys of the sky with radio telescopes such as the Very Large Array in New Mexico have discovered more such radio galaxies in the direction of motion of the solar system than in the opposite direction. This is primarily an active type of galaxy in which a jet or stream of matter that is anchored by twisted magnetic fields in the vicinity of the central black hole ejects matter from the galaxy at relativistic speeds. This produces bright synchrotron radiation that dominates the radio spectrum. The anisotropy of the active galaxies, i.e. their clustering in the direction of motion, is three to four times stronger than researchers had expected from measurements of the radio dipole of the background radiation. The anisotropic distribution of these galaxies therefore persists, even though astronomers have already eliminated all the effects of the Doppler effect from the observational data.
More sensitive telescopes pay off
Thus, unlike past observations with other telescopes, the MALS observations no longer find any anisotropy in the observable universe that cannot be explained by the Earth's proper motion. “This could be because the measurements with MeerKAT are significantly more sensitive than those of previous telescopes, but we don't yet know for sure,” says Jonah Wagenveld, leading scientist of this study and astronomer at the Max Planck Institute for Radio Astronomy. This is because the MALS catalog not only lists a higher number of galaxies than other catalogs, but also other types of galaxies. These include, for example, fainter galaxies that have so far remained undiscovered. These also differ from the more luminous radio galaxies in the shape of their radio spectra. The frequency-dependent Doppler effect would therefore affect the two types of galaxies differently if that effect had not already been taken into account. One possible explanation for the fact that MeerKAT no longer observes anisotropy, could be that the newly discovered galaxies provide a more complete picture of the observed universe. However, this would require the newly discovered and faint galaxies, as observed by MeerKAT, to be unevenly distributed themselves.
Even if the new measurements are fully consistent with the cosmological principle, any doubts will only be dispelled once it has been finally clarified why the deviations between the MALS observations and those of other radio surveys persist. Further observations by MeerKAT or future radio observatories will decide whether we can continue to hold on to our current view of the cosmos.
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Additional Information
The new catalog and accompanying scientific results of this study are described in Wagenveld et al. (2024), accepted for publication in Astronomy & Astrophysics. This is the second of several radio continuum and spectral line data releases to come from MALS and making this data release has been a team effort. The MALS catalogs and images are publicly available at https://mals.iucaa.in. The MALS team is an international collaboration of researchers from around the world. The project is led by Neeray Gupta from IUCAA, India.
The MeerKAT telescope is located in the semi-desert Karoo. It is a facility of the National Research Foundation (NRF) in South Africa and is operated by the South African Radio Astronomy Observatory (SARAO). With 64 dishes, it is the largest radio telescope in the southern hemisphere and one of two precursor instruments of the Square Kilometre Array (SKA) in South Africa.
The SKA Observatory (SKAO) is an intergovernmental organisation bringing together nations from around the world. Its mission is to build and operate cutting-edge radio telescopes to transform our understanding of the Universe, and deliver benefits to society through global collaboration and innovation. The Observatory has a global footprint and consists of the SKAO Global Headquarters in the UK, the SKAO’s two telescopes at radio-quiet sites in South Africa and Australia, and associated facilities to support the operations of the telescopes. Once in operation, the SKAO will be one global observatory operating two telescopes across three continents on behalf of its member states and partners.
