Cosmology and the Universe: The Big Bang, Dark Matter and Dark Energy 263
have developed theories each of which answers these questions
differently. Almost all cosmologists believe that an explanation will
involve some variation of the Big Bang theory proposed by Georges
Lemaitre in 1927. The only alternative, the Continuous Creation or
Steady State Theory, proposed by Bondi, Gold and Hoyle in 1948 has
more or less been dismissed but it is worth mentioning for historic
reasons. According to the latter theory the universe is in a steady state. It
is infinite in extent. Despite its steady expansion, its density remains
essentially constant because of the continuous creation of matter at just
the rate required to produce an equilibrium state, which requires that one
proton per cubic meter, is created every million years.
The Big Bang theory postulates a much more violent universe. The
proponents of this theory believe that the universe began 13.7 billion
years ago as a point of energy and it has been expanding ever since. If
this is true, then projecting backwards in time the universe will shrink.
Eventually one comes to a point in time when the universe collapses
down to a point. If this is true, the collapse to a point is reached by going
13.7 billion years backwards in time, which the proponents of the Big
Bang theory believe was the beginning of the universe. They believe that
all the matter/energy of the universe was contained in this single point
13.7 billion years ago, which exploded because of its inherent instability.
There are various explanations of how this singularity came into being.
The universe has been expanding ever since the Big Bang. Most of
the expansion is due to the fact that chunks of matter were propelled
from the central point with varying relative velocities. These chunks of
matter eventually evolved into clusters. Those chunks of matter, which
were ejected with velocities near the speed of light with respect to us,
have since traveled to the edge of our observable universe 13.7 billion
light years away. Since no matter can travel faster than the speed of light
no matter lies beyond the edge of the observable universe. Ignoring the
acceleration of the expansion the distance of a cluster from us is basically
proportional to its velocity relative to us simply because the faster a
chunk of matter was traveling just after the initial explosion the further it
traveled. Because all velocities are relative, no one cluster may be
considered to be the center of the universe about which all the other
clusters expand. Rather every cluster may be considered the center of the
universe.
The Big Bang theory provides a reasonable explanation of the
universe as well as a number of other cosmological phenomena we shall