by Peter Coles
The Anthropic Principles
Cosmology has always been about Man’s attempts to understand the Universe and his relationship to it. As scientific cosmology has evolved, Man’s role has diminished. Our existence appears accidental, unplanned, and incidental to whatever purpose the cosmos was constructed to fulfil. This interpretation has more recently been challenged by a suggestion called the Anthropic Principle, that there might, after all, be a deep connection between the existence of life and the fundamental physics that governs how the Universe evolves. It was Brandon Carter who first suggested adding the word ‘Anthropic’ to the usual ‘Cosmological Principle’ to stress the fact that our Universe is ‘special’, at least to the extent that it has permitted intelligent life to evolve within it.
There are many otherwise viable cosmological models that are not compatible with the observation that human observers exist. For example, we know that heavy elements like carbon and oxygen are vital to the complex chemistry required for terrestrial life to have developed. We also know that it takes around 10 billion years of stellar evolution for generations of stars to synthesize significant quantities of these elements from the primordial gas of hydrogen and helium that exists in the early stages of a Big Bang model. We know therefore that we could not inhabit a universe younger than about 10 billion years old. Since the size of the Universe is related to its age if it is expanding, this line of reasoning sheds some light on the question of why the Universe is as big as it is. It has to be big, because it has to be old if there has been time for us to evolve within it. This form of reasoning is usually called the ‘Weak’ Anthropic Principle and it can lead to useful insights into the properties our Universe might be expected to possess simply by virtue of our presence within it.
Some cosmologists have sought to extend the Anthropic Principle into deeper waters. While the weak version applies to physical properties of our Universe such as its age, density, or temperature, the ‘Strong’ Anthropic Principle is an argument about the laws of physics according to which these properties evolve. It appears that these fundamental laws are very finely tuned to permit complex chemistry, which, in turn, permits the development of biology and ultimately human life. If the laws of electromagnetism and nuclear physics were only slightly different, chemistry and biology would be impossible. On the face of it, the fact that the laws of nature do appear to be tuned in this way seems to be a coincidence, in that there is nothing in our present understanding of fundamental physics that requires the laws to be conducive to life in this way. This is therefore something we should seek to explain.
In some versions of the strong Anthropic Principle, the reasoning is essentially an argument from design: the laws of physics are as they are because they must be like that for life to develop. This is tantamount to requiring that the existence of life is itself a law of nature, and the more familiar laws of physics are subordinate to it. This kind of reasoning appeals to some with a religious frame of mind but its status among scientists is rightly controversial, as it suggests that the Universe was designed specifically in order to accommodate human life.
An alternative, and perhaps more scientific, construction of the strong Anthropic Principle involves the idea that our Universe may consist of an ensemble of mini-universes, each one having different laws of physics to the others. This may be what emerges from a unified theory, in which the high-energy symmetry is broken in a different way in different patches of the Universe. Obviously, we can only have evolved in one of the mini-universes compatible with the development of organic chemistry and biology, so we should not be surprised to be in one where the underlying laws of physics appear to have special properties. This provides some kind of explanation for the apparently surprising properties of the laws of nature mentioned above. This is not an argument from design, since the laws of physics could vary haphazardly from mini-universe to mini-universe.
This version of the Anthropic Principle is rightly controversial, but it at least addresses the distinction between the ‘how’ and the ‘why’. It remains to be seen whether cosmology can explain why the Universe is the way it is, but we’ve certainly come a long way towards understanding what happened, and how.
Epilogue
Cosmology is in many ways similar to forensic science. Neither cosmologists nor forensic scientists can perform experiments that recreate past events under slightly different conditions, which is what most other scientists do. There is only one Universe, one scene of the crime. In both fields the available evidence is often circumstantial, difficult to gather, and open to ambiguity of interpretation. Despite these difficulties, the case in favour of the Big Bang is, in my view, proven beyond all reasonable doubt.
Of course, important questions remain unresolved. We still do not know the form of most of the matter in the Universe. We do not know for sure whether the Universe is finite or infinite. We do not know how the Universe began, or whether inflation happened. Nevertheless, the points of agreement between theory and observation are so many and so striking that the pieces of a coherent picture seem at last to be falling into place. But, as the saying goes, these are famous last words.
Further reading
General references
Coles, P. (ed.), The Routledge Companion to the New Cosmology (London: Taylor & Francis, 2001).
Gribbin, J., Companion to the Cosmos (London: Orion Books, 1997).
Ridpath, I. (ed.), The Oxford Dictionary of Astronomy (Oxford: Oxford University Press, 1997).
Chapter 1
Barrow, J. D., The World Within the World (Oxford: Oxford University Press, 1988).
Harrison, E., Darkness at Night (Cambridge, Mass.: Harvard University Press, 1987).
Hoskin, M. (ed.), The Cambridge Illustrated History of Astronomy (Cambridge: Cambridge University Press, 1997).
