Is Spacetime Actually a Superfluid?
An article by Marcus Chown
|It’s a radical suggestion, but it would
certainly solve one of cosmology’s greatest mysteries - the
problem of establishing a universal time.
LOOK up at the sky. Almost everything out there is spinning around: stars, galaxies, planets, moons - they are all rotating. Yet physicists believe that the universe itself is not revolving. Why?
It’s a question that Pawel Mazur can’t answer. Mazur, a physicist at the University of South Carolina in Columbia, is one of a number who think it is entirely possible that our universe is spinning on an axis. If these people are right, it could make understanding the universe a whole lot simpler. You could stop worrying about the big problems in cosmology: the origin of the big bang, the nature of dark energy and maybe dark matter too. You could get rid of the strange idea that the universe went through a superfast period of expansion known as inflation. You might even be able to halt the attempt to find a theory that marries together quantum theory ...
At home in the gravastar
It’s a question that Pawel Mazur can’t answer. Mazur, a physicist at the University of South Carolina in Columbia, is one of a number who think it is entirely possible that our universe is spinning on an axis. If these people are right, it could make understanding the universe a whole lot simpler. You could stop worrying about the big problems in cosmology: the origin of the big bang, the nature of dark energy and maybe dark matter too.
You could get rid of the strange idea that the universe went through a superfast period of expansion known as inflation. You might even be able to halt the attempt to find a theory that marries together quantum theory and Einstein’s general theory of relativity. Is it so hard to let the cosmos spin?
Yes, it is, at least while general relativity rules the universe. In order to solve the hideously complex equations of general relativity - Einstein’s theory of gravity - cosmologists assume that the universe is the same in every direction.
Although general relativity can allow the universe to rotate, rotation requires an axis, and a cosmic axis of rotation would bestow a "special" direction on the universe - along the axis. Since there is no observational evidence that such a direction exists, the assumption has always been that the universe is not rotating.
Mazur and his colleague, George Chapline of Lawrence Livermore National Laboratory in California, have a simple response to this: don’t assume that general relativity has all the answers.
Where do they get this heretical idea from?
"From looking at where general relativity breaks down," Mazur says.
General relativity provides an excellent description of what happens in the normal, day-to-day events in the universe, but it fails in "extreme" circumstances. Its equations are unable to tell us anything precise about events such as high-energy particle collisions, for instance, or the collapse of stars into black holes. However, the biggest clue to its limitations, Mazur and Chapline say, is in the way it allows time to break down.
General relativity allows the formation of loops in time in certain circumstances. Sometimes, for instance, a kind of one-dimensional fault line in space-time known as a "cosmic string" can form. When such a string spins rapidly around an axis along its length, it creates a loop in time; travel round one of these "closed time-like curves" (CTCs) and you’ll keep coming back to the same moment in time. Mazur and Chapline contend that, according to general relativity, the same thing can happen with a rotating black hole.
The trouble is, quantum theory requires time to be "universal" - there should never be closed loops of time isolated from the time in the rest of the universe. That means quantum theory can’t work everywhere in a universe governed by general relativity.
And since most physicists reckon quantum theory to be a more accurate description of reality than general relativity, relativity’s view of space and time - what cosmologists call the vacuum - must be wrong.
“What if space-time is actually a superfluid”The way time breaks down around a rotating cosmic string has given Mazur and Chapline a clue to resolving this issue. The CTCs form in regions close to the cosmic string’s axis, which means relativity breaks down in the cores of tiny "gravitational vortices" while continuing to apply everywhere else. "This is very suggestive of a vortex in a superfluid," says Mazur.
Superfluids, such as ultra-cold liquid helium, have very strange properties. They can flow uphill, for example, and without friction. One crucial property of superfluids is that they cannot be made to rotate in the way a bucketful of water will swirl around when stirred. Stir a vat of superfluid and you’ll produce an array of vortices: the superfluidity breaks down within each vortex, but everywhere else the fluid remains still - and superfluid.
Mazur published the analogy between vortices in superfluid helium and the way time breaks down near rotating cosmic strings 20 years ago in Physical Review Letters (vol 57, p 929). Ever since then he has been thinking about what it might mean.
