A journey of 15 billion years back into the past

Paris, May 14, 2002

Two researchers from the Giovanni Domenico CASSINI Laboratory (CNRS – Observatoire de la Côte d’Azur)¹ , in collaboration with an Italian researcher² and a Russian researcher³ , have recently shown that it is possible to go back in time to the beginning of the Universe, about fifteen billion years ago, and to establish, for each galaxy of the current Universe, the place from which its matter comes. This work is published in the May 16, 2002 issue of Nature.

The present structure of the Universe is very unevenly distributed. Astronomical observations reveal that the galaxies are organized into large structures made up of walls and of filaments that are gigantic in extension but relatively small in thickness (figure). In contrast, the early Universe had an almost uniform distribution of matter, with only very slight variations in density from one point to another. These "density fluctuations" can now be detected indirectly through fluctuations in the cosmic microwave background radiation, which keeps a trace of everything that occurred a few hundred thousand years after the beginning of the Universe when the temperature, which was initially very high, dropped, enabling light particles (photons) to escape and to reach us unobstructed.

Would it not be possible to reconstruct these density fluctuations directly by resolving the equations of movement of mass backwards (by going back in time), starting from the currently known positions of the galaxies? To do that, we would also need to know the velocities of the galaxies, which we rarely do. However, the problem does have a unique solution based on the theory of mass transport. The first example of this type of problem was formulated in 1781 by the mathematician Gaspard Monge concerning a civil engineering question: how can earth be transported from one place to another as economically as possible by imposing the volumes occupied by the "cut" and by the "fill?" In this case, the cost of transporting an element of mass is proportional to the distance traveled.

The researchers based their approach on the work of the Russian cosmologist Yakov Zel'dovich dating from the nineteen seventies, and on the more recent work of the Nice mathematician Yann Brenier, to show that the problem of cosmological reconstruction is similar to the problem considered by Monge, but with a cost proportional to the square of the distance traveled. By using an optimization algorithm essentially produced by the Nice astronomer Michel Hénon, they were able, after a few hours of computation on a machine at the Observatoire de la Côte d'Azur, to determine the initial positions and the velocities of several tens of thousands of galaxies. These reconstructions, achieved on computer-simulated artificial universes, showed that the new technique gives excellent results at scales higher than about ten million light-years (a light-year being the distance traveled by light in one year).

The astronomers are currently making great efforts to measure the full positions (direction in the sky and distance) of a large number of galaxies. In a few years from now, we will have catalogues containing about one million galaxies. The new reconstruction technique and improvements to it (aiming to work at scales of a few million light-years) should give us a new window onto the early Universe and thus enable to better understand how it was formed.

Reference: "A reconstruction of the initial conditions of the Universe by optimal mass transportation," by Uriel Frisch, Sabino Matarrese, Roya Mohayaee, and Andrei Sobolevski. Nature, vol. 417. p. 260-262. May 16, 2002.

1 - Uriel Frisch, CNRS Research Director and Roya Mohayaee, European Union Marie Curie post-doctoral scholar
2 - Sabino Matarrese, Cosmologist at the University of Padova
3 - Andrei Sobolevski, Mathematician at the University of Moscow

Researcher contact:
Uriel Frisch
Tel: +33 4 92 00 30 35
e-mail: uriel@obs-nice.fr

Observatoire de la Côte d’Azur contact
Patrick Michel (media relations)
Tel: +33 4 92 00 30 55
e-mail: michel@obs-nice.fr

CNRS press contact:
Martine Hasler
Tel: +33 1 44 96 46 35
e-mail: martine.hasler@cnrs-dir.fr