Black Holes

An evocative name. These are bodies with an escape velocity greater than that of light. They swallow up matter- increasing in mass themselves. However if anti matter falls into a black hole then its mass decreases and eventually the black hole fills up and disappears in a burst of radiation.


where vE is the velocity of escape, c the speed of light, G the gravitational constant, M the mass of the star and R its radius.

The diagram (Figure 1) represents the gravitational fields outside a large star and then a black hole. The depth of the gravitational vortex is much greater for the black hole than for a heavy star – the escape velocity being the speed of light.

Actually only stars much heavier than the Sun will form black holes. The Sun will become a red giant and then shrink away to a white dwarf and finally a black dwarf.



The escape velocity of a body obviously increases the closer to the body that you go, for example for the Earth the escape velocity at the surface of the planet is 11.3 km per second 10 000 km above the surface this will have fallen to just under 7 km per second. Clearly for a Black Hole there will be a distance from the centre of the Black hole where its escape velocity will be equal to that of light – closer you fall in and can never escape, beyond that distance you could theoretically escape if you have a space ship that could travel fast enough.

Although the Sun is too light to form a Black Hole the theoretical "radius" of its event horizon would be just less than 3 km!

The Big Bang

Some fourteen thousand million years ago the Universe as we know it began. Before that time there was no space and no time! We cannot ask what happened before that because there was nothing! As far as we know a cataclysmic explosion began our Universe - matter was created from pure energy in a fireball of unimaginable temperature. As time went by this fireball cooled - particles were created and after some millions of years atoms formed. The cooling process continued and after some 500 000 years the universe had reached a temperature of around 800 K; at this temperature the radiation emitted passed into the infra red and the universe became dark.
See also: The Big Bang (14-16)

The Big Crunch

How much matter is out there? If the density of matter is great enough it is possible that the Universe will stop expanding due to attraction between the matter - in rather the same way as a ball on an elastic cord thrown outwards may eventually come to rest. The point is that the ball starts backwards and the same may happen to the Universe in what astronomers are calling the Big Crunch. For further detils and a much fuller treatment see the section on Cosmology.

Gravity waves and the curvature of space

Light travels in straight lines but gravity will distort space and so light bends near massive objects such as Black Holes.
These packets of curved space - tidal ripples of gravity - should flow through empty space far from the bodies that created them. This has given rise to the search for gravity waves, which would pass through space in much the same way as shock waves pass through the Earth after an earthquake.

Gravitational Lensing

This occurs as the result of the passage of light through the curved space produced by a massive object. It occurs most sharply near a Black Hole. (See also general relativity).