Bose-Einstein offers view on dwarf galaxies

The universe may consist primarily of very small "dark-matter" particles within a very large Bose-Einstein condensate, according to a team of researchers at the Institute for Advanced Studies (Princeton, NJ). The new theory, referred to as "fuzzy cold dark matter," provides a means of explaining discrepancies that have been encountered in describing the structure of dwarf galaxies using the traditional cold dark-matter (CDM) models. "CDM models predict cuspy dark matter halo profiles and an abundance of low mass halos not seen in the rotation curves and local population of dwarf galaxies, respectively," say the researchers.¹

To address this discrepancy without sacrificing the strengths of CDM theory such as its explanations of the microwave background and the clustering patterns and rotation speeds of galaxies, the researchers assumed a very small mass for the dark-matter particles, on the order of 10^-22 eV. Since the de Broglie wavelength of such particles is on the order of thousands of light years, the predicted mass density at the core of a dwarf galaxy would fall more in line with observation. Also the very cold temperatures predicted by traditional CDM theory indicate that these very small particles would actually occupy a galaxy-sized Bose-Einstein condensate.

The fuzzy cold dark-matter approach is just one of several that have been advanced to deal with the CDM theory discrepancy during the last couple of years. And the significance of the discrepancy itself is still open to dispute. One CDM theory modification involves warm dark matter, as opposed to CDM, streaming out of potential wells with mass on the order of thousands of electronvolts. Other modifications include the idea of strong self-interactions among dark-matter particles. Unlike fuzzy cold dark matter, however, other modifications tend to produce further discrepancies between CDM predictions and observations.

The success of the fuzzy-cold-dark-matter approach in explaining the stability of dwarf galaxies is based on applying the uncertainty principle in wave mechanics, which explains the stability of the hydrogen atom on a galactic scale. The hydrogen atom does not collapse because the de Broglie wavelength of the electron is about the same size as the hydrogen atom, according to Wayne Hu, lead author of a paper on this subject. To date the researchers have modeled their theory successfully using one-dimensional simulations, but three-dimensional simulations will be necessary to evaluate it in detail.

REFERENCE

1. W. Hu, R Barkana, and A. Gruzinov, Phys. Rev. Lett. 85(6), 1158 (7 Aug. 2000).