How was the Cosmos born and what exactly is the Big Bang? What part did Einstein play in our present understanding of the universe and how does quantum physics come into the equation? Edgard Gunzig, physicist, author and honorary professor at the Université Libre de Bruxelles, sheds light on these and other questions.
Traditional cosmology, from Copernicus and Newton to the beginning of the 20th century, sought to describe the universe without posing, in scientific terms, the question of its origins. That remained a ‘matter for God’. Is it true to say that Big Bang science laid claim to a field that previously lay within the domain of theology and metaphysics?
The concept of cosmogenesis only entered the field of science at that crucial and surprising moment for physicists when it became clear that the universe was evolutionary and, therefore, had a history. That occurred in 1929 when Hubble made the experimental discovery of the expansion of the universe. Previously, it had been seen as something fixed, static and immutable. The major challenge for the physicists was then to predict how the universe would develop in the future, to forecast our cosmological future, but also to understand its history, its cosmogenesis.
This discovery by Hubble, which marked a radical change in the way we viewed the world, was also at the origin of a ‘drama’ surrounding the representations that Einstein had developed between 1912 and 1915 to describe gravitational phenomena: the theory of general relativity. This theory marked a conceptual revolution in physics as, for the first time, it ‘physicalised’ space and time themselves. They ceased to be a simple passive reference framework, a kind of stage on which the story of the world is enacted. They were suddenly actors in that story. According to this theory, gravity simply translates into a visible form an invisible and previously unsuspected property of space and time, namely spacetime: its curve! The planets in our solar system are travelling along curved trajectories and objects fall to the surface of the Earth, not because the Earth attracts them but because the Earth curves or warps spacetime.
Suddenly, all material bodies, as well as rays of light, were seen to follow ‘natural channels’ within this spacetime curve. But what is responsible for the curve? Why is space not flat as man had always imagined it to be? It is the stars themselves that curve their environment, the content – material or light – curves the container, which is spacetime. Einstein’s equations translate into mathematical terms this link between geometry and matter. This theory thus represents much more than a ‘simple’ explanation of gravity: it is a theory of space and time within which gravity naturally finds its origin.
You spoke of the drama produced by the discovery of an expanding universe by virtue of this remarkable theory, but what are we talking about exactly? What are the changes that this theory brought to our understanding of the universe?
The central concept is that of the Big Bang. It has made such an impression on people that its meaning has often been distorted. Despite its conceptual beauty, Einstein’s new description of the world could only really lay claim to the status of a new physical theory, if it explained not only previously mysterious astronomical phenomena but also forecast totally unforeseen phenomena. General relativity passed this twofold test. It explained a curious and inexplicable phenomenon that could not be explained by Newton’s good old law of universal gravity, one connected to the orbit of the planet Mercury, and it also predicts the new phenomenon of the deflection of light rays by the Sun.
So why not extrapolate to the scale of the universe what functioned so well at the ‘local’ level of the Solar System? Of course, at the time, we did not have any total view of the universe, but the temptation was too strong and Einstein was the first to go down this road. The great Einstein was, nevertheless, a victim of the consensus in opinion of the time and was hoping, through his theories, to provide an explanation of the total structure of the universe… as a fixed and eternal entity without history or origin.
Imagine his disappointment when he realised that his beautiful equations were unsuited to the description of such a universe. If he did not change them, the universe they implied was destined either to collapse in on itself or, on the contrary, expand and thus be evolutionary. As an irony of fate, Einstein realised that there remained one unexplored theory, that is, the introduction of an arbitration addition that would detract somewhat from the simplicity and beauty of his equations: the cosmological constant.
Hence the drama. In 1929, Hubble’s discovery showed that the universe was evolutionary and not static and that Einstein’s ‘correct’ equations were those he had come up with initially, in their simplest form. In other words, Einstein ‘missed’ the greatest theoretical prediction he could have formulated, that of an expanding universe! He would say it was the greatest mistake of his life but he did not know that, far from being monumental mistake, his cosmological constant initiated an adventure with many repercussions that is more pertinent today then ever.
What happened next and how did we then arrive at the Big Bang?
The physicists of the day were convinced they were in possession of unequivocal equations that explained the universe. In particular, this meant it had to be possible to unravel the history of the cosmos by tracing a thread back through time, by analysing in ‘reverse time’ the secrets of their equations. This would inevitably lead to their point of departure and the fundamental question of cosmogenesis.
But this history begins in a finite past, with a singular situation: all the characteristics of the physical (such as temperature, pressure, energy density) and geometrical (curvature) universe become simultaneously infinite. This accident was given the name the Big Bang. It suggests – but this is only an image – a vast explosion involving all the material content and its geometric container.
