Stuart Clark, The Guardian

Gravitational waves are ripples that carry energy across the universe. They were predicted to exist by Albert Einstein in 1916 as a consequence of his General Theory of Relativity. Although there is strong circumstantial evidence for their existence, gravitational waves have not been directly detected before. This is because they are minuscule – a million times smaller than an atom. They are like tiny waves on a lake – from far away, the lake's surface looks glassy smooth; only up very close can the details of the surface be seen.

There is intense speculation among cosmologists that a US team is on the verge of confirming they have detected "primordial gravitational waves" – an echo of the big bang in which the universe came into existence 14bn years ago.

There is intense speculation among cosmologists that a US team is on the verge of confirming they have detected "primordial gravitational waves" – an echo of the big bang in which the universe came into existence 14bn years ago.

NASA says the Geminids will be the most intense meteor shower of the year – and almost everyone can see it

If it survives a potentially disastrous encounter with the sun this week, Comet ISON will put on an impressive early morning display in the run up to Christmas. But anyone hoping for a Bethlehem-style celestial sign on the big day will be disappointed. The comet will probably be too faint to see with a naked eye by then.