Just Six Numbers by Martin Rees

Author:Martin Rees [Rees, Martin]
Language: eng
Format: epub
ISBN: 9780786723584
Publisher: Basic Books

THE TUNING OF THE INITIAL EXPANSION

Ω may not be exactly one, but it is now at least 0.3. At first sight, this may not seem to indicate fine tuning. However, it implies that Ω was very close indeed to unity in early eras. This is because, unless expansion energy and gravitational energy are in exact balance (in which case Ω is, and remains, exactly equal to unity), the gap between those two energies widens: if Ω were to start off slightly less than unity in the early universe, eventually the kinetic energy would completely dominate (so that Ω becomes very small indeed); on the other hand, if Ω were substantially more than unity, then gravity would soon get the upper hand and bring the expansion to a halt.

The range of ‘trajectories’ for our actual universe, consistent with what the dark-matter evidence tells us about the present value of Ω, is shown in Figure 6.1. The figure also depicts some universes in which life as we know it couldn’t have emerged. It highlights a basic mystery: Why is our universe still, after ten billion years, expanding with a value of Ω not too different from unity?

There are, as we’ve seen in the last chapter, good grounds for extrapolating back to when the universe was one second old and at a temperature of ten billion degrees. Suppose that you were ‘setting up’ a universe then. The trajectory it would follow would depend on the impetus it was given. If it were started too fast, then the expansion energy would, early on, have become so dominant (in other words, Ω would have become so small) that galaxies and stars would never have been able to pull themselves together via gravity and condense out; the universe would expand for ever, but there would be no chance of life. On the other hand, the expansion must not have been too slow: otherwise the universe would have recollapsed too quickly to a Big Crunch.

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