General relativity is not the only possible classical theory of gravity. Extensions to general relativity have been proposed, such as 'Higher Derivative Gravity', and 'Scalar-Tensor' theories. (Brans-Dicke theory is one example of the latter). Is there any reason to believe that these other theories are physically relevant? Well, in fact, there are actually some serious discrepancies between the predictions of general relativity and observational data, but physicists have chosen to postulate 'dark matter' and, more recently, 'dark energy', to explain these discrepancies, rather than resort to tinkering with general relativity.
The motion of stars as they orbit a spiral galaxy is not correctly accounted for by general relativity; moving outwards from the centre of a spiral galaxy, the acceleration experienced by the stars in their orbital trajectories decreases, and this decrease follows predictions until the acceleration drops below a certain critical threshold, at which point the decrease in acceleration with radial distance is not as rapid as it should be. The extra acceleration is of the order 1/10^8 cm per second per second. Astrophysicists and cosmologists have postulated the existence of dark matter in order to explain this extra acceleration without having to change their theory of gravity.
Intriguingly, the acceleration experienced by the Pioneer 10 and Pioneer 11 space-probes as they reach the edge of our solar system also fails to agree with theoretical predictions. There appears to be an extra acceleration towards the Sun of the order 1/10^8 cm per second per second. Dark matter cannot be invoked to explain this discrepancy, so this phenomenon remains unexplained.
Yet cosmologists continue to assume that dark matter exists, and in 2005 the results of the 'Millennium Simulation' were triumphantly announced. This was a super-computer simulation of the formation and evolution of galactic structure, which assumed a dark matter model, and, by simulating a cube of space 2.230 billion light-years on a side, was able to obtain a final distribution of matter which agreed with current observations.
In a nice paper this week, Stéphanie Ruphy highlights the assumptions which went into this simulation, (philsci-archive.pitt.edu/archive/00003123/01/Simulation_Phil-Archi.doc):
Quite obviously, a basic presupposition in this model is that there is such a thing as dark matter, that is, some unknown, exotic form of matter which is not seen but which is supposed to dominate the dynamics of the universe. Postulating dark matter is an answer to some puzzling observations of galactic dynamics. But there are alternative interpretations of these observations. Here again, it is worth quoting what cosmologists themselves have to say about dark matter:
“Many attempts have been made to identify its nature, […] but what it is composed of is still unknown. Laboratory searches are under way to try to detect this matter, so far without success. A key question is whether its apparent existence is due to our using the wrong theory of gravity on these scales. This is under investigation, with various proposals for modified forms of the gravitational equations that might explain the observations without the presence of large quantities of dark matter.” (George Ellis, 2006)
And the same goes for the so-called “dark energy” which is another key ingredient of recent cosmological models. There are other ways to interpret observations (in that case observations of very distant supernovae) than to postulate the existence of a form of energy, which we know nothing about, except that its effect would be to accelerate the expansion of the universe.
For in light of what has been said on the existence of alternative sub-models, there is no good grounds to claim that the path actually taken by modelers is the only path leading to a plausible (in the foregoing sense) simulated universe. And note that if the Millennium Run does not have (yet) serious competitors, it is not because alternative paths have been also fully developed and dismissed on empirical grounds. Rather, if only because of the cost in terms of material and intellectual resources of developing alternative simulations built with different modeling ingredients, only one path has been taken to its end, that is, to a level of details and to a scale large enough to allow significant comparison with observations. There is thus no good grounds to exclude that, had the cosmologists the resources to fully develop alternative paths, they would come up with different, but equally plausible representations of the evolution of the universe.
A paper published this week in 'Nature' claims to have mapped the distribution of dark matter in a small region of the sky. The map was produced by analysing the deviation and gravitational lensing of light from background galaxies which cannot be explained by the normal matter in foreground galaxies (under a conventional theory of gravity):
The article on the BBC website states that:
For astronomers, the challenge of mapping the Universe [using the distribution of normal, luminous matter] has been described as similar to mapping a city from night-time aerial snapshots showing only street lights.
Dark matter is invisible, so only the luminous galaxies can be seen directly. The new images are equivalent to seeing a city, its suburbs and country roads in daylight for the first time.
This, of course, is completely wrong. Inferring the existence of dark matter from the trajectories of the light coming from background sources, is analogous to mapping a city from night-time aerial shots of the motion of cars through the city; you still have to make an assumption about the relationship between the trajectories of the cars and distribution of buildings and people. In the astronomical case, the deviations to light which cannot be explained by the normal, luminous matter under a conventional theory of gravity, could well be explained by the normal, luminous matter under a different theory of gravity.