Big Bang riddles and their revelations

We describe how cosmology has converged towards a beautiful model of the universe: the Big Bang universe. We praise this model, but show that there is a dark side to it. This dark side is usually called ‘the cosmological problems’: a set of coincidences and fine-tuning features required for the Big Bang universe to be possible. After reviewing these ‘riddles’, we show how they have acted as windows into the very early universe, revealing new physics and new cosmology just as the universe came into being. We describe inflation, pre-Big Bang, and varying-speed-of-light theories. At the end of the millennium, these proposals are seen, respectively, as a paradigm, a tentative idea, and an outright speculation.