According to the new theory, proposed by South Korean theoretical physicists, dark matter was born from Fermi balls, quantum "bags" of subatomic particles that were packed in dense "pockets" during the dawn of the universe. This theory claims to explain the fact why dark matter began to dominate in the Universe over ordinary, visible matter.
Theoretical physicists at the Center for Theoretical Physics at Seoul National University in South Korea have suggested that the mysterious and elusive dark matter may be composed of so-called Fermi balls left over from the Big Bang. They published their article on the arXiv.org electronic preprint site.
Fermi balls are hypothetical cosmological objects that could appear in the conditions of the earliest Universe due to spontaneous symmetry breaking and the subsequent phase transition. They should not be confused with Fermi bubbles, giant structures in the Milky Way named after the Fermi gamma-ray telescope and formed by the activity of the central supermassive black hole.
Dark matter is a mysterious substance that interacts with ordinary matter only through gravity and does not interact with light. Once upon a time, hypotheses have been made according to which dark matter may consist of tiny black holes permeating the Universe, but numerical estimates excluded such a possibility: the number of such black holes, as well as black holes of stellar mass, is too small to converge the "budget of the Universe", known from various experimental data (from the Planck space observatory, from observations of supernovae, etc.): there should be only 5% of ordinary (baryonic) matter, 27% of dark matter and 68% of dark energy. In the entire history of the Universe, not so many stars have formed that, after their death, they gave rise to a sufficient number of black holes, the mass of which would explain the entire amount of available dark matter. At the moment, the theory of cold dark matter is considered standard, and the most likely candidates for the role of its constituent particles are the as yet undetected WIMPs - weakly interacting massive particles.
According to the new theory, the sought-after black holes could once have arisen from Fermi balls or quantum "bags" of subatomic particles - fermions - that mixed in dense "pockets" during the origin of the Universe. This theory claims to explain the fact why dark matter began to dominate in the Universe over ordinary, visible matter.
“We found that in some cases the Fermi balls could be so tightly packed that the fermions in them were too close to each other, this caused the Fermi ball to collapse and turn it into a black hole,” said a researcher at the Center for Theoretical Physics at the Seoul National Ke-Pan Se University in an interview with Live Science.
Xie and his colleague Kiyoharu Kawanagh of the same Center for Theoretical Physics developed a scenario explaining how dark matter began to dominate space. At a time when the Universe was less than a second, incredible transformations of physical laws took place in it. The particles fell into traps, joining into structures so compact that they could only collapse and turn into black holes. Then these black holes filled the entire Universe, providing the same "budget" established by experimental methods - the obvious dominance of two other as yet undetectable components over ordinary, baryonic matter.
Black holes, like dark matter, do not emit light, so they, in principle, can become a source of hidden mass. “Since black holes are non-luminous and compact objects, their dark matter candidates should be considered in the most natural way,” says Xie.
The extreme conditions that existed in the earliest Universe allow changes in physical processes that are already impossible in the normal conditions of modern space. The first ingredient in the new theory is a scalar field, like the Higgs field, that permeates all space and gives particles their mass. As the universe expanded and cooled, this scalar field underwent a phase transition, transitioning to another quantum mechanical state. This phase transition did not simultaneously affect the entire Universe at once. Initially, only separate areas appeared, in which the transition had already begun, and then it all spread further, akin to how water boils in a saucepan, forming larger and larger bubbles. "This process is called a phase transition of the first kind: water passes from a liquid phase to a gaseous state, but initially the gas appears only in the form of growing bubbles," Ce explained.
The new state of the scalar field, now becoming the ground state, spreads from these points akin to a stream of boiling bubbles. Eventually, the bubbles completely merge with each other, and the scalar field completes its phase transition.
However, to create primordial black holes, which are dark matter, Xie and Kavanagh needed another ingredient. They added a new type of fermion to their model. Fermions are particles with half-integer spin, which include electrons, protons, and neutrons that make up all ordinary atoms.
In the very early universe, these fermions moved freely in a scalar field, but they could not penetrate the small foaming "bubbles" of the new ground state of the cosmos during the phase transition described above. As the bubbles grew, the fermions accumulated in the remaining pockets, becoming Fermi balls. However, there was an additional force acting between these fermions, known as the Yukawa interaction, caused by the same scalar field proposed by South Korean theorists in their paper. Fermions usually avoid falling into the same quantum state and into small volumes, but the scalar field added to them the force of interaction that suppressed this natural repulsion. Let's say protons and neutrons are made of even smaller particles called quarks. Quarks are also fermions, avoiding falling into the same state, but the additional nuclear force (strong interaction) sticks them together. An analogue of such a force is the Yukawa interaction acting in the Se and Kavanagh model.
According to the South Korean theory, once the phase transition was completed, the fate of the Fermi balls was sealed. Squeezed into small "pockets" of the rapidly changing Universe, the clumps of fermions collapsed catastrophically, forming a huge number of tiny black holes. These black holes survived the end of the phase transition and filled the Universe in the form of dark matter.