Collapse of ‘pieces of space-time’ could explain dark matter and the ‘whisper’ of the Universe

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In astrophysics, there are what are called “domain walls”. These hypothetical structures are immense “sheets of space-time” that separate different regions of the universe. In a new study, it is suggested that these elements appeared and disappeared just after the Big Bang and could thus be at the origin of low-frequency gravitational waves, certain types of black holes and even partly explain the black matter.

Domain walls are considered in theories aimed at explaining certain unresolved phenomena in astrophysics, such as the origin of gravity and other fundamental forces. According to some researchers, if domain walls had persisted in the universe, they could have been a major Source of energy and influenced the overall evolution of the universe. They add that if this were the case, we should be able to observe evidence of their existence or effects today. However, no direct evidence supports their presence.

In a new study published on the preprint server
arXiv, a group of researchers led by Ricardo Ferreira, a cosmologist at the University of Coimbra in Portugal, suggests that wall domains formed shortly after the Big Bang, grew rapidly, then mostly destroyed themselves in a very short time. However, their brief existence would have caused notable disturbances in space-time. Additionally, according to the study, some of the domain walls that did not collapse completely may have turned into black holes. These could be numerous enough to constitute a significant part of dark matter.

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Symmetry breaking

According to many researchers, the Standard Model of particle physics (theoretical framework explaining the interactions of elementary particles through fundamental forces) has shortcomings. Furthermore, it does not explain certain cosmological phenomena.

To address this problem, the physicists in the new study proposed extensions to the standard model by introducing “symmetry breaking”. This is a process in which uniform (symmetric) conditions begin to differentiate as the universe cools and expands. The transition can lead to the creation of stable and distinct (initially uniform) cosmic structures. As part of the extension of the Standard Model, domain walls are considered here as consequences of symmetry breaking. They are conceptualized as barriers between different “phases” of the universe, during this transition.

Gravitational waves

In this study, the researchers focused on a version of symmetry breaking theoretically produced in the initial conditions of the universe, just after the Big Bang, where the temperature was around 2 trillion degrees Celsius. The team used computer simulations to model and visualize the evolution of domain walls under this symmetry breaking. By rising and then collapsing, these pieces of space-time would have generated disturbances called “stochastic gravitational waves”, which can propagate through the Universe.

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With time and the continued expansion of the universe, the gravitational waves emitted by these domain walls stretch to longer and longer wavelengths (and therefore to lower and lower frequencies). They would today have frequencies in the nanohertz range, corresponding to the background of gravitational waves recently detected by exploiting millisecond pulsars (neutron stars which rotate extremely quickly on themselves, up to several hundred rotations per second ). However, the exact origin of these waves remains a subject of active research.

Are domain walls the cause of certain black holes?

According to the recent analysis, not all of the domain walls that formed after the Big Bang collapsed. Some would have survived thanks to a sort of space-time “bubble”, larger than the radius of the observable universe at that time. Consequently, as the universe expanded, these walls would eventually become part of the observable universe, where they would manifest as regions of abnormally high energy density. These “overdensities,” as researchers call them, can collapse under their own gravity to form black holes. The latter are called “primordial”, because they would therefore result from phenomena occurring shortly after the Big Bang, and not from the collapse of large stars. The primordial black holes resulting from this hypothetical mechanism would have a low mass (for black holes), namely only a few times the mass of the Sun.

Source: arXiv


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