Case for the Big Bang Joseph Silk

The idea of an expanding universe was a shock to early astronomers, but now the jury is in: the universe is indeed expanding. This is the inevitable consequence of what the America astronomer Edwin Hubble observed (in 1929) as the "redshift" phenomenon. He saw that the light from distant objects in the universe shifted to the red side of the spectrum, which, according to the laws of light waves, means that objects are moving away from the observer.

Curiously, Hubble himself never accepted the radically new idea of an expanding universe, even though it stemmed directly from his work. He rather chose to accept galaxy redshifts as an observable phenomenon without any commitment as to their origin in terms of the properties of space. Perhaps he was confused by the models of other leading cosmologists, who were proposing a static universe. Here, it was suggested that a hypothetical field produced the observed redshift, and indeed in these static models, the universe was seen as devoid of matter, let alone expanding.

The systematic recession of the galaxies is now explained as being due to the expansion of space. Albert Einstein's theory of gravitation, which in 1915 spoke of a curved time-space that could either collapse or expand, certainly predicts this phenomenon. But rather than collapsing, why is space expanding?

This question takes us back to the initial conditions of an infinitesimal patch of matter from which the universe began. That matter must have been in a volatile state, that is, out of equilibrium. This could have been a state of contraction or of expansion. Either way, the density of this primordial patch must have been 1090 grams per cubic centimeter. This is known as the Planck density, after German physicist Max Planck. This density is so high that it takes place only at the interface of quantum theory (in tiny atoms) and general relativity (large-scale gravity). In other words, at the initial conditions, the smallest and largest forces in the universe known today were squeezed together, united and indistinguishable.

The quantum processes were operating in the patch. By quantum jumps, macroscopic clumps of matter could disappear or reappear like the Cheshire Cat in Alice in Wonderland. Black holes, which are so dense with gravity that they attract all the matter around them, could have formed and decayed spontaneously. In this early state, the universe must have been at the most extreme density that can be conceived under known physics. It represents our best guess at the conditions that prevailed near the beginning of time.

After that, the direction of the universe has been quite predictable. It has expanded according to our basic measuring tool, the Hubble diagram, which plots distance compared to the velocity of galaxies as they move away from the central starting point of the universe. We deduce that this expansion began 13.7 billion years ago. The latest data, using supernovae to chart the expansion, have added something surprisingly new to the traditional Hubble diagram: the remotest galaxies are accelerating in recession, speeding up the expansion of the universe, a topic we discuss later.

The ancient age of the universe has also been a surprise to modern science, at least for a century or so. Today, scientists subscribe to the view of a very old universe of about 14 billion years. It is a difficult idea for a substantial minority of the population, especially in North America. Many people prefer a traditional interpretation of the universe drawn from a literal reading of the Bible. In one famous calculation from the King James Bible by seventeenth-century Anglican bishop James Ussher, the universe was created in 4004 BC on Sunday, October 23, at about 7:30 a.m. Today, decades of Gallup polls show that up to 50 percent of Americans think that human life arose fairly recently, according to a literal reading of Genesis, and for many, this would also include the belief in a very young universe.

Fortunately, from the time of Pope Pius XII in the 1940s, guided by the advice of astronomers such as Abbé Lemaître, the Catholic Church and other religious circles have taken a more enlightened approach to modern cosmology, which tries to find a proper balance between theology and science. This view holds that while science is paramount, it presents no challenge to a creed that rests on beliefs that arise from faith. Indeed the converse also applies: the beauty of science and the revelations produced by scientific discovery constitute part of the modern theologian's perspective and toolbox.

Today, for example, the discoveries of modern physics, astronomy, and cosmology reveal intricate details in the physical structure of the universe that seem highly improbable. The proton mass is remarkably close to the neutron mass. Were it very different, stars would not have formed. Further, the force that is accelerating the universe is far weaker than physics leads us to expect. Were this force much stronger, galaxies would never have formed. And in a universe devoid of stars and galaxies, there would not be any observers to marvel at the mysteries of the cosmos. It is not hard to see how theologians might find such discoveries fascinating.

These apparent coincidences in the universe have prompted some to argue that the arrival of human beings on Earth is perhaps not a cosmic accident after all. Indeed, those who employ this reasoning have elevated this human-centered argument into a fundamental principle that governs the universe, which has now been called the anthropic principle, for anthropos, or man. This principle has long held sway in traditional religion. But sadly, in the view of some, the wheel has turned full circle and now physicists too are appealing to the anthropic principle to account for the initial conditions of the big bang. Obviously, the anthropic approach is an unabashedly self-based egocentric worldview.

Following the Evidence

Our concern now is the evidence for the big bang theory of the universe, for we do not want to take it just on hearsay. Four major predictions of the big bang theory have been verified by modern scientific experiments: the recession of galaxies, the abundance of light elements in the universe, the existence of a cosmic background radiation (blackbody) that is uniform, and finally, predicted rates of fluctuations in that same radiation. These four lines of evidence ought to be enough to quench even the most biased critics of what at first sight is a highly implausible theory.

Once the expanding universe had been predicted based on Einstein's theory of...