Cosmology: Scientific Cosmologies
COSMOLOGY: SCIENTIFIC COSMOLOGIES
General speculations about the nature of the world are as old as the Greek pre-Socratic philosophers, but a truly scientific cosmology could not be formulated until there was some knowledge of the basic laws of nature. Isaac Newton's discovery of universal inverse-square-law gravity afforded the first serious opportunity for such an endeavor. Because gravity is attractive, an immediate problem was to explain why the universe did not collapse in upon itself. Planetary motions stopped this happening in the solar system, but what about the "fixed stars"? The answer first suggested was that in a universe of infinite extent, populated uniformly by stars, the attractive forces in different directions would cancel each other out, giving equilibrium.
However, there was a problem with the idea of a limitless cosmos. Every line of sight would have to terminate somewhere on the surface of a star. In 1823 Wilhelm Olbers pointed out that this would imply that the night sky was everywhere uniformly bright. The modern resolution of this paradox relies on the fact that the finite speed of light and the finite age of the universe together mean that only a finite number of stars are actually visible to us.
An important discovery was made at the end of the eighteenth century by Sir William Herschel. He discovered that the band of light known as the Milky Way is actually composed of a multitude of stars, constituting a vast galaxy of which the solar system is only a tiny component. Early speculators, including Immanuel Kant (1724–1804), had proposed that this might be the case. They also suggested that the luminous patches called nebulae might be other "island universes," similar to the Milky Way but at great distances from it. The issue was not finally settled until the twentieth century, but the idea was already in the air that created reality might be much vaster than had earlier been supposed.
Distances to nearby stars can be measured by parallax, the slight shift in apparent celestial position as the Earth moves around its orbit. Beyond that range, estimating distance depends upon establishing a standard candle, a source of light of known intensity whose observed dimming then affords a measure of its distance. Stars of regularly fluctuating brightness, called Cepheid variables, provide this measure, for it is known that their intrinsic brightness is strictly correlated with the period of their variation. In 1924 Edwin Hubble used this method to establish that the Andromeda nebula is a distant galaxy, now known to be about two million light-years away from the Milky Way.
Hubble then went on to make his biggest discovery. Light from distant galaxies is found to be reddened in comparison with the same light from a terrestrial source. This is interpreted as due to the effect of recessional motion, and the degree of reddening induced is correlated to the speed of recession. The effect (Doppler shift) is similar to the change in frequency of an ambulance siren due to the motion of the vehicle. Hubble discovered that the rate at which a galaxy is receding is proportional to its distance. This was then interpreted as an effect due to the expansion of space itself. Just as spots on the surface of a balloon move away from each other as the balloon is inflated, so as space expands it carries the galaxies with it. Hubble's discovery of the expanding universe had a profound effect upon the development of cosmological theory.
Newton regarded space as a container within which the motion of material atoms took place in the course of the flow of absolute time. Albert Einstein's discovery of the theory of general relativity completely changed this picture.
In 1908 Einstein had what he regarded as his happiest thought. He realized that if he were to be falling freely, he would be completely unaware of gravity. This seemingly rather insignificant observation led him to recognize the principle of equivalence, which lies at the root of general relativity. There are two conceptually distinct meanings of mass: inertial mass (measuring a body's resistance to having its state of motion changed) and gravitational mass (measuring the strength of the body's interaction with a gravitational field). Despite their conceptual distinctness, these two measures are always numerically identical. Quantitatively, inertial and gravitational mass are equivalent. This implies that all bodies move in the same way in a gravitational field. Doubling the mass will double the inertial resistance to a change of motion, but it also doubles the gravitational force effecting the change. In consequence the resulting motion is the same. This universal behavior means that the effects of gravity on individual bodies can be reinterpreted as a general consequence of the properties of space itself, or more accurately, taking into account Einstein's earlier discovery of special relativity's close mutual association of space and time, the properties of four-dimensional spacetime. The concepts of space, time, and matter, held quite distinct by Newton, were united by Einstein in a single package deal. He turned gravitational physics into geometry. Matter curves spacetime and the curvature of spacetime in turn affects the paths of matter. There is no time without space and matter, a point Augustine had realized fifteen centuries earlier.
