Has the reign of black holescome to an end? Hazel Muir introduces a new dark lord of the heavens

Nobody has ever seen a black hole. Yet, despite this lack of direct evidence, most scientists believe that a massive star at the end of its life can implode to form an object so dense that nothing-not even light-can escape.

They may be about to change their minds, however. Two researchers in the US are pointing out that physicists have swept some “humiliating” problems with black holes under the carpet. By confronting these problems, they say, they have found an alternative fate for a collapsing star. Emil Mottola of theLos Alamos National Laboratory in New Mexico and Pawel Mazur of the University of South Carolina in Columbia think it mightturn into an exotic bubble of superdense matter, an object they call a gravastar.

According to Mottola and Mazur, gravastars are cold, dense shells supported by a springy, weird space inside. They’d look like black holes, lit only by the material raining down onto them from outside. In fact, they seemto fit all the observational evidence for the existence of black holes.

So far, however, physicists have mixed feelings about the idea of gravastars. Their verdicts range from “outstandingly brilliant” to “unlikely”. What’s certain is that gravastars will rekindle a great debate of the early 20th century: are black holes fact or fantasy?

The idea of black holes dates back to the First World War, when German astronomer Karl Schwarzschild solved the equations of Einstein’s newborn theory of gravity while serving on the Russian front. He showed that space-time around any massive star would be curved. Squeeze a large enough star into a tiny enough space and its density would become infinite and the curvature of space-time would spiral out of control. The gravity near one of these objects would be so strong that nothing-not even photons-could escape its grasp.

Einstein shared the view of most physicists of the time that such objects, later dubbed black holes, were too outrageous to exist. He argued that it was all academic anyway, since stars never shrink this small. But scientists gradually became convinced that they do. If a star is very massive, it will blast apart in a supernova explosion at the end of its life and if a core twice as heavy as the Sun remains, no known force can prevent gravity squeezing it to a point.

The result is a “singularity” with infinite density, where the known laws of physics break down. The singularity’s gravity would be so powerful it would be cloaked in an “event horizon”, a boundary beyond which matter or light couldn’t get out. The dramatic idea of a black hole, which would rip to shreds anyone caught inside it, fired the imaginations of scientists, artists and writers alike. But no one has ever rooted the drama in fact. “So far, there is no direct observational evidence to show that any of the things astronomers call black holes have event horizons or central singularities,” says Neil Cornish, an astrophysicist at the University of Montana in Bozeman.

We know there are compact objects millions of times as heavy as the Sun that hog the centres of galaxies. These black hole candidates give themselves away because hot stars, gas and dust spiralling towards them emit bright X-rays. But that doesn’t mean there’s a cataclysmic black hole in the vicinity; it could simply be a very massive object. The debate petered out decades ago but there’s still no ironclad proof that black holes exist.

But never mind the lack of physical evidence-there are enough problems in black-hole theory itself to make their existence seem implausible to say the least. These problems stem from the fact that our Universe is actually very different from the one that Schwarzschild considered. If we’re to produce a proper description of the Universe we live in, Einstein’s classical theories need to be meshed together with what we know about the quantum laws governing the behaviour of fundamental particles and fields.

Mazur and Mottola have been thinking about quantum gravity for nearly a decade. They began by examining the nature of “quantum fluctuations” in space, time and even in energy fields. Empty space, for example, is never really empty. On the tiniest scales, little particles are popping in and out of existence all the time, creating a seething, fluctuating fluid. “Like a fish in a calm pond, who is not aware of all the incessant jiggling of the water molecules, we are usually not aware of the quantum medium we are immersed in,” says Mottola.

And they have found that quantum fluctuations in the electromagnetic fields that describe tiny things like photons can influence gravitational phenomena on the large scale-such as black holes. So, they reasoned, when early black-hole theorists ignored quantum effects they were creating an unreal space-time.

Information overload

This traditional approach to black holes has produced strange anomalies anyway, and these have remained unresolved, Mazur and Mottola claim. There are problems, for instance, with a black hole’s entropy, a measure of the amount of information it holds. An object that contains many possible states has high entropy, in the same way that a computer with more bits of memory can store more information. When a star forms a black hole, all the unique information about the star-its chemical composition, for instance-appears to be squashed out of existence. Yet current theory suggests black holes have enormous entropy-a billion, billion times that of the star that formed them. No one can fathom where all this extra entropy comes from or where it resides. “Where are all these zillions of states hiding in a black hole?” says Mottola. “It is quite literally incomprehensible.”

