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 impact of the discovery of the Universe’s expansion is that it gives
the Universe a changing history and thereby links the cosmos to the local
conditions needed for the existence of life.
In the 1930s, the distinguished biologist J.B.S. Haldane took an
interest in Milne’s proposal
that there might exist two different time-scales governing the rates of change
of physical processes in the Universe: one, t, for ‘atomic’ changes and
another, τ, for
‘gravitational changes’ where τ = ln(t/to)
with to
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.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 τ 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 τ scale. Thus there would be an epoch of cosmic history on the τ
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. Ultimately, this led to the formulation of
the collection of ideas now known as the Anthropic Principles.
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 one-hundred percent probability if the
spatial structure of the Universe is exhaustively random.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.
As a result, 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.
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, r,
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
|
|
(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 the 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 combination of low temperature and density also ensures that the
sky is dark at night (the so called ‘Olbers’ Paradox’ first noted by Halley,)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 timescale that it takes for stars to reach 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
|
 |
(2) |
where mN is the proton mass, h is Planck’s constant, and c
is the velocity of light.
In expanding universes of the Big Bang type the reciprocal of the
observed expansion rate of the universe, Hubble’s constant Ho≈
70 Km.s-1Mpc-1,
is closely related to the expansion age of the universe, to, by a relation
of the form
|
 |
(3) |
The fact that the age to ≈
1010yr deduced from observations of Ho 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 Ho would have had to regard the
closeness in value of Ho-1 and t*
as a complete coincidence.
Contributed by: Dr. John Barrow