(ORDO NEWS) — Despite decades of research, black holes are still among the most powerful and mysterious celestial objects ever studied. Due to the extreme gravitational forces, nothing can escape the surface of a black hole (including light).
As a result, the study of these objects has traditionally been limited to observing their influence on objects and space-time in their vicinity. It wasn’t until 2019 that the Event Horizon Telescope (EHT) took the first image of a black hole.
This feat was made possible by a technique known as very long baseline interferometry (VBL), which allowed scientists to see the bright ring surrounding the supermassive black hole (SMBH) at the center of the galaxy M87.
A new study by an international team of astronomers has shown how cosmic interferometry can uncover even more secrets lurking behind the veil of a black hole’s event horizon.
The study was led by Leonid Hurwitz, Research Fellow at the Joint Institute for Very Long Baseline Interferometry of the European Research Infrastructure Consortium (JIVE ERIC) and the Delft University of Technology.
He was joined by researchers from the Institute for Radio Astronomy (INAF), the Netherlands Institute for Space Research (SRON), the Center for Computational Astrophysics at the Flatiron Institute, the Harvard-Smithsonian Center for Astrophysics (CfA), the Black Hole Initiative, and a host of universities and research institutes.
As they point out in their study, ultra-high angular resolution in astronomy has always been seen as a path to major discoveries.
In this process, known as interferometry, multiple observatories collect 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 millimeter and submillimeter wavelengths.
Study co-author Dr. Zsolt Paragy, a research fellow at JIVE ERIC, said in an email: “In general, high angular image resolution is achieved in astronomy in three ways: increasing the size of telescopes, observing light at shorter wavelengths, and eliminating (or at least compensation) interference 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 is unhindered (in our terminology: coherently) combined. In this case, the final factor that determines the resolution of the instrument is the distance between the telescopes, which we call the base line”.
A good example of this is the Event Horizon Telescope (EHT), which took the first image of a supermassive black hole (M87) on April 10, 2019.
It was followed in 2021 by an image of the region of the nucleus of the galaxy Centaurus A and the radio emission emanating from it. However, these images were nothing more than faint circles that represented light trapped in the SMBH event horizon – a boundary beyond which nothing (not even light) can escape.
However, the EHT image of M87 was the first direct confirmation of the existence of the SMBH and the first images of the shadows surrounding the SMBH were obtained.
This image also made it possible to see the infalling matter around the supermassive black hole distorted by the extremely strong gravity. In recent years, says Dr. Paragy, there have been other developments in the VLBI field that give us a glimpse of what the future holds:
“Another major achievement in recent years has been the proof of the cosmological origin of the mysterious millisecond radio bursts we call fast radio bursts.
Thanks to its excellent high-resolution imaging capabilities, the European VLBI network has provided the highest accuracy in localizing these very short signals in the sky, which are extremely difficult to capture even with using the latest interferometers.
“These centimeter-wavelength images not only show which galaxy the signals are coming from, but also allow the signal to be narrowed down to small regions within the galaxy, which will be critical to understanding this phenomenon.”
According to the astronomical community, the next logical step is to capture the photon ring. In this region, the gravitational force is so strong that the photons are forced to move in orbits.
In EHT images, most of the light from this ring was scattered before it reached Earth, resulting in relatively blurry images. To build on this success, the Next Generation EHT (ngEHT) will add ten new telescopes and upgrade those already in the network.
However, according to Dr. Paraga, space-based VLBI arrays will provide astronomers with the most detailed images of the photon rings around SMBHs and even the event horizons themselves.
In their study, the team looked at the potential of the future VLBI space telescope, known as the Terahertz Telescope for Astrophysical Research (THEZA), which was the subject of a white paper by Hurwitz, Paraga and many of the team behind this latest paper.
This work was submitted as part of the ESA Voyage 2050 project, an open call for proposals for large class science missions to be conducted over the period 2035-2050.
Like space telescopes that study space in the optical, infrared, X-ray, radio and other parts of the spectrum, this concept provides for the creation of a space interferometer to study the physics of space time in the vicinity of the SMBH. As Dr. Paragy described it:
“Observations from space at very short, millimeter and submillimeter wavelengths will open up new measurements for VLBI.
The benefits of a mission based on the THEZA concept are twofold. A population of supermassive black holes will become available for resolution imaging of black hole shadows that are not visible to these instruments, and will also allow for unique studies of the properties of black hole spin and space-time.”
The team analyzed all elements of the telescope, including antenna systems, receivers, low-noise amplifiers, local oscillators, mixers, transport and data processing.
They concluded that an interferometer based on the THEZA concept would achieve the three main goals of an astronomical mission with ultra-high angular resolution. In short, it will be free from interference from Earth’s atmosphere and will observe black holes at higher frequencies and longer baselines than ever before.
“By studying unique systems composed of close pairs of supermassive black holes, THEZA can uncover the processes that led to the accelerated growth of black holes at the dawn of the universe, which left a vivid imprint on the evolution of galaxies,” Dr. Paraghi added.
“More importantly, THEZA will expand our horizons to measure the shadows of black holes in detail. This will lead to a better understanding of gravity, which is very important since gravity plays a fundamental role in the formation of the universe.”
In the coming years, next-generation observatories will rely on improved detectors and data communications technologies to get an even more detailed picture of some of the most mysterious objects in the universe.
These include proposals such as the proposed Spektr-M space telescope, which is expected to launch by 2030. This device will be equipped with a primary mirror with a diameter of 10 meters, capable of observing space in the submillimeter and far infrared wavelengths.
The James Webb Space Telescope (JWST), which reached its orbit (L2) in January and is almost cool enough (as of late April) to start operations, will soon conduct its own interferometric studies.
As part of the Near-Infrared Imager and Slitless Spectrograph (NIRISS) instrument, the aperture masking interferometer (AMI) will turn the entire aperture of JWST segmented mirrors into an interferometric array.
With NASA‘s plans to send astronauts back to the Moon (as part of the Artemis program) and other space agencies embarking on lunar exploration programs, there are even proposals to build VLBI telescopes on the far side of the Moon, where they will be free from atmospheric or light interference.
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