Black Holes: How Astronomy Reveals the Invisible
Black Holes: How Astronomy Reveals the Invisible
Recent breakthroughs in astronomy promise to reveal the darkest objects in existence
At the heart of Virgo A, a vast galaxy some fifty million light years from Earth, lies a black hole of awesome power. Like many astronomical objects, its sheer size is hard to grasp. In terms of mass, it weighs over six billion Suns; in terms of radius, it stretches four times the distance from Earth to Neptune. Around the black hole is a literal ring of fire; a cloud of dust and gas formed from shredded stars and doomed planets. From this the black hole is feeding, consuming mass equal to ninety Earths every single day.
We know all this because we’ve seen it. Over four days in 2017, radio telescopes across the world pointed their antennas towards Virgo A and captured the most detailed radiowave observation of that galaxy ever made. At the heart of it appeared the black hole, a dark shadow silhouetted against a glowing ring of light.
The resulting photograph, published in 2019, was the first direct image of a black hole ever made. It represented, for many physicists, a final confirmation that black holes - an idea so strange and disturbing that Einstein once denied they could exist - are indeed real. Since then we’ve gone on to image only one other black hole, that at the heart of our own galaxy. No other known black holes, unfortunately, appear large enough from Earth for our telescopes to resolve them.
Still, there are other black holes out there. Astrophysicists are reasonably confident that almost every galaxy hosts one or more at their centres. Others, fortunately far smaller, are scattered throughout galaxies, and, probably, throughout the universe. Some even speculate that untold numbers of tiny black holes formed in the tumult of the Big Bang could be behind the mystery of dark matter.
Even if they aren’t, black holes certainly do hold deep secrets about the evolution of the universe. The details of their birth, in the early years of the cosmos, may one day give us answers about dark matter, dark energy and how, exactly, the modern universe came into being.
Yet to study them we first need to find them, and doing so is easier said than done. Black holes, by their very nature, are invisible objects. They emit no light of their own, no photons that telescopes can pick out amongst the inky darkness of space. Even those we have seen, those in Virgo A and in the heart of our galaxy, appear only as shadows; an inky blackness surrounded by a ring of fire.
The Havoc They Wreak
The easiest way to search for black holes, really, is to look for something that isn’t where it should be. Long before we imaged our galactic black hole, for instance, its presence had been betrayed by stars darting around it at tremendous speed, and by bursts of energy coming from shredded gas clouds. Together those hints pointed to a massive, invisible object at the centre of our galaxy. A black hole, if you like.
In other galaxies the signs are even clearer. Take, for example, the black hole in Centaurus A. There, following a collision with another galaxy, the central black hole has been gorging itself on dust and gas. As all that matter flows around the black hole, it forms a glowing ring moving at over a thousand kilometres per second.
Amidst all that chaos a powerful magnetic field is being generated; accelerating matter towards the two poles of the black hole. This is somehow - physics is not clear how, exactly - sending two powerful jets of charge particles shooting away from the black hole. So immense is the power of the black hole that these jets extend for a million light years or more, creating structures of astonishing beauty.
Not all black holes, of course, display such awesome power. In most nearby galaxies the central black holes are calmer, though not always smaller. Yet they still betray their presence by their impact on matter around them, by the rings of debris they attract and by the powerful radio waves those rings create.
This is what we saw when we photographed Virgo A. Strictly speaking, of course, we didn’t actually photograph the black hole - there is no light from a black hole that can be imaged. Instead we see glowing debris swirling around the black hole; clouds of dust and gas accelerating under a powerful gravitational pull. The black hole itself appears as a shadow, an absence of light, in the centre.
Even that interpretation is slightly deceptive. Black holes are so gravitationally intense that they warp space itself, bending and curving the light we see in hard to grasp ways. The shadow, as a result, is roughly two and half times bigger than the black hole itself, lurking somewhere in its midst. The ring of light, too, is somewhat of an illusion, with the effects of Einstein’s relativity making the lower half look brighter than the rest.
