# Focus: A Galactic Condensate

Phys. Rev. Focus 6, 5
A new theory proposes that dark matter consists of particles so light that their quantum, wavelike nature dominates.

Astrophysicists now largely agree that most of the matter in the Universe is dark matter made of exotic particles invisible from Earth. Many predictions of the conventional cold dark matter theory agree with observations, but one of the theory’s failings is its inability to accurately describe the structure of dwarf galaxies. A proposal in the 7 August PRL suggests that if the dark matter particles were extremely small in mass, their quantum, wavelike nature would automatically spread out the mass in the centers of dwarf galaxies and match observations more closely. According to this idea, dubbed “fuzzy cold dark matter,” such particles would form a titanic Bose-Einstein condensate–a state normally associated with ultracold atoms in physics labs–in and around every galaxy.

The standard cold dark matter theory has successfully accounted for the clustering patterns of galaxies, the properties of the ubiquitous microwave background radiation, and the rotation speeds of galaxies. “The trick,” says Wayne Hu of the Institute for Advanced Study in Princeton, NJ, “is to retain all the nice aspects of cold dark matter” and solve the dwarf galaxy problem at the same time. Hu and his colleagues took a simple approach. Since the mass of dark matter particles is an unknown, adjustable parameter, “why not just take a mass that makes this problem go away?” Hu explains.

Computer simulations of the coalescence of cold dark matter in the early Universe generate dwarf galaxies with steep increases in mass density at their centers, beyond what is observed. But if the dark matter particle has a mass of ${10}^{-22}$ eV, its quantum mechanical, or de Broglie wavelength would be a few thousand light-years, large enough to span the entire galactic core, the researchers suggest. No particle can cluster more tightly than its wavelength allows, according to the uncertainty principle. Hu points out that this effect is essentially the way quantum mechanics solved the stability problem of the hydrogen atom in the 1920s: The reason that the atom doesn’t collapse is that the electron is wavelike; the hydrogen atom is about as big as the electron’s de Broglie wavelength.

Hu and his colleagues propose that, like standard cold dark matter, fuzzy dark matter should be cold, so that none of the particles should have a velocity high enough to escape a galaxy’s gravity. For such small particles, this low temperature means that they would exist as a Bose-Einstein condensate, a state where almost all of the particles occupy their ground states, and they merge into a single, giant quantum state. If this type of dark matter exists, then every galaxy is permeated by this condensate.

With a one-dimensional computer simulation of fuzzy dark matter the team showed that the core of a typical dwarf galaxy would be less concentrated, as observations suggest they are, and that the outer edges of the galaxy would be unaffected by the new, lower mass particles. The fuzzy cold dark matter scenario predicts a specific relationship between the size of a dwarf galaxy’s core and its density, but observations and interpretations of them are not yet precise enough for a true test.

Paul Steinhardt of Princeton University says that the difficulties with standard cold dark matter have become clear only within the past two years, as observations and simulations with high enough resolution have become available. The discrepancies have sparked several new proposals for the nature of dark matter, including fuzzy dark matter, and “people are being very creative,” Steinhardt says. He expects it will take a few years before each idea is thoroughly implemented in simulations and compared with observations, but “when we’re through, we’re going to learn a lot more about the nature of dark matter.”

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