Lightman, A., Ancient Light: Our Changing View of the Universe (Cambridge, Mass.: Harvard University Press, 1991).
North, J., The Fontana History of Astronomy and Cosmology (London: Fontana, 1994).
Chapter 2
Coles, P., Einstein and the Birth of Big Science (Cambridge: Icon Books, 2000).
Pais, A., ‘Subtle is the Lord …’: The Science and the Life of Albert Einstein (Oxford: Oxford University Press, 1992).
Thorne, K. S., Black Holes and Time Warps (New York: Norton & Co., 1994).
Chapter 3
Eddington, A. S., The Nature of the Physical World (Cambridge: Cambridge University Press, 1928).
Trope, E. A., Frenkel, V. Y., and Chernin, A. D., Alexander A. Friedmann: The Man who Made the Universe Expand (Cambridge: Cambridge University Press, 1993).
Chapter 4
Florence, R., The Perfect Machine: Building the Palomar Telescope (New York: HarperCollins, 1994).
Graham-Smith, F., and Lovell, B., Pathways to the Universe (Cambridge: Cambridge University Press, 1988).
Hubble, E., The Realm of the Nebulae (Newhaven: Yale University Press, 1936).
Preston, R., First Light: The Search for the Edge of the Universe (New York: Random House, 1996).
Chapter 5
Barrow, J. D., and Silk, J., The Left Hand of Creation (New York: Basic Books, 1983).
Close, F., The Cosmic Onion (London: Heinemann, 1983).
Davis, P. C. W., The Forces of Nature (Cambridge: Cambridge University Press, 1979).
Pagels, H. R., Perfect Symmetry (Harmondsworth: Penguin Books, 1992).
Silk, J., The Big Bang, rev. and updated edn. (New York: W. H. Freeman & Co., 1989).
Weinberg, S., The First Three Minutes (London: Fontana, 1983).
Chapter 6
Gribbin, J., and Rees, M. J., The Stuff of the Universe (Harmondsworth: Penguin Books, 1995).
Guth, A. H., The Inflationary Universe (New York: Jonathan Cape, 1996).
Krauss, L. M., The Fifth Essence (New York: Basic Books, 1989).
Livio, M., The Accelerating Universe (New York: John Wiley & Sons, 2000).
Overbye, D., The Lonely Hearts of
the Cosmos (New York: HarperCollins, 1991).
Rees, M. J., Just Six Numbers (London: Weidenfeld & Nicolson, 1999).
Riordan, M., and Schramm, D., The Shadows of Creation (Oxford: Oxford University Press, 1993).
Chapter 7
Chown, M., The Afterglow of Creation (London: Arrow Books, 1993).
Cornell, J. (ed.), Bubbles, Voids and Bumps in Time: The New Cosmology (Cambridge: Cambridge University Press, 1989).
Smoot, G., and Davidson, K., Wrinkles in Time (New York: Avon Books, 1993).
Chapter 8
Barrow, J. D., Theories of Everything (Oxford: Oxford University Press, 1991).
——Pi in the Sky (Oxford: Oxford University Press, 1992).
——The Origin of the Universe (London: Orion Books, 1995).
Coles, P., Hawking and the Mind of God (Cambridge: Icon Books, 2000).
Hawking, S. W., A Brief History of Time (New York: Bantam Books, 1988).
Lidsey, J. E., The Bigger Bang (Cambridge: Cambridge University Press, 2000).
