The Big Bang Most scientists believe that a long time ago, the Universe we live in was incredibly small, dense and hot. Many different observations support this view and it now appears that the Universe underwent a very dramatic event right at the beginning, known as the Big Bang. Different estimates put the Big Bang some 13 to 17 thousand million (13 - 17 billion) years ago. Since then the Universe has expanded, cooled down and has become much less dense. For an overall view of the History of the Universe according to the Big Bang model, see the picture. Going back in time The stars in the Universe are assembled in galaxies, each containing of the order of some hundred billion stars. It has been known since the work of Edwin Hubble in the 1920's that the distant galaxies are moving apart. Retracing this Hubble expansion more than twelve billion years, the matter in the Universe would then have been so dense and hot (many thousand degrees) that atoms could not exist, but would have been ionised, i.e., their individual nuclei and electrons would have been separated and surrounded by radiation. Remnants of this cosmic background radiation were discovered by Arno Penzias and Robert Wilson in 1965. Even earlier, when it was only seconds old and its temperature approached a billion degrees, the Universe is believed to have resembled an immense nuclear reactor, with protons and neutrons fusing to make the nuclei of light chemical elements, such as Helium and Lithium. Measurements of the amounts of these elements in the Universe today with astronomical telescopes agree well with calculations of these nuclear processes. The earliest times Back when the Universe was only a few millionths of a second old, even protons and neutrons would have been split into their component quarks, that are produced nowadays in particle accelerators. Experiments with these accelerators are able to reproduce conditions in the early Universe back to when it was about a tenth of a thousandth of a millionth of a second (10-10 s) old. However, there are speculations that, much earlier in the history of the Universe, it might have undergone a brief period of very rapid expansion, called cosmological inflation. Our present knowledge of the underlying laws of physics is inadequate to describe the very beginning of the Universe, or what might have preceded it. Still, we are members of the Universe and Life has formed in this Universe at some moment. There is a direct line from the Big Bang to us, but the amazing development that has led to our existence is still poorly understood.