FASCINATING, bamboozling, vaguely terrifying: black holes are the love-to-hate monsters of the universe. These insatiable cosmic cannibals are concrete predictions of Einstein鈥檚 general theory of relativity, the best theory of gravity we have. Even so, theorists long debated whether they could exist 鈥� until astronomers saw the first signs of them. Now we see black hole paw prints all over: in huge stars collapsing in on themselves, in distant collisions of massive objects that set the universe quivering, and in the dark hearts of galaxies including our own.
This year, we should have the clincher: the first direct image of the supermassive black hole at the Milky Way鈥檚 centre. But as we gear up for that shadowy mugshot, some physicists are entertaining a maverick thought: what if it isn鈥檛 there?
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The new word is that our obsession with black holes might have blinded us to the existence of something even stranger 鈥� a basic phenomenon of particle physics whose significance we have failed to grasp. After all, there鈥檚 good reason to want whatever is at our galaxy鈥檚 heart not to be a black hole. For a start, black holes make a nonsense of quantum mechanics, the best theory of everything-besides-gravity that we have.
It is a speculative idea as yet, to be sure, but there are sound reasons to contemplate it. 鈥淲e scientists tend to be completely arrogant about what we think we know,鈥� says theorist of the Frankfurt Institute for Advanced Studies in Germany. 鈥淚 don鈥檛 want to find myself in 10 years鈥� time saying I was just another arrogant, overconfident scientist.鈥�
For all their complications, the basic notion of a black hole is remarkably simple. In general relativity, the magisterial theory Einstein introduced in 1915, massive objects bend space and time around them, creating the force we call gravity. An object with sufficiently large mass, densely enough packed, bends space and time so much and creates a gravitational field so strong that nothing, not even light, can escape once it crosses the 鈥渆vent horizon鈥�. The black hole is a celestial lobster pot, with the difference that it never gets emptied and just keeps on accumulating lobsters, no trace of which is ever seen again.
鈥淭here鈥檚 still a prize for anyone who can say what a black hole is鈥�
The fact is, no one knows how black holes work on the inside. Relativity suggests that anything that falls in will be crushed by the black hole鈥檚 gravity into a 鈥渟ingularity鈥� of zero volume and infinite density, but the prize is still out there for anyone who can say what that truly means. Meanwhile, theorists鈥� most refined calculations show that black holes must either destroy information 鈥� a complete no-no in quantum theory 鈥� or surround themselves in a seething mass of energy called a firewall, which breaks a tenet of general relativity. Black holes represent the point where the very large, the domain of general relativity, meets the very small, the domain of quantum theory 鈥� and the results are not pretty.
Such problems were already becoming apparent in the 1960s, but that didn鈥檛 stop astronomers imagining ways for nature to produce black holes. When a massive star鈥檚 fuel was spent, for example, it might collapse under its own gravity into a stellar-mass black hole. Such an object would by its nature be pitch black and impossible to distinguish from a pitch-black cosmos, but could make its presence known in other ways. Sure enough, in 1964, a huge outpouring of X-rays was discovered in the constellation Cygnus. Labelled Cyg X-1, it looked exactly like models of the radiation emitted by superheated gas plunging down towards a black hole. By the 1970s, astronomers felt confident enough to proclaim that Cyg X-1 was almost certainly a stellar-mass black hole eating matter in its surroundings.
That wasn鈥檛 the half of it. In the early universe, when giant nebulae were collapsing to form galaxies, the gas at their centres would eventually become dense enough to form monstrous 鈥渟upermassive鈥� black holes, each weighing as much as millions or even billions of suns. The invisible object at the centre of the Milky Way, known as Sagittarius A* or Sgr A*, is thought to be one. It was identified in the 1970s as a particularly strong source of radio signals. Subsequent studies, particularly of the way its gravity pulls nearby stars around, have convinced us that it is indeed a black hole with four million times the sun鈥檚 mass.
