“Most of the matter in the Universe is invisible, composed of some substance that leaves no mark as it breezes through us – and through all of the detectors the scientists have created to catch them. But this dark matter might not consist of unseen particle clouds, as most theorists have assumed. Instead, it might be something even stranger: a superfluid that condensed to puddles billions of years ago, seeding the galaxies we observe today.”
“Superfluid dark matter has important conceptual implications as well. It suggests that the common picture of the Universe as a mass of individual particles bound together by forces – almost like a tinker toy model – misses much of the richness of nature.”
“Most of the matter in the Universe might be utterly unlike the matter in your body: not composed of atoms, and not even built of particles as we normally understand them, but instead a coherent whole of vast extension.”
“Instead of invoking new, undiscovered particles, this different model posits that the evidence for dark matter is instead due to a modification of gravity.”
“The modification of gravity is stunningly successful in some cases, but has problems elsewhere. On the one hand, it fits the rotations of galaxies with remarkably little effort and explains why their brightness-velocity relations all seem alike: modified gravity allows less variation from galaxy to galaxy than does the formation of particle clouds, which could all be different. On the other hand, modified gravity struggles with the observational data for distances much larger or smaller than the size of a typical galaxy. On those scales, it’s the cold dark matter model that works better.”
“It is notoriously difficult to change anything about Albert Einstein’s theory of gravity without ruining it altogether, so most physicists have opted for the safer alternative of particle dark matter. For them, conjecturing new particles is a well-trodden way to solve problems, and the mathematics is familiar territory. But Khoury doesn’t want to pick a side. He wants the best of both, to make the best possible fit with the real Universe.”
“‘Traditionally, people have tried to address the galactic-scale problems by modifying gravity; that’s been the alternative to dark matter,’ Khoury says. ‘For some reasons, maybe sociologically, these two approaches have been considered exclusive: either you’re in the modified gravity camp, or you’re in the particle dark matter camp. But why couldn’t it be both? Of course, Occam’s razor would say it’s less compelling. So the approach we’ve taken is that both phenomena, modified gravity and particle dark matter, could just be aspects of the same theory.’”
“With more data, physicists could also exclude the idea that dark matter consists of unseen clumps of ordinary atoms, like the ones Earth is made of (technically known as baryonic matter). This normal matter interacts too strongly with itself; it would not produce the observed distribution of dark matter.”
“Dark matter also cannot be made of stars that collapsed to black holes or other very dim stellar objects. If that were so, these objects would have to vastly outnumber the stars in our galaxy and cause intense gravitational distortions that could be readily observed.”
“Nor can dark matter be made of other known particles, such as the weakly interacting neutrinos that are emitted abundantly by stars. Neutrinos would not clump enough to create the observed galactic structures.”
“Therefore, to explain what makes up dark matter, physicists instead had to theorise about new, so far undetected, particles. The most widely used ones fall into two broad classes: weakly interacting massive particles (WIMPs) and much lighter axions, though there is no shortage of more complex hypotheses that combine various types of particles. But all attempts to detect any of these particles directly, rather than inferring their presence from their gravitational pull, have so far been unsuccessful. Instead of solving the mystery, the direct-detection experiments have only deepened it.”
“Superfluids do not exist in daily human experience, but they are well-known to physicists. They are analogous to superconductors, a class of materials that moves electricity without resistance. When cooled to temperatures near absolute zero, helium likewise starts flowing without resistance. It will creep through the tiniest pores, and even slide out of trays by moving up walls. Such ‘superfluid’ behaviour isn’t specific to helium; it is a phase of matter that, at low enough temperatures, can be reached by other particles too. First predicted in 1924 by Einstein and the Indian physicist Satyendra Bose, this whole class of ultra-cold superfluids is now known as Bose-Einstein condensates. Liberati realised that dark matter might have a superfluid state as well.”