Lenses of Magnificent Power
That warping of space gives a hint of another way to spot black holes. As every high school student should know, light rays travel in straight lines. Crucially, however, they travel in straight lines through space. If that space is distorted, thanks to the nearby presence of a heavy object, then the light beam will appear to curve slightly. The heavier and denser the object, the greater the distortion, and the more the light beam will curve.
In an extreme, the light beam can enter a region of space so distorted that its curve becomes a circle, and then an ever-tightening spiral. This, then, is the edge of a black hole - a region of space so twisted and distorted by the density of matter that light, and anything else, that falls into it can never escape.
Einstein’s equations predict odd things happen on the boundaries of such places, telling us that time itself can cease to tick onwards. What happens beyond those boundaries, in regions where immense masses are crushed into infinitely small volumes, is so far unexplained by our laws of physics.
Still, on our side of such boundaries, the laws of physics hold. Light rays passing close to a black hole will curve. Should they avoid falling into the black hole, they will continue onwards, with some deflection in their path. In some ways black holes and other massive objects can act rather like lenses, amplifying and distorting light coming from further away objects.
Black holes thus allow us to see things that are otherwise too small and far away to be seen; forming the cosmic version of a magnifying glass. Indeed, many of the oldest known stars and galaxies have been found using this quirk of physics, though often astronomers use far more massive galaxy clusters as lenses, rather than black holes alone.
Of course, we don’t always know where the black holes are. Astronomers therefore run the process in reverse. They look for signs that lensing is taking place, and then search for the object causing it. One can, for example, look for stars that suddenly appear to brighten in the sky, before fading back to normal. This, when it happens, is often the sign of a massive object passing between the star and Earth.
Last year, researchers published a paper outlining the discovery of a black hole within our galaxy using just this approach. They saw a star towards the galactic centre suddenly and unexpectedly brighten. Other observations showed that the position of the star had also seemed to shift slightly, a sure sign of a gravitational lens.
The effect was tiny, but it was enough for the astronomers to conclude they had found a black hole. It is not a big one, measuring just seven times the mass of our Sun. Neither is it that close, lying some five thousand light years from Earth. But it was a clear discovery of a black hole, invisible by other means, with the help of gravitational lensing.
In the next few years astronomers expect to discover hundreds more black holes scattered across the galaxy. That will come thanks to the Gaia space telescope, which has been recording the positions and brightnesses of billions of stars in the Milky Way. Its data, which astronomers are eagerly awaiting, will next be released in 2025.
Ripples in the Cosmos
Black holes don’t just warp the space around them. As they move they also create ripples in spacetime; something cosmologists call gravitational waves. Such waves are constantly washing over the Earth, triggered by the movements of black holes, neutron stars and even the Big Bang itself.
Until recently these waves passed by completely unnoticed. Einstein, of course, predicted them in the early twentieth century, but his calculations suggested they’d be far too small to ever find. Later work, in the seventies and eighties, gave new estimates of their size and, for the first time, hinted at a way to find them.
Yet it was not until 2015 that we built a detector sensitive enough to actually spot one. In September of that year a slight fluctuation in a laser beam was picked up at LIGO, an American institution dedicated to hunting gravitational waves. It was, they found, the trace of a wave sweeping over Earth; an event triggered by two far away black holes spiralling around one another.
Since then LIGO and other observatories around the world have picked up dozens more such waves. Not all come from black holes - many are thought to come from colliding neutron stars - but those that do are conclusive proof that such giants lurk in the heavens above. More importantly, the waves give us another astronomical “sense” - an ability to spot black holes without relying on light.
As the sensitivity of detectors improves - LIGO has just completed a set of upgrades - researchers hope to pick up ever more signs of distant black holes. The spread of detectors around the world - Europe and Japan now host some, as will India in the years to come - also lets them triangulate the sources. That should allow researchers to pinpoint the positions of distant black holes.
The work of observatories like LIGO, coupled with that of Gaia and ever more powerful radio telescopes, means the next decade should reshape our view of black holes. For the first time we’ll be able to map out those that lie hidden around us, invisible against the darkness of space. That, researchers hope, will also point the way to a deeper understanding of our cosmos.