A few weeks ago I briefly described our current best guess as to how our universe will end. Equally intriguing is, of course, the question of how it began. I’m convinced that most readers have at least heard that we believe our universe started with a “Big Bang”—a very hot and dense state. Why do we think that? Because there is real empirical evidence to support this scenario, and because this theory offers a consistent picture of the history of matter and radiation in our universe. Figure 1 is a schematic representation of the entire cosmic evolution. Here is an extremely concise summary of some of the key points concerning the beginning.
Three major observational results have led to the Big Bang theory. First, there was the discovery by astronomers Vesto Slipher, Georges Lemaître, and Edwin Hubble that our universe is expanding. Every distant galaxy is moving away from any other galaxy. The fact that the expansion velocity was found to be proportional to the distance (that is, a galaxy that is twice a far recedes twice as fast), has even allowed for an accurate determination of the expansion age of the universe—our universe is about 13.7 billion years old.
Second, there was the remarkable detection of the “afterglow of creation”—the cosmic microwave background. In 1964–1965, Arno Penzias and Robert Wilson of the Bell Telephone Laboratories discovered that rather than being completely cold, intergalactic space was filled with microwave radiation (like that emitted by microwave ovens) arriving with equal intensity from all directions. This was later identified as the unmistakable relic of the initial, dense cosmic fireball, from which our expanding universe sprouted. Since the primordial photons (the particles of radiation) were predicted to be in thermal equilibrium (due to repeated scatterings and absorptions), the intensity of the radiation at different wavelengths was expected to conform to what physicists call a “black body” spectrum. This has been fully confirmed with an astounding accuracy by two satellites, the Cosmic Microwave Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP).
The third piece of evidence for the Big Bang came from the abundance of the element helium, which comprises about a quarter of the mass of all stars and gaseous nebulae. The point is that most of the elements heavier than helium are fused in the hot nuclear furnaces at the centers of stars. Fred Hoyle, a famous British astrophysicist, built on previous ideas on the source of the Sun’s energy, and proposed in 1946 that all of those so-called “heavy elements” are synthesized from simpler nuclei during stellar evolution. Helium, however, posed a problem. Calculations showed that most of the helium formed in stars is further processed up the periodic table, resulting in the amount of helium being only about 1–2 percent of the cosmic matter. This was in clear conflict with the observations showing that even the oldest stars contained about 25 percent helium. Ironically, it was Hoyle again, himself an opponent of the Big Bang model, who showed (in collaboration with physicists William Fowler and Robert Wagoner) that helium could be synthesized in the hot, dense conditions in the Big Bang. Hoyle’s calculations were based on previous ideas of physicists George Gamow and Ralph Alpher. This amazing theory of “Big Bang nucleosynthesis” further predicted the abundances of the elements deuterium (a heavy isotope of hydrogen) and of lithium. Hard to believe, but we have an extraordinarily successful theory of the nuclear reactions that took place in the first few minutes of the universe’s existence!
Are there any observations that could have refuted the Big Bang model? Absolutely. For instance, had astronomers discovered objects that had a helium abundance considerably lower than 23 percent, this would have violated the absolute minimal value predicted by Big Bang nucleosynthesis. Similarly, the cosmic microwave background could have exhibited a spectrum very different from that indicating a state of thermal equilibrium. No such violations were ever found. Finally, one could show that if neutrinos—those elusive particles that interact very weakly with matter—had a mass of even one-millionth that of a proton, the Big Bang scenario would not have worked (because the universe would have had too much mass). Experiments show, however, that the masses of the neutrinos are much lower, and consistent with the expectations from the primordial fireball picture.
I should note that the modern Big Bang model involves a period of inflation, during which, when the universe was a tiny fraction of a second old (about 10-32 of a second), it underwent a stupendous expansion before settling into a more leisurely pace. I shall describe this inflationary model and the evidence for it in a future blog. I shall also discuss some speculative ideas concerning the appearance of our universe out of nothing. For now, let me end with a quote from British evolutionary biologist J. B. S. Haldane (1892–1964): “My own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.”