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Home SCIENCE The compelling case for axions as our dark matter | by Ethan Siegel | Starts With A Bang! | Apr, 2024

The compelling case for axions as our dark matter | by Ethan Siegel | Starts With A Bang! | Apr, 2024

by California Digital News

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Axions, one of the leading candidates for dark matter, may be able to be converted to photons (and vice versa) under the right conditions. If we can cause and control their conversion, we might discover our first particle beyond the Standard Model, and possibly solve the dark matter and strong CP problems as well. This would imply that we do live in a Universe with strong CP-violation, but only a tiny amount of it: below the experimental and observational thresholds. (Credit: Sandbox Studio, Chicago, Symmetry Magazine/Fermilab and SLAC)

The majority of the matter in our Universe isn’t made of any of the particles in the Standard Model. Could the axion save the day?

Astrophysically, normal matter — even with all the different forms it can take — cannot on its own explain the Universe we observe. Beyond all the stars, planets, gas, dust, plasma, black holes, neutrinos, photons, and more, there’s an overwhelming suite of evidence suggesting that the Universe contains two ingredients whose origins remain unknown: dark matter and dark energy. Dark matter, in particular, has an incredible amount of astrophysical evidence supporting its existence and abundance — outmassing normal matter by a 5:1 ratio. Still, its particle nature remains elusive, though we’re quite certain it must have been cold, or slow-moving at early times, rather than hot, where it would have moved faster in the young Universe.

One of the leading candidates for its nature, the axion, remains compelling more than 40 years after it was first hypothesized, though it’s rarely even presented to the general public. Could this intriguing theoretical particle be the solution to the dark matter puzzle? That’s what Reggie Grünenberg wants to know, asking:

“Axions are speculative particles and hot candidates for dark matter particles that are supposed to have been created primarily in the Big Bang and since then permanently within the cores of stars through a mechanism called the Primakoff effect. This would mean that stars would ‘produce’ dark matter — and that they would need to lose this way much more mass than through nuclear fusion. And that the amount of dark matter in galaxies would grow over time, thus accelerating orbiting stars ever more. Could this model really work?”

There’s a lot to unpack here. But if we go one step at a time, you just might come away thinking that the axion could one day be the solution to the greatest cosmic mystery of all.

The particles and antiparticles of the Standard Model have now all been directly detected, with the last holdout, the Higgs boson, falling at the LHC earlier this decade. Today, only the gluons and photons are massless; everything else has a non-zero rest mass. (Credit: E. Siegel/Beyond the Galaxy)

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