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Prior to the discovery of the expansion of the Universe there was little that cosmology could contribute to the question of extraterrestrial life aside from probabilities and prejudices. After our discovery of the expansion and evolution of the Universe the situation changed significantly. The entire cosmic environment was recognised as undergoing steady change. The history of the Universe took on the complexion of an unfolding drama in many acts, with the formations of first atoms and molecules, then galaxies and stars, and most recently, planets and life. The most important and simplest feature of the overall change in the Universe that the expansion produces is the rate at which it occurs. This is linked to the age of the expanding universe and that of its constituents.

In the 1930s, the distinguished biologist JBS Haldane took an interest in Milne's proposal [1] that there might exist two different timescales governing the rates of change of physical processes in the Universe: one, t, for ``atomic'' changes and another, tau, for ``gravitational changes'' where tau = ln (t / t0) with t0 constant. Haldane explored how changing from one timescale to the other could alter ones picture of when conditions in the Universe would become suitable for the evolution of biochemical life [2], [4]. In particular, he argued that it would be possible for radioactive decays to occur with a decay rate that was constant on the t timescale but which grew in proportion to t when evaluated on the tau scale. The biochemical processes associated with energy derived from the breakdown of adenosine triphosphoric acid would yield energies which, while constant on the t scale, would grow as t2 on the tau scale. Thus there would be an epoch of cosmic history on the tau scale before which life was impossible but after which it would become increasingly likely. Milne's theory subsequently fell into abeyance although the interest in gravitation theories with a varying Newtonian ``constant'' of gravitation led to detailed scrutiny of the paleontological and biological consequences of such hypothetical changes for the past history of the Earth [4]. Ultimately, this led to the formulation of the collection of ideas now known as the Anthropic Principles, [5], [6].

Another interface between the problem of the origin of life and cosmology has been the perennial problem of dealing with finite probabilities in situations where an infinite number of potential trials seem to be available. For example, in a universe that is infinite in spatial volume (as would be expected for the case for an expanding open universe with non-compact topology), any event that has a finite probability of occurring should occur not just once but infinitely often with probability one if the spatial structure of the Universe is exhaustively random [3]. In particular, in an infinite universe we conclude that there should exist an infinite number of sites where life has progressed to our stage of development. In the case of the steady-state universe, it is possible to apply this type of argument to the history of the universe as well as its geography because the universe is assumed to be infinitely old. Every past-directed world line should encounter a living civilisation. Accordingly, it has been argued that the steady state universe makes the awkward prediction that the universe should now be teeming with life along every line of sight [4].

The key ingredient that modern cosmology introduces into considerations of biology is that of time. The observable universe is expanding and not in a steady state. The density and temperature are steadily falling as the expansion proceeds. This means that the average ambient conditions in the universe are linked to its age. Roughly, in all expanding universes, dimensional analysis tells us that the density of matter, rho, is related to the age t measured in comoving proper time and Newton's gravitation constant, G, by means of a relation of the form

Equation 1 (1)

The expanding universe creates an interval of cosmic history during which biochemical observers, like ourselves, can expect to be examining the Universe. Chemical complexity requires basic atomic building blocks which are heavier than the elements of hydrogen and helium which emerge from the hot early stages of the universe. Heavier elements, like carbon, nitrogen, and oxygen, are made in the stars, as a result of nuclear reactions that take billions of years to complete. Then, they are dispersed through space by supernovae after which they find their way into grains, planets, and ultimately, into people. This process takes billions of years to complete and allows the expansion to produce a universe that is billions of light years in size. Thus we see why it is inevitable that the universe is seen to be so large. A universe that is billions of years old and hence billions of light years in size is a necessary pre-requisite for observers based upon chemical complexity. Biochemists believe that chemical life of this sort, and the form based upon carbon in particular, is likely to be the only sort able to evolve spontaneously. Other forms of living complexity (for example that being sought by means of silicon physics) almost certainly can exist but it is being developed with carbon-based life-forms as a catalyst rather than by spontaneous evolution.

The inevitability of universes that are big and old as habitats for life also leads us to conclude that they must be rather cold on average because significant expansion to large size reduces the average temperature inversely in proportion to the size of the universe. They must also be sparse, with a low average density of matter and large distances between different stars and galaxies. This low temperature and density also ensures that the sky is dark at night (the so called ``Olbers' Paradox'' first noted by Halley, [7]) because there is too little energy available in space to provide significant apparent luminosity from all the stars. We conclude that many aspects of our Universe which, superficially, appear hostile to the evolution of life are necessary prerequisites for the existence of any form of biological complexity in the Universe.

Life needs to evolve on a timescale that is intermediate between the typical time scale that it takes for stars to reach a state a state of stable hydrogen burning, the so called main-sequence lifetime, and the timescale on which stars exhaust their nuclear fuel and gravitationally collapse. This timescale, t*, is determined by a combination of fundamental constants of Nature

Equation 2 (2)

where mN is the proton mass, h is Planck's constant, and c is the velocity of light [8], [4]

In expanding universes of the Big Bang type the reciprocal of the observed expansion rate of the universe, Hubble's constant H0 approx 70 km s-1 Mpc-1, is closely related to the expansion age of the universe, t0, by a relation of the form

Equation 3 (3)

The fact that the age t0 approx 1010 yr deduced from observations of H0 in this way is a little larger than the main sequence lifetime, t*, is entirely natural in the Big Bang theory that is, we observe a little later than the time when the Sun forms). However, the now defunct steady state theory, in which there is no relation between the age of the universe (which is infinite) and the measured value of H0, would have had to regard the closeness in value of H0-1 and t* as a complete coincidence [9].

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