Nobel laureate George Smoot looks at exciting times now
and to come in cosmology.
L’âge d’or de la cosmologie
Quatre siècles après Galilée et son télescope, George Smoot, prix Nobel, se penche sur l’époque exceptionnelle que vivent les cosmologistes. Grâce à des observations détaillées, associées à un raisonnement rigoureux et, de temps en temps, à des intuitions brillantes, on a pu produire un modèle élégant et précis du point de vue quantitatif, qui reproduit très précisément tout ce que nous avons observé, depuis des temps très reculés jusqu’à nos jours. La nouvelle cosmologie propose un nouveau laboratoire où des théories fondamentales classiques ou révolutionnaires peuvent être mises à l’épreuve à des échelles qui seraient inaccessibles autrement. Nous nous trouvons ainsi à une époque critique de la cosmologie moderne, en suivant toujours la voie tracée par Galilée.
“La verità è il destino per il quale siamo stati fatti (Truth is the destiny for which we were made)”. This article gives an example of how “truth” is achieved through “discovery” – the method used in science. By revealing nature, discovery is the way in which we can achieve truth, or at least glimpse it. But how can we know or have confidence that we have made a correct discovery? Here we can look to the major architect of the scientific method, Galileo Galilei: “La matematica è l’afabeto nel quale Dio ha scritto l’Universo” (Mathematics is the language with which God has written the universe). A discovery will be described best – and most economically and poetically – mathematically.
Virtual space flight
There has never been a more exciting time for cosmologists than now. Through advanced techniques and ingenious, and often heroic observational efforts, we have obtained a direct and extraordinarily detailed picture of the universe – from very early times to the present. I recently had the pleasure of using a specially outfitted planetarium at the Chabot Observatory Space and Science Center in Oakland, California, and taking a virtual flight through the universe on a realistic (though often faster-than-light) journey based on real astronomical data.
We took off from the surface of the Earth and zoomed up to see the International Space Station at its correct location in orbit. When we first arrived we could only see a dark region moving above the Earth but soon the space station’s orbit brought it out of the Earth’s shadow into direct Sun light. We circled round, looking at it from all sides and then swiftly moved on to see the solar system with all the planets in their correct current locations. After a brief visit to the spectacular sight of Saturn we continued out to see the stars in our neighbourhood before moving on, impatient to see the whole galaxy with all the stars in the positions determined by the Hipparchos Satellite mission. After that we travelled farther out to see our local group of galaxies dominated by our own Milky Way and the Andromeda galaxy.
Moving more and more quickly we zoomed out and saw many clusters of galaxies. I was having trouble deciding quickly enough which supercluster was Coma, Perseus-Pisces or Hydra-Centaurus when viewed from an arbitrary location and moving through the universe so fast. Then, using the latest galaxy survey data, we went out farther to where we were seeing half-way to the edge of the observable universe. All the galaxies were displayed in their observed colours and locations – millions of them, admittedly only a fraction of the estimated 100 billion in the visible universe, but still incredibly impressive in number and scope, revealing the web of the cosmos.
We were actually moving through time as well as space. As we went farther away from the Earth we were at distances where light takes a long time to reach our own planet, so we were looking at objects with a very much younger age (earlier in time). It was fun flying round through the universe at hyperfaster-than-light speed and seeing all of the known galaxies. Soon I asked to see to the edge (and beyond). The operator brought up the data for the cosmic microwave background (CMB) – at the time, the 3-year maps from the Wilkinson Microwave Anisotropy Probe – and it appeared behind the distance galaxies. I asked to move right to the edge, and in the process of zooming out we went past the CMB map surface and were looking back at the sphere containing the full observable universe. Where were we? Out in the part from which light has not had time to reach Earth and – if our current understanding is correct – will never reach us. But still we wonder about what is out there, and we have some hope of understanding.
The second reason why this is such an incredibly exciting time in cosmology is that these observations, combined with careful reasoning and an occasional brilliant insight, have allowed us to formulate an elegant and precisely quantitative model for the origin and evolution of the universe. This model reproduces to high accuracy everything that we observe over the history of the universe, images of which are displayed in the planetarium.
We now have precise observations of a very early epoch in the universe through the images made using the CMB radiation and we hope to start a newer and even more precise and illuminating effort with the launch of the Planck Mission on 14 May. However, we also have many impressive galaxy surveys and plans for even more extensive surveys using new ideas to see the relics of the acoustic oscillations in the very, very early universe, as well as the gravitational lensing caused by the more recently formed large-scale structures, such as clusters of galaxies that slightly warp the fabric of space–time by their presence. Each will give us new images and thus new information about the overall history of the universe.
However, the model invokes new physics; some explicitly and some by omission. First, we put in inflation, the physical mechanism that takes a small homogeneous piece of space–time and turns it into something probably much larger than our currently observable universe but with all its features, including the very-small-amplitude fluctuations discovered with the Differential Microwave Radiometers on the Cosmic Background Explorer, which are the seeds of modern galaxies and clusters. Second, we put in dark matter, which plays the key role in the formation of structure in the universe and holds the clusters and galaxies together. This is a completely new kind of matter – unlike any other with which we have experience. It does not interact electromagnetically with light but apparently does interact gravitationally, precisely the property needed for it to form structure. A third additional ingredient is dark energy, which is used to balance the energy budget of the universe and explain the accelerating rate of expansion observed in the more recent history of the universe. Last, we need baryogenesis, the physical mechanism that explains the dominance of matter over antimatter. We have good reason to believe that there were equal amounts of matter and antimatter at the very beginning, but now matter prevails.
If we add these four extra ingredients in the simplest possible form we can reproduce the observable universe in our simulations or analytic calculations to an accuracy that is equal to (and probably better than) the current observational accuracy – at roughly the per cent level.
There are other things that we don’t put in so explicitly but have reason to suspect might be there. For example, we work with a universe constrained by three large dimensions of space and one of time, even though we know that more dimensions are possible and may be necessary. We do not deal with our confinement to 4D. We also stick with the four known basic forces even though there is plenty of opportunity for new forces; and likewise for additional relics from earlier epochs.
The success of the standard cosmological model has many consequences that puzzle us and also raises several key questions, which are far from answered. The observation of dark energy demonstrates that our well established theories of particles and gravity are at least incomplete – or not fully correct. What makes up the dark side of the universe? What process, in detail, created the primordial fluctuations? Is gravity purely geometry as Albert Einstein envisaged, or is there more to it (such as scalar partners and extra dimensions)? An unprecedented experimental effort is currently being devoted to address these grand-challenge questions in cosmology. This is an intrinsically interdisciplinary issue that will inevitably be at the forefront of research in astrophysics and fundamental physics in the coming decades. Cosmology is offering us a new laboratory where standard and exotic fundamental theories can be tested on scales not otherwise accessible.
The situation in cosmology is rife with opportunities. There are well defined but fundamental questions to be answered and new observations arriving to guide us in this quest. We should learn much more about inflation from the observations that we can anticipate over the next few years. Likewise we can hope to learn about the true nature of dark matter from laboratory and new accelerator experiments that are underway or soon to be operating, as at the LHC. We hope to learn more about possible extra dimensions through observations.
We continue to seek and encourage new ideas and concepts for understanding the universe. These concepts and ideas must pass muster – like a camel going through the eye of a needle – in agreeing with the multitude of precise observations and thereby yield an effective version of our now-working cosmological model. This is the key point of modern cosmology, which is fully flowering and truly exciting. It is the natural consequence and culmination of the path that Galileo started us on four centuries ago.