Now Mazur and Chapline think they might have the answer: what if this similarity between space-time and superfluids is no accident? What if space-time actually is a superfluid?
It’s a radical suggestion, but it would certainly resolve the problem of establishing a universal time. Superfluids are formed when the particles in the fluid lose their individual character and start to behave as if they are one giant particle, known as a "condensate". Space-time being composed of particles that have formed a superfluid condensate would mean it has a universal time built right in.
But that’s only the start of it. Mazur and Chapline have realized that the idea of a superfluid space-time has particularly profound implications when applied to relativity’s breakdown at the edge of a black hole. If our universe is a spinning superfluid, it could explain where everything came from.
Most people are willing to accept that general relativity breaks down at a black hole’s centre - the "singularity" - where the density and temperature of the shrinking star that spawned the hole skyrocket to infinity.
However, according to Mazur, general relativity also breaks down at the "event horizon" of the hole, the imaginary membrane that cloaks the singularity from view and marks the point of no return for in-falling matter. For one thing, he says, the warping of space and time means that light heading for the black hole is accelerated to infinite energy at the horizon, which is physically impossible. Even more serious, says Mazur, is the violation of quantum theory.
In certain circumstances, quantum theory permits a ghostly influence called entanglement to exist between particles.
If one half of an entangled pair of particles were to cross the event horizon and disappear into the singularity while the other did not, then this entanglement would be destroyed, and that is forbidden by quantum theory.
"Since quantum theory is generally considered the more fundamental theory, general relativity cannot provide a true description of gravity close to a black hole," Mazur says. "In other words, horizons do not form."
Instead, he says, space-time undergoes a shift in its fundamental properties.
Mazur’s alternative to black holes arises from the fact that a superfluid can exist in a number of "phases", just as water can also exist as ice or steam. As with water, external factors can change the superfluid phase.
As the star collapses in on itself, the particles within it come ever closer together. Eventually they reach a density that matches the density of the particles that make up the condensate of the superfluid space-time. At this point, Mazur says, the material of the star can interact with the material that makes up space-time, and the result is that the two materials undergo a phase change. Inside a spherical boundary, where conditions "go critical", the stellar matter is converted to energy, and the superfluid changes its phase, just like water turning to steam.
According to Mazur and Chapline’s calculations, the energy associated with this phase of the superfluid space-time has a negative pressure, which manifests as repulsive gravity (see Diagram above right). This gives the space-time vacuum inside the collapsing star enough pressure to halt the gravitational collapse. "A stable object forms in which the repulsive gravity of the vacuum balances gravity," Mazur says. He calls this object a gravastar.
It is not a static structure. Infalling matter from the star that hits this shell is converted into energy, adding to the energy of the superfluid space-time vacuum within the shell.
The conversion of the star’s matter into energy makes the transition layer extremely hot, and quantum uncertainty dictates that, inside the shell, a small amount of that heat will be converted back into matter. As soon as the matter is created, the repulsive force of the internal vacuum energy pushes on the particles, making them race away from each other at ever-increasing speeds.
Hold on - doesn’t that sound familiar? Matter created in a fiery furnace, blowing everything apart?
"It’s the big bang," says Mazur. "Effectively, we are inside a gravastar."
Dark answersAs well as explaining the big bang, the repulsive gravity neatly explains the origin of the dark energy that appears to be expanding our universe at an ever-increasing rate.
Mazur and Emil Mottola of the Los Alamos National Laboratory in New Mexico first published the basics of the gravastar idea in 2001 (New Scientist, 19 January 2002, p 26).
Now, with the superfluid space-time completing the picture, it has become an even more powerful solution to cosmology’s puzzles.
"The gravastar doesn’t work without the superfluid picture, but with it, it resolves CTCs, singularities and the nature of dark energy," Mazur says.
This new picture also does away with the need for inflation. This period of superfast expansion is postulated to have occurred in the first split second of the universe’s existence. No one has yet worked out exactly how this might have happened, but inflation is the best solution we have to a number of cosmological mysteries.
