Challenging the Big Bang: a longer history of time (page 2)

An infinite past

As one tries to describe the early universe in string theory, one finds, not surprisingly, that conventional General Relativity has to be substantially modified at shorter timescales than t_s. Because of their intrinsic scale, strings do not allow density, temperature and curvature to exceed a (large but finite) maximal value and become infinite. Having thus "disposed" of the troublesome singularity, string theory allows us to look back in time, beyond t = 0. The universe becomes more and more cold, empty, flat and non-interacting as time becomes more and more negative, until it reaches complete emptiness and triviality in its asymptotic past.

The universe would thus obey a principle of Asymptotic Past Triviality, emerging from the simplest possible kind of initial states. This is a kind of Copernican Revolution, in time rather than in space, when the Big Bang loses its historical meaning of initial time to become a more modest, though still important, turning point in the history of the universe. The Big Bang was the moment of maximal (yet finite) density, temperature and curvature. Furthermore, the pre-Big Bang turns out to be automatically inflationary, thus solving the problems of standard cosmology in a natural way.

Although it is very rewarding to explain the origin of the universe without having to invoke a very contrived initial state for it, or a bizarre inflationary phase to correct its "bad start", the new pre-Big Bang theory would be science fiction if it had no experimentally observable consequences.

This is not, in fact, the case. Quantum fluctuations are enormously amplified during the pre-Big Bang inflationary phase. Throughout the standard post-Big Bang epoch, these fluctuations generate irregularities in the universe, thus giving rise to a wealth of physical phenomena that today open a window on the pre-Big Bang universe. Among these phenomena are:

• stochastic gravitational waves, which could be measurable by the new generation of gravitational wave interferometric (LIGO, VIRGO) as well as resonant (antenna-like) detectors;
• cosmic magnetic fields, known to be everywhere in galaxies, but the origin of which is still mysterious;
• new, characteristic sources of large-scale structure in the universe, which will be tested through future satellite measurements of cosmic microwave background anisotropy (MAP, PLANCK) or through precise determinations of galaxy and galaxy-cluster distributions;
• new kinds of weakly interacting relic, which could provide the long-sought missing (dark matter) component in the energy budget of the universe.

At a more conceptual level, these quantum fluctuations are responsible for heating up an initially cold and empty universe, and thus for generating, almost from nothing, the hot matter needed to start the physical and chemical reactions to which we ultimately owe our own existence.

In its simplest form, the pre-Big Bang scenario makes definite predictions that may soon be the cause of its downfall. Even so, it would have shown that the most tenacious theoretical dogmas can, and should, be challenged if we want to bring our understanding of the universe as a whole to a level where we can also claim to understand the behaviour of its smallest constituents.