“Bose-Einstein condensates are best understood as a mixture of two components: one that is superfluid and one that isn’t. The two components behave very differently. The superfluid one exhibits long-range quantum effects, no viscosity, and unexpected correlations over large distance scales; it is as if it was made of much larger particles than its actual tiny constituents. The other normal component behaves like the fluids we are used to; it sticks to containers and to itself – it has a viscosity. The ratio between the two components depends on the condensate’s temperature: the higher the temperature, the more dominant the normal component.”
“We are used to thinking that quantum physics dominates only the microscopic realm. But the more physicists have learned about quantum theory, the more it has become clear that this isn’t so. Bose-Einstein condensates are one of the best-studied substances that allow quantum effects to spread widely through a medium. In theory, quantum behaviour can span arbitrarily large distances, provided it isn’t disturbed too much.”
“In a warm and noisy environment such as Earth, fragile quantum effects are quickly destroyed. That is why we don’t normally observe the stranger aspects of quantum physics, such as the ability of particles to behave like waves. But initiate quantum behaviour in a cool, quiet place and it will last. A cool, quiet place like, for example, outer space. There, quantum effects might stretch across vast distances.”
“If dark matter were a Bose-Einstein condensate – one with quantum effects spreading throughout whole galaxies – this state would naturally account for two different behavioural modes of dark matter. Within galaxies themselves, most of the dark matter would be in the superfluid phase. But across galaxy clusters that include much intergalactic space, most of the dark matter would be in the normal phase, giving rise to a different behaviour.”
“According to Khoury and collaborators, it is possible to explain the observed effects of dark matter with a simple model of a Bose-Einstein condensate, one that has only a few open parameters (that is, just a few properties that must have the right attributes to make the model work).”
“According to Khoury, the equations for superfluid dark matter don’t belong to the realm of elementary particle physics. They emerge from theory in condensed matter physics, where they describe not the fundamental particles, but their emergent long-range behaviour. In Khoury’s model, the equations that appear in modified gravity are not those of the individual particles. Instead, they are a description of the particles’ collective interplay.”
“Such equations are unfamiliar to many particle physicists, which is why the relation between superfluidity and modified gravity remained unnoticed for so long. Unlike the equations of modified gravity, however, the equations describing superfluids already have a strong theoretical foundation – just in condensed-matter physics.”
“The superfluid generates patterns of attraction identical to those of the equations of modified gravity, so it can reproduce the observed regularity of galactic rotation curves. However, unlike modified gravity, it behaves this way only in the temperature range in which the superfluid component is dominant. On the larger scale of galactic clusters, the dark matter gets too agitated (that is, too hot) and loses its superfluid properties. In this way, superfluid dark matter could have seeded the formation of visible galaxies while, in its non-superfluid phase, it would match up with the observed structure of clusters.”
“Khoury’s approach explains why astronomers do not see any evidence of modified gravity within the solar system. ‘The Sun itself creates such a huge gravitational field that it would locally destroy the superfluid’s coherence,’ he says. ‘In the vicinity of the solar system, you shouldn’t think in terms of a coherent superfluid. The Sun acts like an impurity. It’s like there’s dirt in the fluid.’”
“If dark matter is a superfluid, the particles it is made of must be lightweight, much lighter than the hypothetical dark particles that have been the targets of most of the searches. The superfluid’s constituents are probably too slight to show up in the experiments currently running.”
“A better and unique prediction of Khoury’s model is that a superfluid’s quantum behaviour should leave a telltale pattern in galactic collisions. When the dark matter condensate from one galaxy runs into that of another, the collision would create interference patterns – ripples in the distribution of matter and gravity, which would affect how the galaxies settle.”
“Superfluid dark matter also makes predictions for the friction between the dark matter components within galaxy clusters; such friction would again produce distinctive patterns of gravitational attraction. Observations of gravitational lensing could detect these fingerprints of superfluid dark matter, provided we know exactly what we’re looking for.”