At the beginning, 13.8 billion years ago, the Universe was concentrated in a very small point, as tiny as an atom. For some reason, it began to expand after the well-known Big Bang faster and faster, without stopping until today. In … In this process, all kinds of simple particles began to be created that, once the conditions cooled enough for complex matter to be stable, created atoms that were then later fused with heavier elements that formed stars, which later in turn they formed star systems, galaxies and everything we can see in the sky.
However, there is a problem: humanity is capable of measuring the mass of the entire Universe. But the accounts don’t work out. The cosmos ‘weighs’ 85% more than it should, so scientists point out that there is something, called dark matter, that we do not see, but it is there. There are multiple theories of where it could ‘hide’, but the suspects with the most ‘proof of guilt’ are primordial or primitive black holes. These are miniature black holes, with masses lower than those of our Sun; Although we should not be fooled by appearances: a black hole the size of an atom would weigh the same as the entire Everest.
For this theory to fit, these primordial black holes would have to be everywhere. However, at the moment, our technology has not been able to find them, so many are wondering if they really exist. Now, a team formed by scientists from the Research Center for the Early Universe (RESCEU) and the Kavli Institute for Physics and Mathematics of the Universe (Kavli IPMU, WPI) at the University of Tokyo (Japan) have discovered that, in reality, it can There might not be so many of these ‘miniature monsters’ after all. The conclusions have just been published in two articles in the journals ‘Physical Review Letters’ and ‘Physical Review D’.
The importance of primordial black holes
Jason Kristiano and his supervisor, Junichi Yokoyama, authors of the papers, have been studying these primordial black holes for some time. «Apart from being candidates to explain the mystery of dark matter, these bodies are also interesting for other reasons, since since the recent innovation in gravitational wave astronomy, binary mergers of black holes have been discovered, which can be explained whether primordial black holes exist in large numbers,” explains Kristiano.
Because now, technology such as global observatories of gravitational waves – deformations in the fabric of space-time that travel throughout the cosmos at the speed of light -, LIGO in the US, Virgo in Italy; as well as KAGRA in Japan, have been able to ‘listen’ to the echoes of phenomena as energetic as two black holes colliding with each other. However, capturing gravitational waves is not definitive proof that primordial black holes exist. In fact, there are inconsistencies: if these bodies existed, they should be reflected in some way in the observations made of the cosmic microwave background (CMB), a kind of trace of the Big Bang explosion that is It has been captured by human technology.
In this work, the authors used a new approach to correct the formation of primordial black holes from cosmic inflation so that it aligns with current observations, as well as being testable against future gravitational wave observations.
“In the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation quickly expanded that by 25 orders of magnitude. At that time, the waves traveling through this small space could have had relatively large amplitudes but very short wavelengths. “What we have discovered is that these small, yet strong waves can translate into an otherwise inexplicable amplification of the much longer waves we see in the current CMB,” explains Yokoyama.
“We believe this is due to occasional cases of coherence between these first short waves, which can be explained using quantum field theory, the most solid theory we have to describe everyday phenomena such as photons or electrons,” he says.
Reconcile the world of atoms with the Universe
For a century, scientists have been striving to ‘reconcile’ Quantum Mechanics or the laws that govern the microworld of particles – in which they have incredible properties such as the ability to be in several places at the same time or to teleport – with Einstein’s General Theory of Relativity, which works perfectly for phenomena that occur at macroscopic scales in space. Quantum field theory is an attempt to reconcile the two.
This hypothesis indicates that the vacuum, in reality, is full of physical activity in the form of energy fluctuations. These variations also have the ability to cause particles to appear that, however, disappear shortly after. In the so-called ‘Planck Epoch’, a period so early in the history of our universe that it spanned only one ten millionth of a billionth of a billionth of a billionth of a second after the Big Bang, our Universe also functioned as a quantum Universe. The collapse of short but strong waves referred to gave rise to primordial black holes whose trace is not found in the CBD. However, the authors say that many of these types of waves in a much more compact cosmos would have had the power to reshape waves much larger than themselves.
“While individual short waves would be relatively powerless, coherent groups would have the power to reshape waves much larger than themselves. “This is a rare case where a theory of something on one extreme scale appears to explain something on the opposite end of the scale,” explains Yokoyama. If, as Kristiano and Yokoyama suggest, early small-scale fluctuations in the universe affect some of the larger-scale fluctuations we see in the CMB, this could upset the standard explanation of the structures of the Universe.
But also, since we can use measurements of wavelengths in the CMB to constrain the extent of the corresponding wavelengths in the early universe, this necessarily limits any other phenomena that might depend on these shorter, stronger wavelengths. And this is where primordial black holes come into play again. “Our study suggests that there should be many fewer of these objects than would be needed if they were truly a strong candidate for dark matter events or gravitational waves,” he says.
However, the theory is still a model. The authors point out that we will have to wait for new observations to corroborate this. Therefore, the case of primordial black holes is still open.
