Perhaps fortunately, most folks haven't noticed that 85% of the Milky Way is missing: The kind of familiar, ordinary matter we know – made up of protons, neutrons, electrons, and their kin – amounts to barely one-sixth of the total mass of our galaxy and others, according to repeated astronomical observations.
Where's the rest? NIST is once again helping to look for it – this time in Seattle, where an ambitious search effort depends critically on a set of ultra-sensitive sensors provided by NIST's Physical Measurement Laboratory (PML).
Whatever it is, the missing mass is in some form that doesn't absorb or emit light and thus can't be seen by telescopes. So it is called "dark matter," and is presumed to surround and pervade the galaxy like an invisible halo, flowing through everything and everybody all the time.
For decades, scientists have been searching for various theorized entities that might fill the bill – with no conclusive evidence to date. One leading prospect is WIMPs: weakly interacting massive particles. The U.S. Department of Energy (DOE) and the National Science Foundation (NSF) are funding two WIMP hunts in North America using deep-underground detectors for which PML provided essential elements in the form of SQUIDs (superconducting quantum interference devices), which are exquisitely responsive sensors and can serve as amplifiers of signals from photon detectors.
But DOE and NSF are also supporting pursuit of another hypothetical particle: the axion. This exotic contender was first postulated in the 1970s as a way to account for a nagging theoretical problem in nuclear physics, but also turns out to have the right characteristics to be a dark matter candidate – if it exists and can be detected.
That's the mission of the Axion Dark Matter Experiment (ADMX), a multi-institution collaboration based at the University of Washington. It too relies on PML's long-standing expertise in fabricating SQUIDs for special purposes. ADMX operates on the theory that once in a while an axion will reveal itself because when it is in a magnetic field and excited by radio waves at the precise frequency that corresponds to the particle's mass, the axion will decay into extremely faint microwave photons.
"Could your cell phone get four bars on Mars? With help from NIST, it probably could."
- Leslie Rosenberg, ADMX dark matter search collaborator, on the sensitivity of NIST SQUIDs
The experiment features a hollow meter-high cylindrical cavity, kept much colder than liquid helium, inside an 8 tesla magnetic field. The interior of the cavity has adjustable tuning rods that incrementally change the resonant frequency in the cavity. The idea is to sweep across a range of frequencies until they hit exactly the right one that prompts the axion decay. The resulting microwave photons should be measurable. Just barely. That's why researchers called on NIST to fabricate a special set of SQUIDs.
"PML's expertise and facilities for fabricating SQUIDs that have world-leading performance specifications enable NIST and collaborators to make unprecedented measurements of low-level signals," says Robert Hickernell, Chief of PML's Quantum Electromagnetics Division.
The axion could have a mass as small as one-trillionth the mass of an electron, and the power it releases in the chamber is projected to be around a yoctowatt: a trillionth of a trillionth of a watt (10-24 W). Detecting that requires some device that can not only register the vanishingly small signal, but amplify it without adding noise.
One frequently used amplifier for this kind of work is the high electron-mobility transistor (HEMT). "But high-performance HEMTs have noise temperatures of about four or five kelvin," says veteran nanofabricator Gene Hilton of PML's Quantum Electromagnetics Division, who recently made the ADMX SQUIDs and, among other accomplishments, made the SQUIDs used in the widely heralded BICEP2 cosmological observations. "Our best is around 100 microkelvin. That's the kind of performance that allows scientists to do the experiment in a few years instead of a few centuries."
ADMX collaboration leader Leslie Rosenberg explains: "Every amplifier introduces noise, and this noise interferes with the desired signal. It happens that quantum mechanics sets a lower limit to amplifier noise. Regular transistors, ubiquitous in electronics, are far from reaching that quantum limit. But NIST's SQUIDs can.
"In general, the time it takes a receiver to detect a signal scales as the square of the noise. NIST's SQUIDs have noise levels 100 times lower that HEMTs, so the time it takes to detect a signal is thereby reduced by around 10,000. Without the NIST SQUIDs, this experiment and others would not be possible."