Masters of the universe
At the heart of galaxies lie supermassive black holes, which, according to a new theory, might hold the key to life itself. Marcus Chown and Caleb Scharf look beyond the event horizon
Twenty-seven thousand light years away in the dark heart of our Milky Way lies a supermassive black hole 4.3 million times the mass of the Sun. Massive as it is, however, Sagittarius A* is an insignificant tiddler compared with its 30-billion-solar-mass cousins lurking in the cores of some galaxies. “The question is: is it a coincidence that we have arisen in a galaxy with a relatively small, benign central black hole?” asks Caleb Scharf of New York’s Columbia University, in his new book Gravity’s Engines. “Or is it a large part of why we’re here rather than someplace else?”
I called Scharf – an English physicist whose art historian father helped found the Open University – at Columbia University, where he is director of astrobiology, to find out more. His specific interest is in exoplanets – of which more than 800 have now been found around nearby stars – and whether they support life. But a few years ago, while writing a textbook on astrobiology and exoplanets, Scharf discovered, quite by chance, that he liked writing the more speculative stuff. Thinking he might write a popular book, he scouted around for a suitable subject and alighted on supermassive black holes, which he had researched in the past. “Loads of exciting stuff has happened in the past two decades. But the whole field is shrouded in impenetrable jargon like ‘accretion disks’, ‘AGNs’ and ‘BL Lacs’,” says Scharf. “I decided to try and demystify it, and tell people about the things that have been going on.”
Gravity’s Engines is the story of how black holes have moved from the edge to the centre of physics research and are now seen to play a crucial role in the sculpting of the cosmos – and possibly a role in the evolution of life on Earth as well.
A black hole is a region of space-time where gravity is so strong that nothing, not even light, can escape – hence its blackness. Before anyone had detected a black hole they existed in theory: they were a prediction of the general theory of relativity, Einstein’s theory of gravity. Black holes are surrounded by an “event horizon”, an imaginary membrane that marks the point of no-return for in-falling matter and light. If an astronaut were able to hover just outside the event horizon, time for them would slow down so much – as Einstein’s theory shows – that it would in principle be possible for them to look outwards and watch the entire future history of the universe flash past their eyes like a movie in fast-forward.
Inside the horizon, the distortion of time is so great that time and space actually swap places. This is why the “singularity” – the point where in-falling matter is crushed out of the existence at the centre of the hole – is unavoidable. It exists not across space but across time and so can no more be avoided than you can avoid your future.
Bizarrely, Einstein, whose theory predicted black holes, could never bring himself to believe in their existence. This is a common problem in physics – overcoming the sheer disbelief and realising that nature really dances to the tune of the arcane symbols scrawled by theorists across a blackboard. “The mistake of physicists,” as Nobel-Prize winning physicist Steven Weinberg has observed, “is not in taking their theories too seriously but in not taking them seriously enough.”
The first stellar-mass black hole, Cygnus X-1, was discovered by the “Uhuru” satellite in 1971. But, actually, something that would turn out to be far more important in the black hole story was found eight years earlier.
Quasars, discovered by Dutch-American astronomer Maarten Schmidt in 1963, are the super-bright cores of galaxies, blazing like beacons at the edge of the universe. Actually, because their light has taken most of the age of the universe to reach us, they are also blazing at the beginning of time. Typically, a quasar pumps out the energy equivalent of 100 normal galaxies like the Milky Way but from a region smaller even than our Solar System. Nuclear energy – the power source of the stars – is woefully inadequate. “The only process that can explain the prodigious energy output of quasars is matter heated to incandescence as it swirls down into a black hole,” says Scharf. “But not a mere stellar-mass black hole – one with the mass of many billion suns.”
For a long time after Schmidt’s discovery, astronomers thought that supermassive black holes were a cosmic anomaly, powering only “active galaxies” – the badly behaved 1 per cent of galaxies of which quasars were the most extreme examples. “But, over the past couple of decades, it has become clear that pretty much every galaxy, including our own Milky Way, has a supermassive black hole at its heart,” says Scharf. “It is just that most are quiescent, having gorged on and exhausted their feedstuff of interstellar gas and ripped-apart stars.”
