Plasma is thought to be the most common form of matter in the universe, but it’s usually pretty hot. The plasma in a solar corona can have a temperature in the millions of degrees.

Researchers at the National Institute of Standards and Technology’s Physics Laboratory, however, have created “ultracold” plasmas—with the electrons about a degree above absolute zero—by cooling neutral atoms to within a hundred-thousandth of a degree of absolute zero and then zapping them with just enough laser energy to separate the electrons and ions to achieve the plasma state.

One of the key measures of any plasma is the recombination rate—how fast the ions and electrons recombine to form neutral atoms.

Theory says there are three main recombination processes, and their efficiency varies in a known way with temperature and density.

However, NIST physicist Steven Rolston says that in practice, an expanding ultracold plasma recombines much faster than expected at very low densities—so much faster that no existing theory describes it.

Rolston and his group are continuing to refine their experiments to explain the behavior of ultracold plasmas, which, although they only exist in earthly labs, are thought to model the interior of white dwarf stars or gas giant planets like Jupiter.

The research also may uncover a path to synthesizing “anti-hydrogen” atoms, the antimatter equivalent of hydrogen. Precise comparisons of the properties of such antimatter twins may probe the fundamental nature of the forces that bind matter and the universe together.

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