“We do not know what dark matter is, but if it has anything to do with any scalar particles, it may be older than the Big Bang,” says astrophysicist Tommi Tenkanen at the Johns Hopkins University, who was not part of a 2019 University of Tokyo study that proposed the axion as a candidate for dark matter. The only fundamental scalar quantum field that has been observed in nature is the Higgs field-a field of energy that is thought to exist in every region of the universe.
Planet-sized Particles to Highly-speculative Dark-Matter Life
Physicists have imagined new kinds of matter ranging from planet-sized particles to highly-speculative dark-matter life, consistent with the known laws of the universe, The Daily Galaxy reported on August 8, 2019, but so far none has been detected or its existence confirmed. The Large Hadron Collider’s discovery of the Higgs boson in 2012 prompted an all too brief burst of optimism that dark matter particles would soon be discovered, but so far none has been seen and previously promising classes of particles have been dashed.
“Inconveniently, dark matter is “dark” in the sense that it hardly interacts with anything, particularly with light. Apparently, in some scenarios it could have a slight effect on light waves passing through. But other scenarios predict no interactions at all between our world and dark matter, other than those mediated by gravity. This would make its particles very hard to find,” observed Sergey Troitsky, chief researcher at the Institute for Nuclear Research of the Russian Academy of Sciences in a March 29, 2019 post in The Daily Galaxy.
ESA’s Euclid satellite will probe the eon before the Big Bang
While dark matter may be too elusive to be found in particle experiments, it can reveal its presence in astronomical observations. We will learn more about the origin of dark matter when the ESA’s Euclid satellite is launched in 2022, which aims at probing the eon before the Big Bang as well as understanding why the expansion of the Universe is accelerating and what is the nature of the source responsible for this acceleration which physicists refer to as dark energy. Euclid, reports the ESA, will explore how the Universe evolved over the past 10 billion years to address questions related to fundamental physics and cosmology on the nature and properties of dark energy, dark matter and gravity.
Dark matter is only known by its effect on massive astronomical bodies, but has yet to be directly observed or even identified. A theory about what dark matter might be suggests that it could be a particle called an axion and that these could be detectable with laser-based experiments that already exist. These laser experiments are gravitational-wave observatories –the most precise machines ever built. These detectors sense fluctuations comparable to subatomic sizes, over their length of kilometers.
At first, we thought it was absurd. How else could you respond to the idea that black holes generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together? We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy,” –theoretical physicist Asimina Arvanitaki
“We don’t know the mass of axions, but we usually think it has a mass less than that of electrons. Our universe is filled with dark matter and it’s estimated there are 500 grams of dark matter within the Earth, about the mass of a squirrel,” says Yuta Michimura with the Department of Physics at the University of Tokyo. “We assume the axion is very light and barely interacts with our familiar kinds of matter. Therefore, it is considered as a good candidate for dark matter.”
Efforts to detect WIMP dark matter
There are many theories as to what manner of thing dark matter might turn out to be, but many physicists believe dark matter is a weakly interacting massive particle, or WIMP. What this means is that it does not interact easily with ordinary matter. We know this to be true because it hasn’t been seen directly yet. But it must also have at least some mass as its presence can be inferred by its gravitational attraction.
There have been enormous efforts to detect WIMP dark matter, including with the Large Hadron Collider in Switzerland, but WIMPs haven’t been observed yet. An alternative candidate particle gaining attention is the axion.
“Our models suggest axion dark matter modulates light polarization, which is the orientation of the oscillation of electromagnetic waves,” explained Koji Nagano, a graduate student at the Institute for Cosmic Ray Research at the University of Tokyo. “This polarization modulation can be enhanced if the light is reflected back and forth many times in an optical cavity composed of two parallel mirrors apart from each other. The best-known examples of these kinds of cavities are the long tunnel arms of gravitational-wave observatories.”
Dark matter research does not get as much attention or funding as other more applicable areas of scientific research, so great efforts are made to find ways to make the hunt cost-effective. This is relevant as other theoretical ways to observe axions involve extremely strong magnetic fields which incur great expense. Here, researchers suggest that existing gravitational-wave observatories such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S., Virgo in Italy or KAGRA in Japan could be cheaply modified to hunt for axions without detriment to their existing functions.
Researchers have designed a way to give the long tunnel arms of gravitational-wave observatories like KAGRA seen above the ability to potentially also detect axion dark matter. © 2019 University of Tokyo Institute for Cosmic Ray Research.
The Axion Hunt Graphic
This chart compares the sensitivity of gravitational-wave detectors suitable for the axion hunt. The project is ongoing at the University of Tokyo Institute for Cosmic Ray Research.
“With our new scheme, we could search for axions by adding some polarization optics in front of photodiode sensors in gravitational-wave detectors,” described Michimura. “The next step I would like to see is the implementation of optics to a gravitational-wave detector like KAGRA.”
One of the biggest outstanding problems in modern physics
This idea has promise because the upgrades to the gravitational-wave facilities would not reduce the sensitivity they rely on for their primary function, which is to detect distant gravitational waves. Attempts have been made with experiments and observations to find the axion, but thus far no positive signal has been found. The researchers’ proposed method would be far more precise.
“There is overwhelming astrophysical and cosmological evidence that dark matter exists, but the question “What is dark matter?” is one of the biggest outstanding problems in modern physics,” said Nagano. “If we can detect axions and say for sure they are dark matter, it would be a truly exciting event indeed. It’s what physicists like us dream for.”
The Daily Galaxy with Miguel Zumalacarregui at the The Max Planck Institute for Gravitational Physics (Einstein Institute). via University of Tokyo
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