BOX 4.1 WHAT WE KNOW ABOUT THE BIG BANG AND HOW WE KNOW IT
The big bang model embodies our accumulated knowledge about how the universe began and evolved to its present state. Like most scientific theories it is not static, but rather is constantly being tested and extended. Nor does it exist in a vacuum—its foundation being Einstein’s theory of general relativity.
Testing the big bang model (or any theory) requires a theoretical framework—in this case, general relativity. If the predictions of the big bang model agree with the data, then both the big bang model and general relativity are being tested (a failure of either one would lead to discrepancies). The fundamental set of observations that support the big bang model are the expansion of the universe; the existence of the cosmic microwave background (CMB) radiation; the 0.001 percent variations in the intensity of the CMB that reflect the primeval lumpiness in the distribution of matter, which seeded all the structure seen today; and the abundance pattern of the lightest elements (hydrogen, deuterium, helium, and lithium) seen in the most primitive samples of the cosmos. A large number of other observations are also consistent with the big bang model.
Within the context of the big bang model, parameters that describe the key features of our universe are measurable. There has been great progress in recent years in improving the precision of these measurements: the temperature of the CMB has been measured to four digits, T = 2.725 K; the expansion rate of the universe (or Hubble constant) has been determined to a precision of 10 percent, H0 = 63 to 77 km/sec/Mpc; the time back to the big bang has been determined to a precision of about 15 percent, t = 11.5 billion to 14.5 billion years; the average density of matter and energy has been measured and found to be between 95 and 125 percent of the critical density; independently, space has been shown to be uncurved (“flat”) to a precision of about 4 percent; and the rate of change in the expansion rate has been measured, indicating that the expansion seems to be speeding up, not slowing down.
Some of these measurements require further assumptions or information beyond the assumption of general relativity; for example, to determine the time back to the big bang requires both the Hubble constant and knowledge about the matter and energy makeup of the universe. The curvature of space was determined from measurements of the size of hot and cold spots on the microwave sky and involves a minor, but nonetheless additional, assumption about the nature of the lumpiness in the distribution of matter. Some of the cosmological parameters test the basic consistency of the framework; for example, the time back to the big bang can be compared to the age of the oldest stars, between 10 billion and 14 billion years. Within the margin of error the universe is older than the oldest objects within it.
Our present understanding of the big bang takes us back to a time when the universe was a soup of elementary particles, a few microseconds after the big bang. Current attempts to extend the big bang model, such as inflation, aspire to describe even earlier moments in the universe and to answer deeper questions, such as, How did the lumpiness arise? What made the universe flat? What