It’s one of the ultimate mysteries of astrophysics: Do black holes really exist? It’s a question that has haunted the scientific community for almost a century and whose definitive answer would have all sorts of far-reaching implications to our current understanding of the Universe. Even though astronomers haven’t yet addressed this major cosmic enigma, they have gathered a strong line of indirect evidence during the past several decades, which have indicated that black holes are indeed real celestial objects and not just a theoretical construct. An international research team has recently utilised the still-under-development Event Horizon Telescope to probe deep into the core of the nearby active galaxy M87. The results of this new study are further strengthening the case for the existence of a black hole hallmark: the boundary also known as the event horizon, which separates these cosmic monsters from the rest of the Universe.
Of all the objects in the Universe, black holes can be considered the most fascinating and mystifying. Enchanting scientists and science-fiction writers alike for decades, they comprise the ultimate cosmic paradox: objects that have an infinite density occupying a zero volume, whose gravity is so immense that nothing, not even light, can escape and where all known laws of physics break down. Even though the possibility for the existence of these cosmic objects had been postulated for more than 200 years, our current understanding of black holes comes from Albert Einstein’s theories of special and general relativity. Einstein showed that, contrary to previous beliefs, the Universe’s geometry consists of a flat, four-dimensional space-time continuum. In this regard gravity, instead of being a separate force, is actually a property of space-time itself. Every object in the Universe that has energy or mass—like stars, planets, and galaxies—curves space-time in much the same way that an iron ball curves an outstretched, flat bed sheet. The greater the mass of an object, the greater its curvature is upon space-time, which in essence defines the object’s gravitational field. Black holes are objects whose entire mass is concentrated in a single point in space, called the singularity. Mathematically this means that their mass has become infinite, making their curvature on space-time infinite as well. Enveloping this singularity is the event horizon, also known as the Schwarzschild radius, which defines the boundary in space where the escape velocity from the black hole equals the speed of light. Just outside this boundary, any infalling object still has the chance to escape the black hole’s immense gravitational pull if its velocity is fast enough, but when something crosses the event horizon it is doomed to a one-way trip toward oblivion.
Black holes fall into two main categories: stellar-mass ones, which are created after the death of massive stars in supernova explosions, and supermassive black holes. The latter have masses hundreds of thousands to billions of times more than that of the Sun, and are thought to be primordial ones, linked with the creation of early galaxies in the Universe. It is now established, from a long line of observational evidence that have been accumulated since the 1970s, that most galaxies in the Universe harbor supermassive black holes at their centers. The study of the central regions of our Milky Way galaxy, which are located approximately 26,000 light-years away at the direction of the Sagittarius constellation, has shown that there lies just such a supermassive black hole, called Sagittarius A*, or Sgr A* for short, with a calculated mass of 4 million times that of the Sun. Yet, even though that may seem as gigantic, the Milky Way’s black hole in reality is small and timid compared to the ones that are thought to lie at the centers of other galaxies like the neighboring Andromeda galaxy, or M31, and the nearby elliptical galaxy M87, as well as those that are found at the centers of quasars. The latter are the centers of active galaxies which are located billions of light-years away, up to the very edge of the observable Universe, and are powered by the constant violent accretion of material around monstrous supermassive black holes that are dozens of billions of times more massive than the Sun.
Even though black holes themselves cannot be observed directly, their immediate surroundings are fascinating natural laboratories for investigating the underlying physics of these mysterious objects. The immense gravity of supermassive black holes can attract immense amounts of matter which creates an accretion disk around the event horizon prior to falling inward, while being subjected to very extreme temperatures and pressures in the process. As a result, the accretion disk can glow very brightly throughout the entire electromagnetic spectrum, as it releases immense amounts of energy. One phenomenon that is not very well understood is the creation of polar jets of material that is being emitted along the axis of rotation of these accretion disks which has been observed in many galactic centers, like that of the nearby galaxy M87. These jets are streams of matter that are ejected from the black hole, often at relativistic velocities that reach the speed of light. In order for astronomers to study these high energy processes, they need to be able to directly observe the immediate surroundings of supermassive black holes at the centers of galaxies with angular resolutions at the scale of the Schwarzschild radius—the event horizon itself.
Until recently, such an accomplishment was considered impossible to be achieved. Yet a series of important technical advances that have taken place within the last decade have brought such a prospect almost within reach, namely with the use of a technique called Very Long Baseline Interferometry, or VLBI for short. By linking together many separate smaller radio telescopes, VLBI allows astronomers to create one bigger virtual radio telescope with a size as big as the greatest distance between the individual radio telescopes. The net result is that such a VLBI array provides astronomers with angular resolutions that would have been impossible to achieve with a single telescope. The application of VLBI has allowed astronomers to see right through the dust and gas that surrounds the center of the Milky Way galaxy, as well as those of nearby galaxies, helping them to put strident constraints on the mass and size of the supermassive black holes that lie there.
