ÐÓ°ÉÔ­´´

In recent years, cosmology has come close to converging on the model that
Fred Hoyle dubbed the ‘big bang’. The reason for this convergence is not a
shared philosophy, nor even peer pressure, but the weight of many detailed
measurements. The relatively simple big bang model fits these observations
well, whereas they require very complex explanations in other cosmological
models.

My view, and that of my colleagues at Chicago, Brian Fields and Craig Copi,
is that the quality of the latest observations places strict constraints on
all cosmological models and requires any new cosmological model to be as
precise as the big bang model itself.

Three observational ‘pillars’ drive most cosmologists towards the big bang:
the expansion of the Universe, the existence and character of the microwave
background radiation, and the primordial abundances of the light elements. A
cosmological theory must explain these three pillars in quantitative detail,
using theoretical models that agree as far as possible with known
laboratory physics. Beyond this, the simplest theoretical extensions should
be attempted before making more complex assumptions. Most active work in
cosmology is done within the big bang framework because of its continuing
success in meeting these criteria, as observational data become more and
more accurate.

Threefold achievement

The big bang’s success lies in the way it accounts for the three pillars.
The expansion of the Universe is a natural consequence of very general
considerations of a relativistic theory of gravity applied to the Universe
as a whole. The microwave background is the cooled relic of a once-hot
state, telling us that the Universe was once hot and dense. The big bang
theory does not predict its temperature, but it does predict that the
present-day background radiation will have, to high precision, thermal
distribution. This prediction for the microwave spectrum is quite
restrictive. Physicists have known since the 19th century that bodies in
thermal equilibrium radiate electromagnetic radiation whose intensity
spectrum is uniquely determined by the temperature of the body. Only thermal
bodies radiate this type of intensity spectrum. Remarkably, that predicted
distribution agrees with observations over the entire microwave frequency
band.

The primordial abundances of the light elements come from an early hot-dense
Universe that at one time acted as a giant thermonuclear reactor converting
hydrogen to helium with traces of deuterium and lithium. By adjusting only
the density of normal matter, the light element abundances are predicted.
These predictions are in very good agreement with increasingly precise
observations which range over 10 orders of magnitude.

Not only is the big bang quite successful in explaining the three pillars,
but the pillars themselves force virtually all cosmological models to invoke
an environment similar to that of the big bang. The first, and oldest, of
the three observational pillars is not unique to the big bang; in fact, it
is also part of the old steady state theory. However, the second and third
pillars force cosmologists to accept the idea of a hot, dense early
Universe that is relatively uniform.

Such a Universe is what we mean by the big bang. Even alternatives such as
the quasi-steady state cosmology (QSSC) proposed by Jayant Narlikar and his
collaborators attempt to invoke high densities and energies (temperatures),
albeit less uniformly than the big bang. However, the new COBE data on the
microwave background radiation show that the spectrum is thermal to better
than 1 part in 10 000 and isotropic (the same in all directions) to 1 part
in 100 000. So any sources of the microwave background radiation would have
to be distributed with remarkable spatial uniformity.

It is important to remember here that the term big bang refers to a whole
class of cosmological models which all predict a hot and dense early
Universe, but which also have considerable differences. Critics of the big
bang often yoke together the expansion of the Universe, the existence of
the microwave background radiation and big bang nucleosynthesis with more
speculative features that include variations on the basic model – in
particular, those for dark matter and structure formation. These last two
depend much more than the first three on speculative physical and
mathematical assumptions.

Some people also have the mistaken impression that big bang models claim to
explain the whole history of the Universe, even back to the very instant of
creation itself. Classical general relativity in a big bang framework does
evolve from some initial ‘singularity’. But most physicists assume that
classical general relativity must merge with quantum mechanics to yield some
theory of quantum gravity, since on a suitably microscopic scale, quantum
mechanics is found to be relevant for all physical phenomena.

The microscopic level where quantum mechanics is expected to affect gravity
is still beyond the reach of all current experiments, but would nevertheless
have been relevant in the early Universe. Most proposed quantum gravity
theories have no initial singularity.

The key point is that such questions are irrelevant to the big bang
framework. None of the three observational pillars measures events in the
Universe that occurred at times approaching the realm where quantum
mechanics and general relativity would merge. Some proposed variations do
make speculations about arbitrarily early times, but the generally accepted
part of the framework is only that determined by working backwards from the
observations.

The problems of structure formation and dark matter do seem to require new
physics. But such new physics is relatively independent of the basic
framework. It is true that models which try to avoid the big bang framework
and ignore the observational constraints do not solve the problems of
structure formation as well as models based within the big bang framework.
For example, alternative cosmologies such as the QSSC or ‘plasma cosmology’
have not produced numerical galaxies and structures that fit the
observations as precisely as models which are currently being studied within
the big bang framework. Even so, these are not arguments for or against the
framework itself.

Exotic objects

Similarly, arguments about producing specific, well studied, exotic objects
such as quasars are not of cosmological importance. Detailed, successful
models for these objects exist within the confines of traditional physics,
which are not directly tied to the cosmological framework other than through
the expansion.

Red herrings such as the ‘initial singularity’, structure formation, or
quasars lead many people astray when trying to determine the best
cosmological model. I would argue that the simplicity with which the big
bang explains observations has not yet been matched, or even approached, by
any alternative model.

Let me give you an example. One suggestion for avoiding a hot, dense state
in the early Universe as a natural precursor to the microwave background
radiation is that of iron needles of just the right dimensions (to act as
small antennae in the desired wavelength range), scattered throughout the
Universe. But such iron needles also make the Universe opaque to radio
waves. This conflicts with observation, as distant radio sources have been
observed at wavelengths that would have been absorbed by iron needles.

It is always worthwhile to try to find alternative views. But in my view,
other attempts to fit what is observed, for example the QSSC, do not do so
as neatly or as economically as the big bang. So far none of the big bang’s
predictions has failed. Of course, we still have unsolved problems and we
still have new measurements to make. But that is the nature of a vital
field of science.

David N. Schramm is Louis Block Professor in the physical sciences at the
University of Chicago.