Cracking the Dark Code: A Treasure Hunt for a Hypothetical Particle

Dark energy and dark matter are called "two dark clouds in the sky of physics in the early 21st century." However, Gao Yu, an associate researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, and Yang Qiaoli, a professor at the School of Science and Engineering of Jinan University, both pointed out in an interview with Science and Technology Daily that after years of exploration, physicists have begun to realize that hypothetical particles called axions can not only explain the basic symmetry problems of dark matter and strong interactions, but also explain the mystery of dark energy and even the imbalance of matter and antimatter.

Given that axions are responsible for solving many cosmological mysteries, the "treasure hunt" for axions has been carried out in full swing in many parts of the world.

What is the Axion?

In 1977, physicist Frank Wilczek took a routine walk that forever changed the pace of exploration for some scientists.

During that walk, he came up with two ideas: one about how a theoretical particle that would later be called the Higgs boson might interact with other particles, and the other about using axions as a solution to a theoretical physics problem called strong charge-parity (CP).

Gao Yu explained: "What Wilczek called 'axion' is based on the new elementary particle predicted by theoretical physicists to solve the symmetry theory in quantum chromodynamics. The English name of 'axion' is a word game: AXI-ON = axis + particle. Wilczek thought that a detergent brand with the same name was very suitable to describe the properties of the particle, so he chose this name."

The researchers pointed out that if axions exist, they will obey the strange rules of quantum mechanics. This means that they can be both waves and particles. As particles, their mass will be very low, about 10-11-10-9 times the mass of an electron. Its macroscopic wavelength is even comparable to the width of a galaxy, up to 3,000 light years.

Hopefully it will reveal several mysteries

Scientists introduced axions because they wanted them to be very weak, thus representing an extremely weak CP violation. This results in axions having very small mass and theoretically almost no interaction with other particles, which makes axions an ideal candidate for dark matter research because these properties are exactly the same as dark matter.

"The reason why dark matter is dark is that it hardly interacts with light. It neither reflects light nor produces light. This coincides exactly with the characteristics of the weak interaction of axions," said Yang Qiaoli.

The early universe was filled with a lot of energy. As the universe cooled, the axion field began to oscillate, releasing energy in the form of pulsating light and heat, and the energy density carried by these oscillations evolved just like dark matter.

Yang Qiaoli explained: "When the mass of the axion is very light, the age of the universe may even be less than the time required for the axion field to vibrate once, so the axion can be regarded as a kind of dark energy. In this way, the more the universe expands, the greater the energy of the axion field, which can drive the universe to continue to expand."

Axions also hold promise for providing clues to two other mysteries of the universe. The first is the Hubble constant crisis. The Hubble constant that scientists derive from combining measurements of the cosmic microwave background with the current standard model of cosmology is consistently significantly lower than observations based on Type Ia supernovae and other astrophysical markers. Scientists predict that if there were certain axion-like particles in the early universe, they could change predictions based on the cosmic microwave background, thereby eliminating the Hubble constant crisis.

The second mystery is the imbalance of matter and antimatter. When the universe was born, equal amounts of matter and antimatter should have been produced, and they should have annihilated immediately after meeting each other, but in fact matter dominated. An article published in Physical Review Letters in 2020 pointed out that at the beginning of the Big Bang, the movement of the axion field could produce an imbalance of matter and antimatter, making the matter in the universe that has evolved to this day far more than antimatter, so everything may have originated from the axion field.

The “hunting” operation has various tricks

Detecting axions is extremely challenging given that they hardly interact with other particles and have an extremely low mass, but some hunts have already begun.

The largest experiment at present is the Axion Dark Matter Experiment (ADMX) at the University of Washington in the United States, which aims to use magnetic fields to capture axions that decay into photons. In theory, axions in the universe can be converted into low-energy microwave photons in a microwave resonant cavity surrounded by superconducting magnets, which are amplified by the resonant cavity and then detected by the detector.

The "Axion Solar Telescope" of CERN takes a different approach, using an X-ray telescope to detect axions produced by the sun. Nuclear reactions in the sun produce a variety of particles such as neutrinos and high-energy photons, and may also produce axions. The kinetic energy of the axions produced is extremely high, and the energy of the photons produced by their conversion is in the X-ray band, which can be observed with an X-ray telescope.

In addition, axion-photon oscillations can change the shape of the energy spectrum of distant stars, and its "clues" may be captured in high-precision or high-energy astronomical observations such as my country's hard X-ray modulation telescope, 500-meter aperture spherical radio telescope, gravitational wave burst high-energy electromagnetic counterpart all-sky monitor satellite, and high-altitude cosmic ray observatory.

In recent years, various new small experiments have emerged that take advantage of the wide range of axion masses. A new experiment called "Hidden Region Quantum Sensing" at the University of Sheffield in the UK will start in 2024. Compared with other axion search experiments, this experiment can be operated at a temperature close to absolute zero, which will make the experiment essentially in a quantum state, so it is expected to be more sensitive than other experiments.

Yang Qiaoli said that the increasingly mature axions still have a wide parameter space to be explored. World-class universities and research institutions such as the Massachusetts Institute of Technology and Yale University in the United States, the Italian National Institute of Nuclear Physics, and the Korean Axion and Precision Physics Center are actively conducting relevant research.

Gao Yu and Yang Qiaoli both believe that the resonant cavity experimental technology represented by ADMX has led to a wave of searching for dark matter axions internationally. At present, China has the experimental conditions to catch up with the international advanced level, and many detection schemes such as spin magnetic measurement and various resonant cavities are being actively promoted, some of which are original schemes independently proposed in China.