As described in a previous AmericaSpace article, astronomers have been developing an ambitious international project in recent years, in order to advance Very Long Baseline Interferometry to the next level, with the implicit goal of directly imaging the event horizon of Sgr A* at the center of the Milky Way. Named the Event Horizon Telescope, or EHT for short, the project aims to link together some of the most advanced radio telescopes around the globe in order to achieve an angular resolution similar to that of a single giant telescope the size of the entire Earth. Despite being still in development, EHT has already showcased its potential by providing some important early observations, including the imaging of the vicinity of Sgr A* with an angular resolution of 4 Schwarzschild radii—an impressive result considering that the EHT has been mainly operating for years with just three stations that are located in Hawaii and the continental U.S. These observations have been the most detailed yet of Sgr A*, while providing further evidence that the latter is indeed a supermassive black hole.
Now, a research team led by Dr. Avery Broderick, an Assistant Professor at the University of Waterloo’s Department of Physics & Astronomy in Canada, have used the EHT’s unique capabilities to probe deep into the center of M87, a nearby elliptical galaxy which is located approximately 53 million light-years away. Despite its great distance, the supermassive black hole in M87 is much bigger than that of the Milky Way, sporting a mass 6 billion times that of the Sun, which gives it a much larger apparent size in the sky that makes it possible to be observed with the Event Horizon Telescope. The goal of the new study by Broderick’s team was to examine if this very compact object at the heart of M87 truly exhibited the hallmarks of black holes as predicted by theoretical models and wasn’t of a different nature instead. More specifically, if the central object at the core of M87 were found to exhibit a luminosity above a certain level, then that would indicate that it has a solid stellar-type surface where all the infalling material from the surrounding accretion disk falls upon, which in turn would result in the release of large amounts of energy at a predicted manner. If in the other case M87 harbors a black hole, it should exhibit a much smaller luminosity due to the fact that, according to theory, any infalling material would get redder and dimmer as it approached the event horizon before vanishing entirely by the moment it would cross it. “Accretion onto compact objects with a surface, e.g., white dwarfs, neutrons stars, results in the formation of a boundary layer in which any remaining kinetic energy contained within the accretion flow is thermalized and radiated,” writes Broderick’s team in their study which was published online on arxiv earlier last month. “In contrast, gas accreting onto a black hole is free to advert any excess energy across the horizon without further observational consequence. If the mass accretion rate can be independently estimated, this difference provides an observational means to distinguish between the presence of a surface, or more accurately a ‘photosphere’, and a horizon.”
By utilising the EHT and based on previous extensive studies of M87, the researchers set out to calculate the total mass of the accretion disk at the center of the galaxy, as well as the energy output of the relativistic jet of material that is ejected from the latter. Their observations showed that the luminosity of the compact object at the center of M87 was indeed several orders of magnitude lower than what would be expected if the galaxy didn’t harbor a black hole. “Despite the [current] astrophysical uncertainty in the relationship between the jet power and mass accretion rate, we have shown in this paper that, within the context of current jet launching paradigms, we can rule out the existence of an observable photosphere in M87* within which the kinetic energy of the accreting gas is deposited,” writes Broderick’s team. “The implication is that the kinetic energy of the gas is advected past an event horizon, beyond which it is no longer visible to distant observers. In other words, M87* must have an event horizon.” Nevertheless, the researchers are quick to point out that the current observing resolution of the EHT still isn’t adequate enough to pinpoint the exact mass of the accretion disk around M87, resulting in several intrinsic uncertainties in the team’s estimated values. However, the lack of detection of any stellar-type photosphere at the inner edge of the accretion disk constitutes a strong indirect evidence that M87 indeed harbors a supermassive black hole, as predicted.
More definitive results are bound to come in the next few years, as the Event Horizon Telescope is linked up with more stations around the planet. “Our argument will be strengthened in the near future as key components to the current jet paradigm are critically tested by mm-VLBI observations of M87,” concludes Broderick’s team in its study. This advancement of the EHT to full strength is already well underway, with the project having met important milestones earlier this year. After years of extensive development and testing, the powerful 10-m South Pole Telescope, which is located at the Amundsen–Scott South Pole Station in Antarctica, was finally linked up with the EHT in January, thus greatly expanding the array’s coverage and resolution. “Now that we’ve done VLBI with the South Pole Telescope, the Event Horizon Telescope really does span the whole Earth, from the Submillimeter Telescope on Mount Graham in Arizona, to California, Hawaii, Chile, Mexico, Spain and the South Pole,” says Dr. Dan Marrone, an Assistant Professor at the University of Arizona’s Department of Astronomy who led the effort. “The baselines to the South Pole Telescope give us two to three times more resolution than our past arrays, which is absolutely crucial to the goals of the EHT. To verify the existence of an event horizon, the ‘edge’ of a black hole, and more generally to test Einstein’s theory of general relativity, we need a very detailed picture of a black hole. With the full EHT, we should be able to do this.”
Despite having being studied intensively for decades, black holes have remained as elusive and mysterious as when they were first proposed a century ago. The Event Horizon Telescope project promises to change all that during the next few years, by finally shedding light to these fascinating dark cosmic objects, while allowing scientists to make the ultimate test to Einstein’s long-held theory of general relativity and even uncover some unknown new physics in the process.
Video Credit: Harvard-Smithsonian Center for Astrophysics