Researchers believe they have identified a lead that could finally help us resolve one of the most stubborn enigmas in all of astrophysics: in a recent study, they explain that it is increasingly likely that black holes are the origin of dark energy, which itself is suspected of playing a determining role in the expansion of the universe.
In 1929, the illustrious astronomer Edwin Hubble — the same one who gave his name to the famous telescope — made a discovery that would revolutionize cosmology: he was the first to demonstrate that our Universe was expanding. Since then, many researchers have tried as best they can to calculate the speed of this expansion, also called the Hubble-Lemaître observation… and this is where the problems begin.
The expansion of the Universe, a stubborn mystery
Initially, traditional Newtonian physics suggested that expansion should slow over time, held back by gravitational interactions between extremely massive objects like galaxies. But the situation changed in the 1990s with the arrival of new methods which brought to light a much more nuanced situation. We now know that the expansion of the Universe is accelerating, instead of slowing down as previously thought. The problem is that the different measurement methods cannot agree on the value of this acceleration. They all produce results that appear individually consistent, but are fundamentally inconsistent. We find ourselves with a major inconsistency which we today call the Tension d’Hubble.
To put things into perspective, several eminent astrophysicists have approached the problem from the other end. Instead of calculating the Hubble constant from observations, they sought to identify the driving force behind the expansion of the universe. For this, they introduced a completely new concept: dark energy. This term designates a form of energy still unknown, but omnipresent in the cosmos. According to some models, it represents up to 70% of the total energy of the Universe. And above all, it plays a determining role in its expansion. Its influence would be such that it would counterbalance the influence of gravity, forcing the different celestial bodies to move away more and more quickly.
The more time passes, the more convincing this idea becomes. In fact, it works so well that its three main architects, Saul Perlmutter, Adam Riess, and Brian Schmidt, were awarded Nobel Prizes for this revolutionary contribution to the theory. But there remains an absolutely huge hole in this model: the exact nature of dark energy remains completely unknown. No one knows exactly what it is made of or where it comes from. To this day it remains one of the greatest mysteries of all astrophysics and even science in general.
An extension of the cosmological coupling »
But recently, other astronomers have begun to move closer to a convincing explanation; according to a recent research paper, it is increasingly likely that this dark energy comes directly from black holes.
This work starts from a central hypothesis according to which these objects could be direct repercussions of the mechanisms which allowed the Universe to take off with the Big Bang.
This theme emerged spectacularly last year, with the publication of a very important study on the subject. Its authors looked at supermassive black holes which reside at the centers of so-called “dead” galaxies, that is to say they have completely exhausted the raw materials supposed to fuel the growth of their stars and their black hole. In theory, the latter should therefore no longer be able to grow; yet, the authors observed that they continue to gain mass.
To explain this apparent inconsistency, they introduced a new concept: a « cosmological coupling “, through which the properties of objects would be directly affected by the expansion of the universe (see our article).
This is a fairly revolutionary notion, because it directly opposes the idea that parameters like mass are intrinsic properties of objects; instead they would be intimately linked to the way in which the Universe evolves on a global scale. The researchers behind this new study saw it as an opportunity to extrapolate to advance the quest for dark energy.
« If you ask yourself the question: “Where in the universe do we see gravity as strong as it was early in its history?” the answer lies at the center of black holes », Says Gregory Tarlé, professor of physics at the University of Michigan and co-author of this new study. “ It is possible that what happened during the initial inflation is happening in reverse; the matter of a massive star would revert to dark energy during gravitational collapse, like a small Big Bang played in reverse. »
Data consistent with theory
To try to verify this, the five research teams involved in this project based themselves on DESI (Dark Energy Spectroscopic Instrument). It is a state-of-the-art observatory that, as its name suggests, is entirely dedicated to dark energy. Using a set of more than 5,000 detectors, it constantly probes distant galaxies to better understand the way in which the universe expands and, by extension, the mechanisms that allow dark energy to make its office.
The observatory recently completed the first cycle of this scientific campaign. And despite his relative youth, he has already proven himself to be incredibly productive. It made it possible to collect a mountain of data which notably brought to light a trend with serious implications.
Indeed, we have known for years that black holes become more and more numerous and massive over time. However, at the same time, DESI readings clearly show that the density of dark energy is also increasing… but not at any rate; It turns out that the rise of dark energy corresponds surprisingly well to that of black holes. In fact, the connection seems so strong that statistically speaking, this is probably not a coincidence. The more data accumulates, the more closely the two parameters seem to be linked.
« The two phenomena are consistent with each other: as new black holes were created following the death of massive stars, the amount of dark energy in the universe increased by the right amount said Duncan Farrah, associate professor of physics at the University of Washington–Hawaii and co-author of the study. “ The fact that black holes are the source of dark energy is becoming more plausible. »
A new era for the hunt for dark energy
It should be clarified that although the consistency of the data is very encouraging, it is only a hypothesis at the moment. It will still take a lot of data and work to achieve rigorous confirmation. But the good news is that DESI seems well on its way to providing us with new pieces of this great puzzle. He has not yet shown everything he is capable of and will continue to gain momentum. It is therefore a safe bet that new, increasingly convincing elements will emerge over the next four years. A very exciting prospect for the researchers involved, because it marks the beginning of a new era in the hunt for dark energy: that of concrete experimentation.
« This will only bring more depth and clarity to our understanding of dark energy, whether or not it continues to support the black hole hypothesis », Rejoices Steve Ahlen, another co-author of the study. “ I think it's a wonderful experimental effort. We may or may not have preconceived ideas, but we are guided by data and observations. »
« Fundamentally, the question of whether black holes are dark energy, coupled with the universe they inhabit, is no longer just a theoretical question », concludes Tarlé. “ Now this is an experimental question! »
The text of the study is available here.
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