The persistent mystery surrounding dark matter has led physicists to propose that it may have formed at a distinct moment from ordinary matter. Their new hypothesis suggests, in particular, that there could have been a second Big Bang, referred to as a “dark” one, occurring after the one proposed by the standard cosmological model, and that this event could be the origin of dark matter. While this may seem implausible, the hypothesis could potentially explain why the detection of dark matter remains elusive.
According to the standard cosmological model, the Universe underwent a period of inflation shortly after the Big Bang. This transitional phase witnessed a rapid expansion of space-time, driven by vacuum energy prevalent in space. It preceded the “hot” Big Bang, characterized by a series of energy transformations leading to the stabilization of expansion and the release of vacuum energy in the form of a hot plasma of particles. This process gave rise to particles of ordinary matter (photons, leptons, quarks, etc.) as well as those constituting dark matter (such as the hypothetical Weakly Interacting Massive Particles or WIMPs).
However, this model faces challenges in explaining the formation of dark matter, particularly our inability to detect it. There may be another significant gravitational source, according to cosmological phenomena that defy explanation by the presence of visible matter alone. Galaxies rotating rapidly within clusters and the peripheral regions of galaxies rotating as fast as their centers indicate the presence of substantial amounts of invisible matter generating additional gravity, likely five times more abundant than visible matter.
Researchers from the University of Texas at Austin propose that the difficulty of dark matter interacting with the weak nuclear force, one of the four fundamental forces of nature, could hinder its detection. In this context, gravity alone would connect it to ordinary matter. Given the weakness of gravity at the scale of individual particles, their detection would become nearly impossible.
These hypotheses lead them to suggest a revision of the standard cosmological model. They propose that dark matter might have formed separately from visible matter during a second Big Bang. This event would have occurred within a month after the hot Big Bang, with a minor influence on the structure of galaxies.
A “Dark” Big Bang at the Origin of Strange Particles
The alternative cosmological scenario proposed in this new study suggests that the formation of visible matter and dark matter is entirely distinct. According to this theory, the hot Big Bang would have produced only radiation and visible matter, excluding any dark matter. However, while the dark sector would initially be cold, it would contain only a small amount of vacuum energy compared to the existing radiation density.
Vacuum energy, not undergoing redshift, could become significant later, although it would never dominate the energy density of the Universe. When this energy decays during a dark-phase transition, it could generate large amounts of dark matter and potentially dark radiation. Researchers have named this major phase the “Dark Big Bang.”
They posit that this second Big Bang could have created at least three types of distinct dark matter particles, each stranger than the other. If the phase transition were sudden, it would have produced massive particles via expanding bubbles, converting the system from one state to another, akin to boiling water bubbles. During collisions between these bubbles, they would burst, releasing their energy.
In a dark Big Bang scenario, the bubbles would be so energetic that they would produce enormous particles, about 10,000 billion times the mass of a proton. These hypothetical particles are dubbed “darkzillas,” referring to the giant fictional Japanese monster.
However, if the phase transition were gradual, the dark Big Bang would give rise to more modest particles, similar to WIMPs. These would interact with dark versions of fundamental forces, such as dark electromagnetism, which produces dark photons. However, in the absence of these forces, the generated particles could not balance their energy by absorbing or creating dark photons. To compensate, they would become “dark cannibal particles,” devouring each other in an attempt to maintain equilibrium.
Researchers estimate that phase change bubbles associated with the dark Big Bang could leave distinct ripple imprints in spacetime in the form of gravitational waves. They believe that the analysis of these waves, first detected in 2016, could potentially corroborate their theory. Details of the study are available in preprint on arXiv.