do we live inside a gravastar?
An incredible finding could mean that we’re not only questioning the existence of black holes, but also the structure of the universe as we know it
In 2001 two physicists proposed a solution to 70 years of debate around black holes, the most extreme, mindbending objects in the universe, by replacing them entirely. Their alternative, named a gravastar, could solve fundamental paradoxes raised by Stephen Hawking. Some even believe our entire universe may exist inside one. Now, after time out of the spotlight, interest in gravastars is returning, fuelled by the real possibility we could soon prove their existence once and for all.
An insatiable appetite and the ability to bend reality, virtually stop time and turn people into spaghetti, or perhaps just transport them to new parts of the universe or a whole new dimension? The possibilities associated with black holes since they were first proposed challenge even the most imaginative mind’s ability to comprehend what our best scientific theories tell us should be so.
It's not surprising they have given science-fiction writers and Hollywood the vehicles and inspiration for some of their most elaborate plot lines.
A product of the violent deaths of the largest stars in the cosmos, the resulting runaway collapse of the stellar core under gravity eventually produces a singularity, a single point of infinite density and curvature of space-time. Its unimaginable attractive force means any dust, gas or even light that crosses a specified perimeter of no return, the event horizon, is sucked in, like a great whirlpool of seafarer legend.
But do black holes actually exist? To this day we have to infer them indirectly through orbital irregularities which only make sense with the insertion of large centres of mass where none can be seen. Beyond that everything we know about black holes comes from theory and simulation – the extrapolation of Einstein’s general relativity to extreme but logical ends.
This has opened the door to alternative theories, particularly when the centrepiece of black hole theory, the singularity, stretches not only time and space, but also our fundamental laws of physics. “Singularities are, generally speaking, a breakdown in your physical model,“says Emil Mottola, who along with Pawel Mazur proposed the alternative gravastar theory at the start of the millennium.
An example of this theoretical conflict is the information paradox, which concerns not the stuff of the universe itself, but the facts about these objects, the properties of their identity. The laws of the universe suggest information cannot be destroyed, however, a black hole singularity provides no escape mechanism for the information it acquires as it swallows up matter. The radiation continuously coming off black holes, also known as Hawking radiation, shouldn’t contain any information. But because energy and mass are equivalent according to Einstein, this radiation means black holes are forever losing mass and will eventually run out. In death they take with them any last chance this captured information could be returned to the wider cosmos.
If a black hole has no singularity then information needn’t be trapped and could instead leak out, and yet singularities are an inevitable consequence of our black hole models.
“Once you get inside the horizon there is no way out in the logical sense and the literal physical sense. You are stuck by causality,” says Mottola.
For many years it was assumed something must intervene, a mysterious influence often vaguely ascribed to ‘quantum effects unknown’.
“Black holes have been around for seven to eight decades and people have been trying to resolve these issues for roughly the same time,” says Raúl Carballo-Rubio at the Perimeter Institute in Ontario, Canada. “These questions have led people to try and find alternatives.”
“Black holes have been around for seven to eight decades and people have been trying to resolve these issues”
Raúl Carballo-Rubio
Mazur and Mottola’s contribution took inspiration from recent research into the outward-pushing forces of dark energy that are believed to be accelerating the expansion of the universe. They realised the repulsive properties of dark energy, known as negative pressure, could counter the effects of gravitational collapse that were sending black hole models down the singularity path.
In their gravastar model a classical gravitational collapse during the death of a giant star is halted early enough that neither a singularity, nor a horizon can form. This initial interruption is provided by the build-up of quantum effects which produce an explosive event that dramatically changes the structure of the collapsing star’s interior, and a bubble of vacuum is formed, held in place by a dark energy-like negative pressure.
This ‘repulsive’ core extends out to where the horizon of the black hole would be in the classical model. It replaces the horizon’s fictional surface, a purely mathematical point of no return, with a true, solid shell made of ultra-stiff collapsing matter, wrapped around the empty vacuum interior. “You could hit your head, or probably much worse if you struck such a surface,” says Mottola.
Their newly imagined objects were named gravitational vacuum condensate stars before the snappier gravastar name was adopted, and were proposed to be almost, but not quite as dense as a black hole.
Mottola laughs when asked about the reaction their ideas received. “It backed directly into the common wisdom about what the interior of a black hole is, and some people are just not willing to entertain this possibility.”
Not content with merely rubbing up against black hole modellers, the paper also included what Mottola calls, almost dismissively, “some intriguing speculation” that if correct would “override our entire understanding of a good part of cosmology, inflation and the Big Bang.”
