03 September 2010

Modern cosmology and universal history

5.15 The beginnings of scientific cosmology

The first step towards a scientific cosmology was taken in 1823 when the German astronomer Wilhelm Olbers discussed a paradox that has subsequently been associated with his name. He simply asked ‘Why is the sky dark at night?’ The paradox becomes apparent when you calculate the brightness that should be expected given the assumptions that were current about the overall structure of the universe. If the universe is infinitely large, Euclidean (i.e. the shortest distance between two points is a straight line) and stars (or galaxies) are distributed evenly throughout it, the sky should not be dark at all but as bright as the surface of the average star!

This might have been explained by arguing that the universe is relatively young so that light from distant stars has not had time to reach us. However, by the nineteenth century it was widely accepted that the Earth (and, hence, the universe) was very old. Thus a more popular explanation was that the universe consisted of a finite number of stars concentrated into a finite region of an infinite space – the island universe model of cosmology in which the Milky Way (our own galaxy) constituted a unique island of matter and energy in an infinite void.

In the 1920s astronomers were able to show that some nebulae (clouds of luminous gas and dust) were too far away to be part of the Milky Way – they were island universes or galaxies in their own right. One of the discoverers of extragalactic objects, Edwin Hubble, went a stage further. In 1924 he announced the discovery that light from distant galaxies was systematically redder than light from nearby galaxies and that the degree of red shift was proportional to the distance. This provides a simple explanation for Olbers’ Paradox: if light from distant galaxies is redder, it contributes less energy to the overall brightness of the night sky than light from nearby galaxies. Eventually there comes a point where a galaxy is so distant that it is simply invisible (the ‘event horizon’).

The simplest explanation for this red shift is that it is a case of the Doppler effect. This is the phenomenon that causes the pitch of a train whistle to vary as the train approaches or recedes. According to this explanation, the light is reddened because the galaxies are moving away from us. Since the degree of reddening is also a measure of the speed of recession, Hubble was able to show that more distant galaxies are receding from us faster than nearby ones.

5.16 The Big Bang

At first sight this observation might suggest that the Earth was located at the centre of some cosmic explosion. However, the fact that all motion is relative implies that observers elsewhere in the universe would make similar observations. This observation is consistent with an expanding universe. To illustrate this one might paint spots on a balloon and blow it up. As the balloon expands, the spots recede from each other and more distantly separated spots recede more rapidly.

Extrapolating backwards in time from the observation that the universe is expanding leads to the suggestion that there might have been a time in the distant past (between 10 and 20 billion years ago) when the entire universe was concentrated into a single point. This point would be unimaginably hot and dense. At this ‘t = 0’ the universe would begin to expand rapidly, if not violently. As it expands and cools, matter as we now know it begins to appear. Small variations in the density of that matter lead to condensation and the eventual formation of stars, galaxies and planets. Gradually the mutual gravitational attraction of matter slows the expansion of the universe. The result is the basic picture of the universe as portrayed by modern cosmology.

5.16.1 Evidence for a Big Bang?

Taking the Big Bang as our educated guess about the origin of the universe, we naturally ask what would such a universe look like? Can we deduce potential observations from the hypothesis of a primordial fireball? The answer is ‘yes’.

Since light travels at a finite velocity, observations of distant objects are also observations of conditions in the past. In the distant past, the universe was smaller and therefore denser than it is today. We would therefore expect distant objects to be closer together than those nearby – there is some evidence from radio astronomy that this is the case. We would also expect observations of very distant objects to be consistent with a younger, hotter universe.

In an effort to discredit this theory, Fred Hoyle and some colleagues calculated the chemical composition of a Big Bang universe. This is relatively straightforward since the bulk of the chemical elements would be generated in the first couple of seconds of violent expansion and cooling. Much to their surprise, the outcome of their predictions was very similar to the observed chemical composition of the universe (about 80 per cent hydrogen and 20 per cent helium – all the rest is a mere trace explicable as the result of supernova explosions at the end of the first generation of stellar evolution).

But the most convincing evidence for the Big Bang came from an accidental discovery in 1965. Two young American astronomers, Penzias and Wilson, were attempting to pioneer astronomy in the microwave part of the spectrum. They picked up a very faint signal which seemed to be coming from every part of the sky. At first they thought it was a problem with the telescope. Only when they had thoroughly checked all their equipment did the full significance of their observation became apparent. In the 1940s, George Gamow had predicted that the Big Bang should have left a trace of itself in the form of microwave radiation spread evenly across the sky. Furthermore, the predicted strength of this radiation was comparable with observed results.