Einstein set to work to discover the equations that would give quantitative expression to his idea. The search was long, but in November 1915 he hit upon them. Immediately he was able to show that they predicted a small deviation in the behavior of the planet Mercury, which had already been observed but which had defied Newtonian explanation. Later, in 1919, observations of a total solar eclipse confirmed another prediction, relating to the bending of starlight by the Sun. Overnight Einstein became in the public's imagination the iconic scientific hero.
This integration of space, time, and matter in a single theory afforded the opportunity to construct a truly scientific account of the whole universe. However, there seemed to be a problem. At the time, physicists still believed that cosmological theory should yield a static picture. Physics was to be the last of the sciences to recognize the true significance of temporality and unfolding process. The geologists had got there at the end of the eighteenth century, and by mid-nineteenth century the biologists, with the publication of Charles Darwin's Origin of Species in 1859, had followed suit. In the early twentieth century, the physicists still held the Aristotelian notion of an eternally changeless cosmos. Einstein could not find a static solution of his equations. Consequently, when he published his cosmological proposals in 1918 he tinkered with the equations, adding an extra term (the cosmological constant). It represented a kind of antigravity, a repulsive force designed to counterbalance over great distances the attractive force of conventional gravity.
Einstein later called this addition the greatest blunder of his life. He had missed the chance to predict an expanding universe, for his unmodified equations had solutions (discovered by the Russian meteorologist Alexander Friedmann and the Belgian priest Georges Lemaître) that corresponded to the behavior later observed by Hubble. Moreover, his proposed static solution did not really work, for it was unstable and would have collapsed under disturbance.
Big Bang Cosmology
If the galaxies are presently moving apart, then in the past they must have been closer together. This leads to the conclusion that the universe we observe today appears to have emerged from the Big Bang, a primeval state of immensely condensed and energetic matter. Current estimates date this emergence at 13.7 billion years ago.
Taken literally, the Big Bang itself is an instant of infinite density and energy, a singularity that is beyond the power of conventional science to analyze. (Some highly speculative ideas about the very early universe, close to the Big Bang, will be discussed below.) Although some religious people (including Pope Pius XII) succumbed to the temptation to speak of the Big Bang as "the moment of creation," this was clearly a theological mistake. The Judeo-Christian-Islamic doctrine of creation is concerned with ontological origin (why is there something rather than nothing?), rather than temporal origin (how did it all begin?). God is as much the Creator today as God was 13.7 billion years ago. Big Bang cosmology is very interesting scientifically, but not critically significant theologically.
Nevertheless, three cosmologists, Hermann Bondi, Fred Hoyle, and Thomas Gold, feared that Big Bang cosmology might favor religion, and so in the 1960s they proposed an alternative steady state theory, the picture of an everlasting universe always broadly the same. This return to Aristotelian ideas was reconciled with the recession of the galaxies by the supposition of the continuous creation of matter, taking place at a rate too small to be observed but sufficient over time to fill in the gaps left by the motion of the already existing galaxies. Further observational results have disposed of this idea.
As the universe expands, it cools. By the time it was a microsecond old, its temperature was already at the level where the cosmic processes taking place had energies sufficiently low for scientists to possess a reliable understanding of their nature. Discussion is further simplified by the fact that the early universe was almost uniform and structureless, making it a very simple physical system to consider.
