Black holes are one of the most fascinating existences in the universe. Whatever the nature of black holes, these exotic cosmic objects will continue to attract the attention of scientists and laypeople alike.
But where do black holes come from? How do they form, and what gives them such destructive energy? Before answering these questions, we must first answer a more fundamental question: What is a black hole? “Essentially, this is an object or a point in the universe whose gravity is so strong that nothing can escape it,” says astrophysicist Neta Bahcall of Princeton University. Even light will be absorbed by the black hole, which is why black holes cannot be seen.
The emergence of these strange celestial bodies is like a phoenix reborn from the ashes of a dead star. When a massive star approaches the end of its life, its interior is nearly depleted of the hydrogen, it needs to fuse to form helium. These stars then began to burn helium, fusing the remaining atoms to form heavier elements until the formation of iron. At this point, nuclear fusion is no longer sufficient to provide enough energy to support the outermost layers of the star. The outermost layers of the star begin to collapse inward, and then explode in an extremely bright supernova.
However, there are still a small number of stars left at this time. Albert Einstein’s theory of general relativity predicts that if these remnants have a mass about three times that of the sun, the gravitational pull of the remnant star will overwhelm everything else, and the matter that makes up the star will collapse into an infinitesimally small and infinitely dense point. In fact, the known laws of physics cannot fully explain these infinities. “They’re going to break apart at some point, and we don’t really know what’s going on.”
If the stellar remnant was left alone, the black hole would usually stay put and have no effect. But if there is gas and dust around, then these materials will be sucked into the black hole like the water flow in the sinkhole. In the process, the gas and dust heat up, producing bright light, and the black holes take their mass for themselves and grow bigger.
If two black holes meet, their strong gravitational forces will attract each other, causing them to orbit and move closer and closer. Their combined mass would shake the web of space-time around them, producing gravitational waves. In 2015, astronomers discovered the existence of such gravitational waves with the Laser Interferometer Gravitational-Wave Observatory (LIGO).
”That was the first time we actually ‘saw’ black holes and confirmed their existence,” Bascall said, adding that the results also further confirmed the equations predicted by Einstein.
Scientists have found indirect evidence of black holes before, pointing to the fact that the stars at the center of the Milky Way orbit a massive, invisible object. The formation of such supermassive black holes — billions of times the mass of the sun — is a very challenging problem.
The researchers believe that these supermassive black holes were originally much smaller than they are now, and they may have been only modest in size when they formed in the early universe. Over time on a cosmic scale, these black holes have continuously absorbed gas and dust, collided with and merged with other black holes, and finally formed the scale of a giant. Still, Bascall said, many details of the theory remain murky.
Astronomers have observed objects known as quasars, which are extremely bright active galactic nuclei that may be powered by matter-eating supermassive black holes. Quasars date back to the first billion years after the Big Bang. Why did such massive objects form in such a short time when the universe was just forming? Scientists are still very confused about this. “It certainly highlights and adds to the complexity of the issue,” Bascall said.