Black holes are astronomical objects with gravitational pulls so powerful that nothing can escape them — not even light. “The surface” of a black hole, called the event horizon, represents the boundary where the velocity required for escape exceeds the speed of light, which is the speed limit of the cosmos. It is impossible for matter and radiation to escape.
There have been extensive observations of two main types of black holes. Our Milky Way galaxy is dotted with stellar-mass black holes, while supermassive monsters weighing 100,000 to billions of solar masses are found at the centers of most of the large galaxies, including our own.
Existence of black holes
Astronomers have long suspected the existence of intermediate-mass black holes, which have a mass between 100 and 10,000 solar masses. There have been a handful of candidates identified with indirect evidence, but the most convincing instance was on May 21, 2019, when the National Science Foundation’s LIGO, located in Livingston, Louisiana, and Hanford, Washington, discovered gravitational waves from two stellar-mass black holes merging. GW190521, also referred to as GW1905, produced a black hole weighing 142 Suns.
If a star with a mass greater than 20 solar masses runs out of fuel in its core and collapses under its own weight, it forms a stellar-mass black hole. Supernova explosions are triggered by collapses that destroy stars’ outer layers. There is no known force that can stop the collapse of a crushed core if it contains more than three times the mass of the Sun. We know supermassive black holes exist from the very beginning of the life of a galaxy, but we don’t know how they got there.
Black holes grow by consuming matter that falls into them, including gas stripped from nearby stars and even other black holes.
Merging black holes
Until now, astronomers have only been able to see the merger of black holes by observing a subtle ripple in spacetime caused by their emission of gravitational waves. A boom, flash, supernova, or any other light emanating from these mergers were not visible.
Black holes merging were relatively small affairs, their masses not exceeding a few dozen times the mass of the sun. Supermassive black holes, on the other hand, are more likely to form when two black holes merge. We would gain a whole new perspective on extreme gravity if we were able to capture both gravitational and electromagnetic waves from the same event.
According to a new paper appearing on the preprint journal arXiv recently, the easiest way to spot merging giant black holes is to identify the bright accretion disks surrounding each of them (known as active galactic nuclei, or AGN). The radio galaxy 0402+379 may already be a candidate for joining us soon. It takes a lot of time to monitor birds closely, hours of detailed observation, and a few breaks to find those pairs.
Variability in light output from an AGN is another method of estimating variability. The total light output will change in an almost-regular pattern as the black holes orbit and grow closer to each other. Blazar OJ 287, which brightens about every 12 years, is one candidate with this approach.
The Doppler shift of light generated by merging black holes might also enable astronomers to detect the merging black holes even if they are unable to recognize the individual black holes. Identification of exoplanets around distant stars uses the same technique.
Researchers in the paper point out that “gravitational wave astronomy” is only just getting started, and we have more work to do before we expect to see a massive merger between black holes. If we discovered a kilonova, it would be like finding a goldmine – an opportunity to study gravity in one of the most extreme environments within the universe.
What does a black hole look like?
Black holes have event horizons, which are boundaries around their mouths beyond which light cannot escape. Particles cannot pass through event horizons once they pass through them. When particles cross an event horizon, gravity remains constant.
One point in space-time where the mass of the black hole is concentrated is called a singularity, or the inner region of a black hole where its mass lies.
One of the most important characteristics of black holes is their singularity, which is where the mass of the black hole is concentrated in space-time.
Black holes cannot be seen by scientists the way stars can be seen. As gas and dust are drawn into the dense creatures, astronomers must instead detect the radiation black holes emit. But supermassive black holes in the center of galaxies can be obscured by thick dust and gas surrounding them, making their telltale emission impossible to detect.
As matter is pulled toward a black hole, it can ricochet off the event horizon and be hurled outward instead of being drawn in. The material is accelerated to near-relativistic speeds, creating bright jets of material. These powerful jets are visible from great distances, even though the black hole itself is invisible.
An image of M87’s black hole (released in 2019) was obtained by the Event Horizon Telescope after two years of research. The collaboration of telescopes, which extends over many observatories worldwide, produces an overwhelming amount of data, which cannot be transmitted via the internet.
Researchers expect to image other black holes over time and compile a repository of what the objects are like.