Black holes are one of the most enigmatic cosmic phenomena in our fascinating universe. Most are formed from the remnants of massive stars that have collapsed under their own gravity, but a few black hole types even defy that explanation. The existence of black holes was first implied in the early 20th century by Albert Einstein’s theory of general relativity, but it wasn’t until several decades later that the term ‘black hole’ was coined by physicist John Archibald Wheeler. The first confirmed discovery of a black hole occurred in 1971 when intense x-ray emissions from the binary star system Cygnus X-1, combined with its size and mass measurements, could only be explained by the existence of a black hole. Since then, astronomers have discovered different types of black holes including stellar mass black holes, supermassive black holes and some even theorise the existence of primordial black holes. On April 10, 2019, the first image of a black hole was announced, captured by the Event Horizon Telescope collaboration. The image showed the supermassive black hole at the centre of galaxy M87 and comprised of glowing matter surrounding the shadow of the black hole’s event horizon. The study of black holes continues to thwart our understanding of gravity and the fundamental nature of the universe, as we discover stranger, unexpected and seemingly unexplainable occurrences.
Figure 1: A radio image of the black hole at the centre of galaxy M87. Image Credit: Event Horizon Telescope collaboration et al.
The hypothetical primordial black holes mentioned above are one of the most interesting theories challenging our understanding of the formation of our universe. Some astrophysicists even believe the dark matter in our own galaxy could be made up of clusters of tiny primordial black holes. Thought to have formed less than one second after the Big Bang, when the universe was still so compact that pockets of subatomic matter would undergo gravitational collapse, forming black holes as small as 10-8 kg. Any black holes as small as this would be extinct today due to Hawking radiation, causing the black hole to evaporate in a time much shorter than the age of the universe. However primordial black holes could also range to thousands of solar masses, and in the compact early universe, collisions would have been common, causing primordial black holes to merge. Many scientists believe these hypothetical primordial black holes to be the seeds of the, so far unexplained, existence of intermediate-mass and supermassive black holes. Supermassive black holes usually have a mass above 100,00 solar masses, with some having a mass of several billion. Observations show that almost every large galaxy has a supermassive black hole at its centre. However classical models of black hole formation (caused by the remnants of a star) cannot explain the size and mass of supermassive black holes. Although most astrophysicists agree black holes can ‘grow’ from mergers and accretion, many different theories have been suggested as to how supermassive black holes may have formed. Some suggest the first ever stars formed were supermassive, some models show direct collapse of massive gas clouds can skip the star phase and collapse directly into a black hole, others corroborate the above-mentioned theory that primordial black holes formed seconds after the Big Bang would have had sufficient time to accrete to the observed mass today. Either way, it’s still an intriguing and active area of research.
Figure 2: computer-simulated image of a supermassive black hole at the centre of a galaxy. Image Credit: NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (STScI).
Intermediate-mass black holes are even more puzzling. Too large to be formed from the collapse of a star but lacking the high-density and velocities observed at the centre of galaxies which seemingly lead to the formation of supermassive black holes. Three main scenarios for the formation of intermediate-mass black holes have been postulated. The first suggests intermediate-mass black holes are formed by the merging and accretion of standard stellar-mass black holes. The second is a runaway collision theory which suggests stars in dense stellar clusters continuously collide resulting in the product collapsing to form an intermediate-mass black hole. Lastly, theory suggests the collapse of one supermassive star could form an intermediate-mass black hole. However, such high mass stars are yet to be observed.
Binary black holes are another source of mystery. Their existence is intuitive, since we have observed many binary systems of massive stars, it follows that at the end of their lives the stars will form a system of binary black holes orbiting each other. Binary supermassive black holes also exist as a product of two galaxies merging. The counterintuitive part is what’s known as the final parsec problem. When two galaxies collide, the supermassive black holes are extremely unlikely to hit head on and will instead shoot past each other. Eventually dynamical friction will cause the two supermassive black holes to settle into a bound orbit a few parsecs from each other. However, during this process, as they become closer together, material is either ejected from their surroundings or accreted until the environment around them is no longer dense enough for momentum to be lost to fusion. Subsequently, there is not enough matter left to cause a merger within the known age of the universe. Despite this problem, supermassive black holes appear to have merged in the history of the universe causing a search for an explanation which would bring the two black holes together in an astronomically reasonable time. One proposed explanation suggests a second galactic collision, with a third galaxy, leading to a third supermassive black hole in close proximity causing chaotic orbits and additional mechanisms for energy loss.
Figure 3: Artist's illustration of supermassive black hole pair. Image credit: NASA/CXC/A. Hobart.
Size variations among black holes is another fascinating phenomenon. The largest black hole ever found lies at the centre of the quasar TON 618, measuring in at over 40 billion solar masses and is known as an ultra massive black hole. In contrast, the smallest black hole ever discovered only weighs in at around 5-10 solar masses. Predictions surrounding black hole formation suggest a star could collapse to form a black hole as small as 3 solar masses. According to Einstein’s equations, a black hole containing this mass would only be 15 miles across. This one-minute YouTube clip from NASA gives a good comparison of black hole sizes using animation: (284) NASA Animation Sizes Up the Biggest Black Holes - YouTube.
Although we have achieved significant milestones in the research, discovery and even imaging of black holes. As per the norm with most studies of the universe, each new discovery raises more questions. The study of black holes continues to unveil the extraordinary and mind-bending features that makes them some of the most captivating objects in the universe. Exploring the strange and awe-inspiring natures of black holes opens doors to new frontiers and stretches the limits of our imagination, forever expanding the understanding of the cosmos we inhabit.
Written by Aliss Rodricks, Engagement and Outreach Coordinator Intern