Cosmology and the Universe: The Big Bang, Dark Matter and Dark Energy 273
rarefied explaining how the universe went from a homogeneous
distribution of energy and matter into one that is lumpy, i.e. one
consisting of superclusters, clusters, galaxies, nebulae and stars. The
existence of dark matter is required to explain a number of other
observations including the lack of uniformity of the cosmic microwave
background. Dark matter has to be invoked to explain how clusters of
galaxies are able to remain gravitationally bound to each other despite
their relatively high velocities. There is not enough observable matter to
hold these clusters of galaxies together indicating the existence of dark
matter. Finally the phenomenon of gravitational lensing in which the
bending of light by a gravitational field has also revealed the existence of
dark matter. Although it is quite certain that dark matter exists it is still a
mystery as to its actual make up.
According to inflationary theory which explains a number of
cosmological features the universe at approximately 10-36 seconds after
the Big Bang underwent an extremely rapid exponential expansion in
which its volume increased by a factor of at least 10^78. It is postulated by
some cosmologists that the Big Bang that created our observable
universe which extends out to 13.7 light years was not the only Big Bang
but there have been other and that there are in fact other universes that lie
beyond our observable universe. In some versions the laws are the same
in these other universes as they are in our universe but the distribution of
matter, dark matter and dark energy could be different. And in some
versions the laws and the physical constants in the other universe are
different from our. The multiverse is the set of all these universes.