Scientific areas
Violent phenomena in the Universe are connected to the presence of very compact objects, neutron stars or black holes. It is only very recently that black holes have acquired the status of astrophysical objects, and not just curiosities of the theory of gravitation (singularities protected by a horizon, whereas the big bang is a naked singularity). Indeed, black holes seem to be ubiquitous in the Universe: they represent the final stage of a star, are present in many binary systems, but also at the centre of most galaxies (such as our own). And, even though they are rather simple objects from the point of view of gravity, their environment is very complex. Indeed, the understanding of how high-energy phenomena recycle energy into their environment and the feedback to large distances of powerful explosions, jets, and the impact of their companion high-energy particles are keys to understanding the emergence of new structures (clouds, stars, galaxies) in the Universe. All require high precision observations and elaborated modeling.
We start with the Universe in its largest dimensions. The question of the emergence of space, popularly known as the big bang, is obviously a central one and requires one to study the Universe at its earliest times, i.e. the most distant Universe (because of the finiteness of the speed of light). The Universe has been transparent to light since about 380 000 years after the big bang. This first light is in the present epoch observed in the form of a microwave background, the so-called cosmic microwave background (CMB). Investigating times close to the big bang (the so-called inflation period) requires the identification of very tiny details on maps of fluctuations of the CMB (those for which G. Smoot received the Nobel Prize in 2006) and thus precision measurement methods. Another promising approach is the detection of deformations of spacetime known as gravitational waves. These waves could have formed immediately after the big bang and would provide the only direct test of the content of the Universe emerging from the big bang. Since those early epochs, the expansion of the Universe has been decelerating. It was discovered in 1998 that the expansion has been accelerating again for the past 4 billion years. This is attributed to some new form of energy, called dark energy. Identifying dark energy has been singled out by both the astrophysics and fundamental physics communities as the priority task of the coming decade. It is obviously related to the question of the future of our Universe.
The research consists in locating the remaining witnesses of these early periods, collecting rare field samples and measurements, analyzing them with the most exquisite modern techniques of imagery, isotope geochemistry, mineralogy in extreme pressure and temperature conditions, as is beginning to be carried out on cores from Australia and South Africa that date back to 2.7 billion years and more. Because we need to go back even further in the past, to the first billion years of Earth’s history, it requires pushing back the resolution limits of many current instruments and also improving our understanding of some basic physical, chemical and even biological principles, and producing when needed efficient numerical modeling of massive amounts of data.
But the Present and the Past are keys to one another, and understanding the distant past of the Earth requires even better understanding of the processes which are still currently active on Earth. This is for instance the case for active subduction of plates, a generator of dangerous, powerful earthquakes and volcanoes, but also the key operator allowing the formation of granitic crust, that is the precursors to continental fragments that generally remain afloat at the top of the mantle, to form buoyant continental plates. A case in point is the subduction of the American plates under the Caribbean, forming the active arc of the Antilles. Active volcanoes provide there field laboratories where large parts of our scientific endeavour can be tested. The processes of rock generation, fluid-rock interaction, alteration and erosion, and chemical input into the world’s ocean can be studied near the three volcanological observatories of Martinique, Guadeloupe and Montserrat, three cousin volcanoes (which our teams continuously monitor) in different stages of their volcanic life cycle. This also leads to yet another kind of catastrophe, with potentially dangerous consequences to the local populations, bridging for us the gap between fundamental research and applications in the form of protection to the local populations and advice to the authorities.
The formation of planets, in particular the Earth and the Moon, through the process of accretion and proto-planetary impacts, controls their chemical state and thus their present dynamics. The understanding of the emergence of planets from the pristine solar disk, relies on detailed observations of remote solar systems with different degrees of maturation, in particular the observation of exoplanets, and on high resolution isotopic measurements of the composition of chondrites, the initial building blocks of planets. This is where the world views and approaches of the astro- and geo-sciences meet.
The accretion of the Earth forming objects, the Moon-forming impact, the nature and duration of a magma ocean, the formation of the Earth’s core, the appearance of the Earth’s magnetic field, early mantle convection and the emergence of the first oceans, atmosphere and continental fragments, the effect of the late heavy bombardment, the emergence of life, the growth of the inner core, the birth of plate tectonics are all key events in the history of the young Earth occurring in the first one or two billions of years, which have left precious few traces that modern geosciences are increasingly able to uncover and unravel. The early history of the Earth has not been a “long quiet river flowing” (as the title of a famous French movie goes) but it has been punctuated by catastrophic events separating periods when the rules of the game may have changed, with the emergence of new media and structures after each revolution. This new view of a non-linear evolution with “phase changes” is in line with progress in non-linear physics and chaos theory. There were a “before” and an “after”, with often very different characteristics, regarding the appearance of many of the features recalled above.