Since the 1930s, researchers have discovered evidence suggesting the Universe’s mass is composed of approximately 85% of a perplexing and yet unexplained kind of matter presenting itself at various scales (i.e., out to tens of kpc from galactic rotation curves measurements, out to 200 kpc from microlensing measurements and out to cosmological scales from cosmic microwave background measurements). There are numerous possibilities surrounding the genesis of this component, but physicists have yet to reach a definitive conclusion on the subject.
Because the mystery substance did not emit light, it was dubbed “dark” matter. Other traits observed in dark matter particles include disappearing electric charge and stability, or at the very least a very long lifetime. Because dark matter only interacts with other particles by gravity and perhaps weak interactions, it is extremely difficult to investigate it experimentally, because particles are often discovered through electromagnetic and strong interactions, which are usually far more intense.
Thus, no dark matter constituents have yet been seen, despite several attempts using ground-based detectors as well as particle colliders and satellites. Nonetheless, astrophysicists have conducted substantial research into this subject and have presented numerous ideas for the role of dark matter particles, some of which are beyond the boundaries of the Standard Model of fundamental particles. These include potential primordial black holes created in the early Universe, weakly interacting heavy particles, ultralight axions, and a plethora of other phenomena.
A few years ago, Klaus Werner led a study in which the authors postulated that the elementary dark matter components could be compact configurations of gravitons — the particle that mediates the force of gravitation — connected to each other by gravity. This hypothesis is appealing to scientists since it does not necessitate the consideration of undiscovered particles, as is done in many other speculative models of dark matter, and simplicity is often a key quality in physics.
They employed the general theory of relativity, a geometric theory that treats gravity as a deformation of spacetime caused by particle energies and masses, to investigate graviton interactions. These particles include gravitons, which have enough energy to interact with other gravitons in the same way that planets and stars do. This mechanism allows particles mediating gravitational interactions to be attracted to each other and create limited systems, analogous to the solar system.
Their reasoning, however, contained a flaw: general relativity is known to be an imperfect theory of gravity since it excludes quantum effects. There is currently no widely recognized “quantum gravity” theory, but scientists have created a number of potential options.
In a recent study published in The Journal of High Energy Physics, a group of physicists led by Leonardo Modesto of China’s Southern University of Science and Technology improved on Werner’s analysis by taking into account the interaction of gravitons in various quantum gravity theories (string theory being the most well-known example) that generalize general relativity.
Physicists discovered through analytical calculations that gravitons can attract each other and form compact forms that can constitute dark matter in practically all of the theories they studied. These objects were dubbed Planckballs because their size, according to the scientists’ calculations, was of the order of the Planck length — a typical scale of any quantum gravity theory and about equal to 1035 m.
The energies of gravitons had to be exceedingly high for Planckballs to form — often on the order of the Planck mass, a typical mass scale of quantum gravity that is approximately 22 orders of magnitude heavier than an electron. It means that only in the very early Universe, when the temperature was exceedingly high, could bound systems of gravitons have evolved (in some theories of quantum gravity it is of the order of 1032 degrees).
While the team’s idea is intriguing from a theoretical standpoint, only future more precise experimental studies will be able to determine which of the proposed dark matter theories is correct, or whether the reality is even more complex and dark matter is made up of objects that scientists haven’t even considered yet.