Abstract
After presenting a historical overview of man's efforts to understand the Universe, the second part of this lesson reviews the factual and speculative aspects of modern cosmology. This lesson begins with a description of the history of man's attempts to understand the Universe. Ancient civilizations had their mythologies, which were entirely speculative. As science progressed, the need was felt more and more to back up speculations with observational facts. Thus, highly hypothetical theories (such as the Pythagorean anti-Earth, Aristotelian circular orbits, or Ptolemaic epicycles) had to be abandoned, because they were contradicted by observational facts, more and more numerous as a result of improved techniques.
A major step forward in our understanding of the cosmos came in the 17th century, with the Newtonian law of gravitation. Although its starting point was highly intuitive and speculative, many of its applications to the Universe were successful, and established its credibility as a scientific theory. For example, it explained the movements of the Moon and the planets, predicted the passage of the comet now known as Halley's, and led to the discovery of the planet Neptune. Another example of success in astrophysics is the theory of the internal structure of stars and their evolution, which explains the observed properties of stars, as has been repeatedly verified. In truth, to be acceptable, a scientific theory must have undergone repeated verification of its predictions. How does modern cosmology stand up to such a test?
Modern cosmology really dates back to 1929, when Edwin Hubble discovered the expansion of the Universe. But a few years earlier, Alexandre Friedmann had proposed models of the Universe which explained such expansion, and which led to the conclusion that the Universe had originated in a gigantic explosion, the so-called "Big Bang". In the 1940s, George Gamow and his young colleagues Ralph Alpher and Robert Herman extrapolated Friedmann's models to the earliest epochs, when the Universe was extremely hot and expanding very rapidly. They were able to explain the origin of the light elements found in the Universe, synthesized in the 2-3 minutes after the big bang. Alpher and Herman also predicted the existence of fossil background radiation, a sort of signature of this extremely hot epoch. Such radiation was subsequently discovered in 1965. This work can be seen as an essential success of this cosmology, and gave it unquestionable credibility. Subsequent work focused on studying the Universe from its earliest instants, going back to a... micro-micro-micro-micro-micro-micro-second after the big bang. At that time, the nature of physics was unknown and unverifiable. The Universe itself could not be directly observed using astronomical methods. What's more, the events supposed to have taken place then cannot be repeated. So the claim to have achieved some success in these recent investigations rests solely on a number of highly speculative hypotheses, which can only with great difficulty satisfy the requirements that one is entitled to impose on a scientific theory.