Index
A
Abell clusters 95–6
accelerating universe 11, 56, 91–2
age of the Universe 54–6
Almagest 5
Alpher, R. 62
Anaximander 4
Andromeda Nebula 47, 94–5
anthropic principle 125–7
antimatter 68, 72–3
Aquinas, T. 5
Aristotle 5, 7
arrow of time 117–20
B
Babylon 2–3
baryons 64, 68, 72–3, 80
baryon catastrophe 83
baryon number 72
Bethe, H. 62
Big Bang 8–11, 37–8, 57–73, 115–17
black body 59–61
black holes 24–6, 115–16, 120
Bondi, H. 58
BOOMERANG 105–6
bosons 67–70
Brahe, T. 6, 89
branes 123
C
C-field 58
carbon 125
carbon dating 54
Cepheid variables 51–3
CERN 65
Chandrasekhar mass 90
China 2–3
classical physics 114
closed Universe 10, 34, 78, 118
clusters of galaxies 81, 95–6
COBE (Cosmic Background Exlorer) 98–100, 102
continuous creation 58
Coma cluster 17–18, 96
Copenhagen interpretation 111–12
Copernican Principle 29–30, 43–4, 81
Copernicus, N. 5, 7
cosmic microwave background 8–9, 30, 59–61, 73
cosmological constant 31, 91–2
cosmological flatness problem, see flatness problem
Cosmological Principle 29–30, 32–3, 43–4, 75, 98
Perfect Cosmological Principle 58–9
cosmography 93–106
curved space 20–26, 77–8
D
dark matter 11, 71, 81–5
cold dark matter 101–2
de Sitter, W. 8, 46
decelerating universe 54–6, 77
determinism 111–12, 114, 124
deuterium 62–3
deuterium bottleneck 64
Dicke R. 59
dipole anisotropy 98
Dirac, P. A. M. 67
distance indicators 51–3
Doppler effect 41–2, 98
Doppler shift, see Doppler effect
E
Eddington, A. S. 43
Einstein, A. 7, 11, 14–26, 28–38
‘biggest blunder’ 30–1
electromagnetism 12, 15, 67, 70, 113–14
electrons 61, 68–9
electroweak theory 70–1, 113, 122
energy 75–6
entropy 119
Enuma Elish 2
equivalence principle, see principle of equivalence
escape velocity 76
Euclid’s geometry 10, 21–2, 35
expanding Universe 8, 39–53
F
fermions 67–70
Feynman, R. 67
filaments 96
flat Universe 10–11, 33, 78
flatness problem 86–7
fractal 96
Friedmann, A. 8
Friedmann models 32–4, 45–6, 75–8, 86, 99
G
galaxies 7–8, 26, 40, 47, 51–2, 58
galaxy clusters, see clusters of galaxies
Galileo 15
Gamow, G. 62
gauge bosons, see bosons
gedanken experiments 15–19
general relativity, see general theory of relativity
general theory of relativity 7–8, 20–6, 28–9, 74–5
Glashow, S. L. 70
globular clusters 56, 79
gluons 70
Gödel, K. 124
Gold, T. 58
gravitons 115
gravity 12–14, 67, 112
gravitational instability 99
gravitational lensing 84
gravitational waves 115
grand unified theory (GUT) 71
Great Wall 96
Greek cosmology 4–5
GUT, see grand unified theory
Guth, A. 87
H
Ho, see Hubble’s constant
hadrons 68
Hartle, J. M. 120
Hawking, S. 37, 116–18, 120
Heat Death of the Universe 6
Heisenberg, W. 108
see also uncertainty principle
helium 62–4, 125
Herman, R. 62
Hipparchos 51
Higgs 71
homogeneity 29, 45, 58
see also, Cosmological Principle
Hoyle, F. 57–8
Hubble, E. 7–8, 43
Hubble’s constant 48–56, 75, 78
Hubble’s law 39–47
Hubble Deep Field 10
Hubble Space Telescope (HST) 10, 53, 101–2
hydrogen 62, 125
I
incompleteness theorem 124
inflation 11, 30, 73, 87–8
isotropy 29–30, 45, 58, 61
see also Cosmological Principle
K
Kepler, J. 6, 7
Kepler’s star 89
L
Laplace 25
Large Magellanic Cloud 90
Lemaître, G. 8, 32, 43–4
leptons 67–8
Local Group 95
large-scale structure 93–106
leptons 67–9
Lick Map 93, 95
life in the Universe 125–7
lithium 62
M
M theory 123
Mach’s Principle 29
Many Worlds interpretation 112
MAP (Microwave Anisotropy Probe) 10, 84, 104
Marduk 2–3
MAXIMA 105
Maxwell, J. C. 15
Maxwell’s theory 15, 64, 70
Michell, J. 25
Milky Way 7–8, 30, 50, 52, 89
mythology 1–4
N
Narlikar, J. V. N. 58
nebulae, see galaxies
neutrinos 67–8
neutrons 62–4
Newton, I. 6
Newtonian mechanics 13, 27–8, 110
no-boundary hypothesis 120–1
nucleosynthesis 62–4, 80
O
Olbers’ Paradox 7
Omega (Ω) 74–92
open Universe 10, 34, 56, 78
oxygen 125
P
Pan Gu 2–3
parallax 50–1
parallel universes, see Many Worlds interpretation
Peebles, P. J. E. 59
Penrose, R. 37, 116–17
Penzias, A. 8, 59
phase transitions 11
photons 67–70, 108–11
Planck, M. 107
Planck time 115; 117
Planck length 115
Planck Surveyor 10, 84, 104
Plato 4–5
Platonism in cosmology 6, 123
Principia 6
principle of equivalence 17–19
principle of relativity 7–8
protons 62–4
Ptolemy 5
Q
QCD 69–70
QED 67–70, 114
quantum chromodynamics, see QCD
quantum electrodynamics, see QED
quantum physics 107–14
quantum gravity 66, 88, 112–14, 117
quarks 68–70, 88
quasars 66
R
radioactivity 54–5
recombination 61
redshift 42–7, 90, 94–5, 102
relativity, general theory of, see general theory of relativity
relativity, principle of, see principle of relativity
relativity, special theory of, see special theory of relativity
ripples 10, 98–100, 102–4
Ryle, M. 58
S
