Physics Articles

At the beginning of 2014, nine researchers of renown were gathered in Oslo to receive the prestigious Kavli Prize. Awarded every two years since 2008 and endowed with a million dollars, it rewards exceptional contributions to the domains of astrophysics, neuroscience and nanoscience. The recipients of the prize for astrophysics : the three fathers of cosmic inflation. Encounter with those who have described precisely how the Universe was born : the Russians Andrei Linde and Alexei Starobinsky and the American Alan Guth.

You are the three laureates of the Kavli Prize in astrophysics for your pioneering role in the elaboration of the theory of cosmic inflation. What are your respective contributions?

AS At the end of the 1970s, at the Landau Institute for Theoretical Physics, near to Moscow, I tried to explain how the Big Bang was produced by employing Quantum Gravity (a discipline which attempts to reconcile Quantum Mechanics and General Relativity). In 1979 my calculations led me to the conclusion that at the time of the first instants of the Universe, it underwent an exponential expansion. I also realized that this violent phase must have produced wavelets in space-time (a little like waves on a lake when one throws a pebble in it) and that these waves, called gravitational waves, must be detectable in the sky today

AG Given that exchanges between Russian and American researchers were almost non-existent at the time, I was not aware of the work of Alexei, despite it being so remarkable. In 1979, I was then a post-doctoral worker at the Stanford Linear Accelerator Center (California). I came by accident upon a work by Robert Dicke, a great cosmologist at Cornell University. He presented the problems which the Big Bang theory encounters and notably that of the "flatness" of the Universe. In parallel I worked with Henry Tye, likewise a post-doctoral worker at Stanford, on another puzzling problem for the Big Bang theory - that of monopoles. The equations describing the entirety of the first instants of the Universe indicate that it was filled with exotic particles representing only a single pole ("a little like a magnet which would have, uniquely, a north pole or a south pole"). However these particles are non-existent today.

We attempted therefore to understand what would make it necessary to change the classical theory of the Big Bang, so that the monopoles disappear. We realized that a phase of cooling had exactly the anticipated effect. It was only a few months later that we understood that such a cooling would go hand-in-hand with an exponential expansion and that this phase, on stretching the Universe in a flash, resolved the problem of its flatness.

I thought to get the perfect theory, that which explains how the Universe was born, in total accord with observations. But I fell down on an oversight : inflation must necessarily result in the combination of particles and therefore in the creation of the matter which became the galaxies, stars, planets. Now the theory which I proposed ended with the appearance of a multitude of bubbles in expansion. A chaotic situation which avoids all formation of matter. The scenario therefore failed miserably.

AL In 1981, Steve Hawking affirmed that inflation could not function, precisely because of its problem with chaotic bubbles. But I found a solution. I worked at the time in the same group as Alexei, in the domain of superconductivity. I realized that inflation could work if one made it require a "scalar field" to produce the inflation (to represent a scalar field, imagine a water surface, a lake e.g. when this field - this lake - is agitated , waves are created, which form particles ). At the end of inflation this scalar field, called inflaton, oscillates (a little bit like if the lake continued to fill up and empty ) which created waves, then particles - our matter. I remember that after calling a colleague to verify with him that this solution could be a good one, I woke my wife up telling her ecstatically : "I believe that I know how the Universe was born"

In February 2014 the BICEP2 experiment, based in Antartica, announced they had discovered the proof of existence of gravitational waves, predicted by Alexei. But some days after the awarding of your Kavli Prize, this discovery has been brought into question by the measurements from the satellite Planck. Is your theory in difficulty?

AG The new article by the Planck team confirmed more or less what the majority of us expected : the signal detected by BICEP2 is maybe entirely due to the dust of our galaxy, but at this stage we are still not certain. The teams from Planck and from BICEP2 are working even at this moment on a joint analysis, which must say to us, with the utmost precision possible, if the collected data implies 'yes' or 'no' to the trace of primordial gravitational waves.

If the answer is 'yes' - and I hope it is - this will be evidently very exciting, but if no signal is found, this will not be an argument against inflation. Several versions of the theory, which work quite well, predict a weak intensity of gravitational waves, too weak to be detectable by BICEP2. We shall therefore have to eliminate the versions which predict the strongest waves.

AS The signals announced by BICEP2 in February 2014 were very strong, much more than those that we were expecting. That was very surprising. We now know that the signals we are looking for could not be so intense because a great part finds dust to be impenetrable. But we would be very content if the observations discover some signals ten times, even a hundred times less than these original announcements.

