Despite decades of study, black holes remain one of the most powerful and mysterious celestial objects ever studied. Because of the extreme gravitational forces involved, nothing can escape the surface of a black hole (including light). As a result, the study of these objects has traditionally been confined to observing their influence on objects and spacetime in their vicinity. It was not until 2019 that the first image of a black hole was captured by the Event Horizon Telescope (EHT).

This feat was made possible thanks to a technique known as Very-Long Baseline Interferometry (VLBI), which allowed scientists to see the bright ring surrounding the supermassive black hole (SMBH) at the center of the M87 galaxy. A new study by an international team of astronomers has shown how a space-based interferometry mission could provide reveal even more secrets hiding within the veil of a black hole’s event horizon!

The research was led by Leonid Gurvits, a researcher with the Joint Institute for Very Long Baseline Interferometry European Research Infrastructure Consortium (JIVE ERIC) and the Delft University of Technology. He was joined by researchers from the Institute of Radio Astronomy (INAF), the Netherlands Institute for Space Research (SRON), the Flatiron Institute’s Center for Computational Astrophysics, the Harvard-Smithsonian Center for Astrophysics (CfA), the Black Hole Initiative, and multiple universities and research institutes.

As they indicate in their study, ultra-high angular resolution in astronomy has always been seen as a gateway to major discoveries. In this process, known as interferometry, multiple observatories gather light from a single object that would otherwise be very difficult to resolve. In recent years, astronomers have relied on VLBI to detect radiation at the millimeter and submillimeter wavelengths. As study co-author Dr. Zsolt Paragi, a fellow researcher with JIVE ERIC, explained to Universe Today via email:

“In general, high angular resolution imaging is achieved in astronomy in three ways: by increasing the size of our telescopes, observing light at shorter wavelengths, and eliminating (or at least compensating for) the disturbances caused by the Earth’s atmosphere.

“Radio astronomy pioneered the development of imaging techniques based on interferometry, when the signal from different telescopes at large distances are seamlessly (in our terminology: coherently) combinedIn this case, the ultimate factor that determines the resolving power of the instrument is the distance between the telescopes, which we call baselines.”

A good example of this is the Event Horizon Telescope (EHT), which captured the first image of a supermassive black hole (M87) on April 10th, 2019. This was followed in 2021 by an image of the core region of the Centaurus A galaxy and the radio jet emanating from it. However, these images were little more than faint circles, which represented the light trapped within the SMBHs’ event horizons – the boundary from which nothing (even light) can escape.

Different photon paths create layers of light. Credit: George Wong (UIUC) and Michael Johnson (CfA)

Nevertheless, the image of M87 acquired by the EHT constituted the first direct confirmation of the existence of SMBHs and was the first time the shadows surrounding one were imaged. This image also provided a view of the infalling matter around the supermassive black hole, distorted by extremely strong gravity. In recent years, said Dr. Paragi, other developments have occurred in the field of VLBI that offer a taste of what’s to come:

“Another keystone result in recent years was proving the cosmological origin of the mysterious, millisecond-duration radio flashes we call fast radio bursts. Due to its excellent high-resolution imaging capability, the European VLBI Network provided by far the highest accuracy sky localization of these very brief signals, that are extremely difficult to catch even with the most modern interferometers.

“These centimeter-wavelength images not only show which galaxy the signals come from, but they can also narrow down the position of the signal to small regions within the galaxy
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