JILA researchers used an advanced atomic clock to restrict the search for elusive dark matter, an example of the value of continuous improvements in clocks beyond timekeeping.
Older atomic clocks operating at microwave frequencies have already driven out dark matter, but this is the first time that a newer clock, operating at higher optical frequencies, and an ultra-stable oscillator to ensure stable light waves, have been exploited to define more precise limits. on research. The research is described in Physical examination letters .
Astrophysical observations show that dark matter makes up most of the “substance” of the universe, but so far it has escaped capture. Researchers around the world have searched for it in various forms. The JILA team focused on ultralight dark matter, which in theory has tiny mass (much less than a single electron) and enormous wavelength – how far a particle travels in space – which could be as large as the size of dwarf galaxies. This type of dark matter would be linked by gravity to galaxies and therefore to ordinary matter.
Ultralight dark matter is expected to create tiny fluctuations in two fundamental physical “constants”: the mass of the electron and the fine structure constant. The JILA team used a strontium lattice clock and a hydrogen maser (a microwave version of a laser) to compare their well-known optical and microwave frequencies, respectively, to the frequency of light resonating in an ultra-stable cavity made from a single crystal. of pure silicon. The resulting frequency ratios are sensitive to variations over time of the two constants. Relative fluctuations in ratios and constants can be used as sensors to relate cosmological models of dark matter to accepted physical theories.
The JILA team set new limits on a floor for “normal” fluctuations, beyond which any unusual signal discovered later could be due to dark matter. The researchers constrained the coupling strength of ultralight dark matter to electronic mass and fine structure constant to be in the range of 10-5 (1 in 100,000) or less, the most accurate measurement ever made of this value.
JILA is jointly managed by the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.
“No one knows at what level of sensitivity you will start to see dark matter in lab measurements,” said Jun Ye, NIST / JILA member. “The problem is that physics as we know it is not quite complete at this point. We know something is missing, but we don’t yet know how to fix it. “
“We know dark matter exists from astrophysical observations, but we don’t know how dark matter connects to ordinary matter and the values that we are measuring,” Ye added. “Experiments like ours allow us to test various models of theory that people have put in place to try and explore the nature of dark matter. By setting better and better limits, we hope to eliminate some incorrect theoretical models and possibly make a discovery in the future. “
Scientists don’t know if dark matter is made up of particles or oscillating fields affecting local environments, Ye noted. The JILA experiments aim to detect the “pull” effect of dark matter on ordinary matter and electromagnetic fields, he said.
Atomic clocks are first-rate probes for dark matter because they can detect changes in fundamental constants and rapidly improve in accuracy, stability and reliability. The stability of the cavity was also a crucial factor in the new measurements. The resonant frequency of light in the cavity depends on the length of the cavity, which can be traced back to the Bohr radius (a physical constant equal 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 electronic mass. Therefore, changes in resonant frequency relative to transition frequencies in atoms can indicate fluctuations in these constants caused by dark matter.
The researchers collected data on the strontium-to-cavity frequency ratio for 12 days, with the clock running 30% of the time, resulting in a data set of 978,041 seconds. The data from the hydrogen maser lasted 33 days, with the maser running 94% of the time, resulting in a record 2,826,942 seconds. The hydrogen / cavity frequency ratio provided useful sensitivity to electronic ground although the maser was less stable and produced louder signals than the strontium clock.
JILA researchers collected dark matter research data during their recent demonstration of an improved timescale – a system that integrates data from multiple atomic clocks to produce a single, highly accurate timing signal for distribution. As the performance of atomic clocks, optical cavities, and time scales improve in the future, frequency ratios can be reexamined with ever higher resolution, further expanding the scope of research on the black matter.
“Anytime you use an optical atomic timescale, there’s a chance to set a new limit or make a discovery of dark matter,” Ye said. “In the future, when we can put these new systems into orbit, it will be the largest ‘telescope’ ever built for dark matter research.”
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