By the time it was about three minutes old, the universe had cooled to the extent that nuclear interactions ceased on a cosmic scale. As a result the gross nuclear structure of the world got fixed at what it still is today, three-quarters hydrogen and one-quarter helium. By the time the cosmos was about half a million years old, further cooling had taken it to the point where radiation was no longer energetic enough to break up any atoms that tried to form. Matter and radiation then decoupled and the latter was left simply to cool further as cosmic expansion continued. Today this radiation is very cold, three degrees above absolute zero. It was first observed in 1964 by Arno Penzias and Robert Wilson. Known as cosmic background radiation, it forms a fossilized deposit left over from the big bang era, telling us what the universe was like when it was half a million years old. One of the things we learn is that the cosmos was then very uniform, with fluctuations about the mean density amounting to no more than one part in ten thousand. This background radiation put paid to the steady state theory, which could not explain its properties in the natural way that was possible for Big Bang cosmology.
Gravity has the long-term effect of enhancing small fluctuations. A little more matter here than there produced a little more attraction here than there, thereby triggering a snowballing effect by which the universe eventually became lumpy with galaxies and stars. By a cosmic age of one billion years this process was in full swing. As stars condensed, they heated up and nuclear reactions began again on a local scale. Initially, stars burn by converting hydrogen into helium. At a later stage of stellar development, heavier elements, such as carbon and oxygen, are formed by further nuclear processes. Inside a star this sequence cannot get beyond iron, the most stable of the nuclear species. At the end of their lives, however, some stars explode as supernovae, not only scattering the elements they have made out into the environment, but also, in the explosive process itself, generating the missing elements beyond iron. In this way the ninety-two chemical elements eventually became available. One of the great triumphs of twentieth-century astrophysics was unraveling the details of the delicate processes of nucleosynthesis. When a second generation of stars and planets formed, there was available a chemical environment sufficiently rich to permit the development of life. Thus began one of the most remarkable developments in cosmic history known to us. With the eventual dawning of self-consciousness the universe became aware of itself.
The Anthropic Principle
As scientists came to understand the evolutionary processes of cosmic history, they began to realize that the possibility for the development of carbon-based life depended critically on the details of the laws of nature actually operating in the universe. The collection of insights pointing to this conclusion has been given the name of the anthropic principle, though carbon principle would have been a better choice as it is the generality of life, rather than the specificity of Homo sapiens, that is involved. Many examples have been given of these anthropic "fine-tunings."
One is provided by the stellar processes by which the elements necessary for life have been formed. Every atom of carbon in every living body was once inside a star, and the process by which that carbon was made depends critically on the quantitative details of nuclear physics. Three helium nuclei have to combine to make carbon. One would expect a two-step process, two heliums first fusing to form beryllium, and then a third helium being added on to make carbon. However, there is a problem because beryllium is very unstable and this makes the second step problematic. In fact it is only possible because there turns out to be a substantial enhancement effect (a resonance) occurring at exactly the right energy. If the nuclear forces were different from what they actually are, this resonance would be in the wrong place and there would be no carbon at all. When Hoyle discovered this remarkable coincidence, he felt it could not just be a happy accident but there must be some Intelligence lying behind it.
Examples can be multiplied. Developing life on a planet depends upon its star providing a long-lived and reliable source of energy. Stars burn in this way in our universe because the force of gravity is such as to permit it. The most exacting anthropic fine-tuning relates to Einstein's cosmological constant. Modern thinking has revived this notion, but its strength has to be extremely weak to prevent the universe either collapsing or blowing apart. Many cosmologists believe the force (usually called dark energy ) is actually present, but at a level that is only 10−120 of what one would regard as its natural value. Anything larger than this tiny number would have made the evolution of life, or any complex cosmic structure, quite impossible.
These scientific insights are uncontroversial, but what their deeper, metascientific significance might be held to be has been highly contended. Few are prepared to treat these anthropic coincidences as merely happy accidents, and so two contrasting explanatory proposals have been widely canvassed. One views the universe as a divine creation, explaining its finely tuned specificity as an expression of the Creator's will that it should be capable of having a fruitful history. The other is the multiverse approach, supposing that this particular universe is just one member of a vast portfolio of different existing worlds, each separate from each other and each possessing its own natural laws and circumstances. Our universe is simply the one in this immense cosmic array where, by chance, the development of carbon-based life is a possibility. Although there are highly speculative scientific ideas that might to a degree encourage multiversal thinking (see below), the unobservable prodigality of the multiverse approach makes it seem a metaphysical proposal of considerable extravagance, which appears to do only one piece of explanatory work in defusing the threat of theism.
