Photo credit: N. Hanacek / NIST
JILA researchers have used a state-of-the-art atomic clock to narrow down the search for elusive dark matter. This is an example of how continuous improvements in clocks beyond the measurement of time have value.
Older atomic clocks that work with microwave frequencies looked for dark matter before. However, this is the first time a newer clock with higher optical frequencies and an ultra-stable oscillator have been used to ensure that steady waves of light are used to set more precise limits on the search. The research is described in Physical Examination Letters.
Astrophysical observations show that dark matter makes up the majority of the “material” in the universe, but has so far eluded detection. Researchers around the world have looked for it in various forms. The JILA team focused on ultra-light dark matter, which theoretically has a tiny mass (much less than a single electron) and a huge wavelength – how far a particle propagates in space – that could be as large as the size of dwarf galaxies . This type of dark matter would be bound to galaxies by gravity, and thus to ordinary matter.
Ultralight dark matter is expected to produce minute fluctuations in two basic physical “constants”: electron mass and fine structure constant. The JILA team used a strontium lattice clock and a hydrogen maser (a microwave version of a laser) to compare their known optical and microwave frequencies to the frequency of light vibrating in an ultra-stable cavity made from a single crystal of pure silicon. The resulting frequency relationships are sensitive to fluctuations in both constants over time. The relative fluctuations in ratios and constants can be used as sensors to link cosmological models of dark matter with accepted physical theories.
The JILA team has set new limits for “normal” fluctuations on a floor, above which unusual signals that are later discovered may be due to dark matter. The researchers limited the coupling strength of ultra-light dark matter to the electron mass and the fine structure constant to about 10-5 (1 in 100,000) or less, the most accurate reading ever taken for this value.
JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.
“Nobody really knows what level of sensitivity you will see dark matter in laboratory measurements,” said Jun Ye, Fellow of NIST / JILA. “The problem is that physics as we know it is not yet complete at this point. We know something is missing, but we don’t yet know exactly how to fix it. ”
“We know from astrophysical observations that dark matter exists, but we don’t know how dark matter combines with ordinary matter and what values we measure,” added Ye. “Experiments like ours allow us to test various theoretical models that humans have put together to study the nature of dark matter. We hope that by setting ever better limits, we can rule out some false theoretical models and make a discovery in the future. ”
Scientists aren’t sure if dark matter is made up of particles or oscillating fields that affect the local environment, Ye noted. The JILA experiments are designed to capture the “pulling effect” of dark matter on ordinary matter and electromagnetic fields, he said.
Atomic clocks are the main probes for dark matter because they can detect changes in the basic constants and improve rapidly in precision, stability and reliability. The stability of the cavity was also a decisive factor in the new measurements. The resonance frequency of the light in the cavity depends on the length of the cavity, which can be traced back to the Bohr radius (a physical constant that corresponds to the distance between the nucleus and the electron in a hydrogen atom). The Bohr radius is also related to the values of the fine structure constant and the electron mass. Therefore, changes in the resonance frequency compared to transition frequencies in atoms can indicate fluctuations in these constants caused by dark matter.
Researchers collected strontium / cavity frequency ratio data for 12 days, with the clock running 30% of the time, resulting in a data set of 978,041 seconds. The hydrogen maser data spanned 33 days with the maser running 94% of the time, resulting in a record of 2,826,942 seconds. The hydrogen / cavity frequency ratio provided useful sensitivity to electron mass, although the maser was less stable and produced louder signals than the strontium clock.
JILA researchers collected the dark matter search data during their recent demonstration of an improved time scale – a system that incorporates data from multiple atomic clocks to generate a single, highly accurate timekeeping signal for distribution. As the performance of atomic clocks, optical cavities, and time scales improve in the future, the frequency relationships can be reexamined with ever higher resolution, further expanding the range of the dark matter search.
“Every time an atomic optical time scale is run, there is an opportunity to push a new frontier or discover dark matter,” Ye said. “If we can put these new systems into orbit in the future, it will be the largest ‘telescope’ ever built to search for dark matter.”
Reference: “Precision Metrology Meets Cosmology: Improved Constraints on Ultra-Light Dark Matter From Atom-Cavity Frequency Comparisons” by Colin J. Kennedy, Eric Oelker, John M. Robinson, Tobias Bothwell, Dhruv Kedar, William R. Milner and G. Edward Marti, Andrei Derevianko and Jun Ye, November 12th 2020, Physical Examination Letters.
DOI: 10.1103 / PhysRevLett.125.201302
Funding was provided by NIST, the Agency for Advanced Defense Research Projects, and the National Science Foundation.
