To the Editors of 'The Observatory' Chess-playing armchair cosmologists

摘要

I've often used Rees's description, mentioned in the Gerald Whitrow Lecture on 2019 February 81, of the observable Universe shrunk to the size of the Solar System. The interesting thing, though, is that the density would be within an order of magnitude or so of that of a neutron star, thus not extremely far removed from our experience. Of course, that demonstrates how empty the Universe is,mainly between stars and within atoms: the Hubble Deep Field, where one sees the sky almost covered in galaxies, is about the same angular size as a grain of rice held at arm's length; a sky of such fields, compressed to what is still a familiar density, would form a ball of radius not much larger than that of the asteroid belt.