Interview with Antonio Riotto, Professor of Theoretical Physics and President of the Physics Section at the University of Geneva, awarded the 2018 Buchalter Cosmology Prize for his innovative contributions to cosmology
How did the universe originate?
We do not know for certain, but there are several hypotheses. The most widely accepted is that about 14 billion years ago the universe emerged from nothing, as the result of a quantum fluctuation of the vacuum – a concept we will come back to later – which would have produced an energy variation sufficient to trigger cosmic expansion. Alternatively, another possibility is that the universe is eternal, that it has no origin, and that what we perceive as its beginning by tracing events backwards in time, is in fact merely the end of another epoch, of a universe that expanded and then contracted, only to expand again afterwards. Unfortunately, none of these hypotheses is experimentally testable. According to the modern cosmological view, immediately after its initial phase, the universe underwent a period of extremely rapid expansion, known as inflation, which increased its volume exponentially within a fraction of a second, erasing any pre-existing information. We can no longer recover the content of the universe prior to inflation; for this reason, today the term “big bang” is used – somewhat imprecisely – to refer to the transitional phase between inflation and the moment when the universe began to fill with a thermal bath, the so-called primordial soup.
How did the universe evolve after this first phase?
The inflationary phase – driven by vacuum energy of still unknown origin – lasted only a fraction of a second. When it ended, thanks to a phase transition, it generated a thermal bath that filled the universe with relativistic particles, that is, particles travelling at speeds close to that of light: photons, electrons, neutrinos… all the particles we study today in laboratories, and perhaps a few more that we have not yet managed to detect with current accelerators. Then the evolution described by the “standard cosmological model” began. About one minute after the thermal bath was created, nucleosynthesis occurred, meaning that the first atoms were generated, chemical elements such as hydrogen, helium and lithium; and meanwhile the primordial soup of particles – interacting extremely rapidly and thus sharing a common average energy, called temperature – began to cool as the universe expanded, just like any fluid which, when compressed, remains hot and when expanded, cools. Until, about 380,000 years after the big bang, photons, the particles of light, ceased interacting with surrounding matter. Electrons and protons, with which photons normally interacted, began to combine into stable, neutral hydrogen atoms, and light, which interacts only with charged particles, started to propagate freely to us: the universe thus changed from opaque to transparent, and we were able to take a “picture” of it in the 1960s, when the first radio telescopes captured the cosmic microwave background radiation (CMB), the fossil trace of the moment when light became free from matter. During this period, the universe expanded at a decreasing rate and its energy ceased to be dominated by the radiation of relativistic particles in favour of non-relativistic matter (that is, matter travelling at speeds lower than that of light), such as baryons, protons, neutrons, and heavy particles in general. This allowed the small fluctuations created during inflation to clump into larger inhomogeneities through the effect of gravity, eventually forming the structures we observe today: galaxies, galaxy clusters, dark-matter haloes, and so forth. Then, even more recently, a dramatic shift occurred: the universe began to expand in an accelerated rate, with a speed that increases over time. We realised this in 1998 by observing supernovae, extremely luminous objects, the remnants of exploding stars, which can be detected at enormous distances. These supernovae appeared dimmer than expected, and therefore farther away than predicted, and the only way to explain this discrepancy was that the universe had reversed its previous trend and had begun to accelerate its expansion.
What is causing this acceleration?
Its cause remains unknown. One possibility is that, very recently in the universe’s history, its energy density ceased to be dominated by non-relativistic matter and came instead to be dominated by another kind of fluid of unknown origin, which we call dark energy. This fluid has a counter-intuitive property: it exerts negative pressure. In other words, when compressed, it does not push back, but behaves in a sort of anti-gravitational way, expanding even more outwards and therefore favouring the accelerated expansion of space. But dark energy is only a hypothesis. So too is the idea that gravity at very large – cosmological – distances may not follow Newton’s law exactly (according to which its force falls off as the inverse square of the distance). On such enormous scales, gravity would be even weaker, as would its capacity to slow expansion, thus allowing the universe to accelerate its expansion naturally. Clearly, as I mentioned earlier, this remains a theory that is, at present, experimentally inaccessible.
Speaking of untestable hypotheses, let’s take a step back to the hypothesis of the universe originating from vacuum quantum fluctuations. What are these fluctuations?
To answer this question, we must enter the world of quantum mechanics. At very high energies or at very small distances, the laws of classical physics give way to quantum laws, which can at times be quite counter-intuitive. For example, in quantum mechanics the position of a particle cannot be determined exactly, because the concept of particle is intrinsically associated with the concept of wave – a wave of light that is not localised at a single point but distributed over space. Because of this dual nature, the particle-wave cannot be described uniquely, but only probabilistically. The same probabilistic logic also permeates the quantum description of the vacuum. Imagine creating a classical vacuum in the laboratory by sucking out all the contents of a box, and observing it with a microscope capable of probing extremely small distances. We thus discover that the vacuum is not empty: there exists a certain probability that, at a certain moment and in a certain region of space, pairs of particles and antiparticles, so-called virtual pairs, spontaneously appear. These pairs form and annihilate incessantly – hence the term virtual –, filling the quantum vacuum, whose minimum energy state is never zero. However, if we move the walls of the box apart extremely rapidly, exponentially – mimicking the inflationary expansion of the universe – something new happens: the virtual particle pairs separate too quickly to meet again and annihilate; they lose the possibility of returning to the previous state, and from virtual they become real. This phenomenon, in cosmology, is called quantum fluctuation, and it implies that particles, inhomogeneities emerge from the vacuum. In physics, anything that has energy, such as these particles, modifies in some way the geometry of the spacetime in which it is created. Indeed, these inhomogeneities, originating in the microscopic realm, persist and are even amplified during the inflationary period, their wavelength grows as the universe expands, until gravity draws them together to form the classical structures (galaxies, solar systems) that we observe today on macroscopic scales. This is the most fascinating representation of how quantum physics operates: the macroscopic structure of the universe, and everything that follows from it – such as the fact that life was able to develop – has its origin in the microscopic quantum world, in the minute fluctuations that arose in the very first instants of the universe.
