Einstein’s Theory of General Relativity was a new way of describing gravity, and it had some unexpected consequences.
One of these regarded the description of an object collapsing under its own gravitational pull. If the object is sufficiently dense, no force is strong enough to counteract the attraction of gravitation, and it collapses all the way down to a point. According to Einstein’s theory, the endpoint of this collapse is a singularity, which can be described as a ‘tear’ in spacetime. A singularity is often hidden inside a black hole whose size is set as the region where the gravitational pull is so strong that even light cannot escape from it.
Einstein thought this oddity of his theory was unappealing and potentially a mistake. He abhorred singularities because they seemed to imply that his theory was inadequate because it did not apply to the singularity itself. He argued that black holes and singularities do not occur in nature, but only in some artificially simple cases studied by physicists. Decades later, physicists including Stephen Hawking and Roger Penrose developed better mathematical tools for studying Einstein’s theory, and they discovered that singularities are difficult to avoid. In fact, our best understanding is that the entire universe began about 14 billion years ago as a singularity that expanded explosively in what is called the Big Bang.
Even more surprisingly, astronomers have discovered that black holes are abundant in nature. They come in two general classes.
The largest are called ‘supermassive,’ and they have masses of a million to a billion times that of our Sun. Supermassive black holes are always found in the centres of galaxies, and our own Milky Way has a supermassive black hole. Though the black hole itself is invisible, we can measure its mass by following the motions of stars close to it. In the same way that the orbits of planets in our solar system allow us to measure the mass of the Sun, the tracks of these stars provide accurate measurements of the mass at the centre of the Milky Way.
Deep images of the Milky Way’s centre show us what looks like empty space, and yet we know there’s something massive and very compact there because of the motions of the stars. A black hole is found in every large, nearby galaxy, and so growing a big black hole seems to be common practice among big galaxies.
When they are growing, supermassive black holes are called quasars. Streams of gas from the galaxy get pulled into orbits around the black hole. As the material swirls around before plunging into the black hole, it gets quite hot and glows. The black hole at the centre of this glowing disk is invisible, but the disk can outshine the trillions of stars in the galaxy a thousand times over. Quasars are so bright, they can be seen to huge distances. As we look far away in the universe, we are looking back in time because it takes so long for the light to reach our telescopes.
So, we see quasars when the universe was a fraction of its current age – only a few billion years old. We know there is a black hole powering each quasar, because of their very small sizes and the high speeds of the gas swirling around it. Normal matter could not be compressed into such a small space and create so much gravity.
A much smaller black hole can be formed at the end of the lifetime of a massive star (many times more massive than our Sun) after it explodes as a supernova. Such a black hole traveling through space on its own would be invisible, but massive stars are often formed in pairs.
As the stars age, the more massive one will burn through its fuel first and become a supernova. After this first star explodes, it often keeps its partner. In such systems, the orbit of the second star can reveal the mass of its black hole companion. In some cases, the second star is quite close to the black hole, and the stronger gravitational pull on the near side of the star can siphon off the outer layers of the star and pull them towards the black hole. As the outer layers fall into the black hole, this material will form a swirling disk of hot gas with such high temperatures that it glows in X-rays.
So, though the black hole itself is invisible, the material feeding can be seen up to millions of light years away in galaxies outside our Milky Way.
Now that we know that black holes are common throughout the universe, we have new questions. One of the current puzzles of black hole science is how to go from the small black holes formed after the explosion of a single massive star to the supermassive black holes seen in the centres of galaxies. The biggest black holes must have started from smaller seed black holes, but how do they grow so fast? Were the seeds formed from the first generation of stars, or from some other process? Are black holes spinning? Einstein’s unappealing singularities are keeping the current generation of black hole researchers busy.
Physics and Astronomy professor Sarah Gallagher focuses her research on active galaxies, black holes and compact groups of galaxies.