Cosmology and the Universe: The Big Bang, Dark Matter and Dark Energy 257
orbit about the galaxy is 240 million years. With the observation of the
galactic universe the perceived dimensions of the universe greatly
increased. The Sun was dethroned from its special position at the center
of the universe. It became one of 10^11 stars occupying some obscure
corner of the galaxy-universe. The new universe from an aesthetic point
of view was not pleasing because of its lack of uniformity and
homogeneity.
This notion of the universe was not destined to have a long life,
however. Just as the nature of the Milky Way Galaxy was being
understood, Edwin Hubble began his study of nebulae, the fuzzy cloud-
like non-stellar objects found in various parts of the sky. He found that
not all of these fuzzy patches of light are the same type of objects. Many
are luminous clouds of gas several light years across lying within our
galaxy. Other nebulae lie outside our galaxy and are galaxies in their
own right. Their distance from us varies from 1 million light years to
13.7 billion light years, the edge of the observable universe. These
distant galaxies also contain millions and millions of stars gravitationally
bound to each other and orbiting their galactic nuclei.
It is estimated that the observable universe contains approximately
1011 galaxies, which is as many as the number of stars contained in a
single galaxy. The observable universe, therefore, contains 10^22 stars
altogether. The galaxies are found to form clusters and superclusters in
which the galaxies are gravitationally bound to each other. The number
of galaxies per cluster varies from 20 in our own local group to 10,000
found in a cluster 1.23 billion light years away in the constellation Como
Berenices. There is some evidence that clusters of galaxies form
gravitationally bound super clusters. For the purposes of our discussion,
however, we will take the clusters as the largest sub units of the universe.
Galaxies and their clusters are distributed homogeneously throughout
space suggesting that our universe is symmetrical and uniform after all.
Perhaps the most interesting feature of the clusters is the fact that every
single observed cluster is moving away from our local group or cluster
with a velocity proportional to its distance from us. The universe seems
to be expanding with us at the center. Because of the proportionality of
the distance to a cluster and its recessional velocity, the universe viewed
from any other cluster will appear the same as it does to us. It too will be
the center of an expanding universe. All the clusters will be receding
from it with velocities proportional to their distances. The motion of the
clusters in three dimensions may be likened to the two dimensional