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  Though, a possible gain would be to fulfil that ancient scientific dream of Thales by identifying the ultimate building block of nature. And don’t discount the wow factor. It would certainly give new meaning to the idea of being one with the universe.

  What if we’re living inside a black hole?

  If you ever happen to fall into a black hole, you’ll be spaghettified. Yes, this is the actual scientific term for what will happen. The crushing gravity of the black hole will simultaneously stretch and compress you, moulding you into a stream of subatomic particles resembling long, thin noodles. These particles, which once were you, will then plummet down to the centre of the hole, where they’ll be further compacted to a density so extreme that mathematics can’t even quantify it.

  The one silver lining in all of this is that, if you happened to be conscious when you fell into the black hole, the spaghettification would occur so quickly that you’d scarcely feel a thing. All in all, not a bad way to go.

  Given that black holes are such hostile environments, they would seem to be unlikely places for life to exist. After all, what could possibly survive in them? Well, there’s a popular theory that contradicts this logic. It argues that, improbable as it might seem, not only can life exist in such a place, but we’re proof of it, because the universe we’re living in is a giant black hole.

  A black hole is defined as an object whose gravity is so extreme that nothing can escape from it, not even light. The suspicion that such objects might exist dates back several centuries, though for most of that time scientists were unwilling to accept that a phenomenon so bizarre could possibly be real. The first person to predict their existence was the English clergyman John Michell. He submitted a paper to the Royal Society in 1783 in which he speculated that, if a star were 500 times larger than our own sun, the strength of its gravitational field would prevent light from escaping. Despite its vast size, the star would disappear from sight, becoming a black hole (although Michell didn’t use that term).

  The scientific community dismissed this hypothesis as wild conjecture. It didn’t seem plausible that any star could be that large. And, anyway, the prevailing belief was that light wasn’t affected by gravity. So, the odd idea of black holes was filed away and forgotten.

  It wasn’t dusted off and revived until 1915, when Albert Einstein published his general theory of relativity. This convinced scientists that gravity wasn’t a force, but instead represented the curvature of space and time. In which case, light would be affected by gravity, because it would follow the curve of space–time as it travelled.

  General relativity also implied that an object of sufficient mass and density might distort space–time so severely that it would form a well from which nothing, not even light, could climb out. At the centre of this well, a singularity would form – a point at which the strength of the gravitational field became infinite.

  Even though this is what the theory implied, most scientists, including Einstein himself, continued to regard black holes as a crazy idea. The problem was that the laws of physics would break down in a singularity, and physicists instinctively shied away from accepting that this could happen. The idea that matter could be infinitely compressed also seemed wrong, because how can something that’s finite acquire an infinite value? Scientists assumed that, at some point, the subatomic particles making up matter would find a way to resist further compression.

  It wasn’t until the 1960s that the scientific community finally came around to accepting the existence of black holes. In fact, it was only in this decade that the term was coined; science journalist Ann Ewing gets credit for first using it in a 1964 article. The complex mathematics of how space would curve around black holes had come to be better understood, which made physicists feel more comfortable with the whole idea of them. Also, the new technologies of radio and X-ray astronomy were revealing that the universe contained some very strange, high-energy and extremely dense objects, such as quasars and pulsars. By comparison, black holes no longer seemed as improbable.

  Once black holes had been accepted as a plausible phenomenon, it didn’t take long for people to start wondering if we might be living inside one.

  The black-hole universe hypothesis wasn’t the brainchild of a single theorist. There was no one figure who emerged as its champion. Instead, it was a concept that began circulating like a meme within the scientific community during the late 1960s and early 1970s, and then caught the popular imagination.

  The physicist Roger Penrose may have been the first to speculate about the idea in print. He mentioned the possibility in a 1967 essay he submitted for Cambridge University’s Adams Prize. The hypothesis reached a wider audience five years later when the physicists Raj Pathria and Irving Good each authored a brief article about it, which appeared, respectively, in the journals Nature and Physics Today. By the 1980s, the hypothesis was well established as a trendy, if unorthodox, idea, and discussions of it regularly appeared in books and articles.

  The hypothesis itself occurred independently to a number of people because, within the context of astrophysics, it’s actually a somewhat obvious idea to arrive at. There are only two places in nature associated with singularities: the centre of a black hole and the start of the Big Bang that created our universe. So, it’s logical to wonder if the two might be related.

  Once you start comparing the features of our universe with a black hole, other similarities suggest themselves. There’s the fact that a black hole has an event horizon. This is a point of no return, an invisible line around the black hole that, once crossed, forbids exit for anything, even light. Whatever passes over that line is entirely within the clutches of the gravity of the black hole and, from the point of view of an external observer, it vanishes completely, effectively ceasing to exist as part of the universe. The event horizon acts as a barrier. Nothing that is within its circumference can ever travel outside of it.

  Similarly, we’re trapped within a cosmological event horizon defined by the limits of how far we can see out into the universe (about forty-six billion light years in each direction). We can never travel beyond that horizon. We’re trapped inside it just as completely as objects within the event horizon of a black hole are trapped.