This even more contentious idea comes from the following logic: in the interior of gravastars we are invoking the same repulsive negative pressure associated with dark energy. Dark energy is believed to exist throughout the universe at large and account for its seemingly magical quickening expansion. Therefore, could it not be possible that our entire universe might reside within such a gravastar interior?
The suggestion was not without precedent. In the 1960s many, including eminent Russian nuclear physicist and Nobel Peace Prize winner Andrei Sakharov, noted that general relativity produces
“Differences between classical black holes and gravastars are profound, but not easily spotted”
singularities, both from gravitational collapse inside black holes, and also in cosmology with the Big Bang. Mazur and Mottola took these observations one step further, while the replacement of classical black holes with gravastars came with the added bonus of a repulsive force to account for our universe’s apparent expansion.
As you may have guessed the paper’s reviewers had a few questions, and critics pointed out that they were unable to underpin their gravastar with the equations to explain its formation or ongoing dynamic behaviour.
“We didn't know how to do it so we did the simplest thing first: construct a static, spherically symmetric solution that is just sitting there, without telling you how it got there,” explains Mottola.
They did eventually get a version of the paper published in 2004 but, after dealing with referees for several years, both were happy to leave it there. “We were tired of fighting,” admits Mottola.
However, they couldn't leave it alone completely. And others didn't either. Theoretical work progressed developing models of more realistic, slowly rotating gravastars that could accrete matter, as well as working out what the observational consequences of this behaviour might be.
“The development of the theory has been slow. We have received criticism and I would accept and agree with the criticism,” Mottola continues. Part of the reason for the slow progress was the difficulty in combining theoretical work with observational data.
The differences between classical black holes and gravastars with their solid surface are profound, but not easily spotted. If either object is just sitting there minding its own business there will be no clues as to which theoretical object you are looking
at – or not looking at in this case. Just like black holes the compact, intense gravity of a gravastar means any light from it would be deflected by the gravitational field so that it would never reach us.
However, hopes have been raised by theoretical hints that the behaviour of a black hole-gravastar candidate should differ in a way that could be detected when agitated.
“You have to ping it somehow, hit it with something, accrete matter onto it or collide it,” says Mottola, though more theoretical work on the gravastar surface is required before we can really start looking in detail.
A more immediate possibility comes from collisions. In 2015 the Laser Interferometer Gravitational-Wave Observatory (LIGO) observed the first direct gravitational-wave signal from what was interpreted as the merger of two black holes. While the scientific world celebrated a stunning vindication of Einstein's theory 100 years later, fans of gravastars were querying the source of these 'ripples' in the fabric of space-time. Mottola believes the low quality of the data leaves enough doubt for alternative interpretations to hold weight. So could the LIGO result really be evidence of gravastar mergers instead? “It could be,” he suggests.
With the poor quality of those first datasets we may never know for sure. Instead the hope is that better measurements of future events could allow scientists to eavesdrop on the resulting colliding forces to differentiate between a black hole and gravastar merger.
This hope is encouraged by a particular prediction from gravastar theoretical models – a phenomenon known as gravitational echoes. Simulations of black hole mergers suggest gravitational-wave bursts quickly die out. However, gravastar models see gravitational waves bouncing around inside them and leaking out over a series of progressively weakening spurts. This search should be further aided by the possibilities of multi-messenger studies which observe the same event simultaneously in gravitational waves and across the electromagnetic spectrum.
For the moment Mottola is focusing on tightening up those accretion models. However, for Carballo-Rubio the focus is on the birth of these objects. “How you form this gravastar rather than the gravastar’s properties themselves might be the best angle to try and test this observationally,” he says.
But what about that original speculation that our universe might exist within a gravastar?
Mottola remains positive but cautious. “I would think it is very likely something of that kind is eventually going to be correct,” though he admits it is “immature” to talk about the cosmological implications of gravastars when our theoretical understanding of these objects within our own universe is still so incomplete.
Carballo-Rubio is less convinced such a grand idea is worth pursuing at all. His own work, published in early 2018, suggested that rather than a pure vacuum, the internal composition of gravastars is more likely to be a combination of matter and vacuum polarisation. “There is no longer a clear relation between the internal geometry and cosmological solutions,” he explains.
For Mottola on the other hand, there are perhaps pragmatic and scientific reasons to focus his energies on the gravastars residing inside, rather than outside our universe. “You would have to overturn a big a good part of cosmology and inflation and the Big Bang, and I am sure that would be even more controversial. But this is not totally off my mind,” he concludes.