The Universe

The universe comprises matter, light, and other forms of
radiation and energy. It includes the earth and everything
on it. It also includes everything in the solar system. 

The size of the universe is unknown. Most astronomers think
that bright, unusual galaxies called quasars may be the
most distant objects in the universe. It is believed that
they be as far away as 10 billion light-years from the
earth. 

Most cosmological theories about the universe are based on
the idea that any part of the universe is like any other
part having the same age. The basis for theories about the
behavior of the universe is formed in part by Albert
Einstein's general theory of relativity. This theory is
based on tow beliefs; (1) that no signal can travel faster
than the speed of light, and (2) that the laws of physics
are the same everywhere in the universe. 

Theories resulting from these ideas involve a universe that
expands and contracts. From observing the red shift in
distant galaxies, scientists conclude that the universe is
expanding. But the overall behavior of the universe depends
on the average density of matter in the universe at the
present time. The other theory is based on the belief that
any part of the universe is like all other parts at all
times. According to this theory, matter is continuously
created and formed into new galaxies that replace those
which recede to infinite distances. 

The development of the telescope, the spectroscope, and the
photographic plate led to a great expansion of astronomical
knowledge.