5.17 The shape of things to come

The fact that mutual gravitational attraction is causing the expansion of the universe to slow down suggests three possible future scenarios, depending on how much matter there is in the universe. The more matter, the greater the gravitational attraction and the more rapidly the expansion of the universe will slow down. If there is sufficient matter, the gravitational attraction will eventually overcome the expansion and the universe will begin to collapse again. This leads to a family of so-called closed cosmological models. If the total mass of the universe is less than that critical mass, expansion will continue indefinitely – an open universe. At the critical mass itself, the expansion will cease in the infinitely far future.

But what is the mass of the universe? Direct observations of luminous objects suggest a mass that is only a tiny fraction of the critical mass. This would suggest an open universe. However, studies of galaxy clusters reveal that their masses are much greater than what we might expect from their luminosity. In other words, much of the mass of the universe is in the form of ‘dark matter’ that is observable only through its gravitational effects. Estimates of the amount of dark matter vary but many sources suggest that it is sufficient for the actual mass of the universe to be quite close to the critical mass.[1] We discuss the implications of these predictions for theology in 10.20.

5.18 Is the Big Bang a moment of creation?

Strictly speaking the point associated with the Big Bang itself is a singularity – a point at which our laws of physics break down. In itself, this does not imply an absolute beginning. Nevertheless, it is tempting to read the Big Bang as having theological significance. After all, it does seem remarkably like a moment of creation.

This temptation received strong papal endorsement in 1951. Pope Pius XII announced that ‘everything seems to indicate that the universe has in finite times a mighty beginning’. He went on to claim that unprejudiced scientific thinking indicated that the universe is a ‘work of creative omnipotence, whose power set in motion by the mighty fiat pronounced billions of years ago by the Creating Spirit, spread out over the universe’. To be fair, he did also admit that ‘the facts established up to the present time are not an absolute proof of creation in time’.

Such pronouncements are guaranteed to provoke controversy. Even members of the Pontifical Academy of Sciences were divided over the wisdom of the Pope’s remarks. While Sir Edmund Whittaker could agree that the Big Bang might ‘perhaps without impropriety’ be referred to as the Creation, George LemaĆ®tre, one of the pioneers of the Big Bang theory, felt strongly that this was a misuse of his hypothesis (see 1.15).

Beyond the Christian community there was even greater unease. One of the fundamental assumptions of modern science is that every physical event can be sufficiently explained solely in terms of preceding physical causes. Quite apart from its possible status as the moment of creation, the Big Bang singularity is an offence to this basic assumption. Thus some philosophers of science have opposed the very idea of the Big Bang as irrational and untestable.

One popular way to evade the suggestion of an absolute beginning has been to argue that the universe must be closed. If it will eventually return to a singular point, why should it not then ‘bounce’? This is the so-called cyclic universe (see 5.22). Other astronomers opposed to the Big Bang, proposed instead a steady state theory. Fred Hoyle took a lead in this proposal. As we indicated in 1.15, his motives were explicitly theological. The steady-state theory argued that, in spite of appearances, the universe was infinitely old and did not evolve over time. Although defended by some very able scientists, this theory suffered a number of major setbacks which led to its demise. In order to maintain a steady state in the face of universal expansion it was necessary to postulate the continuous creation of matter from negative energy – ingenious, but contrived. There was the embarrassment of Hoyle’s failed attempt to show that the Big Bang could not account for the chemical composition of the universe (5.16.1). Finally, the steady state theory was not able to accommodate the new data that appeared – particularly the existence of the microwave background (5.16.1).

5.19 From Big Bang to inflation

The Big Bang theory has been very effective in predicting phenomena that have subsequently been observed by astronomers (5.16.1). However, the theory also raises a number of questions that it is unable to answer.

