And only at the end did the Big Bang arrive. The initial explosion, the matter concentrated in an infinitely small point that expands in an instant, the fateful event before which nothing existed: this is not how our story began. In fact, that’s not even how the real story happened.
There is a chapter preceding the Big Bang that science begins to furnish with details and which Gian Francesco Giudicedirector of the department of theoretical physics at CERN in Geneva and academic of the Lincei, says in “Before the Big Bang. How the universe began and what came before” (Rizzoli, 252 pages, 19 euros).
If we don’t even know how the Big Bang happened, how do we understand what happened before?
“Science has made enormous progress in recent years. By combining astronomical observations with knowledge of particle physics, a convincing picture of the processes that started the Big Bang has been reconstructed. I have tried to offer the reader the most complete story that science allows us to tell today.”
What then is the first chapter in the history of the universe?
“The empty space. But vacuum in physics is a different concept from nothing. The primordial void hides a strange form of energy in the fabric of space-time, which we call vacuum energy.”
And what does this vacuum energy do?
“Vacuum energy has a characteristic that makes it unique: it exerts reverse gravity. Instead of being attractive, gravity becomes repulsive. As a result, space begins to expand more and more rapidly, in a chain process that fuels furious expansion. In a quadrillionth of a nanosecond, a space no larger than a Covid virus extends as far as the observable universe today. This phase is called inflation and precedes the Big Bang.”
Gian Francesco Giudice next to the Cern particle accelerator
What happens in this phase of very rapid expansion of empty space?
“At first glance, not much. In reality, the ingredients for a complex universe like ours are silently being prepared. And here quantum mechanics comes into play, the theory that governs the infinitely small but which also becomes important for understanding the history of the cosmos. Quantum mechanics imprints microscopic inhomogeneities on the vacuum energy, which however are magnified by the expansion of space. These are the primordial seeds that will later grow to germinate into galaxies, stars, planets and all the structures present in the sky today. Without these tiny disturbances caused by quantum mechanics the universe would be a monotonous homogeneous gas and life could not exist.”
How would Einstein accept the theory of inflation?
“I think he would be satisfied to know that the current understanding of cosmic history is founded on his theory of relativity. I imagine he would burst out laughing in surprise when he heard that the initial engine of the Big Bang was vacuum energy. After all, it is he who introduced this concept (even if for the wrong reason), and then ended up repudiating it. If I finally got around to telling him about the vital role of quantum mechanics in the universe, perhaps he would abruptly end the conversation and walk away impatiently. In his entire life Einstein never digested the idea of quantum mechanics.”
Inflation continues driven by vacuum energy. Then?
“Vacuum energy changes slowly over time. Then it reaches a critical value and the structure of space-time suddenly changes. It’s the moment of the Big Bang. The frantic expansion of the universe then comes to an end. The space continues to expand, but in an increasingly gentler way.”
What exactly is the Big Bang then?
“It is the moment in which the energy accumulated in the vacuum is transformed into hot and dense matter. It is the moment in which the energy of the vacuum, at the end of its slow evolution, disintegrates and gives life to all the particles that form matter, from which stars, planets and finally life will be born.”
Is it like a wave breaking when its energy turns into foam?
“Yes and it is a moment of transition that almost simultaneously involves an enormous, perhaps even infinite, space. It has little to do with the image of a localized explosion with which the Big Bang is often erroneously portrayed.”
If every form of energy corresponds to an elementary particle, does vacuum energy also have its own particle?
“And identifying it is the dream of many physicists. Unfortunately we know very little about her appearance and characteristics. We are faced with a haystack without even knowing the shape of the needle. Finding this particle would tell us profound secrets about the dawn of cosmic history.”
The book is structured like a mystery, with the Big Bang posing a series of mysteries and only one explanation that manages to solve them all: inflation.
“The story of how science arrived at the Big Bang is a thriller full of unexpected events, brilliant intuitions and huge blunders, surprising discoveries and erroneous interpretations. It contains all the elements that make scientific research so fascinating, but also so human. Once we get to the bottom, the solution to the mystery is astonishing and reveals an unexpected image of the primordial universe. It is also a starting point for new adventurous hypotheses.”
Which?
“For example, the idea that our universe is just a drop of an immense ocean: the multiverse. Humanity began its journey of discovery by considering itself at the center of the cosmos and then, piece by piece, this conception collapsed. We would end up discovering that our universe is not unique either.”