It is necessary primarily because different regions of the universe that today have the same temperature - as indicated by the cosmic background radiation left over from the big bang - are accelerating away from each other too slowly to have been in contact when the universe began, and if they weren’t in contact, there is no reason why they should be at the same temperature. Inflation solves the problem by making the universe much smaller earlier on so heat could easily flow around it, equalizing the temperature.
Mazur says the superfluid universe idea makes inflation redundant because one object, the collapsing star, contained all of space-time.
That means all the matter within the gravastar had already been in contact for a significant length of time.
"In our picture, there is a long pre-big-bang period - there is plenty of time for everything to come to the same temperature," he says.
This explanation of our universe seems radical - implausible, even - but Mazur thinks it makes a lot of sense: the recipe, a small amount of matter and a whole lot of energy, fits the observed facts.
"Only 4 per cent of the mass-energy of the universe is in the form of the ordinary, light-emitting matter, and 73 per cent is dark energy," he says.
It’s worth pointing out that the remaining 23 per cent of unaccounted-for matter in our universe - what cosmologists refer to as dark matter - is also unaccounted for in the superfluid universe scenario. However, Mazur and Chapline think it curious that dark matter is always found near ordinary matter.
Perhaps, they say, the dark matter may not be matter at all, but the result of some interaction of ordinary matter with dark energy.
The gravastar has become a powerful solution to cosmology’s puzzlesSo why do we need our universe to spin?
Simply because the star that collapsed would have been spinning, and its angular momentum can’t just disappear. Although you can’t stir a superfluid into spinning, the formation of the gravastar - our universe - through interaction with the matter of the rotating, collapsing star will impart a spin to it. That, of course, means there should be an axis - the dreaded "preferred direction" in the cosmos. So, is there one?
Although most physicists would say there isn’t, Mazur and Chapline speculate that a very puzzling feature of the cosmic background radiation could be explained by an axis of cosmic rotation. The hot and cold spots in the radiation should be randomly distributed across the sky, but Kate Land and João Magueijo of Imperial College London have highlighted a curious alignment of the biggest hotspots in the data from NASA’s Wilkinson Microwave Anisotropy Probe (New Scientist, 2 July 2005, p 30). According to Mazur and Chapline, if the universe is rotating slowly, its axis might explain the alignment.
Could Mazur and Chapline’s radical revision of standard cosmology be right? Eric Poisson of the University of Guelph in Ontario, Canada, doesn’t think so.
"My reaction is that their ideas are not sound. This is definitely not the great new idea," he says.
Avi Loeb of the Harvard-Smithsonian Center for Astrophysics is cautious too, but not so dismissive.
"Mazur and Chapline’s suggestion is interesting," he says, "but much more work needs to be done in order to demonstrate that it is a viable alternative to the standard big bang plus inflation model."
Specifically, Loeb wants to see what kind of structures would appear within the superfluid universe.
If the universe is a rotating superfluid, then close to the boundary of the universe tiny vortices will be spawned as the fiery shell of the gravastar imparts energy to the superfluid within. Mazur and Chapline say these vortices may have "seeded" the formation of galaxies. The seeds are today seen as fluctuations in the temperature of the cosmic microwave background (CMB) radiation.
It is here that Mazur and Chapline’s qualitative picture of the universe must confront the quantitative. Cosmologists believe that it was random quantum fluctuations in the vacuum of space that gave rise to the CMB variations. According to standard inflationary theory, these quantum fluctuations would have been of all sizes, or "scale-invariant", and CMB data appears to back that idea up.
The question is, do Mazur and Chapline’s vortices also fit the data?
"They need to demonstrate that one gets a scale-invariant spectrum of density fluctuations as well as a ’flat’ cosmology as one gets from inflation," Loeb says. "We have evidence for both features from the CMB."
Mazur agrees that this is a crucial test.
"We need to be able to predict the structures we see around us today better than the current model," he says. "Then, and only then, we will know whether we are really on to something."
If they turn out to be right, it will be reassuring: looking up at the night sky, we’ll know that the universe is not an aberration; like everything else in sight, it does have a spin. On the other hand, the superfluid universe raises a disturbing question.
Are alien races staring out from within what we think of as black holes?
Somewhere out there, within a fiery shell, someone may be gazing up at the impenetrable border of a universe contained within our own.