Among the long list of mysteries of the universe is the mystery of the formation of supermassive black holes. How these monstrous objects, with masses that could be as much as 40 billion times the mass of the Sun, came to be remains truly misunderstood, even as many research groups are busy creating scenarios. The problem is this: how did such colossi, the oldest of which we know existed barely 800 million years after the Big Bang, managed to form – and therefore collect so much matter – in such a short time?
Dark matter as a fundamental “ingredient”
Theories are regularly published in peer-reviewed journals, providing food for thought for physicists and cosmologists around the world. In December 2020, an astrophysicist at the University of La Plata in Argentina, for example, formulated a hypothesis in a study that “clumps” of dark matter, present in greater or lesser amounts in certain places, could contribute to the extremely rapid growth of supermassive black holes. The researcher demonstrated that the cores of galaxies, consisting of dark matter and surrounded by a halo of dilute dark matter, can be stable and, therefore, theoretically exist. Based on this principle, the center of these structures could become so concentrated that when the critical density threshold was reached, they would collapse into giant black holes, making these objects the very first inhabitants of galaxies.
Exhibited in a study published February 23, 2022. Physical Review Letters, another theory confirms this hypothetical connection between primordial black holes – the name given to black holes thought to have formed during the primordial universe – and dark matter. Three physicists from Brookhaven National Laboratory (BNL) inEState of New York, this time argue that the early universe would have undergone a rapid and violent transformation that would have caused its densest regions to collapse into black holes. This transformation is called the “cosmological phase transition” by Human Davudiasl, Peter B. Denton and Julia Gerlein. However, to fully understand their reasoning, it is necessary to go back.
Understand the invisible
In the first moments, the Universe was immeasurably dense and hot. A tiny dot where the temperature was 100,000 times hotter than at the center of the Sun. Matter, consisting of elementary particles that had no connection with each other, was then structureless, comparable to soup. Then the expanding universe gradually cooled, quarks and gluons condensed, forming particles called hadrons (meaning protons and neutrons). The first atoms appeared after 380,000 years; stars and galaxies in 200 million years. Since it involves known particles interacting with each other, physicists have been able to trace this history. A story in which supermassive black holes struggle to find their place. “How to trace the events associated with particles whose existence still eludes us and which behave differently?”, asks Human Davudiasl, whom he interrogates Science and the future. Representing and checking by means of calculations the plausibility of the scenario.
“If there really was a ‘dark sector’ with ultralight dark matter, the early universe could provide ideal conditions for a very efficient form of collapse.”
We know that baryonic, or visible, matter is only 5% of the matter that makes up the universe. The remaining 95% will consist of dark energy and dark matter, up to 68% and 27% respectively. Since it is thought to be virtually invisible and interacts weakly, if at all, with ordinary atoms, dark matter remains a mystery for now. However, its existence has been conclusively proven from its gravitational effects. In addition, Hooman Davudiasl’s team started from the following starting point: if almost all ordinary matter is made up of electrons and two types of quarks, these particles are only part of a sector much larger than other visible particles. “Therefore, there is a chance that dark matter, even if it itself consists of only one type of particle, is a constituent of a whole range of as yet unknown particles, which we call the “dark sector”.”explains Human Davudiasl.
Violent change in the “Dark Sector”
Thus, the researchers concluded that the existence of a “dark sector”, the structure of which does not differ from the structure of our visible sector, can explain the formation of supermassive black holes. “The frequency of interactions between known particles suggests that matter as we know it could not collapse into black holes very efficiently.”Peter B. Denton said in a statement. “But if there really was a dark sector with ultralight dark matter, the early universe could provide ideal conditions for a very efficient form of collapse.”
Precisely these ideal conditions would be ensured by this phase transition, comparable to the well-known transformation of boiling water into steam. “But vice versa and on the scale of the Universe”, says Human Davudiasl. This sudden and rapid mutation thus, the densest regions of the early universe are more likely to collapse into black holes. Since water behaves differently before and after the phase transition, then the particles present in the sources should have been the same. Therefore, it is this rapid mutation that will be the key to the formation of supermassive black holes.
But their demonstration goes further: in their opinion, this cosmological phase transition could also lead to the formation ultralight particles of dark matter – particles whose mass would be infinitely lighter than the mass of a neutrino, the lightest particle known today. “Such an event could provide the right ingredients”concludes Human Davudias before qualifying the mysterious growth of supermassive black holes and dark matter as “two possible sides of the same coin.” According to the trinity of scientists, if dAlthough other groups have already studied the implications of this type of transition, their study is the first to establish a link between supermassive black holes and dark matter in this way.
The effects are still being seen
After the theoretical demonstration, the time for empirical proof inevitably comes. The good news is that even though we are about 13 billion years away from this event, it would be possible to observe the gravitational waves that were likely created by such a strong event. “Because the phase transition and the formation of supermassive black holes would occur everywhere in the universe, gravitational waves would radiate in many places, in many directions”adds the physicist.
“Of course, such a signal weakens over time, but our calculations show that these waves have a characteristic shape that allows us to make a prediction for this signal and its expected frequency range.” On the other hand, the residual gravitational wave signal from such a violent cosmic event may well be within reach of future pulsar synchronization experiments.