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The world's largest radio telescope under construction

It looks like several huge fields of gleaming Christmas trees in the middle of Australia's red outback – at least once it's finished. And that's just one of two parts of the Square Kilometer Array radio telescope, or SKA for short. It is considered one of the greatest research instruments and most ambitious scientific projects of the 21st century. The telescope is called "Square Kilometer" because its more than 130,000 antennas will offer a reception area of half a square kilometer after completion. Construction began in early December 2022 after decades of diplomatic negotiations over funding.

On two continents

When fully expanded, the telescope with 131,072 fir-tree-like antennae two meters high in Australia and a further 130 parabolic antennae, each 15 meters in diameter, in South Africa will be able to decipher the secrets of our universe.

The two antenna fields extend over large distances. In Australia, the smaller wire antennas are distributed over 65 kilometers, the parabolic mirrors in South Africa over 150 kilometers. The two antenna fields scan the sky in different frequency ranges and look in different directions during regular operation. But if something special happens in space, both telescope fields can be coupled with each other and at the same time take a look at the same object.

Radio telescopes also look for light

Visible light makes up only a small portion of the electromagnetic spectrum. Above the visible light, i.e. at higher vibration frequencies of the electromagnetic waves, the areas of ultraviolet radiation as well as X-ray and gamma radiation begin. The infrared range of the electromagnetic spectrum begins below visible light, i.e. at a lower frequency. The James Webb Space Telescope, for example, works in the infrared. Even lower wave frequencies are referred to as radio emissions.

So the radio telescope also looks at light, but light at a frequency other than the visible one. It is a kind of technical "eye" for radio emissions. If people were to have "radio eyes," said Bonn astrophysicist Michael Kramer in an interview with SWR, "then the universe would look completely different to us."

Deep look into the past of the universe

The researchers want to use the new radio telescope to find out how the first stars and galaxies formed. "To this day we don't know: What is dark energy? What is dark matter?" says Michael Kramer, astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn. "Are the gravitational laws in Albert Einstein's theory of relativity correct or not? All of these are questions that we cannot yet answer. And the telescope will be very helpful in bringing us at least closer to the answer."

According to Kramer, the universe was initially very hot after the Big Bang, but then cooled down, but remained starless. Only later did gravity begin to "clump" the hydrogen present everywhere. This is how the first stars finally formed. With their radiation, they ionized the hydrogen in their environment, i.e. the electrons were "snatched" from the hydrogen atoms.

Information about formation of galaxies

The astrophysicists want to use the SKA space telescope to receive and examine the weak, very low-frequency radio signal that is generated. This is intended to give a better understanding of how the first galaxies, stars and black holes formed after the Big Bang from ordinary hydrogen as the starting material.

"There is no other method of actually making these waves visible," says Kramer, "or measuring them and looking for the signal to see how the universe has developed. There are many objects that can only be observed in the radio range can, for example, radio pulsars or the light from black holes."

Up to a hundred times more sensitive than previous telescopes

According to the astrophysicist Kramer, the telescope will be up to a hundred times more sensitive in the final stage than the radio telescopes currently available to the researchers. In particular, the SKA can also capture radio emissions from the early days of the universe. Radio waves, which have been traveling through space for billions of years, gradually decrease in frequency as the universe expands. If you want to look far into the past, according to Kramer, you have to do it at low frequencies. That's what the antennas are for.

In the most remote places in the world

The regions in which the antennas are set up are among the most remote places in the world. Because such a sensitive telescope also captures many signals that interfere with research, such as mobile phone signals and radio stations. "If you were to position a cell phone on the moon, even at the Earth-moon distance, the cell phone would be the third strongest radio source in the sky," Kramer said.

However, the sources that the researchers want to look at, such as the signals from the early universe, are up to a billion times weaker. In order to avoid interference, the telescopes are therefore built in regions with little civilization. Both locations, in Australia and in South Africa, are in so-called "radio silent zones". In them it is forbidden by law to set up radio stations.

German research policy is alienated from the project

Germany was one of the founding members of the SKA consortium in 2011. In 2015, however, the federal government withdrew from the project for financial reasons. This was generally regretted in the radio astronomy community – especially since Germany played a leading role in an important predecessor project in South Africa. Since 2019, the Max Planck Society has represented the interests of German researchers in the circle of SKA nations. Not an ideal solution, but in this way the German radio astronomers can contribute further and use the data to the full extent.

Astrophysicist Kramer expects the first antennas to be put into operation in 2027. The SKA is also a special telescope in that you can work with it while it's still under construction: "I'm really looking forward to seeing this data then."

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