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I was once talking to a freshman studying political science. He just happened to have read a popular book on quantum theory and was shocked that in physics it is not the majority that decides which theories should be accepted, and that the universe does not alter when accepted views change - happily enough, because substantial changes in our scientific image of the world happen quite often.

In 1998, two teams of astrophysicists, investigating supernovae with the help of ultra-modern technology, announced an astonishing discovery: the overall recession of galaxies is not slowing down, as it should under the influence of gravity, but is actually accelerating. The consequences are tremendous. To account for this phenomenon, one has to admit that about 75 per cent of the material content of the universe is composed of some unknown anti-gravitating entity, dubbed "dark energy". Scientists are not easily prone to accept ideas that overturn their established views. But observational evidence, from independent sources, started to accumulate, and cosmologists had to face the gigantic problem of the "dark" component of the universe.

Is this indeed an entirely new, unprecedented problem? The history of physics teaches us that "equations are wiser than those who wrote them down". In 1917, Einstein applied his gravitational field equations to model the global structure of the universe, and was very upset when it turned out that the universe, as described by his model, is gravitationally unstable: it either collapses or expands, but is never static. To prevent this catastrophe, Einstein added to his equations a famous cosmological constant whose aim was to stabilise the universe. However, a few years later, a Russian mathematician, Alexander Friedman, discovered that Einstein's equations with the cosmological constant admit a host of cosmological models with various "stability properties", among them many expanding universes (with different rates of expansion). This embarras de richesses became the fundamental strategy of modern cosmology: physical theory presents a plethora of possible cosmological models (in principle, an infinite number of them), and it is up to observational data to select among them a model, or a class of models, that best corresponds to observational results.

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