This situation, whereby all physical sizes become infinite, is essentially a non-physical situation. It does not correspond to any state of the universe whatsoever and, on the contrary, indicates precisely that the mathematicians are taking the lead and no longer describing anything tangible. This Big Bang is the mathematical detonator of a physical history. And this physical history, that of our universe, which is misleadingly referred to as ‘the Big Bang theory’, begins in this equation just a few minute fractions of a second afterwards. The equations, at this stage, thus provide us with a history but without giving us the key to its origin. In other words, the Big Bang represents an admission of theory’s impotence.
Can science go beyond this impotence and move towards an understanding of the origins?
Yes, and for that it is essential to understand the origin of this impotence, and thus of the Big Bang. Despite its enormous contribution, general relativity has a major limitation: it describes links that exist between the content and container but remains silent as to the origin of this content. The presence of matter (and of radiation) contained in the universe is one of the things that the theory cannot explain. If follows from this, when the universe is viewed in reverse, that any quantity of matter and radiation becomes progressively compressed into an increasingly small volume, resulting in an increase in its pressure, its temperature, its energy density and the curving that Einstein’s equations attribute to it. When this volume is reduced to a single point (not a particular point in space but all of space compressed into itself!) the inevitable is produced: all these physical and geometric quantities become simultaneously infinite and we have the Big Bang! While this Big Bang is thus rendered inevitable, it is also demystified. It is seen as an intrinsic limitation precisely because of its inability to explain the origin of its constituents and even more so cosmogenesis. It is precisely this that bears the misleading name of the ‘Big Bang theory’ .
So science is unable to decipher cosmogenesis?
One of the most fascinating aspects of physics over the past three decades is that it progressively provided a negative response to this question. This response comes from the confrontation between general relativity and the second fundamental pillar on which the whole of contemporary physics is based: quantum theory. It is an extension of Einstein’s theory that takes from quantum theory that which it lacks: the possibility to create and destroy matter. The central player in this is none other than the vacuum! Not the (intuitive) vacuum of traditional physics that is equivalent to the absence of all things, but the quantum vacuum that is the site of uncontrolled activity, of constant and, in principle, irremovable fluctuations. This fluctuating quantum vacuum is the most fixed state that nature can accommodate, whose degree of emptiness cannot be taken any further. And this vacuum – this is the essential point – possesses a non-zero energy (unlike the traditional vacuum), which is associated with all these fluctuations. This vacuum is a strange player that could produce the very worst or the very best outcome, a disaster for contemporary physics or the path to a possible representation of cosmogenesis.
Let us speak about the best and leave the worst for another time…
It is essentially the unexpected possibility of conceiving of the emergence of the universe out of the quantum vacuum or a process that avoids any recourse to the Big Bang. This is an extraordinarily subtle interplay between the quantum fluctuations of the vacuum and the expansion of the space that gives birth to the universe: the expansion of space forces the vacuum to transform its fluctuations into particles, thus to create matter, and this matter, in turn, forces space to expand. An extraordinary snake that bites its own tail! The universe would then result from a mechanism that unfolds in a self-consistent manner, with no need of any external help. The universe that is thereby created discovers in itself the energy necessary to activate its own creation. What is more, the initial creative stage that results from this phenomenon corresponds to this initial period of vast expansion and inflation that physicists need so much today in order to reconcile experimental observations and theoretical predictions.
Most of these experimental data result from the observation of what is today the true memory of our cosmological past: the fossil electromagnetic radiation that floods the entire universe and that was produced several thousand years after its creation. This is the genuine remnant of the primordial universe that is imprinted on its most intimate properties that we can observe today.
The other Gunzig Uncertain relationships are a principle of quantum physics: it is impossible to know simultaneously all the characteristics of a particle. Come to that, how does one ever grasp the complexities of a human being? Relations d'incertitude is a ...
The other Gunzig
Uncertain relationships are a principle of quantum physics: it is impossible to know simultaneously all the characteristics of a particle. Come to that, how does one ever grasp the complexities of a human being? Relations d'incertitude is a jointly penned autobiography. Elisa Brune, a journalist, tries to pin down a researcher whose life, from the very beginnings, reads like a novel: the little Jewish boy born in Spain during the civil war when his parents were fighting for the International Brigades, a father executed by the Nazis, a mother who took him to Poland where he grew up under the shadow of Stalin. And then came science. Is there a link between the chaos of life and the questions that fascinate us, and perhaps the manner of asking them? Cosmogenesis, the backdrop to the book, also allows the authors to lift the veil on some of the mysteries surrounding the Big Bang, the bootstrap, the quantum vacuum, etc.