The Very Early Universe
The closer scientists try to press to the Big Bang, the more extreme are the regimes involved and therefore the more speculative their thinking.
Many believe that when the universe was about 10−36 seconds old, a kind of boiling of space occurred, called inflation, which expanded the universe very greatly and with immense rapidity. The idea is not only supported by some theoretical arguments, but also gains credibility through its ability to explain some significant facts about the universe. One is cosmic isotropy: the background radiation appears virtually the same in all directions despite the fact that the sky contains many regions which, on a simple extrapolation back to the Big Bang, would never have been in causal contact with each other. On an inflationary picture, however, these different regions derive from an initially much smaller domain where there would have been the causal contact necessary to produce uniformity of temperature and density. Inflation would also have had a smoothing effect, thereby explaining the large-scale homogeneity of the universe and the close balance between expansive and gravitational effects that is actually observed (and which, in fact, is another anthropic ne-cessity).
Much more speculative is the attempt to understand the Planck era, before 10−43 seconds, when the universe was so small that it has to be understood quantum mechanically. The proper unification of quantum theory and general relativity has not been achieved. In consequence there are many different hypothetical accounts of quantum cosmology. A frequent theme is that universes may continually arise from the inflation of fluctuations in the ur-vacuum of quantum gravity, and our universe is just one member of this proliferating multiverse. The assertion that this process would represent science's ability to explain creation out of nothing, is merely an abuse of language. A quantum vacuum is a highly structured and active medium, very different from nihil.
On the largest scale, the history of the cosmos involves a tug of war between the expansive tendencies of the Big Bang and the contractive force of gravity. If in the end gravity wins, what began with the Big Bang will end in the big crunch, as the universe collapses in upon itself. If expansion wins (the currently favored option), the universe will continue to expand forever, becoming progressively colder and more dilute, eventually decaying in a long drawn out dying whimper.
In its eschatological thinking, theology must take account of these reliable scientific prognostications of the eventual futility of current process. Ultimately, a simple evolutionary optimism is not a viable possibility.
Barrow, John, and Frank Tipler. The Anthropic Cosmological Principle. Oxford, 1986. An encyclopedic survey of anthropic insights and arguments.
Drees, Willem. Beyond the Big Bang: Quantum Cosmologies and God. La Salle, Ill., 1990. A careful and quite technical survey of possible connections between quantum cosmologies and theology.
Hawking, Stephen. A Brief History of Time: From the Big Bang to Black Holes. London, 1988. Famous exposition of the author's particular version of quantum cosmology.
Leslie, John. Universes. London, 1989. A concise and careful account of scientific and philosophical issues relating to the anthropic principle.
Leslie, John, ed. Physical Cosmology and Philosophy. New York, 1990. A useful collection of reprinted papers.
Miller, James, ed. Cosmic Questions. New York, 2001. A wide-ranging collection of papers given at a conference sponsored by the American Association for the Advancement of Science.
Polkinghorne, John. Science and Creation: The Search for Understanding. London, 1988. A scientist-theologian looks at the universe considered as a creation.
Polkinghorne, John, and Michael Welker, eds. The End of the World and the Ends of God: Science and Theology on Eschatology. Harrisburg, Pa., 2000. A collection of papers considering eschatological issues in the light of modern science.
Rees, Martin. Before the Beginning: Our Universe and Others. London, 1998. Readable account of modern cosmological ideas; supportive of the idea of a multiverse.
Weinberg, Steven. The First Three Minutes: A Modern View of the Origin of the Universe. 2d ed. New York, 1988. Classic and moderately technical account of early universe cosmology.
Worthing, Mark. God, Creation, and Contemporary Physics. Minneapolis, 1996. Creation considered in the light of modern physics.
John Polkinghorne (2005)