What will become of the universe? Which cosmological scenarios are considered most plausible today?
Before the supernovae observations of 1998, it was thought that the universe was not only expanding but also decelerating. This was because it was assumed that its energy content was dominated by non-relativistic matter, that is, by the matter we ourselves are made of and by what we call dark matter, which is different from the dark energy we believe dominates today. Dark matter is made of particles that we have not yet discovered, but whose existence is certain because they influence gravity: if the universe consisted only of the matter we know – roughly 5% of its total content – this matter would not exert sufficient gravitational force to aggregate inhomogeneities and form the structures we observe today. Dark matter, in this sense, enhanced gravity, acting as a catalyst for the formation of these structures; and being non-relativistic, meaning that it travels at speeds much lower than that of light, it was thought that the universe was therefore decelerating. Given this, there were two possible fates for the universe. The first envisaged the universe continuing to expand forever while decelerating, without any particular consequence, since galaxies would recede from us at ever lower speeds and would remain observable. The second suggested that the universe would eventually reach a maximum extent and then begin to contract in what is known as the “big crunch”: the galaxies would move ever closer to us until reaching a final contraction and producing an anti–big bang. But everything changed after 1998, when observations of supernovae at very great distances showed that the universe is in fact accelerating because of an unknown dark energy. This new understanding eliminated the big crunch scenario, which is no longer considered viable, and introduced a new hypothesis: the universe will continue to expand at an ever-increasing rate, and galaxies will recede from us ever more rapidly. Light will take increasingly longer to reach us, and the images will become ever dimmer, ever fainter, until galaxies cease to be visible. This is what is known as “cosmic darkness”, and it is currently the scenario supported by the data. Of course, this too may one day be disproved – if, for example, we were to discover that dark energy, just as it appeared, might also disappear –, but at present it is the most plausible scenario.
Which experimental confirmations are you hoping to see?
For a theorist like me, three confirmations would be truly exciting. The first could come from Euclid, an experiment in which INFN plays a key role and which is already collecting data. Euclid has been designed to explore the composition and evolution of the dark universe and should provide an answer regarding the so-called “cosmological constant”, that is, dark energy, which we currently imagine as a fluid with constant pressure and constant energy density over time (hence the name constant). If Euclid’s data were instead to indicate that this dark energy has a time-varying density and pressure, we might discover that dark energy is not a fluid, does not have static properties but evolves over time, and consequently the universe itself might evolve into a different state, opening a scenario distinct from the otherwise inevitable cosmic darkness. This confirmation would be interesting not only from an experimental perspective but also theoretically, because it would imply that this fluid is a fundamental object, akin to a scalar field such as the Higgs field, and possesses identifiable properties from the standpoint of particle physics. In other words, we could finally ascribe definite characteristics to dark energy, and this would amount to a revolution in cosmology. The second confirmation I would like to see in the near future would be the detection of gravitational waves generated during the primordial evolution of the universe. This too would be revolutionary, not because we would be measuring gravitational waves from a new source – we already know they exist and have already detected them –, but because primordial gravitational waves would carry information about what really happened close to the big bang, as they reach us undisturbed, unaffected by any interaction. It would probably be the only way to test what occurred a billionth of a second after the big bang, during the transition from inflation to the thermal bath. Indeed, there is a theory suggesting that at that moment a phase transition took place in which bubbles formed and the symmetry of the Standard Model broke. Just as when you boil water to make pasta and the water changes from a liquid to a vapour, the fluid filling the universe would have shifted from one energy state to another, and the particles of the Standard Model as we know them today would have changed from being massless to possessing mass. This phase transition would have been violent, and bubbles would therefore have formed, expanded to fill the entire universe and completely altered its energy state. The collision between these bubbles may have produced testable signatures, namely, gravitational waves, and we hope to measure them with experiments such as LISA, a space-based interferometer to which INFN is making a fundamental contribution. Last but not least, the third confirmation I hope for concerns the measurement of the polarisation of the cosmic microwave background radiation. This background radiation, generated when the universe was about 380,000 years old, should in fact exhibit a certain polarisation, meaning that the light should in some sense rotate due to the gravitational waves generated during inflation. If we were able to measure it, we could trace the history of the universe back to its very earliest moments, before the phase transitions, adding a crucial piece to the theory of inflation. We already have abundant data confirming that inflation took place, but this final measurement would really allow us to close the chapter.
BIO
Antonio Riotto is currently Professor of Theoretical Physics and President of the Physics Section at the University of Geneva. He works in the fields of cosmology, black hole physics and gravitational waves. In the past, he was postdoctoral researcher in the cosmology group at Fermilab in Chicago, CERN fellow in Geneva, research director at INFN and staff member in the theoretical division at CERN. In 2018, he was awarded the Buchalter Cosmology Prize for his innovative contributions to cosmology.
