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Sjoert van Velzen receives Vidi grant to solve 'riddles from the universe'

Minuscule elementary particles from space colliding with Earth can give us an insight into the distant objects they come from. But first, you need to know how to catch them. With a Vidi grant from NWO, researcher Sjoert van Velzen will 'hunt' for neutrinos coming from exploding black holes.

Neutrinos are peculiar particles. They are so small that they cannot be split any further, they have no mass and behave a bit like light. And they pass through pretty much everything. Researcher Sjoert van Velzen calls them the 'riddles' of the universe. ‘Neutrinos bring more questions than answers,’ he says.

Neutrinos: potential treasures of information

Van Velzen is particularly fascinated by neutrinos that have extremely high energy levels. ‘These come towards us at almost the speed of light and contain more energy than we can generate with the particle accelerators on Earth. But remarkably, we know almost nothing about their exact origin.’ And yet they are potential treasures of information. ‘If we can find out where they came from, we could learn a lot about the extreme conditions in which they formed.’

The past few years have already shown a tip of the iceberg. In 2019 and again in 2021, a neutrino was observed while a large explosion in which a black hole swallowed a star occurred around the same time. This led Van Velzen to suspect that one might be related to the other. It is exactly this hypothesis that he will investigate further with the Vidi grant from NWO.

Neutrino telescope at the bottom of the sea

But ‘catching’ neutrinos is anything but easy. Only a handful have been observed in the past few years, with a special telescope at the South Pole. A telescope currently under construction at the bottom of the Mediterranean Sea, the KM3NeT, should make more measurements possible.

Heart of darkness: A view of the accretion disc around the supermassive black hole, with jet-like structures flowing away from the disc.

These neutrino telescopes look very different from the dish or spherical telescopes we are used to. They consist of dozens of sensors placed side by side over an area of a kilometre. ‘Moreover, they have to be in places that are dark and yet transparent,’ Van Velzen explains about the strange locations. ‘Because the telescope measures the light trail left by the neutrino, you quickly end up with a dark environment under water or ice.’

When a black hole swallows a star

Despite these obstacles, the researcher hopes to make new observations of neutrinos with the telescope in the Mediterranean Sea. Afterwards, he wants to use a regular telescope to see if he can link them to a black hole swallowing a star, just like in 2019 and 2021. ‘In such an event, a lot of matter in the black hole disappears in a short time and a big explosion of light takes place.’

Thanks to the 800,000 euros from the Vidi grant, Van Velzen can work with two PhD candidates and a postdoc for the next five years. ‘We now have a hint that exploding black holes and neutrino observations are related, but I want to get to the level where we know that almost without a doubt,’ he says, ‘with the ultimate goal of gaining a better understanding of the most extreme objects in our universe.’

Tekst: Samuel Hanegreefs

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