The cosmic chicken-or-egg dilemma: Did stars or black holes come first?

We live surrounded by situations in which this dilemma can be applied: what came first, the chicken or the egg? Here I'm going to address a very current one, in the field of astrophysics, involving the processes of galaxy formation. We do expand on that. We present the problem using telegraphic concepts, which we've already discussed, and we invite the reader to refer to other articles in this section.

Galaxies didn't always exist. There was a time, almost 13 billion years ago, when there were no stars, just a soup of matter, light, and dark energy . A non-static universe wasn't to Einstein's liking, but his own equations—in the hands of Lemaître, Leavitt, and Hubble—proved him wrong.

Galaxies are dying. Galaxies like our own, those beautiful nearby spirals and ellipticals, are dying —or are already quite dead. Just compare how many stars are forming today with how many they host. The math doesn't add up : at the current rate, it would take the Milky Way several lifetimes to make the ones it already has. Therefore, galaxies in the past must have been much more active in star formation, something we have confirmed by studying the Universe when it was about half its current age.

Something is stopping star formation. Since only a small percentage of the available material—predominantly hydrogen—ends up becoming stars, something must be halting that process. And the culprit , according to the most widely accepted theory among astrophysicists, is the supermassive black hole that inhabits the core of all galaxies. These supermassive black holes typically have a mass that is one-thousandth of the stellar mass of their host galaxy, yet they have an extraordinary influence on the evolution of the cosmos.

That theory establishes that at some point in the early universe, perhaps 100 or 200 million years after the Big Bang—we still don't know exactly when—the first stars began to form as the primordial gas (mainly hydrogen, but also some helium and a very small amount of other elements like lithium ) condensed. And so the first galaxies were born, where, somehow, supermassive black holes appeared and grew. Through processes we call negative feedback, these supermassive black holes eventually prevented the gas from continuing to have the right properties to form stars, because the energy released by the black hole prevents the gas from condensing into galaxies. So what we would have is a process of star formation, then the formation of a supermassive black hole, and eventually, this would stop star formation.

But the JWST space telescope is changing this perhaps very naive view. We are discovering a multitude of very distant galaxies—that is, galaxies that existed when the universe was very young—that already had supermassive black holes. And they were very, very large when the universe was less than 10% of its current age! Even among, say, the ten most distant galaxies we know of, with a fully confirmed distance, half could have black holes as massive as the Milky Way itself—around a million suns, or even somewhat larger. They are so large that their mass could be greater than the star-like mass of the host galaxies in that young universe. Note the conditional verb: we are still checking our calculations. We have also discovered objects that could have even greater masses just a little later, in the first 15% of the history of the universe, the so-called little red dots . It is very difficult to explain how such large black holes could have formed in such a short time, and in galaxies with so few stars.

And here comes the chicken-and-egg dilemma. Was the process really that stars formed, then black holes created, then black holes grew, and then black holes finally stopped star formation? Or perhaps black holes preceded the first stars, then suns began to form, then black holes grew, and then galaxies died? Call black holes the egg and call stars the chicken, or the other way around.

The question is not trivial. If supermassive black holes were there before stars, perhaps they favored the formation of suns in the first place, only to do just the opposite, preventing more from forming. If black holes were already there, their origin can't be stellar, which is the origin for which we have a good deal of data; we know which stars form black holes (but tiny ones compared to these supermassive ones). So what happened? Were they always there? Did they form through a process not yet observed, and perhaps completely unknown, or known but unproven?

The chicken-and-egg dilemma isn't new. It seems Aristotle already addressed it, in his quest to identify causes and effects and search for the first unmoved mover. All chickens come from eggs, and all eggs produce chickens. It doesn't seem possible to have one without the other and vice versa, leaving aside disquisitions about the evolution of species (yes, there were eggs before chickens). There is an infinite sequence, and Aristotle concluded that there is no origin; nothing can come first in that sequence. If nothing can come first, then we question the concept of time. In fact, Plutarch seems to have wondered, while thinking about the same problem, whether the world had a beginning. And from there we can move on to discussing creation and jump into theology. That's not my area of expertise, nor is it the area of expertise for this section.

Cosmic Void is a section that presents our knowledge of the universe in both qualitative and quantitative terms. It aims to explain the importance of understanding the cosmos not only from a scientific perspective, but also from a philosophical, social, and economic perspective. The name "cosmic void" refers to the fact that the universe is, and is, for the most part, empty, with less than one atom per cubic meter, despite the fact that, paradoxically, there are quintillion atoms per cubic meter in our surroundings, which invites reflection on our existence and the presence of life in the universe. The section is composed of Pablo G. Pérez González , researcher at the Center for Astrobiology, and Eva Villaver , deputy director of the Instituto de Astrofísica de Canarias.