Big Bang "Problem"
The hot big bang theory has been extremely successful in correlating the observable
properties of our Universe. However, there are some difficulties associated with
the big bang theory. These difficulties are not so much errors as they are
assumptions that are necessary but that do not have a fundamental justification.
The required discussion is technical, so we will be content with a rather
superficial statement of the three basic problems that are associated with
the big bang and how they might be cured by a new idea that arises from considering
the implications of elementary particle physics for cosmology.
The Horizon Problem
We have already encountered the horizon problem in conjunction with the
discussion of the
cosmic microwave background: when we look at the
microwave background radiation coming from widely separated parts
of the sky, it can be shown that these regions are too separated to have been able
to have ever
communicated with each other even with signals travelling at light velocity.
Thus, how did they know to have almost exactly the same temperature? This general
problem is called the horizon problem, because the inability to have
received a signal from some distant source because of the finite speed of light is
termed a horizon in cosmology. Thus, in the standard big bang theory we must simply
assume the required level of uniformity.
The Flatness Problem
The experimental
evidence is that the present Universe has very low geometrical curvature in its
spacetime (it is nearly flat). Theoretical arguments that are well established but
too complex to go
into here suggest that this is a very unlikely result of the evolution of the
Universe from the big bang, unless the initial curvature is confined to an
incredibly narrow range of possibilities. While this is not impossible, it does
not seem very natural.
The Monopole Problem
The only plausible theory in elementary particle physics for how nuclei in the
present universe were created in the big bang
requires the use of what are called
Grand Unified Theories (GUTs).
In these theories, at very high temperatures such
as those found in the instants after the Universe was created the strong, weak, and
electromagnetic forces were (contrary to the situation today) indistinguishable
from each other. We say that they were unified into a single force.
Although there is as yet no certain evidence for
the validity of such theories, there is strong theoretical reason to believe that
they will eventually turn out to be essentially correct.
Our current understanding of elementary particle physics indicates
that such theories should produce very massive particles called
magnetic monopoles,
and that there should be many such monopoles in the Universe today. However,
no one has ever found such a particle. So the final problem is: where are
the monopoles?