  The reason we can’t travel beyond our cosmological horizon is because the universe is expanding, and this causes the horizon to move away from us faster than we can travel to catch up with it. It’s actually receding faster than the speed of light, and, although it’s not bound by light’s speed limit, we are. This means that the laws of physics forbid us from ever travelling beyond that horizon.

  This all may sound contradictory. Why can’t we travel faster than the speed of light, when the cosmological horizon can? It’s because space is expanding, and it’s doing so everywhere, which means that the expansion is cumulative. The more distance lies between two objects, the more units of space there are that are simultaneously expanding, without limit. Add together enough points of space and the expansion rate will eventually surpass the speed of light. No one ever said astrophysics was simple or intuitive!

  Then there’s the issue of the Schwarzschild radius. In 1915, soon after Einstein published his theory of general relativity, the German physicist Karl Schwarzschild used its equations to compute what the strength of the gravitational field would be surrounding any uniform ball of matter. This was an impressive feat, not only because Einstein’s general relativity equations are famously difficult to solve, but also because Schwarzschild was in the German army, dodging bullets on the Russian front, when he did this. Plus, he had contracted an incurable skin disease which was rapidly killing him. He mailed his calculations off to Einstein, and then he died.

  Schwarzschild’s analysis came to a seemingly bizarre conclusion. It suggested that any object would become a black hole if it was squeezed small enough, because the pull of gravity at its surface would increase as its mass grew denser. The strength of an object’s gravity is inversely related to your distance from it –
specifically, to the distance between you and the centre of its mass. So, if you reduce the distance to its centre by compressing the entire mass into a smaller diameter, its gravity will grow correspondingly.

  Let’s look at an example. If mad scientists were able to take the entire mass of the Earth and compress it down to slightly smaller than a golf ball, the pull of gravity on its surface would become inescapable. It would turn into a black hole. Similarly, if these scientists could shrink you down to a tiny speck, smaller than the nucleus of an atom, you’d also become a black hole. The size at which an object becomes a black hole is now known as its Schwarzschild radius, and this radius can be calculated for any object.

  Scientists initially dismissed Schwarzschild’s finding as a theoretical oddity, because they were reluctant to believe that black holes were a genuine phenomenon. But, once they had come around to accepting their reality, it occurred to some to ask what the Schwarzschild radius of the observable universe might be. Which is to say, how small would you need to shrink the observable universe in order to transform it into a black hole?

  The mass of the observable universe can be estimated by observation, and we know its size. (We can see forty-six billion light years in any direction, so it’s ninety-two billion light years across.) When these numbers were run through the equations, the disturbing result emerged that the observable universe lay within its Schwarzschild radius. It was already at its black-hole size!

  This conclusion may sound impossible, because surely an object must be enormously dense to have the gravity of a black hole, and yet, looking at the universe around us, there’s plenty of empty space. But this highlights another peculiarity of Schwarzschild’s calculations. His analysis revealed that the more massive an object is, the less dense it needs to be to become a black hole. For instance, if those mad scientists were able to compress the entire Milky Way galaxy within its Schwarzschild radius, its density would be less than that of water in the ocean. And the mass of the entire observable universe, confined within its Schwarzschild radius, would not be very dense at all. In fact, it would be exactly as dense as we observe it to be.

  So, the argument goes that, if you consider all these factors together – the singularity associated with our universe through the Big Bang, its cosmological event horizon and the fact that our observable universe lies within its Schwarzschild radius – you’re led to the seemingly inescapable conclusion that we must be living inside a black hole.

  Of course, most astrophysicists aren’t about to concede this. For a start, they note that the singularity of the Big Bang isn’t comparable to the singularity of a black hole, because it’s in the wrong place. If you were to fall into a black hole, the singularity would lie unavoidably ahead of you, in your future, but in our universe the singularity lies in our past, when the Big Bang occurred. That’s a big difference. Our universe seems to have emerged from a singularity, but it’s not heading towards one.

  Also, the event horizon of a black hole is a fixed boundary in space, whereas the cosmological horizon is relative to one’s position. A civilization twenty billion light years away sees a distinctly different cosmological horizon than we do.

  And, as for the observable universe being within its Schwarzschild radius, it’s true that it is. This doesn’t necessarily mean we’re in a black hole, though. What it means is that the rate of expansion of our universe has closely matched its escape velocity, and we should be thankful for this. If the universe had expanded slower, it might have collapsed back in on itself, and, if it had expanded faster, it would have been impossible for structures such as galaxies and solar systems to form. Instead, it’s been expanding at just the right rate to allow us to come into existence.

