Once Before Time - Martin Bojowald [84]
Possible tests of the theory, in this case loop quantum gravity, then arise. The effects of quantum gravity are usually small, even near the big bang, but in some cases they can add up during long cosmological stretches of time. If this does happen, observations may indeed come within reach. Should that be realized, it would constitute a major milestone in modern theoretical research, for so far no experimental indications exist for the exact form of quantum gravity. Just as experimental data—for instance, atomic spectra—were always important guidelines during the development of quantum mechanics, observations will play an equally essential role in completing the currently discussed models of quantum gravity to a successful version.
In loop quantum gravity, observations, even if only indirect ones, would have another exhilarating advantage. They would give insights into the repulsive force that at the big bang gave rise not only to accelerated expansion but also to the turning point of the previously contracting universe to our expanding one. From precise observations of the repulsive force and its action during the big bang phase, we could draw conclusions about the ancestor universe itself. We would be granted a first, though indirect, glance at the universe before the big bang!
It is not only that the data of the cosmic microwave radiation have to be much improved, for which the Planck satellite would already be a large step forward, but also other independent sources of signals will have to be explored. Electromagnetic radiation simply cannot reach our measurement devices from sufficiently deep stages of the big bang phase; the more penetrating messengers of neutrinos or gravitational waves will be required.
In addition, these messages scatter, limiting how far back in time we will be able to see. With neutrinos, one can view a moment in time about a second after the big bang, much earlier in the universe than the slightly less than half a million years after the big bang that is granted by microwave radiation. But even this blink of a second does not penetrate deeply enough: A second still contains an unimaginably large number (extending to 42 digits) of discrete time steps of quantum gravity. Gravitational waves could come from even earlier times, but here quantum gravity still has to show clearly just how far back we can see. Even with gravitational waves, no arbitrarily distant hindsight will be possible. The principle of cosmic forgetfulness also limits a direct view of the preceding universe.
How, then, would it be possible to gain indirect insights into properties of the universe before the big bang? Here, the earlier comparison of the singularity with the splitting off of water droplets is useful again. The splitting off is similar to the big bang singularity and its prevention in loop quantum cosmology, with the continuous water drop playing the role of the smooth geometric rubber band of curved space-time in general relativity. The splitting off, in this view, presents a singularity, easily explained and resolved in an atomic theory of water—just as the discrete time of loop quantum cosmology opens up access to the universe before the big bang. (In the water drop, there is, however, no repulsive force that would prevent the singular split.) What is torn apart is the rubber band of the classical picture, not the real atomic world.
Just as it is difficult, if not impossible, for us to see directly back before the big bang,