To many people, astronomy's description of the big bang is more fantastic than any creation myth. As critical thinkers it is important to evaluate this scientific creation story and ask the questions: how secure is the big bang model? and Are there any other ways of explaining the universe we live in that match the data so well? As is discussed in other articles, there are three well documented lines of evidence for the big bang, and as we discuss below, competing theories were not as well supported by the data.
In the late 1940s, when our only evidence of the big bang was the red shift of most galaxies, a group of theorists proposed an alternative cosmology called the steady state model. The steady state model proposed that the universe was homogeneous and isotropic; it is the same in all directions and if viewed with a large enough perspective is without structure. This use of the cosmological principle as an assumption is consistent with the premise of the big bang. Steady state then goes on to state that there was no origin to the universe, and no singularity from which everything emerged. To account for the Hubble relation — the increasing recession velocity of galaxies with distance — the theory proposed that atoms were created steadily and spontaneously to fill the voids created by the recession of galaxies. Many scientists saw this hypothesis as an outrageous violation of the principle of conservation of mass, but defenders of the theory pointed out that it is hardly more outrageous than imagining that the whole mass of the universe appeared instantly, as in the big bang model! Fred Hoyle, one of the steady state theorists, coined the term "big bang." He intended it as a put-down, but his move backfired because the catchy name stuck. Astronomers also prefer not to capitalize Big Bang, perhaps as an ironic comment on the enormity of the idea. The steady state model died after the discovery of the cosmic background radiation, which had no natural explanation in a universe of essentially constant density.
Other alternatives to the Big Bang have been proposed and have also come up short. The tired light model holds that light loses energy as it crosses the vast reaches of space. Red shifts are caused by this effect, rather than by the expansion of space. Observations rule out the tired light model, with no such shifts being observed under controlled conditions. British physicist Paul Dirac, one of the founders of the quantum theory, proposed the idea of variable gravitation. If the force of gravity had decreased over billions of years, it would simulate some of the effects of an expanding universe. This theory has also been ruled out through modern observations of the distant universe; no differences in orbital motions of stars and galaxies within clusters is seen over time. In fact, there are quite strict limits on variations in the strengths of several fundamental constants over time.
In the 1930s, Caltech physicist Richard Tolman proposed an oscillating universe model, where expansion and contraction follow each other in an endless cycle. However, Roger Penrose and Stephen Hawking later showed that the increase in entropy applies to an oscillating universe too. After many cycles, the universe would be much more chaotic than the one we observe. There is also no physical mechanism to account for the oscillation (although there is also no physical mechanism to account for the initiation of the big bang). Also, the oscillating model was more attractive when astronomers thought there might be enough mass in the universe to cause expansion to a maximum size followed by collapse — a closed universe model. The universe we live in is open and will expand forever, to the oscillating universe model is disfavored.
The failed models show that scientists have continued to be ingenious in thinking up ways to explain the universe. The big bang model has survived because it has explanatory and predictive power. A "web" of evidence supports the idea of a dense and hot early phase of the universe. Not only do we see Hubble expansion, we see radiation and light elements that originate from a hotter state, and we see cosmic evolution of galaxies and large scale structures. The big bang follows the precepts of a good scientific theory: it makes predictions, it is testable, and it can be refuted. However, it would be arrogant of scientists to believe they have the final verdict on the creation event. The history of cosmology indicates that there are likely to be surprises in store for us.
What is the frontier of research on the big bang? We have evidence for the big bang that dates back to 380,000 years and a temperature of thousands of Kelvin (the cosmic microwave background) and a few minutes and a temperature of ten million Kelvin (the abundance of light elements). The next landmark would be an understanding of the physics a microsecond after the big bang, when the matter in the universe came into existence and the asymmetry between matter and antimatter originated. Another goal is verification of the inflation idea, dating back to an incredible 10-35seconds and a temperature of 1027 Kelvin. Beyond that lies the Planck era, the limit of our knowledge, at 10-43seconds and a temperature of 1033 Kelvin. The ultimate questions in cosmology are: what caused the big bang and what, if anything, came before the big bang?
Sir Fred Hoyle. Click here for original source URL.
Temperature map of the universe, as measured by WMAP. Click here for original source URL.