P.J.E. Peebles

Remarks presented by Uriel Frisch, Nice September 2, 2004

For the award of the ADION Medal

The Observatoire de Nice was born, in a sense, twice: founded in the 19th Century, it suffered great decay after World War I, and only in 1962 was it born again under the directorship of Jean Claude Pecker. It was his suggestion to have the Association pour le Développement International de l'Observatoire de Nice (in short ADION) award a yearly medal to "an astronomer known worldwide both for his work and for his role in the international cooperation in astronomy." In 1988 the Observatoire de Nice merged with the more recent Centre d'Études et de Recherches Géodynamiques et Astrométriques to form the Observatoire de la Côte d'Azur, now directed by Jacques Colin. From that time on the ADION Medal was awarded to "a scientist whose work had or has significant impact on the scientific activities at Observatoire de la Côte d'Azur, in astronomy as well as in connected sciences."

In other words, with our modest means we wish to thank those who have been a great source of inspiration for many of us here. Not surprizingly most of them have received a great deal of international recognition. This is definitely the case with Philip James Edwin Peebles (I shall simply call him "Jim"), the quintessence of all cosmologists, who got the A.C. Morrison Award, the Eddington Medal, the Heineman, the Robinson and the Gruber Prizes and will go to Hong Kong next week to receive the first Shaw Prize in astronomy.

If I were now to review Jim's achievements and tell why he really deserves to be called the father of modern cosmology, I would need far more time than is reasonable in such introductory remarks; also I might give the wrong impression that all these prizes are just some kind of background radiation emanating from a distant past when Jim had barely decoupled from being a graduate student and was interested in reconstructing the thermal history of the Universe right up to the Big Bang.

But as all of you here know he was given the ADION Medal also for recent and ongoing work on reconstructing the dynamical history of the Universe around us. In a seminal paper "Tracing galaxy orbits back in time," published in the Astrophysical Journal in 1989, Jim outlined a procedure for the reconstruction of galaxy orbits, which is known as the Least Action or the Numerical Action Method. This work gave rise to the field of (dynamical) reconstruction of the Universe in which we have been involved here for four years and which is at the centre of a workshop held this week.

Although I have known Jim for many years and I have spent a lot of time with his books, I was not aware of the 1989 paper until Roya Mohayaee and Sabino Matarrese told me about it and I was immediately intrigued: it seemed to imply that for a prescribed cosmological model the mere knowledge of the present positions of the galaxies without knowledge of the peculiar velocities would be enough information to trace the orbits back in time, although with a small degree of non-uniqueness which can be removed using addtional observational data. Mathematicians such as Yann Brenier and Grégoire Loeper have now proven that unique reconstruction can indeed be performed if we know the present distribution of dark matter on scales where multistreaming is negligible, so that we can in principle generate maps of the density fluctuations in the early universe, one of the very few ways of complementing CMB measurements. With Roya, Michel Hénon, Sabino and Andrei Sobolevsky we have now developed efficient algorithms for performing the reconstruction when the number of objects used to trace the dark matter is large.

But let me return to Jim's original approach. It was characteristically rooted in the observational aspects. Basically, Jim's procedure poses orbit reconstruction as a mixed boundary condition problem, allowing solutions to be found by making physical assumptions about the initial conditions. If enough information on the current three-dimensional positions and velocities of galaxies can be provided by observations then the problem becomes sufficiently constrained that a reliable model can be constructed of the underlying mass distribution. The method has the great advantage that it is applicable even in highly non-linear regimes.

Through application of the Numerical Action Method, first in the region of the Local Group and later across the Local Supercluster, Jim concluded that the mean matter density of the universe must be well below the critical value required for closure. At the time, this result found disfavour because it was inconsistent with the then popular Einstein-de Sitter world model. Only in the last six years has the accummulation of evidence resulted in recognition that the early claims by Jim about the mean matter density of the universe were correct.

Of course reconstruction has moved a lot since 1989. It is at the centre of the Dynamical Observation of Galaxies project of the Space Interferometry Mission (SIM) which aims to obtain many peculiar velocities of galaxies and then using reconstruction obtain information eg on dark matter. Already Brent Tully and Roya have beeen able to show that the Virgo cluster contains a lot more mass than previously thought.

But now, it is time to hear Jim's views on "Open issues in cosmology and structure formation."

Addendum: List of key contributions of P.J.E. Peebles

• With Dicke, Roll and Wilkinson, Peebles predicted the existence of the cosmic background radiation and planned to seek it just before Arno Penzias and Robert Wilson found it accidentally. Peebles has investigated characteristics of this radiation and showed how it may be used to constrain models of the universe.
  [Dicke, Peebles, Roll & Wilkinson, 1965, ApJ, 142L, 414]
• Peebles has calculated the universal abundances of helium and other light elements, demonstrating agreement between the Big Bang theory and observations.
  [Peebles, 1966, ApJ, 146, 542]
• Peebles with Yu suggested in 1970, independently from Harrison and Zel'dovich, the scale-invariant primordial power spectrum. Peebles' work showed that the primordial acoustic oscillations should have stamped the cosmic microwave background (CMB), which is confirmed by CMB explorers.
  [Peebles & Yu, 1970, ApJ, 162, 815]
• Peebles' works on mass distribution in galaxies led to the idea that galaxies are embedded in dark matter halos.
  [Ostriker, Peebles & Yahil, 1974, ApJ, 193L, 1]
• The work of Peebles and Groth on two- and three-point correlation functions and on the power spectrum of two- and three-dimensional galaxy catalogues provided evidence of the existence of large quantities of dark matter in the halos of galaxies. Peebles continues to work on the origin of galaxies.
  [Peebles & Groth, 1975, ApJ, 196, 1]
• With Marc Davis, Peebles integrated the BBGKY equations for the development of strongly nonlinear clustering in an expanding universe.
  [Peebles & Davis, 1977, ApJS, 34, 425]
• Peebles is the founder of the cold dark matter model of the Universe.
  [Peebles, 1982, ApJ, 263L, 1]
• The work of Davis and Peebles on the two-point velocity and position correlation lead to determination of the cosmological parameter Omega_m=0.3.
  [Davis \& Peebles, 1983, ApJ, 267, 465]
  Later on, Peebles determined the presently accepted mean mass density of the Universe (Omega_m=0.3) using galaxy clustering.
  [Peebles, 1986, Nature, 321, 27]
• Peebles was the first to resurrect Einstein's cosmological constant and the concept of its time variation, which became known later as the Quintessence.
  [Peebles & Ratra, 1988, ApJ, 325L, 17]
• Peebles used the fact that the abundance of clusters is exponentially sensitive to the normalization factor sigma_8 to constrain the amplitude of primordial density fluctuations.
  [Peebles, Daly & Juszkiewicz, 1986, ApJ, 347, 563]
• Peebles' work on the orbit reconstruction with Brent Tully and others led to a correct determination for the value of cosmological parameters.
  [Peebles, 1989, ApJ, 344L, 53; Shaya; Peebles, Tully, 1995, ApJ, 454, 15]
• Peebles' books on physical cosmology have had a significant impact in convincing physicists that the time has come to study cosmology as a respectable branch of physics.
  [Peebles, 1980, The large scale structure of the Universe; Peebles, 1993, Physical Cosmology]