  The theoretical physicist Sean Carroll has pointed out that, if one is really keen on the idea that we’re stuck in some kind of gigantic cosmic hole, a better argument might be made for the idea that we’re living in a white hole, which is the opposite of a black hole. More specifically, it’s a time-reversed black hole out of which matter spews unstoppably rather than falling inwards. But white holes come with their own set of problems. They’re theoretically possible – because the laws of physics work equally well whether one goes forwards or backwards in time – but only in the same way that it would be theoretically possible for a broken egg to spontaneously reform into a whole egg. Physics allows it, but the odds of ever seeing such a thing happen seem close to zero.

  So, perhaps we’re not living in a black hole. The fact alone that our universe isn’t collapsing inwards towards a singularity surely proves this. But, in astrophysics, things are never quite that simple. Fans of the black-hole universe hypothesis insist there are ways around all the contrary arguments.

  One way it could still work is if a singularity does lie inescapably in our future. If, billions of years from now, the expansion of the universe were to halt and then reverse, leading to a ‘big crunch’, this, they say, might satisfy the definition of a black hole.

  Theoretical physicist Nikodem Poplawski has proposed another way. He argues that, inside a black hole, matter might actually expand outwards, rather than contracting inwards. This could be the case if matter reaches a point at which it can be crushed no further, and it then bounces back out explosively, like a coiled-up spring. The event horizon would continue to separate the contents of the black hole from the wider universe, and the expansion would then occur into an entirely new dimension of time and space, exactly resembling the sudden expansion of time and space during the Big Bang. Poplawski goes so far as to propose that all black holes form new universes, and that this is the way our own universe came into existence, as a black hole that formed in some larger universe.

  Of course, this raises the question of how the parent universe might have formed, which would seem to be a mystery. Unless it too was originally a black hole. Perhaps the cosmos consists of a series of black-hole universes, nested within each other like Russian dolls, extending infinitely all the way up and down. A mind-boggling thought, perhaps, but arguably not inherently more far-fetched than any other theory about the origin of our universe.

  Weird became true: dark matter

  When you look up at the night sky, you see thousands of stars twinkling above you, but mostly you just see darkness. You might imagine this blackness is empty space, but not so. Astronomers now believe it’s full of invisible ‘dark matter’. Particles of this stuff may be drifting through you right now, but you’d never be aware of it because dark matter is fundamentally different from ordinary matter. The two barely interact at all. There’s also way more of it than there is matter of the visible kind – over five times as much.

  Given the oddness of this concept, it shouldn’t come as a surprise that, for a long time, astronomers themselves were reluctant to believe in dark matter. In fact, there was a gap of almost half a century between when its existence was first proposed and when mainstream science accepted it as real.

  The man credited with discovering dark matter was Fritz Zwicky, a Swiss astrophysicist who moved to the United States in 1925 after accepting a position at the California Institute of Technology. He spent part of his time there doing sky surveys; this is routine astronomical work that involves cataloguing objects throughout the cosmos. In the course of this activity, his attention was drawn to the Coma Cluster, which is an enormous group of galaxies located about 320 million light years away from Earth.

  Zwicky noticed that the galaxies in this cluster were moving extremely fast – so fast that, by his reckoning, they all should have scattered far and wide long ago. Instead, they remained gravitationally bound to each other as a cluster, and this puzzled him because he knew it would require a lot of gravity to counteract their speed. He did the calculations to figure out exactly how much, and that’s when things got weird. He estimated it would take as much as fifty times more gravity than all the visible galaxies in the Coma Cluster together could produce.

  Most astronomers probably would have assumed they had
made a mistake in their calculations, but Zwicky was confident in his work, and he also wasn’t shy about leaping to grand theoretical conclusions. He decided there had to be massive amounts of hidden extra matter that was keeping the entire cluster gravitationally bound together. In a 1933 German-language article, he described this as ‘dunkle Materie’ or ‘dark matter’.

  Zwicky’s theory fell on deaf ears. This was partly because he had published in a German journal, which meant it fell under the radar of much of the English-speaking astronomical community. But, more significantly, his idea was outrageous. It wasn’t controversial to claim there was stuff in the cosmos that optical telescopes couldn’t see, such as planets or burned-out stars, but Zwicky was claiming there was far more of this dark matter than there was visible matter. Huge amounts more. Naturally, astronomers wanted more proof before accepting such a radical concept.

  Zwicky also found it difficult to find allies willing to take his strange idea seriously for a more prosaic reason – because his colleagues didn’t like him much. He had a reputation for being abrasive, cranky and highly opinionated, often referring to those who dared to disagree with him as ‘spherical bastards’ (his term for a person who was a bastard from whatever angle you looked at them). Behaviour of this kind didn’t endear him to his colleagues. As a result, dark matter languished in obscurity. It wasn’t until the 1970s that the concept finally emerged into the light, thanks to the work of Vera Rubin.

  Like Zwicky, Rubin experienced social challenges in having her work accepted, but this, more frustratingly, was due to her gender. When she began her career, she was one of the few female astronomers and had to struggle to be taken seriously in the male-dominated profession. Where Zwicky raged at those who disagreed with him, Rubin patiently collected more and more data until it eventually became impossible to ignore what she was saying.