Further convincing, but still circumstantial, evidence for black holes came last year with the announcement of the first detection of gravitational waves. These ripples in space-time emanate from very massive objects that are accelerating 鈥� two bodies spiralling inwards to smash into each other, for example. The signal observed by the LIGO collaboration in September 2015 was exactly that predicted for the collision and merger of two stellar-mass black holes. LIGO has since seen two more signals, each compatible with the mergers of stellar-mass black holes.
鈥淏oson stars would hang there like doughnut-shaped cosmic couch potatoes鈥�
Case closed? Not so fast, says Rezzolla. 鈥淎lthough the presence of a binary black hole system is an obvious and simple explanation, it鈥檚 not the only explanation,鈥� he says. In particular, the signals might not come from black holes, but from an entirely different theoretical invention: boson stars.
Let鈥檚 back up a bit. The fundamental particles that make up most matter 鈥� you, me, those supposed black holes 鈥� all belong to a class known as fermions. Their signature characteristic is that they obey the Pauli exclusion principle, which says that particles cannot occupy the same quantum energy state as one another. The Pauli principle explains the appearance of the material world: it determines how electrons arrange themselves in different energy states around an atomic nucleus, and thus the properties of the various chemical elements.
Bosons are a different kettle of fish. The Higgs boson, discovered to great fanfare in 2012, is perhaps the most notorious example. It provides matter particles with their mass; other bosons carry the forces that allow matter particles to interact. Bosons aren鈥檛 exotic. In fact, we see them all the time, quite literally: photons of light are bosons.
The thing about bosons is that they can cram together with virtually no limits. Rather than forming some kind of uncontrollable subatomic mosh pit, they become what is in effect a collective particle, a state of matter known as a Bose-Einstein condensate.
We know how to make Bose-Einstein condensates in the lab. We also now know that, given the right bosons, there鈥檚 nothing to stop them forming something on a bigger scale 鈥� perhaps much bigger. Some physicists even think they can form stars, although not as we know them. 鈥淲hen we say star, we are basically just saying a collection of stuff that holds together,鈥� says theorist of Long Island University in New York.
When normal matter forms a star, gravitational pressure heats it so it ignites into nuclear fusion, pouring out light. In contrast, boson stars would just hang there like cosmic couch potatoes. Doughnut-shaped couch potatoes: simulations suggest that if boson stars rotate as conventional stars do, centrifugal forces would give the bosonic matter that form.
These celestial doughnuts would be transparent. Emitting no light of their own, they would be invisible, and the primary thing that would give them away would be their intense gravity. Sound familiar? 鈥淏oson stars could mimic black holes,鈥� says Liebling. 鈥淎nd it is possible that we are getting tricked.鈥�
The idea of boson stars isn鈥檛 new, but astrophysicists pooh-poohed it because no one could think what sort of boson might be used to make one 鈥� the particles such as photons that transmit the fundamental forces don鈥檛 cut the mustard. Then came the discovery of the Higgs boson. It revived interest in novel bosons 鈥� not least because they could be a boon for particle physics, too (see 鈥�The right sort of boson鈥�). That in turn has spurred physicists to ask how we might seek evidence of boson stars.
LIGO is one obvious way, although the current detector can鈥檛 tell whether gravitational waves come from two black holes merging, or from two boson stars. The place to look for the difference is not in the pre-merger phase, the inward spiralling that gives out the waves observed so far. Rather, it is in the aftermath, when the two objects have coalesced and are still quivering from the shock, like a rung bell. 鈥淎s with bells, each object will have its own frequency and tone,鈥� says Rezzolla. 鈥淏lack holes of a given mass will have their own tone, boson stars will have another.鈥� Unfortunately LIGO is not yet able to hear this 鈥渞ingdown鈥� signal with sufficient precision, and upgrades that will let it do so are probably at least five years away.
