The radio astronomer Antony Hewish has lectured several times at the Lindau Meetings and this is his second lecture. In all his lectures one can hear him stress the fact that physics can learn from astronomy. From the historical point of view, considering, e.g., the relevance of Johannes Kepler’s work for Isaac Newton’s theory of gravitation, this may seem somewhat unnecessary. Also recently, the realization that particle physics plays an important role in space, has again made astronomy and physics come together in the branch of astro-particle physics. Hewish, of course, also makes the point that radio astronomy is an ideal way to study physical processes in the Universe. His lecture brings together several threads of astronomical observations and theoretical ideas, from the discoveries of radio galaxies and quasars, the microwave background radiation, the clustering of galaxies, the theoretical maps of the sky and the need for dark matter. The main point, though, is to explain how we can understand what we observe and measure today using the theory of the expanding Universe. How can we, e.g., from what we observe today say about the density fluctuations that must have been present about 105 years after creation? This is the time when the electrons and the protons recombined to form hydrogen atoms and the Universe became transparent. There are many theories about this, the foremost today is probably the theory of inflation, in which the Universe undergoes an extremely rapid expansion following the Big Bang. Hewish argues that one could get help in deciding on different theories by using radio telescopes, maybe of a new kind. It is interesting to consider that when Hewish gave his lecture, the results of the COBE mission was not yet published. In particular the fluctuations in the microwave background registered by one of the COBE detectors in principle answers Hewish main question!