From the astronomers came to a consensus in the 1980s that most of the mass of the universe is invisible – that “dark matter” must glue galaxies together and by gravity sculpt the cosmos as a whole, the experimenters looked for non-luminous particles.
They first set out in pursuit of a heavy, slow form of dark matter called a Weakly Interacting Massive Particle, or WIMP – the top favorite candidate for the missing matter in the cosmos because it could solve another unrelated puzzle in particle physics. Over the decades, teams of physicists have set up larger and larger targets, in the form of huge crystals and multi-ton vats of exotic liquids, in hopes of catching the rare earthquake. an atom when a WIMP struck it.
But these detectors have remained silent, and physicists increasingly envision a wider spectrum of possibilities. On the heavy side, they say the invisible matter of the universe could clump into black holes as heavy as the stars. At the other extreme, dark matter could spread in a fine mist of particles trillions of billions of times lighter than electrons.
New hypotheses are accompanied by new detection methods. Kathryn Zurek, theoretical physicist at the California Institute of Technology, said that if the current WIMP experiments see nothing, “then I think there will be a substantial part of the field that is going to move into these new types of experiments. “
Already, the work has started. Here are some of the many new fronts in the search for dark matter.
Between an electron and a proton
The WIMPs would have enough weight to occasionally pass over an entire atom. But in case the dark matter is lighter, some experimenters are installing smaller pins.
A softer rain of dark matter particles weighing less than protons could occasionally release electrons from their host atoms. The first experiment designed specifically to capture this dark matter is the CCD Sub-Electron-Noise Skipper experimental instrument (Sensei), which uses technology similar to digital cameras to amplify signals of emancipated electrons unexpectedly inside materials.
When a Sensei prototype ignited with only a tenth of a gram of silicon, it found no dark matter. Even so, the team’s results, published in 2018, instantly excluded some models.
“We just turned on and we had the best limits in the world,” said Tien-Tien Yu, a physicist at the University of Oregon and a member of the Sensei Collaboration, “because there was no limits previously. ”
Recent results from a 2 gram version of Sensei extend those limits, and now Yu and his colleagues are preparing to deploy a 10 gram version in an underground lab in Canada, away from interfering cosmic rays. Other groups are designing low-cost alternative experiments targeting the same fruit at hand.
If dark matter is even lighter or blind to electric charge, it may not release an electron. Zurek reflected on ways in which even these pipsqueaks could betray their presence by influencing the behavior of groups of particles.
Imagine a block of silicon, for example, like a mattress with springs representing atomic nuclei. Bounce a quarter on the mattress, Zurek says, and while none of the springs will move much, the part could trigger a ripple that goes through many springs. She proposed in 2017, a similar disturbance of a dark matter interaction could generate sound waves that could slightly heat the system.
A project taking this route, Tesseract, is currently running in a basement at the University of California at Berkeley, looking for ripples from dark particles similar in weight to those Sensei targets. However, future sensitive upgrades could theoretically find particles up to a thousand times lighter.
But there are still more possibilities for Lilliputian particles. The axion – an entity so light that it is more of a wave than a particle – could include dark matter and simultaneously resolve a mystery about the strong nuclear force. the Axion Dark Matter Experiment (ADMX) recently began researching axions decaying into pairs of photons inside a strong magnetic field, and more similar research is underway.
Some experiences aim for even lighter things. The lightest dark matter can be is about a trillionth of a trillionth of the electron’s mass – which would make a particle that looks like a very low energy wave, with a wavelength about the size of d ‘a small galaxy. Lighter (and therefore longer) entities would be too diffuse to explain why galaxies stick together.
Clues from above
As experimenters prepare for the next generation of devices looking for direct contact with dark matter, others plan to scour the skies for indirect signposts.