One of these questions is the so-called ‘horizon problem’. Observations reveal that above a certain scale (about 1024 metres[2]) the universe is highly uniform in structure. However, this degree of uniformity is an embarrassment to cosmologists. According to relativity theory, there should be no causal connection between points separated by distances greater than c multiplied by t (where c is the velocity of light and t is the age of the universe). Extrapolating this back to the Big Bang suggests that the primordial universe was partitioned into about 1080 causally separate regions (Barrow and Tipler, 1986: 420). Nevertheless, all these disconnected regions had to expand at the same rate to maintain the observed degree of uniformity!
Equally embarrassing for conventional Big Bang theory is that fact that although the universe is highly uniform it is not perfectly uniform. According to current theories, galaxy formation depends upon the existence of small initial irregularities in the Big Bang itself. These are amplified by cosmic expansion to the point where gravitation can begin the process of stellar condensation (Barrow and Tipler, 1986:417). If the initial irregularities are too large, the result is the rapid and widespread formation of black holes instead of stars. If the initial irregularities are sufficiently small, the precise expansion rate of the cosmos becomes critical – too rapid and the irregularities will not be amplified enough for galaxy formation to occur; too slow and the cosmos will be closed with a lifetime too short to permit biological evolution. Evidence of the existence of such irregularities in the early universe has been supplied by the COBE (COsmic Background Explorer) satellite’s observations of small irregularities in the cosmic microwave background. There is no mechanism within conventional Big Bang theory to account for these primordial irregularities. This is often called the ‘fine-tuning problem’.

            The widespread expectation among cosmologists that the actual mass of the universe is close to the critical mass (5.17) is a further problem for Big Bang theory in that it offers no explanation of this coincidence. Yet another difficulty arises from the fact that, according to particle physics, the cooling of the early universe after the Big Bang should lead to the production of topological anomalies, particularly magnetic monopoles. Indeed, monopoles should be the dominant matter in the universe. And yet, no monopole has ever been observed, directly or indirectly.

            In the 1980s dissatisfaction with these shortcomings of conventional Big Bang theory led Alan Guth to propose an alternative – the inflationary universe theory. According to this theory, in the earliest moments of its existence the region we now think of as the universe contained an excited state known as a false vacuum. This false vacuum possessed a repulsive force that caused this region to expand far more rapidly than would be possible in conventional Big Bang theory. In an unimaginably short period (perhaps 10−37 s) this region doubled in size at least 100 times. However, due to the peculiar properties of false vacuum, its energy density remained unchanged. In other words, the total energy contained in the region grew enormously. This false-vacuum state is extraordinarily difficult to imagine, but the best non-mathematical account available is provided by Brian Greene (2004: Chs. 9-10).

            The period of inflation must have been extremely short because the false vacuum is unstable. A false vacuum ‘decays’ into other forms of matter, converting its energy into a tremendously hot gas of elementary particles; essentially the same conditions as are predicted by conventional Big Bang theory for the period before the formation of the first atoms of hydrogen and helium. However, the inflationary phase means that the universe was originally much smaller than had previously been thought (perhaps a billion times smaller than a proton); small enough for it to have become uniform before inflation began. This primordial uniformity would then have been preserved during the inflationary phase and beyond, thus solving the horizon problem. Again unlike conventional Big Bang theory, the inflationary approach leads to an explanation of why the actual mass of the universe is close to the critical mass. Inflationary models may also account for the fine-tuning problem, if the irregularities in the cosmic background are understood as quantum fluctuations blown up by inflation (Greene, 2004:305-10).

            Guth’s original version of inflation was later shown to be unsatisfactory but this approach has spawned an entire family of inflationary models. One popular version is that developed by Andrei Linde. In this ‘chaotic’ version, different regions of the false vacuum decay at different times; each region becoming a separate ‘bubble universe’. However, because the false vacuum as a whole is expanding at an exponential rate, it is outgrowing the decay process. In other words, it is spawning ‘bubble universes’ ad infinitum. For comment on the relation of inflationary models to the problem of God’s action, see 10.15. Greene is one of those working on a ‘superstring’ account of the early universe, using descriptions in eleven dimensions. This approach shows some promise, but also reflects just how difficult it is to devise experimental tests of phenomena which only occur in the very special conditions of the early universe (see Greene 2004:Chs 12-13).

[1] At the moment of writing (August 2004) the prevailing view is that the so-called ‘cosmological constant’ which would accelerate the universe’s expansion (see Guth, 1997:37–42 for the history of this term) may be non-zero, and sufficiently large to guarantee an ever-expanding universe.
[2] The scientific convention for writing very big and very small numbers is used here. 102 is 10 multiplied by itself, or a hundred – 106 is ten multiplied by itself four more times, or a million. 1024 is 10 multiplied by itself twenty-four times, or a million million million million. Numbers less than 1 are written with negative indices – so 10-9 would be one-billionth.

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