Another seemingly impossible feature is that photons falling into a black hole would gain an infinite amount of energy by the time they reach the event horizon. But the gravitational effects of this enormous energy are ignored in the classical theory. Mottola says these problems have forced physicists to dream up far-fetched excuses. They say, for example, that some of the black hole’s entropy might be hidden in other universes. Mottola doesn’t buy these “esoteric assumptions”, and concludes that black holes are a bag of contradictions that don’t make a good case for their own existence at all.

But is there an alternative? Could it be that when a star collapses, something happens to prevent a black hole forming? Mazur and Mottola think so. They have shown that quantum effects can make space-time change into a new and curious state that would lead to the formation of a strange new object.

That change is a phase transition, like liquid water turning into a solid block of ice. They believe that in the extreme conditions of a collapsing star, space-time undergoes a quantum version of a phase transition. The phenomenon is nothing new. The Nobel Prize for Physics in 2001 was awarded for the observation of just such an event in the lab: the transformation of a cloud of atoms into one huge “super-atom”, a Bose-Einstein condensate (BEC). This clump of atoms, which all share the same quantum state, forms at temperatures within a whisker of absolute zero.

When an event horizon is about to form around a collapsing star, Mazur and Mottola believe that the huge gravitational field distorts the quantum fluctuations in space-time. These fluctuations would become so huge they would trigger a radical change in space-time, very similar to the formation of a BEC. This would create a condensate bubble. It would be surrounded by a thin spherical shell composed of gravitational energy, a kind of stationary shock wave in space-time sitting exactly where the event horizon of a black hole would traditionally be. The formation of this condensate would radically alter the space-time inside the shell. According to Mazur and Mottola’s calculations, it would exert an outward pressure. Because of this, infalling matter inside the shell would do a U-turn and head back out to the shell, while matter outside the shell would still rain down on it.

In a paper submitted to Physical Review Letters, Mazur and Mottola have shown that, like classical black holes, gravastars are a stable solution of Einstein’s equations. What’s exciting, they say, is that gravastars don’t suffer any of the mathematical ailments of black holes. There’s no riotous singularity where the laws of physics break down. There’s no event horizon to imprison light and matter. And the entropy of a gravastar would be much lower than that of any star that might collapse to form it, dodging the problem of excessive entropy that plagues black holes.

Take a gravastar with a mass 50 times that of the Sun, for example. Like the event horizon of a black hole with the same mass, the shell would be roughly 300 kilometres in diameter. But it would be around just 10-35 metres thick. Just a teaspoonful of the material would weigh about 100 million tonnes. But Mazur and Mottola have shown it would have a temperature of only about 10 billionths of a degree above absolute zero. And it wouldn’t emit any radiation, making it as black as any black hole would be.

Dark energy

Gravastars would be just as much fun for sci-fi buffs-in fact they’d be even more ruthless. Imagine a black hole of a million solar masses, like the one thought to sit in the centre of our Galaxy. You could cross its event horizon without feeling a thing: it’s only as you approached the singularity that you’d be torn apart by the huge gravity gradient. But if you were drifting towards a gravastar of the same size, you’d never get anywhere near its centre. As soon as you hit the shell you’d explode into pure gravitational energy.

Marek Abramowicz, an expert on black holes at Gothenburg University in Sweden, calls the idea of gravastars “outstandingly brilliant”. “Their unique and remarkable properties could explain several high-energy astrophysical phenomena that now are puzzling.” He thinks they might explain gamma-ray bursts-ultra-intense flashes of gamma radiation from a distant source that appear somewhere in the sky about once a day.

Astronomers aren’t certain what causes gamma-ray bursts. It might be the formation of a black hole in a supernova explosion, but this process would struggle to muster enough energy. The birth of a gravastar, on the other hand, would be extraordinarily violent and might shed enough energy to account for gamma-ray bursts.

