dark matter dark energy
June 25–29, 2001
Conference
WHERE IS THE MATTER?
During the week of June 25–29, 2001, an international conference took place in Marseille, bringing together 200 participants, including 140 speakers, with the theme "Where is the matter?" A subtitle was also provided:
Tracing Bright and Dark Matter with the New Generation of Large Scale Surveys
Translation:
Mapping observable matter and dark matter using the new generation of large-scale surveys.
These words carry great significance. They immediately situate this conference within a specific paradigm. It is assumed that the universe is a single entity possessing two components: one accessible to our observational methods (in the visible, ultraviolet, infrared, and generally electromagnetic wave-based observations), and another that, for now, eludes such investigation and is described under the generic term "dark matter" (in English, "dark matter"). How can we observe and map this dark matter? On one hand, by relying on gravitational lensing effects, which are generally found to be too strong to be explained by visible matter alone—whether galaxies or galaxy clusters. On the other hand, by analyzing the kinematic parameters of observed objects.
Even before the discovery of these "anomalously strong" gravitational lensing effects, scientists like Fritz Zwicky, based on the analysis of galactic rotation curves or the measurement of galaxy velocities within clusters, had concluded that the optically detectable matter could not account for the cohesion of these systems. By "rotation curves," we mean measurements taken on interstellar gas orbiting within the overall gravitational field, using the Doppler-Fizeau effect. This gas, clearly, was rotating too quickly—especially at its periphery (characterized by a distinct "plateau"). Galaxy clusters are likened to "gas clumps," with galaxies acting as "molecules." The "peculiar velocities" of galaxies, as astronomers call them, become equivalent to the thermal agitation velocities of gas molecules—randomly distributed in all directions. Maintaining this gas analogy, the combination of "thermal agitation" velocities and density yields what is known as pressure (a measure of the kinetic energy density of agitation per unit volume). An interstellar gas cloud does not disperse because gravitational forces balance the pressure forces that tend to disperse it. If we consider a cluster of a thousand galaxies as a kind of gas clump, we can also say that the pressure forces tending to disperse it—calculated from the measured velocities of galaxies and the detected mass—are too large to be counterbalanced by gravity alone. Conversely, knowing the cluster's mass, one can calculate an escape velocity. As Zwicky noted, individual galaxy velocities exceed the escape velocity of the cluster to which they belong. If no additional force were acting, these galaxies would have long since left the cluster. The same applies to stars within galaxies. The problem is thus very real. The question lies in the interpretation of this phenomenon.
The unanimous response among astronomers today is "dark matter," although no one has yet been able to specify the nature of this "dark matter." Nevertheless, no one has ever doubted that the observed effects must stem from the presence of an unobserved component—massive, positive, and quietly residing within our (single) universe. Within this context, work on "mapping the invisible" has already begun. Initially, researchers simply stated: "In such a galaxy cluster, a certain mass M must exist to prevent the cluster from exploding—or, equivalently, to explain the strong gravitational lensing effects it produces (multiple images, background galaxies distorted, deformation extending to stretching into arcs)." Subsequently, astrophysicists such as Albert Bosma from the Marseille Astrophysics Laboratory, to which I belong, empirically added "dark matter halos" of unspecified nature to fit the rotation curves—using the anglicism "fitting the rotation curves," meaning the empirical method of matching observed velocity values with theoretical laws. A number of researchers are now fully dedicated to calculating the distribution of matter within these invisible dark matter halos. This is known as "zeroth-order theories." This activity, requiring no particular expertise, is purely technical. Those engaged in it do not aim to provide insights into the nature of the "dark matter" they empirically map, let alone into the processes that may have led to its presence in galaxies. Since the nature and origin of this component remain unknown, constructing a "galactic dynamics" model is, a fortiori, impossible. At the conference in question, I heard an American present an overview of models attempting to describe galaxy formation (based on cold dark matter, or "CDM"). Naturally, all these models rely solely on Newton's law (plus an enormous computer crunching the data). Yet they remain largely unconvincing, as regardless of the initial conditions introduced, the resulting "proto-galaxies" possess far too little angular momentum. Thus, one of the questions raised by these "new theorists" (Newton's law plus a large computer) was: "Where does the angular momentum of galaxies come from?" We are clearly still at the level of "zeroth-order theories," whether in mapping or in simulation attempts.
Since 1999, astrophysicists such as Yannick Mellier and Fort, followed by half a dozen teams around the world, have produced the results of six years of work, corresponding to a large-scale program. If dark matter exists in the universe—particularly within galaxy clusters—it would produce gravitational lensing effects. At the extreme, this can produce almost kaleidoscopic images, as the Hubble Space Telescope has shown us, where background objects behind a cluster explode into multiple images, sometimes forming one or more gravitational arcs. One then understands that when the supposed concentrations of dark matter are less significant, they induce only minor distortions in galaxy images, adding an extra ellipticity to their appearance—purely an optical effect that superimposes on their actual ellipticity. Mellier, Fort, and those who followed them developed an image-processing method in which the computer detects local anisotropies in the images (based on the assumption that in the absence of gravitational lensing, the major axes of elliptical galaxy images should be randomly distributed...