A roulette wheel and dark magic

In my blog "On a rubber mat" I briefly described how space-time is determined by its mass energy contents. For instance, a star curves the space around it creating a gravitational pot hole where other matter falls and gets imprisoned. The bigger is the mass the more curvature is resulted. In a galaxy there are billions of starts each contributing to the total curvature of the space around the galaxy. Stars have motion that prevents them to fall in to the center of galaxy. In the center of a galaxy there is likely always a super massive black hole. You can think a roulette where you throw multiple balls that start spinning around the center. In contrast to the roulette wheel, there's no friction in the space reducing the speed of the stars, which lets them to orbit the center for ages. If we were able to add more enough speed to a ball while spinning, it would eventually reach the edge of the wheel and rolls out with its current direction. If a star had high enough velocity it would escape the gravitational well of the galaxy same way. 

The motion of stars has been researched for a long time. In 1922, Dutch astronomer Jacobus Kapteyn found that there needs to be much more mass in the universe than is visible to explain the velocities of the stars. He called this invisible mass as dark matter. In 1933, Swiss astrophysicist Fritz Zwicky studied galaxy clusters, and found that the gravity effect of the visible part of galaxies was far too small to keep galaxy clusters together. Rubin and Ford's results published in 1980, concluded that most of galaxies must contain about six times as much dark as visible mass. A stream of observations in the 1980s supported the presence of dark matter, including gravitational lensing of background objects by galaxy clusters, which resulted in a consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle. 

Today the dark matter is expected to account for approximately 85% of the matter in the universe. Its subatomic form is still a mystery. Seems that dark matter barely interacts with any other way than by gravity, which makes its direct detection very hard. There are several theories presented. In the most cases dark matter is though to be non-baryonic because of lack of interactions. Hypothetical weakly interacting massive particles (WIMP) became a favorite candidate for the dark matter in the 1980s. They are typically predicted to be 1–1,000 times heavier than protons and to interact with matter only feebly. Because of their large mass, they would be relatively slow moving and therefore "cold". Simulations of a universe full of cold dark matter produce galaxy distributions that are roughly similar to what is observed. By contrast, hot dark matter would smear out the large-scale structure of galaxies and thus is not considered a viable cosmological model. Besides the gravity WIMP may have the weak nuclear interaction. XENONnT is the world’s largest and most sensitive detector for direct searches of WIMPs, located in Gran Sasso (LNGS), in Italy. It contains six tons of xenon atoms waiting a hit of a WIMP, which would cause burst of light and an emission of electron that the detector would pick up. While WIMPs were not found by the previous smaller detectors such as XENON1T, their mass has been already limited to below 10 protons. If XENONnT won't make a discovery it will push the limit further. Construction work for the next generation detector Darwin reaching up to 50 xenon tons is already planned to start in 2025. Going beyond that size is improbable since the detector would reach the neutrino floor, at which WIMPs cannot be anymore distinguished from neutrinos. 

Though no WIMPs were found, XENON1T provided valuable data, for instance, about neutrinos. In June 2020, the XENON1T research group published a possible discovery of hypothetical axion particle. Axions were originally theorized in 1970 for explaining the CP symmetry problem of the strong interaction. But as they are invisible and have very weak interaction with matter they have been a cold dark matter candidate too. In contrast to WIMPs, axions are very light (a fraction of an electron mass) so there needs to be orders more of them to explain dark matter. In one model the axion field (aka Peccei-Quinn field) went through a phase transition shortly after the cosmic inflation. As a result the axion field stuck to different values within patches of the universe creating axion strings that were topological defects between these patches. Before the first microsecond of the universe axion strings decayed to axions, which is proposed to be source of dark matter. The recent simulation of axion strings (Buschmann et.al in 2022) predicts axion mass to range from 40 to 180 microelectronvolts. This type of mass predictions help to focus the experiments of the direct axion detection in future. 

In 2021, ENS scientists constructed so far the largest and most detailed dark matter map of the universe by analyzing gravitationally lensed shapes of the millions distant galaxies. Dark matter seems to be distributed more evenly in the universe as believed before, which even questions the current cosmology theory based on the work done by Einstein. The map shows that a web of dark matter spreads across the entire universe. Dark matter is concentrated in "halos" where galaxies are centered but there are also vast areas of nothingness, where laws of physics could be different. If axion field really exist and it has patched the universe with different field values, could this explain imparity with the prevailing theory and what is recently observed? There has been earlier direct observations of filaments that are the largest constructions of the universe coupling galaxy clusters together, which also supports cold dark matter theory. It was predicted that 60% of all hydrogen is located in filaments. Dark matter plays a key role in determining the structure of filaments and galaxies in the largest scale. Could filaments be relics of axion strings that has left finger prints in the cosmic web? The popularity of axion research has partially raised since a lack of predicted signals from other dark matter candidates. Many WIMP models are meanwhile ruled out by exploring cosmic rays. Like normal matter WIMP is expected to have a particle and anti particle, which annihilate each other when they collide. Since the expected radiation from annihilation has not been been discovered from the space, it has turned investigations to other dark matter candidates. Most notable other candidates under research are sterile neutrons and primordial black holes that hypothetically formed right after the big bang.