physical constants. In a small number, the values will allow the
existence of objects like carbon atoms, which can act as the
building blocks of living systems. Since we must live in one of
these universes, we should not be surprised that the physical
constants are finely tuned. If they weren’t, we wouldn’t be
here. The strong form of the Anthropic Principle is not very satisfactory. What operational meaning can one give to the existence of all those other
universes? And if they are separate from our own universe, how
can what happens in them, affect our universe. Instead, I shall
adopt what is known as the Weak Anthropic Principle. That is, I
shall take the values of the physical constants, as given. But I
shall see what conclusions can be drawn, from the fact that life
exists on this planet, at this stage in the history of the universe.
There was no carbon, when the universe began in the Big Bang, about 15 billion years ago. It was so hot, that all the matter would have been in the form of particles, called protons and neutrons. There would initially have been equal numbers of protons and neutrons. However, as the universe expanded, it would have cooled. About a minute
after the Big Bang, the temperature would have fallen to
about a billion degrees, about a hundred times the
temperature in the Sun. At this temperature, the neutrons will
start to decay into more protons. If this had been all that
happened, all the matter in the universe would have ended
up as the simplest element, hydrogen, whose nucleus
consists of a single proton. However, some of the neutrons
collided with protons, and stuck together to form the next
simplest element, helium, whose nucleus consists of two
protons and two neutrons. But no heavier elements, like carbon or oxygen, would have been formed in the early universe. It is difficult to imagine that one could build a living system, out of just hydrogen and helium, and anyway the early universe was still far too hot for atoms to combine into molecules.
The universe would have continued to expand, and cool. But some regions would have had slightly higher densities than others. The gravitational attraction of the extra matter in those regions, would slow down their expansion, and eventually stop it. Instead, they would collapse to form galaxies and stars, starting from about two billion years after the Big Bang. Some of the early stars would have been more massive than our Sun. They would have been hotter than the Sun, and would have burnt the original hydrogen and helium, into heavier elements, such as carbon, oxygen, and iron. This could have taken only a few hundred million years.
After that, some of the stars would have exploded as supernovas, and scattered the heavy elements back into space, to form the raw material for later generations of stars.
Other stars are too far away, for us to be able to see directly, if they have planets going round them. But certain stars, called pulsars, give off regular pulses of radio waves. We observe a slight variation in the rate of some pulsars, and this is interpreted as indicating that they are being disturbed, by having Earth sized planets going round them. Planets going round pulsars are unlikely to have life, because any living beings would have been killed, in the supernova explosion that led to the star becoming a pulsar. But, the fact that several pulsars are observed to have planets suggests that a reasonable fraction of the hundred billion stars in our galaxy may also have planets. The necessary planetary conditions for our form of life may therefore have existed from about four billion years after the Big Bang.
Our solar system was formed about four and a half billion years ago, or about ten billion years after the Big Bang, from gas contaminated with the remains of earlier stars. The Earth was formed largely out of the heavier elements, including carbon and oxygen. Somehow, some of these atoms
came to be arranged in the form of molecules of DNA. This has the famous double helix form, discovered by Crick and Watson, in a hut on the New