The Event Horizon Telescope 鈥� an international project to look directly into the maw of a black hole 鈥� might deliver clarity sooner. Radio telescopes across the globe have been wired up with the aim not just of detecting emissions from Sgr A* as superheated gas spirals across its event horizon, but of mapping them out. Do that with enough precision, and the shape of the black hole itself should show up as鈥� well, a black hole in the middle of the image.
This is a tall order, the equivalent of sitting in London and taking a picture of a mustard seed in New York. For that reason, Heino Falcke of Radboud University in Nijmegen, the Netherlands, who is one of the driving forces of the Event Horizon Telescope, cautions against overexcitement 鈥� although he is enthusiastic about the data gathered so far. 鈥淚t鈥檚 not going to be a beautiful sharp image. It鈥檚 likely to be an ugly peanut,鈥� he says.
Or perhaps an ugly doughnut. Opinions differ as to whether it will be easy to distinguish between images of a black hole and a boson star. Calculations by of the Paris Observatory in France suggest that the gravity of a compact boson star will bend light around itself, creating an empty region that for the shadow of a black hole event horizon. 鈥淎 boson star is really different to a black hole and yet still it produces features that look like a black hole,鈥� he says.
鈥淚t is like sitting in London and taking a picture of a mustard seed in New York鈥�
Rezzolla thinks this analysis is overly pessimistic. Like a black hole, a boson star will be sucking in matter from its surroundings, but the boson star鈥檚 transparency means this matter will be visible at its centre. It is also likely to heat up and start emitting light or some other form of electromagnetic radiation. 鈥淭his light might remove the presence of a shadow all together,鈥� says Rezzolla.
Vincent agrees that the behaviour of matter inside a boson star is an area that needs investigation. 鈥淚t鈥檚 a programme of research that I am trying to develop. We are developing the codes to do this from scratch,鈥� he says.
Falcke is not expecting any surprises when the first image from the Event Horizon Telescope finally comes together. 鈥淚 am afraid it is nothing other than a boring black hole,鈥� he says. That choice of words indicates just how deep-seated the belief in black holes now is. Even those working on boson stars admit they are a long shot. 鈥淚 am open to arguments but still they are pretty exotic,鈥� says Liebling.
Then again, the reward for killing off black holes is potentially immense. Embodying the conflict between general relativity and quantum theory as they do, they are a massive roadblock to progress on an overarching theory of nature. Put like that, it seems this is a puzzle where keeping our options open would be wise. 鈥淚t is best to stay open minded,鈥� says Rezzolla. 鈥淭hen let the experience tell you what you really have there.鈥�
The right kind of boson
Boson stars depend on there being bosons to make them. In 1955, US physicist John Wheeler wondered whether stars might be made out of photons of light, rather than matter. He called these objects gravitational electromagnetic entities, or geons. But it soon turned out that geons made of 鈥渟pin-one鈥� bosons such as photons would be unstable and evaporate away.
In the 1960s, theorist David Kaup of the University of Maryland showed that spin-zero bosons . At the time, though, no spin-zero bosons existed. That changed on 4 July 2012, with the discovery of the Higgs boson. To those who had never lost faith in boson stars, it came as a huge fillip. 鈥淲e knew that there is at least one such boson in nature, so it was a motivation to go further in that direction,鈥� says Fr茅d茅ric Vincent of the Paris Observatory in France.
There is still a big hurdle to clear. The way bosons all clump together means that the smaller their mass, the bigger the star that they form. Very massive stars mean very light particles. The Higgs, at 125 gigaelectronvolts, or about 250,000 times the mass of the electron, was simply too heavy.
A possible alternative is an axion. This hypothetical particle has been proposed since the 1970s and is a candidate for dark matter, the mysterious glue that astronomers believe holds galaxies together. That idea, in turn, opens up the possibility that boson stars might account for at least some of the dark matter.
Although searches for axions have yet to bear fruit, the discovery of a boson star could help things along: by telling us the axion鈥檚 likely mass, it could tell us where to focus in experiments to make the particles on Earth.
This article appeared in print under the headline 鈥淗oly moley!鈥�
