‘Things are more like they are now than they ever were before.’
Dwight D. Eisenhower
‘The four ages of Man: Lager, Aga, Saga, and Gaga.’
Anonymous
When we think about the age and the size of the Universe we generally do so using measures of time and space like years, kilometres or light years. As we have already seen, these measures are extremely anthropomorphic. Why measure the age of the Universe using a ‘clock’ that ‘ticks’ once every time our planet completes an orbit around its parent star? Why measure its density in terms of atoms in a cubic metre? The answers to these questions are of course the same: because it's convenient and we've always done it like that. But here is a situation where it is especially appropriate to use the ‘natural’ units of mass, length and time that Stoney and Planck introduced to help us escape from the strait-jacket of a human-centred perspective.
If we adopt Planck's units then we see that the
present age of visible universe ≈ 1060 Planck-times
The size of the visible Universe is similarly huge:
present size of visible universe ≈ 1060 Planck-lengths
and so is its mass:
present mass of visible universe ≈ 1060 Planck-masses
Thus we see that the very low density of matter in the Universe is a reflection of the fact that
present density of visible universe ≈ 10 –120 of the Planck density
and the temperature of space, at three degree above absolute zero, is
present temperature of visible universe ≈ 10 –30 of the Planck temperature
These extremely large numbers and tiny fractions show us immediately that the Universe is structured on a superhuman scale of staggering proportions when weighed in the balances of its own construction. By its own standards the Universe is old. The natural lifetime of a world governed by gravity, relativity and quantum mechanics is the fleetingly brief Planck time. Somehow our Universe has managed to keep expanding for a vast number of Planck times. It seems to be much older than it should be. Later we shall see that cosmologists think they know how this came about. Yet, despite the huge age of the Universe in ‘ticks’ of Planck time, we have learnt that almost all this time is needed to produce stars and the life-supporting chemical elements.
‘At the End of the Universe you have to use the past tense a lot … everything's been done you know.’
Douglas Adams1
Why isn't our Universe much older than it appears to be? It is easy to understand why the Universe isn't much younger. Stars take a long time to form and to produce the heavier elements that biological complexity requires. But old universes have their problems as well. As time passes in the Universe the process of star formation will slow down. All the gas and dust that forms the raw material for stars will have been processed by stars and ejected into intergalactic space where it is unable to cool down and coalesce into new stars. Few stars means few solar systems and planets. Any planets that do form are less active than those formed earlier. The production of radioactive elements in the stars will diminish and those that are formed will have longer half-lives. New planets will be less geologically active and will lack many of the subterranean movements that power vulcanism, continental drift and mountain uplifting on the Earth. If this also makes the presence of a magnetic field less likely on a planet then life will be very unlikely to evolve into complex forms. Typical stars, like our Sun, emit a wind of electrically-charged particles from their surface which will strip off the atmospheres of orbiting planets unless the wind can be deflected by a planetary magnetic field. In our solar system the Earth's magnetic field has protected its atmosphere from the solar wind but Mars, unprotected by any magnetic field, lost its atmosphere long ago.
Long life on a solar system planet is probably not easy to sustain. We have gradually come to appreciate how precarious it is. Putting to one side the attempts that living beings persist in making to extinguish themselves, exhaust natural resources, spread lethal infections and deadly poisons, there are serious outside threats as well. The movements of comets and asteroids are a serious hazard to the development and persistence of intelligent life in its early stages. Impacts are not uncommon and have had catastrophic effects on Earth in the distant past. We are fortunate to be doubly shielded from these impacts – by our small near neighbour, the Moon, and by our giant distant neighbour, Jupiter. Jupiter is a thousand times more massive than the Earth and sits on the outskirts of the solar system where its powerful gravitational pull can capture incoming objects heading for the inner solar system. In July 1994 we were able to witness the fragmentation and capture of comet Schumacher-Levy 9 by Jupiter.2 In the twentieth century we had two significant impacts on Earth, one in South America and the other in Tunguska in northern Russia. We have been cheating the law of averages and one day our luck will change. Some governments are already investing effort in monitoring asteroids and planning countermeasures against Earthbound objects. Clearly, the longer a planet is around the greater its chances of being hit (see Figure 7.1).
These outside interventions upon the evolution of the Earth have a curious flip-side. True, they can produce global extinctions and set back the evolution of complexity by millions of years. But, in moderation, they may have a positive, accelerating effect upon the evolution of intelligent forms of life. When the dinosaurs were extinguished by the impact of a large meteor or comet striking the Earth on the Yucatan peninsula 65 million years ago at the end of the Mesozoic Era, the Earth was rescued from an evolutionary dead end. The dinosaurs seem to have evolved along a track that developed physical size rather than brain size. The disappearance of the dinosaurs, along with most other life-forms on Earth at that time, opened up space for the emergence of mammals. It also cleared niches of competitors for natural resources.
Figure 7.1 The average frequency of meteor impacts of different sizes on the Earth's atmosphere. Also shown is the diameter of the meteor and the diameter of the crater left on the Earth's surface together with the likely effects. 3
Figure 7.2 The pattern of response to an environmental crisis that produces a mass extinction event on Earth.4
This spurred a rapid acceleration in the development of diversity. Perhaps impacts play a vital role in kick-starting evolution when it gets stuck along unpromising paths. Without impacts the development process may settle into a stable, but unexciting byway with steady extinctions reducing species diversity constantly (see Figure 7.2). Harsh, fast-changing conditions stimulate adaptation and accelerate the evolutionary process. They also increase diversity and the best life insurance a planet can take out against total extinction of its biology by a future impact is to create diversity. You won't see it that way if you're a dinosaur though.
In our solar system life first evolved surprisingly soon after the formation of a hospitable terrestrial environment. There is something unusual about this. Suppose the typical time that it takes for life to evolve is called t(bio), then from the evidence of our solar system, which is about 4.6 billion years old, it seems that the time it takes for stars to settle down and create a stable source of heat and light, t(star), is not very different to t(bio) because we have found simple forms of terrestrial bacterial life that are several billion years old.
This similarity between t(bio) and t(star) seems like a coincidence. At first sight we might assume that the microscopic biochemical processes and local environmental conditions that combine to determine the magnitude of t(bio) are independent of the nuclear astrophysical and gravitational processes that determine the typical stellar lifetime of a star. However, this assumption leads to the striking conclusion that we should expect extraterrestrial forms of life to be exceptionally rare. The argument, in its simplest form, introduced by Brandon Carter5 and developed further by myself and Frank Tipler,6 and still investigated minutely today7 goes like this. If t(bio) and t(star) are unconnected with one another then the time that life takes to arise will be random with respect to the stellar time scale t(star). So it is most likely8 that we should either find that t(bio) is much bigger than t(star) or find that t(bio) is much less than t(star).
Now let's take stock. On the one hand, if t(bio) is typically much less than t(star) we need to ask why it is that the first observed inhabited solar system (ours!) has t(bio) approximately equal to t(star). By our logic, this would be extraordinarily unlikely. On the other hand, if t(bio) is typically much greater than t(star) then the first observed inhabited solar system (ours) is a statistical fluke and most likely to have t(bio) approximately equal to t(star), since systems with t(bio) much greater than t(star) have yet to evolve. Thus we are led to conclude that we are a rarity, one of the first living systems to arrive on the scene.
In order to escape from this conclusion we have to undermine one of the assumptions underlying the argument that leads to it. For example, if we suppose that t(bio) is not independent of t(star), then things look different. If the ratio t(bio)/t(star) increases with t(star) then it may become likely that we will find t(bio) approximately equal to t(star). Mario Livio9 has suggested how t(bio) and t(star) could be related by a relation of this general form if the evolution of a life-supporting planetary atmosphere requires an initial phase during which oxygen is released by the photodissociation of water vapour. On Earth this took 2.4 billion years and built up the atmospheric oxygen to about one thousandth of its present value. The length of this phase might be expected to be inversely proportional to the intensity of radiation in the wavelength interval 1000–2000 ångstroms, where the key molecular levels for water absorption lie. Further studies of stellar evolution may allow us to determine the length of this phase and so establish a link between the biological evolution time and the main-sequence stellar lifetime.
This model indicates a possible route to establishing a link between the biochemical time scales for the evolution of life and the astrophysical time scales that determine the time required to create an environment supported by a stable hydrogen-burning star. There are obvious weak links in the argument, though. It provides only a necessary condition for life to evolve, not a sufficient one. We could imagine an expression for the probability of planet formation around a star. It would involve many other factors which would determine the amount of material available for the formation of solid planets with atmospheres at distances which permit the presence of liquid water and stable surface conditions. Furthermore, we know that there were many ‘accidents’ of the planetary formation process in the solar system which have subsequently played a major role in the existence of long-lived stable conditions on Earth. For example, as Jacques Laskar and his coworkers10 have shown, the presence of resonances between the precession rates of rotating planets and the gravitational perturbations they feel from all other bodies in their solar system can easily produce chaotic evolution of the tilt of a planet's rotation axis with respect to the orbital plane of the planets over times much shorter than the age of the solar system. The planet's surface temperature variations and sea levels are sensitive to this angle of tilt. It determines the climatic differences between what we call ‘the seasons’. In the case of the Earth, the modest angle of tilt (approximately 23 degrees) would have experienced this erratic evolution had it not been for the presence of the Moon. The Moon is so large that its gravitational effects dominate the resonances which exist between the Earth's precessional rotation and the frequency of external gravitational perturbations from the other planets. As a result the Earth's tilt wobbles only by half a degree around 23 degrees over hundreds of thousands of years.
This shows how the causal link between stellar lifetimes and biological evolution times may be rather a minor factor in the chain of fortuitous circumstances that must occur if habitable planets are to form and sustain viable conditions for the evolution of life over long periods of time.
‘Life is not for everyone.’
Michael O'Donoghue11
One of the assumptions that arguments for the inevitability of a large and cool universe are implicitly making is that all life is very much like us. Biologists seem happy to admit the possibility of other forms of life but are not so sure that it is likely to evolve spontaneously, without a helping hand from carbon-based life-forms. Most estimates of the likelihood of extraterrestrial intelligences existing in the Universe focus upon life-forms similar to ourselves who live on planets and need water, gaseous atmospheres and so forth. It is worth stretching our imaginations a little to think about what life might be like if it was space-based rather than planet-based. The astronomer Fred Hoyle created an interesting example which he hoped would evade the usual unfavourable conclusions about likelihood that had been made for planet-based ETIs. Not content with successful careers as astrophysicist and populariser of science, Hoyle branched out into science fiction, with notable successes. His most famous story, The Black Cloud,12 was a big publishing success that created a plausible contemporary thriller involving scientists not dissimilar to Hoyle himself. Indeed, despite his assurances that the characters are entirely fictitious, it is hard not to identify the hero with Hoyle himself. The Black Cloud was written in 1957, just a few years after the discovery of coincidences concerning the values of the constants of Nature that have important implications for the possible existence of carbon and oxygen, and hence for life in the Universe. There was much discussion about the likelihood of life elsewhere and the first two Soviet Sputnik space probes were launched in 1957. The scenario is set for Earth to face the approach from space of a cloud of gas, of which there are many in interstellar space, which is on course to pass between the Earth and the Sun. If it does then the heat and light from the Sun will be cut off for a period, after being amplified for a while by reflection from the cloud, with potentially calamitous consequences for Earth. Events take an unexpected turn. The cloud turns out to be intelligent, an amorphous life-form existing as a huge system of complex molecular correlations moving through space. After much intrigue and excitement the Earth survives its brief encounter with the passing cloud but not before it has established a dialogue with it and learned to decode the signals it uses to speak to us. Yet the most important message that Hoyle was trying to get across in this story was the possible error of assuming that life lives on solid planets. Perhaps the chemical complexity needed to qualify as ‘life’ could exist in huge molecular clouds, stabilised by the binding force of gravity. Even carbon might not be needed in these nebulous cradles of life. Thirty years later Hoyle was to return to this theme in his scientific work and science fiction, imagining that self-reproducing molecules could have evolved within cometary interiors and then spread around the galaxies by the motion of the comets.
Other science fiction writers had explored the possibilities of alternatives to carbon chemistry. Silicon was known to form chain molecules a little like carbon does, but unfortunately they tend to be like quartz and sand, rigid and uninteresting as a building block for biology.
Ironically, the computer revolution has since shown that it is silicon physics rather than silicon chemistry which holds the most promise as another basis for life. But such artificial forms of life and intelligence are not spontaneously evolved. They have required the help of carbon-based organisms to bring together the highly organised, and hence extremely improbable, configurations needed for their persistence and development. These more abstract alternatives to life in flesh and blood form are now rather familiar to us and science fiction writers have to be considerably more subtle than merely to imagine aliens with odd chemistries and new bodily forms. But back in 1957 Hoyle's idea was a novel one. It played an important role in widening the spectrum of possibilities for life beyond what most astronomers had in mind. The probability of life was not to be determined solely from the statistics of habitable planets with temperate atmospheres and surface water in orbit around friendly stars.
‘If you're killed, you've lost a very important part of your life.'
Brooke Shields13
There is one further curiosity about the coincidence that exists between the biological evolution time and astronomy. Since it is unsurprising that the ages of typical stars are similar to the present age of the Universe there is also an apparent coincidence between the age of the Universe and the time it has taken to evolve life-forms like ourselves. If we look back at how long our intelligent ancestors (Homo sapiens) have been on the scene we find that it is only about 200,000 years, which is much less than the age of the Universe, 13 billion years. Our human history has lasted for less than two hundred thousandths of the history of the Universe. But if our descendants could go on indefinitely into the future the situation for them would become very different. Suppose they were still thinking about these questions when the Universe was one thousand billion years old. Then they would calculate that their intelligent ancestors had been around for 1000 billion minus 13 billion plus 200,000 years. The answer 987.2 billion years is very similar to 1000 billion years. Our descendants would not think that the history of their civilisation lasted for just a tiny fraction of the history of the universe. Brandon Carter and Richard Gott have argued that this appears to make us rather special compared with observers in the far distant future. If you believe that our location in cosmic history should not be special in this way then you are led to a dramatic conclusion. In order to make sure that we and our descendants in the near future do not have a special view of cosmic history, thinking our own history is vastly less than the total history of the Universe, we need to have no far future descendants. If life on Earth disappeared in a few thousand years then all our descendants would observe roughly the same number for the fraction of cosmic history that has seen human civilisation exist. Gott estimated that by this argument we should be 95 per cent confident that life on Earth will end between 5000 and 7.8 million years in the future.
There is no reason to confine this argument to such cataclysmic events as the extinction of human life. It is based upon the simple statistical fact that if you observe something at a random time there is a 95 per cent chance that you will be observing it during the middle 95 per cent of the period when it can be observed.14 To show the versatility of this simple piece of statistics, Gott was asked to prepare a series of predictions for the January 1st, 2000 issue of the Wall Street Journal. The ones chosen are shown in Figure 7.3.
It is easy to work out these sorts of statistics for the precarious things of your choice. If the present time is to be random with respect to the total time over which something is observable then with 95 per cent confidence its future is expected to lie within a time interval bigger than 1/39th and 39 times its past age. If we only want 50 per cent confidence then its future will extend between one third and three times its past age.
Figure 7.3 With 95 percent confidence these are the shortest and longest times that we expect the following structures and organisations to have lasted for or to last for in the future according to Richard Gott's predictions15 on New Year's Day 2000.
‘Moriarty: “All that I have to say has already crossed your mind.”
Holmes: “Then possibly my answer has crossed yours.”
A. Conan Doyle16
Dicke's response to the problem of the Large Numbers had many important consequences. He showed that the approaches of Eddington and Dirac had been extreme and unwarranted. They had tried to explain the Large Number coincidences by making major changes to our theories of physics. Eddington wanted to create an ambitious new fundamental ‘theory of everything’ from which he imagined would flow new equations linking the constants of Nature in unsuspected ways, showing the Large Number coincidences to be consequences of a deep-laid scheme of Nature. Likewise, Dirac responded by giving up the constancy of one of the traditional constants of Nature, G, so as to allow the coincidences between different large numbers to be consequences of an as yet unknown theory of gravity and atomic phenomena. Dicke, by contrast, took a less iconoclastic approach. He recognised that not all moments of time are equal: we should only expect to be looking at the Universe when it is old enough for living beings to exist within it. As a result there is an irremovable bias besetting our astronomical observations that we do well to be aware of. This bias ensures that Dirac's coincidence between different Large Numbers will be observed by beings like ourselves. Dicke's lesson for scientists is a powerful and simple one and if you don't take it on board then, like Dirac and Eddington, you may be doomed to embark upon an unwarranted wild-goose chase, giving up well-established theories for speculative new possibilities. Critics who have not understood Dicke's contribution sometimes object it is ‘not a scientific theory’ because it makes no predictions and so ‘cannot be tested’.
This is a serious misunderstanding. The recognition of observer bias is not somehow a rival scientific theory that needs to be tested. It is a principle of scientific methodology of which we remain unaware or wilfully ignore at our peril. It is just a sophisticated version of a principle that experimental scientists are very familiar with – experimental bias.
When you carry out an experiment or seek to draw conclusions from observational data the most important insight that the experimenter requires is the awareness of what biases beset the experiment. Such biases will make it easier to gather certain sorts of evidence than others and produce a misleading result. An interesting case that came to light in the newspapers was in regard to the controversial issue of mathematical achievement levels in tests by school children in different countries. For many years it had been claimed that the average achievement by pupils in some South-East Asian countries was significantly higher than in the United Kingdom. Then it came to light that the weakest pupils in that country were removed from the total who were evaluated at an earlier stage in the educational process. Clearly, the effect of their removal is to skew the average attainments to be higher than they would otherwise be. Another recent example that caught my eye was an American survey to discover if people who attended church also tended to have better health. A serious bias beset the final results because people who were extremely sick would be unable to attend church.
What these examples show is that scientists of all sorts must strive to be aware of any bias that might skew their data to produce a conclusion that is not present in the underlying evidence. Dicke noticed something similar in the astronomers' view of the Universe. Ignore the lesson of observer selection and wrong conclusions will be drawn.
The challenge of the Large Numbers played an important role in the development of our efforts to understand the structure of the Universe and the range of possibilities available for the constants of Nature that supply the skeleton on which the outcomes of Nature's laws are fleshed out. It encouraged a serious questioning of the constancy of the traditional constants of Nature, especially Newton's ‘constant’ G, and led to the formulation of new theories of gravity which enlarged Einstein's theory to include this possibility. This also precipitated a broad change of outlook. Suddenly, subjects like biology and geology which had traditionally had very little to do with astronomy and cosmology, were seen to be of cosmic importance. A broadened perspective to cosmological thinking appeared. Some cosmological theories might be tested by geophysical or palaeontological evidence or lead to histories in which the evolution of life by natural selection could not have occurred. Astronomers became used to asking how finely balanced a situation existed in the Universe with respect to the existence of life like us or life of any other conceivable sort. The observed values of many of the fundamental constants of Nature or of the quantities describing the properties of the Universe's global properties – its shape, its speed of expansion, its uniformity – also seemed quite delicately poised. Quite small shifts in the status quo would render all conceivable complexity impossible. Habitable universes came to be seen as rather a tricky balancing act to accomplish.
‘It is more important that a proposition be interesting than that it be true … But of course a true proposition is more apt to be interesting than a false one.’
Alfred North Whitehead17
It is interesting to end our look at Dicke's way of treating the Large Number coincidences between constants of Nature by taking a look back at a very similar type of argument made by Alfred Wallace in 1903. Wallace was a great scientist who today receives little of the credit he deserves. It was he, rather than Charles Darwin, who first had the idea that living organisms evolve by a process of natural selection. Fortunately for Darwin, who had been thinking deeply and gathering evidence to support such an idea independently of Wallace over a very long period of time, Wallace wrote to him to tell him of his ideas rather than simply publishing them in the scientific literature. Yet today, ‘evolutionary biology’ focuses almost entirely upon the contributions of Darwin.
Wallace was far broader in his interests than Darwin and was interested in most areas of physics, astronomy and earth sciences. In 1903 he published a wide-ranging study of the factors that make the Earth a habitable place and went on to explore the philosophical conclusions that might be drawn from the state of the Universe. His book went under the resonant title Man's Place in the Universe. 18 This was before the discovery of the theories of relativity, nuclear energy and the expanding universe.19 Most nineteenth-century astronomers conceived of the Universe as a single island of matter, what we would now call our Milky Way galaxy. It was not established that there existed other galaxies or what the overall scale of the Universe was. It was only clear that it was big.
Wallace was impressed by the simple cosmological model that Lord Kelvin had developed using Newton's law of gravitation. It showed that if we took a very large ball of material then the action of gravity would cause it all to collapse towards its centre. The only way to avoid getting pulled into the centre was to orbit around the centre. Kelvin's universe contained about one billion stars like the Sun in order that their gravitational forces would counterbalance motions at the observed speeds.20
What is intriguing about Wallace's discussion21 of this model of the Universe is that he adopts a non-Copernican attitude because he sees how some places in the Universe are more conducive to the presence of life than others. As a result, it is only to be expected that we lie near, but not at, the centre of things.
Remarkably, Wallace produces an analogue of Dicke's argument for the great age of any universe observed by humans. Of course, in Wallace's time, long before the discovery of nuclear power sources, no one knew how the Sun was powered. Kelvin had argued for gravitational energy, but it was not adequate for the job. In Kelvin's cosmology, material would be attracted by gravity into the central regions where the Milky Way was situated and fall into the stars that were already there, generating heat and maintaining their luminous power output over huge periods of time. Here, Wallace sees a simple reason for the vast size of the Universe:
‘Here then, I think, we have found an adequate explanation of the very long-continued light- and heat-emitting capacity of our sun, and probably of many others in about the same position in the solar cluster. These would at first gradually aggregate into considerable masses from the slowly moving diffuse matter of the central portions of the original universe; but at a later period they would be reinforced by a constant and steady inrush of matter from its outer regions possessing such high velocities as to aid in producing and maintaining the requisite temperature of a sun such as ours, during the long periods demanded for continuous life-development. The enormous extension and mass of the original universe of diffused matter (as postulated by Lord Kelvin) is thus seen to be of the greatest importance as regards this ultimate product of evolution, because without it, the comparatively slow-moving and cool central regions might not have been able to produce and maintain the requisite energy in the form of heat; while the aggregation of by far the larger portion of its matter in the great revolving ring of the galaxy was equally important, in order to prevent the too great and too rapid inflow of matter to those favoured regions… . For [on] those [planets around stars] whose material evolution has gone on quicker or slower there has not been, or will not be, time enough for the development of life.’22
Wallace sees clearly the connection between these unusual global features of the Universe and the conditions necessary for life to evolve and prosper:
‘we can dimly see the bearing of all the great features of the stellar universe upon the successful development of life. These are, its vast dimensions; the form it has acquired in the mighty ring of the Milky Way; and our position near to, but not exactly in, its centre.’23
He also expects that this process of infall and solar power generation from gravitational energy will probably have a staccato form with long periods of infall driving heating of the stars followed by periods of quiescence and cooling, a period which we have just begun to experience:
‘I have here suggested a mode of development which would lead to a very slow but continuous growth of the more central suns; to an excessively long period of nearly stationary heat-giving power; and lastly, an equally long period of very gradual cooling – a period the commencement of which our sun may have just entered upon.24
Wallace completes his discussion of the cosmic conditions needed for the evolution of life by turning his attention to the geology and history of the Earth. Here he sees a far more complicated situation than exists in astronomy. He appreciates the host of historical accidents that have marked the evolutionary trail that has led to us, and thinks it ‘in the highest degree improbable’ that the whole collection of features that are conducive to the evolution of life will be found elsewhere. This leads him to speculate that the huge size of the Universe might be required in order to allow life a reasonable chance of developing on just one planet, like our own, no matter how conducive its local environment might be:
‘such a vast and complex universe as that which we know exists around us, may have been absolutely required … in order to produce a world that should be precisely adapted in every detail for the orderly development of life culminating in man.’25
Today, we might echo this sentiment. The large size of the observable Universe, with its 1080 atoms, allows a vast number of sites for the statistical variations of chemical combination to be worked through.
Yet, despite his interest in the huge size of the Universe in making it probable that we evolved, Wallace was averse to the idea of a Universe populated with many other living beings. He believed that the uniformity of the laws of physics and chemistry26 would ensure that
‘organised living beings wherever they may exist in the universe must be fundamentally, and in essential nature, the same also. The outward forms of life, if they exist elsewhere, may vary, almost infinitely, as they do vary on earth … We do not say that organic life could not exist under altogether diverse conditions from those which we know or can conceive, conditions which may prevail in other universes constructed quite differently from ours, where other substances replace the matter and ether of our universe, and where other laws prevail. But within the universe we know, there is not the slightest reason to suppose organic life to be possible, except under the same general conditions and laws which prevail here.’27
Wallace provides an intriguing bridge between the pre-evolutionary way of thinking and the modern perspective brought about by the discovery that the Universe is changing. His approach to cosmology shows how the consideration of the conditions necessary for the evolution of life is not wedded to any particular theory of star formation and development but must be used in each context as appropriate. For Wallace it was a new picture of the Universe developed by Kelvin. For modern astronomers it is the well-tested theory of the expanding Universe in which the energy generation by the stars is almost completely understood. Both theories were dynamic: Kelvin's model allowed material to fall from great distances into the centre of the star system under the influence of gravitational attraction whilst the Big Bang theory of Dicke's expanded to increasing size with the passage of time. In both scenarios size and time were linked and the vastness of the Universe had unusual indirect consequences for what could happen within it, consequences that had a crucial bearing on the possibility of life and mind emerging in the course of time.