It should be pointed out that the origin of supermassive black holes – unlike their stellar-mass black cousins, which are believed to form from the catastrophic shrinkage of stars – is a total mystery. Perhaps they form when stellar-mass black holes collide and coalesce in the crowded heart of a galaxy. Or perhaps they form directly from the shrinkage of a giant gas cloud. “The trouble is they have to grow extremely big extremely quickly,” says Scharf. “We see supermassive black holes which have already reached billions of solar masses by the time the universe is about 5 per cent of its current age – a mere 500 million years after the Big Bang.”
A supermassive black hole undoubtedly appears impressive on a human scale. But, actually, it is extremely small compared with its parent galaxy, and it has a very small mass compared with the mass locked up in a galaxy’s stars. The surprise, however, is that, everywhere, supermassive black holes have left their indelible imprint. For instance, the mass of the stars in the core of the galaxy is commonly about 1,000 times the mass of the black hole. Clearly, there is an intimate connection between a supermassive black hole and its parent galaxy. “Think how amazing this is,” says Scharf. “It’s as if something as small as a bacterium has orchestrated the building of something as big as New York City.”
The means by which tiny supermassive black holes project their power over vast reaches of space are “jets”. Propelled by twisted magnetic field in the gas swirling down to oblivion, the jets – channels of super-fast matter – stab outwards from the poles of the spinning black hole. They punch their way through the galaxy’s stars and out into intergalactic space, where they puff up titanic balloons of hot gas – some of the largest structures in the known universe.
The balloons of gas were, in fact, the first cryptic hint science got of the existence of supermassive black holes. In the 1950s, radio astronomers, using equipment adapted from war-time radar, discovered that the radio emission observed from some galaxies came not from the central knot of stars, as expected, but, mysteriously, from giant, radio emitting “lobes” on either side of the galaxy.
The thread-thin jets that are feeding the lobes were imaged for the first time, by the Very Large Array telescope in New Mexico, only in the early 1980s. They mock our puny attempts at accelerating matter. Whereas the multi-billion-euro Large Hadron Collider near Geneva can whip a nanogram or so of matter to within a whisker of the speed of light, nature can accelerate many times the mass of the Sun each year along cosmic jets.
According to Scharf, the jets help to control the structure of their parent galaxies because, in the inner regions where the jets are still fast and powerful, they drive out all the gaseous raw material of stars, snuffing out star formation. “In the outer regions, however, where they are slower, they impact gas clouds, the concussion may trigger them to collapse under gravity to give birth to new stars,” he says.
But supermassive black holes do not just sculpt galaxies by starting and stopping star formation. According to Scharf, they may determine the very character of the stars that form. Galaxies with the biggest supermassive black holes – so-called giant ellipticals – have a much greater proportion of cool, red, long-lived stars, and there is evidence, says Scharf, that the black hole may be responsible. Such stars spawn planets with few of the heavy elements, such as carbon and magnesium and iron, necessary for life. There is evidence that the kind of chemistry needed for biology might not happen on their surfaces. “Life may have arisen on Earth only because the Milky Way has a relatively small black hole in its core,” he says. “If this had not been the case, the Sun and Earth would never have been born.”
When we look out across the universe, we see a whole zoo of different galaxies. If Scharf is right, the ones with small supermassive black holes may be filled with planets teeming with life. But others – the ones with biggest supermassive black holes - may be dead, filled with countless sterile planets.
But every cloud has a silver lining.
One day, when the Sun swells to a red giant, the Earth will become uninhabitable. Our descendants, if there are any around, will have no choice but to abandon Earth and find another star. But exactly the same fate will befall all other stars in our galaxy. Eventually, our descendants may look outward across the gulf of intergalactic space to the galaxies of the biggest black holes. There, conveniently, will be trillions upon trillions of planets – new homes, empty and waiting.
Gravity’s Engines: The Other Side of Black Holes by Caleb Scharf is published by Allen Lane.