| 12 | If you throw a ball in the
air it slows down, stops and turns round, all because it is attracted to
the Earth by gravity. If you were to throw it fast enough (11 km/s),
gravity wouldn't have managed to slow it to a standstill before it had
got so far away (because it is moving quickly) that it would be
effectively out of reach of gravity, which diminishes with distance (see
box 6). 11 km/s is known as the escape velocity for the Earth. Every object has its own escape velocity, including the Universe. If the outer galaxies are receding faster than the Universe's escape velocity then they will never stop, and the expansion will go on for ever. If they are receding slower than the escape velocity, then they will eventually turn round, the expansion will turn into a contraction, and we will be in for a Big Crunch. The escape velocity of the Universe is obviously a key factor in all this (as is the current recessional speed of the distant galaxies), and no-one knows its value. It depends partly on the mass of the Universe, and this is not yet known. However, it is clear that a more massive universe will exert a bigger gravitational pull on the edge galaxies and make it more likely that they will turn round to engage in a big crunch. |