So what did happen during the
big bang? We were not privileged to witness the big bang, so to
answer this
question astronomers must look at the current state of the universe and work backwards.
The cosmic background radiation tells us that the current temperature of the universe is about 2.7 degrees Kelvin. The Hubble plot tells us how fast the universe is expanding. It is extremely difficult to do so, but astronomers also attempt to look at the Hubble plot for the most distant galaxies to see if the universe was expanding more rapidly in the past.
Astronomers can also
measure the relative abundance of elements and isotopes that formed during the initial big bang to deduce how long the right conditions for forming these particular atoms during the big bang lasted.
Combining these observational clues with the theoretical framework of general relativity, cosmologists attempt to work out the
history of the universe and the physical properties such as temperature and density at the different
times in this history.
Before the Beginning
The universe begins with the big bang at time zero. It is natural to ask: what about before time zero? In the context of the big bang theory, there is no scientific answer to that question. That is, at least at this point, a fundamental limitation of the big bang theory. Some individuals asking that question provide a theological answer. Others accept that the question has no answer. A few are seeking a scientific answer, but they have not made much progress.
The Planck Time
The big bang theory also runs into a fundamental limitation during the first 1E-43 seconds (1E-43 refers to the power of 10 i.e. 1 times 10 to the minus 43rd power.) after time zero. This time is called the Planck time and arises from quantum mechanics.
Without going into detail, quantum mechanics predicts that for anything smaller than a certain scale, chance and uncertainty win out over Newtonian determinism. We can therefore predict or measure the path of a planet or a baseball, but we can only estimate probabilities for an electron. Subatomic particles are smaller than Planck''s scale, so chance and uncertainty dominate. This inability to predict or measure their paths results not from faulty instruments or techniques but from a fundamental limitation of nature.
The Planck time is this limiting scale translated into time units. For times in the history of the universe less than 1E-43seconds, quantum mechanics limits our ability to predict or measure the conditions. Our history of the big bang must therefore begin at 1E-43 seconds. At this time the universe had an estimated density of 1E96times the density of water and an estimated temperature of 1E32 degrees Kelvin. Our universe began in an unimaginably hot dense state. From this initial state the universe began to expand in the big bang.
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