At the dawn of the 20th century
Major questions about the universe
What was the picture of the universe that science was proposing at the beginning of the 20th century? Our Galaxy, with its Milky Way, was already fairly well known, and both its size and the diversity of its stars had been explored throughout the previous century, since William Herschel's measurements. Emmanuel Kant had speculated on the existence of other similar galaxies, or "island universes", but observation had not been able to settle the matter. The question of what lay beyond our own Galaxy remained a major one. Nevertheless, one fundamental result seemed certain. Since Galileo's work nearly three centuries earlier, the idea that the laws of physics discovered in laboratories on Earth applied throughout the universe had become the most satisfactory working hypothesis. This hypothesis had been Newton's, assigning a universal property to the gravitation he observed on Earth and in the solar system. Similarly, spectroscopy, developed since Fraunhofer's work (1814), also suggested that matter, with its diversity of chemical elements, was fundamentally the same everywhere in the universe.
Second question: where does the Sun get its energy from? Geological studies had shown that the Earth was at least several tens, if not hundreds, of millions of years old, and the presence of ancient plants in fossils showed that the Sun must have shone almost uniformly for a very long time. It was easy to show that, if the Sun had been made of combustible matter, it could not have lasted this long. Other attempts, such as attributing the energy radiated by the Sun to a permanent fall of meteorites, did not stand up to analysis. The source of solar energy remained a mystery.
Third question: what was meant by the universe, and what was its future? In his Introduction to the System of the World (1796), Pierre Simon de Laplace had proposed, under the name of primitive nebula, a vision of the formation of the solar system under the effect of gravitation, from an earlier cloud of gas. But deeper cosmological questions about the extension of the universe, its apparent permanence, its age, and possibly its future, were very fragile. Isaac Newton, considering the gravitational force between stars, which is always attractive, had correctly concluded that it alone could not lead to an eternally stable state. He therefore postulated the existence of a repulsive force to balance it, of unknown origin and endowed with an almost theological status. This was the price to pay for lasting stability in the universe.
