Black holes seem like mystical objects, but they are an integral part of current astrophysics and are at the center of attempts to unify quantum physics and general relativity. While black holes are invisible; they can be observed by the effects they have on their direct environment and even on larger scales. Radboud University has a unique expertise in black hole research; initiating the Event Horizon Telescope collaboration to create the first ever image of a black hole, and continuously working to expand our knowledge about black holes, which in turn can tell us more about the fundamentals of our universe.
The event horizon of black holes
The defining feature of a black hole is the event horizon: the 'dividing line' within which nothing, not even light, can escape the gravitational pull of the black hole. Recently, advancements in mm-wave astronomy have allowed us to actually capture images of the event horizon of two of the largest black holes in the sky: Sgr A* and the one in M87. By studying the precise measurements of the black hole shadow, we can confirm one of the fundamental predictions of Einstein's theory of general relativity. This also gives us the opportunity to test and narrow down various alternative theories of gravity.
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Using black holes to study the universe
To better understand black hole astrophysics, we compare the collected data with detailed computer simulations that take into account factors like the bending of light due to gravity and the transport of radiation. These simulations help us examine accretion flows and jets: two types of gas flows that have a fundamental impact on the evolution of stars and galaxies, the acceleration of high-energy cosmic rays and the observational signatures of extreme events such as X-ray transients and gravitational-wave mergers. This level of analysis allows us to study and test models of black hole astrophysics in unprecedented detail.
Event Horizon Telescope
Black holes are either rather small or very large and far away. Imaging these thus required a very high resolution. In 2000, Radboud University professor Heino Falcke proposed to combine telescopes around the world to form a virtual telescope: the Event Horizon Telescope the size of the Earth using the imaging technique Very Long Baseline Interferometry (VLBI). Interlinking the eight telescopes has resulted in unprecedented sensitivity and resolution. In 2019, the EHT consortium presented the first ever picture of a black hole.
It was ground breaking in many ways:
It presents the direct evidence for the existence of black holes, showing that Einstein (and hence the theory of General Relativity) was right,
It shows the first ever image of lensed and relativistic beamed emission on the scale of the event horizon in extreme gravity,
And it helps to constrain models for jet formation and energy extraction from black holes
The first ever picture of a black hole was a ground breaking scientific acomplishment. However, to secure long-term future science observations, a crucial piece is missing: a millimetre-wave radio telescope in Africa. Astronomers from Radboud University have teamed up with the University of Namibia to realize such a telescope: the Africa Millimetre Telescope.
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