The BICEP2 episode has ultimately caused much harm to our theory. In February, all the media had the title The proof of inflation finally discovered and since the observations are now in question, inflation is equally placed in doubt, as if the existence of gravitational waves was vital to the solidity of the theory. If they were proved to exist, this would not be the cherry on the cake. In effect, we have no need of them. The theory of cosmic inflation is solid. It has already been confirmed by numerous observations.

Can you mention which?

AG The theory predicts for example that the Universe must have a "critical" density which renders it uniform and flat. This density which we assign the value 1, has been verified by observation with a precision of 0.5%. The models predict also that the irregularities in the cosmological background radiation has a distribution in size and a particular frequency. Now, that is precisely this distribution and this frequency which has been measured with excellent precision by Planck and by numerous other experiments.

AL In the 1970s, we still did not know what the size of the Universe was. There was a possibility that it could be closed and relativity compact. Inflation, and the exponential expansion which it engenders, implies on the contrary an extraordinarily large Universe, which the observations (by Hubble, notably) have confirmed. By virtue of its exponential expansion, inflation implies a uniform Universe which permits us to explain why, whichever direction you look, there is the same temperature and the same density. But at the epoch when the theory was formulated, the measurements of uniformity were not good. Today, by virtue of COBE, WMAP and Planck, we know that it is in effect uniform with a precision of 0.01%.

Inflation appears to be engendered by a repulsive force, equally invoked for explaining the acceleration of the expansion of the Universe discovered in 1998 (and baptised therefore dark energy). These two episodes of history of the Universe - do they proceed from the same phenomenon?

AG We think in effect that the inflation, like the acceleration of the current cosmic expansion, are caused by an effect of repulsive gravity. The theory of general relativity predicts that matter endowed with a negative pressure can create a repulsive gravity. But in the case of inflation, the motor is inflaton. Whereas for the accelerated expansion it is rather the energy of the vacuum which is involved (which remains to be confirmed). Otherwise, the motor of inflation is immediately more powerful than that of current cosmic acceleration. Note that the discovery of dark energy has largely reinforced the credibility of inflation. Firstly because it has shown the possibility of a repulsive gravity, such as we proposed with inflation. But most importantly it has confirmed that the famous critical density which renders the Universe flat and homogeneous is equal to 1 as our theory claims. Before 1998, the best observations gave a critical density of 0.25. The theory therefore had some serious problems . But henceforth, when you add the dark energy to the total balance of the Universe, you obtain well and truly the density as expected : 1.

Nevertheless, the theory is still not accepted by all the cosmology community. Certain people criticize it animately, notably because it requires a multitude of different versions. The Frenchman Marc Lachieze-Rey even goes far as comparing it to epicycles.

But it has no comparison to the case of epicycles, where people persisted in maintaining the circular orbits theory of the planets [before Kepler showed that they are elliptic], even to the extent of adding complicated epicycles to them, with the aim of agreeing with observation. In the case of inflation, it is the opposite, the observations fit the theory perfectly. If there exist several "sub-theories", it's because of several parameters needing to be defined. For example, we still do not know exactly at what level of energy inflation operates, or when exponential inflation stops. It is the observations which will help us to select the good version among all those which are still possible.

AL I know that the theory with its terrifying exponentail expansion, the fact that it predicts some multiverse etc., sounds like science-fiction to certain people. But on the other hand, if 40 years ago someone had said to me that the Universe had known an episode so extravagant in the first instants of its history, I would have said to them : Come back when you have sobered up.

Andrei, during one of your talks at Oslo, within the framework of the Kavli Prize, you declared that that was a "fantastic epoch to be a cosmologist". Can you explain to us why?

AL We live in an era of great discoveries! A hundred years ago, we did not know that the Universe was in expansion, that the galaxies existed.... Now with our prodigious instruments, we can see a good part of our Universe, we are only limited by the maximum distance that the light has been able to traverse since its birth, 13.8 billion years ago. And so the new knowledge that we receive today permits us to think about the Universe in its entirety. And not solely at ours! We can also formulate some theories over other possible Universes, like the multiverse. Fantastic, I would say to you

AG I share totally this point of view. I think that Andrei, Alexei and I myself are very fortunate to have embraced a career as a cosmologist in the 1970s. The first light of the Universe (i.e. cosmological microwave background) came to be discovered and it was actively studied. At that time nevertheless we did not collect much data. When I commenced to calculate what were the properties of the undulations (or irregularities) of the microwave background, that appeared to me to be a game, a mathematical exercise to see if we were fully in agreement over our results. I did not imagine for a second that it could be measured a fortiori with an extraordinary precision. What's more these observations are all in accord with this "crazy" theory which we have brought into focus and which described the behavior of the Universe before even the first second. That is staggering.