What is General Relativity?
General Relativity (GR) is the modern theory of gravity: it is an
improvement over Newton's theory of gravity where gravity is strong, but
agrees with it where gravity is weak. In GR, space and time are combined
into a 4-dimensional space-time. The basic concept is that gravity is not
a force, but rather it is the curvature of space. For example, the space
(and space-time) near a `black-hole' are so strongly curved that light
cannot escape. The Einstein field equations are
| Gμν is a matrix (tensor) derived from the curvature of space-time |
| Tμν is a matrix (tensor) describing the distribution of matter (density, pressure, etc) |
| κ = 8πG/c4 is a constant. |
What is Cosmology?
GR provides the basis for modern cosmology - the study of the Universe as
a whole. (In Newtonian gravity, cosmology was not really possible as the
maths does not work for an infinite distribution of matter.) GR has led to
the standard `big bang' model of the cosmos. This states that the universe
is expanding - observations do indeed show that all the galaxies are moving
away from each other - and that about 14 billion years ago, there was
something like a gigantic explosion - a `big bang' - that started the
expansion. But, unlike a normal explosion, the space and the matter in
it expand together. (Whether this big bang was really the beginning of
spacetime itself is a more metaphysical question).
A more recent concept is the idea of `inflation', in which quantum fields caused a period of very rapid expansion that occurs very early on.
The latest observations seem to show that the matter content of the universe is about 70% "dark energy", %27% "dark matter", and 3% normal matter. There is much debate about the nature of "dark matter" and "dark energy", but as yet no certainty. It is quite worrying that we don't really know the physics behind 97% of the universe!
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