N.B. This question was asked in 2007, and my reply was written on 30 October 2007. They took a while to print it!
What was it that started the big bang?
This question has a definite and very simple answer: we do not know!
To put things in context, by the "Big Bang" we refer to the general idea that the universe is expanding and began in a hot dense phase in which it was initially filled with radiation. Our detailed theoretical calculations succesfully describe the universe in broad terms from the time when it was of order a second old until the present. The theory is supported by the observed match of abundances of light elements to the predictions of primordial nucleosynthesis calculations, and the evidence of the cosmic microwave background (CMB) radiation. But the "Big Bang" is not a complete theory if taken to earlier and earlier times.
Just as the temperature of gas in a container will decrease if you slowly increase the volume of the container, so the temperature of the stuff in the universe decreases as the universe expands. Initially it was very hot, so hot that the average kinetic energy of particles was above the binding energy of hydrogen. This meant that matter existed as a plasma of free electrons and protons mainly. Like the interior of the sun it was opaque, because light scatters off free electrons by "Thomson scattering". When the universe was a few hundred thousand years old it cooled below 3000 degrees Kelvin, meaning that the average kinetic energy of the gas fell below the binding energy of hydrogen. The electrons became bound to protons: the first atoms formed. The light that scattered off the last free electrons has travelled to us ever since, forming the CMB, which is as far back into the past history of the universe that we can directly see. From light element abundances and the details of the CMB we can infer the nuclear processes that went on back at earlier times when the universe was a few seconds to several minutes old.
If we go back to earlier times when the universe was fractions of a second old then the universe was smaller and hotter still. We have an essential problem since this means we are dealing with higher and higher energies, where physics is less well established. Such energies are not available to us in laboratories. Next year the Large Hadron Collider (LHC) at CERN in Geneva will start colliding protons together with an energy of 14 trillion electron Volts, the largest energies ever seen on Earth. That will certainly give us new insights into what went on when the universe was fractions of a second old. But if you break those fractions into even smaller fractions, you will always arrive at huge energy scales compared to which 14 trillion electron Volts is small beer. The birth of the universe was the biggest particle collider experiment ever; but not an experiment that can be repeated!
To understand what went on at the earliest times, we have to think laterally. What is clear is that our present theories of physics are not complete. Very high energy physics would seem to demand that we be able to describe gravity by a quantum theory; something that has eluded physicists for decades. Our present observations indicate that the universe initially went though an "inflationary period" during which it expanded exponentially fast in those first fractions of a second. But there are perhaps 200 different possible models of an inflationary universe, and no obvious way to choose between them. The CMB observations do place contraints on inflationary models which rule some out already, but ultimately much tighter constraining priniciples are required. A successful quantum theory of gravity might provide such principles. Although we cannot see beyond the opaque CMB with electromagnetic waves, gravitational waves - that is ripples in space - would have been produced at the earliest times, and the echo of these might be detected by future space-based gravitational wave detectors, such as the LISA mission next decade. That would provide important data and clues to nail down a quantum theory of gravity.
One ultimate limit is that if we try to break a second into ever smaller fracions, then the definition of time becomes unclear. According to our present understanding, time does not really have a meaning if you try to break it into bits smaller than one Planck time: that is, 0.00000000000000000000000000000000000000000006 seconds. That being the case, the notions of "cause" and "effect" cease to have a meaning when we are talking about times that small. Some serious physicists would say that the universe only exists because there are living creatures such as us here to observe it, an idea known as "the anthropic principle". Until we have a successful experimentally verified quantum theory of gravity (at least!) questions about the ultimate beginning of the universe will remain speculative.
Dr David Wiltshire
Department of Physics and Astronomy
University of Canterbury