Mottola points to another possible connection between gravastars and astronomical observations. Three years ago, data from distant stellar explosions suggested that the expansion of the Universe is getting faster all the time (New Scientist, 11 April 1998, p 26). Many physicists ascribe this acceleration to a mysterious “dark energy” that gives space an outward pressure. Mottola says that if you scale the size of a gravastar up to around the size of the visible Universe, the pressure of the vacuum inside roughly matches the pressure that seems to be accelerating the expansion of the Universe. So our Universe might be one cosmic gravastar: a giant shell trapping the Milky Way and all the other galaxies we see. “We might be able to entertain the really radical notion that we-and everything we see in the Universe-could be inside such an object,” Mottola speculates.

It’s a bold claim, and he and Mazur are still working out whether it’s justifiable. Unlike their hypothetical gravastar, the Universe contains copious ordinary matter and its visible edge is always ballooning outwards. But they’re keen to see what happens when they modify their gravastar model to include these complications. “It is certainly premature at this point, but the seeds of a possible new cosmological model are contained in the gravastar solution,” says Mottola.

Fact or fantasy?

In the meantime, they are trying to figure out how they could tell ordinary-sized black holes and gravastars apart. The differences might be subtle-after all, in isolation, they’re both dark and the gravitational fields outside a black hole event horizon and the gravastar shell would be the same. But a good guess would be that gravastars would shine more brightly, since matter falling onto one would be turned into radiation. Black holes would gobble all the matter, but a gravastar would let its energy escape.

The next step is to identify the telltale signs of a gravastar, Mottola says. “It is the only way to convince the sceptical-including ourselves-that nature really behaves this way.” Yet physicists aren’t even sure what black holes look like. In October last year, they reported seeing what appeared to be a heavyweight black hole, but material falling onto it is emitting far brighter X-rays than theories predict. The excess energy is roughly equivalent to the output of 10 billion Suns. If it is a black hole, it’s not clear why it’s so bright.

The object may be whirling round and dragging magnetic fields at the event horizon with it, and these could generate the extra energy by whipping up and heating nearby gases. But Mazur thinks there’s a better explanation for that extra energy. The “black hole” could be a gravastar, he says. Stars, gas and dust raining down onto its shell would violently dissolve into pure gravitational energy that might emerge as bright X-rays.

To try to resolve this issue, Mazur is working out what a rotating gravastar might look like. Like every other compact object in the Universe, a gravastar would almost certainly be spinning rapidly.

Not all astronomers are as enthusiastic about gravastars. Cornish questions whether an exploding star could really lose enough entropy to form a gravastar, given that the second law of thermodynamics says that the entropy of an isolated object will always tend to increase. “In other words, a cup can break into a thousand pieces, but it is highly unlikely that a thousand shards of pottery will spontaneously come together to form a cup,” says Cornish. “Mazur and Mottola talk about a star shedding entropy in some way to make the formation of a gravastar possible, but I don’t think that is a likely scenario.” But Mottola points out that when exploding stars form other remnants, such as neutron stars, they do shed entropy.

And although Cornish admits that black hole singularities are mathematically troublesome, he also believes that a satisfactory quantum theory of gravity will cure this problem. Then there’ll be no need for gravastars, he says. Robert Wald of Chicago University adds that Mottola and Mazur have put forward no arguments about how gravastars could form in the devastating collapse of a massive star. Even if they did form, how would they survive the onslaught of matter raining down on them? “What happens if a gravastar has accreting matter showered upon it? Won’t it collapse to a black hole?” he says.

“The gravastar is stable,” counters Mottola. He says that matter falling onto the shell could make it wiggle and radiate away energy, but because the gravitational pull of the shell balances the force of the springy vacuum inside, it couldn’t actually collapse. Any matter that fell onto the shell would simply become part of it, he says.

All the same, Mottola and Mazur admit there are still unsolved issues with the formation of gravastars. “We must have a better idea of how this phase transition actually occurs in the gravitational collapse process,” says Mottola. The exact nature of the exotic stuff inside the gravastar shell is still open to debate, and they hope to find out whether gravastars can really form in the mayhem of a star’s violent death-and whether gravastars could merge to form the heavyweight objects that sit at the centre of galaxies. They are encouraging others to join the investigation. “There are many unanswered questions and we are really just opening a new direction for future research,” says Mottola.

But if gravastars can weather the controversy, then maybe there’ll no longer be any need for black holes-maybe they really are pure fantasy. It wouldn’t be the first time that Einstein’s dazzling intuition has been proved correct.

Further reading: www.arxiv.org/abs/gr-qc/0109035

New Scientist issue: 19th January 2002