All civilizations have wondered how their world began, but cosmology as a science is generally presumed to have its origins in the twentieth century. The first mathematical models, incorporating the ideas of general relativity, were proposed by Albert Einstein and quickly elaborated by a series of authors, e.g. Friedmann, Lemaitre, and de Sitter, who showed the possibility of an expanding Universe or, as it is now known colloquially, one that originated in a Big Bang. This possibility was confirmed in the 1920s by Edwin Hubble's observation that distant stars were receding with a velocity proportional to their distance from us, strongly suggesting a homogeneous isotropic expanding Universe.
The field of cosmology took a giant step forward in the early 60s with the observation by Penzias and Wilson of the microwave radiation emitted at the time of formation of atoms, some 300,000 years after the Big Bang. The essential truth of this picture of the Universe's genesis was enhanced soon thereafter by the formulation of a credible scenario for primordial nucleosynthesis, the formation of the nuclei of light atoms in the first minutes after the Big Bang. The confirmation of the predictions of primordial nuclear abundances made in this model confirmed its essential correctness and took our knowledge of the Universe's origins back to an era just seconds after the Big Bang.
That is where matters stood in 1980. A number of puzzles remained, but it was unclear whether these ever could be answered or were simply to be taken as initial conditions in our description of the Big Bang. Two of the most celebrated puzzles were the so-called homogeneity problem and the flatness problem. The first relates to the uniformity in the cosmic microwave radiation in all directions, a puzzle because regions separated by more than a degree were not in causal contact at the time the radiation was emitted and therefore should exhibit independent spectra, whereas they are identical to one part in a hundred thousand. The second relates to the lack of curvature of space, a phenomenon only possible if the energy density of the Universe is finely tuned to a value corresponding to flatness.
In 1981 Alan Guth wrote a paper which became an instant sensation in cosmology. He proposed that the Universe had undergone a period of exponential expansion at a very early time, somewhat less than a millionth of a billionth of a billionth of a billionth of a second after the Big Bang. In this so-called inflationary model, the curvature of the Universe was ironed out and the growth was so quick that our present Universe originated from a minuscule region of space that was in causal contact. Not only did he show how the two major problems as well as some others could be solved, but also he provided detailed mechanisms for their working.
This inflationary scenario, even though its details are still not all known, has become the commonly accepted picture for the very early Universe, has pushed our knowledge of the Big Bang back to almost unimaginably small times, has stimulated experiments still being performed by telescopes, by satellites and from balloons, and has focused scientific attention on the study of the very early Universe with its attending complications. Guth's work has invigorated cosmology, astrophysics and particle physics and has had a lasting profound influence on our view of the Universe's origins.
Information as of April 2001