Once Before Time - Martin Bojowald [81]
As one can see from the argument, the existence of the big bang singularity plays an important role, for it is taken as the starting point of the universe. But on account of the breakdown of the theory, the universe cannot be considered to have begun at the singularity. In a reasonable theory, the singularity is instead eliminated by mechanisms such as the repulsive forces of quantum gravity. Then, we automatically solve the horizon problem in a way different from that proposed by Guth: If the universe existed before the big bang, signals had more time, in fact as much as they needed, to connect all places in the early universe; a homogeneous distribution could easily be achieved.
As a motivation for inflation, the classical horizon problem thus disappears. Instead, inflation has a further, much more important advantage not yet seen by Guth’s original considerations. Only the application of detailed calculations to determine how ripples in space-time propagate and evolve—based on methods worked out in 1981, mainly by Slava Mukhanov and G. V. Chibisov, even before the advent of inflation theory—showed that inflation leads to a special kind of very nearly homogeneous matter distribution. This form of the distribution is exactly what is also indicated by current observations. Although the accelerated expansion of the early universe, in contrast to that of the late one with dark energy, cannot be seen directly, there are strong, if indirect, indications for inflation, that is, for negative pressure in the early universe.
The explanatory power of inflationary models is much larger yet. They not only can show why matter is so homogeneous, but they also provide a mechanism for understanding why matter as we know it arose at all from the hot, dense, mangled big bang phase: In a universe expanding with acceleration, matter particles are continuously being created out of the vacuum—from perfect emptiness! As in our earlier description of the quantum mechanical wave function as ocean waves that are piled up strongly by a quaking ground, wave functions of matter are being excited by the “trembling” stage of an accelerated space-time. An initially empty vacuum state of matter is being filled more and more with real matter particles during inflation.
In cosmology, this means that even a state of perfect vacuum at the big bang does not remain empty as inflation goes on, but is increasingly being enriched with matter. For a sufficiently uniform expansion, one with almost constant acceleration, the matter distribution toward the end of inflation automatically takes a nearly homogeneous form, in excellent agreement with measurements of cosmic background radiation.
Such a process of matter creation is possible only if gravity is included in the equation; otherwise it would be in contradiction with energy conservation. The energy carried by newly emerging particles must come from some source; it cannot just arise from nothing. Negative pressure also plays a role here: Normally, when the gravitational force is weak, matter under negative pressure must become denser; it can lower its energy by contracting, and consequently it does so as much as possible. Only if matter is surrounded by sufficiently strong walls can this contraction be prevented. In the universe, matter is distributed homogeneously and uncontained; nothing should prevent it from contracting. But in its idiosyncratic way, gravity does not respect this tendency and instead reacts to negative pressure with accelerated expansion: With positive pressure, we expect matter to drift apart, but since pressure contributes to energy, it causes gravitational attraction. Negative pressure means a change of sign, reversing the effects: We do expect contraction, but instead, enhanced expansion occurs under gravity. Rather than being lowered, matter energy is increased, sponsored by gravity. All this becomes perceptible by the creation of matter excitations, or particles, even if the initial state was vacuous.
Explicitly, particle creation happens as follows (in this