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By Root 658 0

context, matter must be described by quantum theory, since only this allows a full understanding of its vacuum state). Like any state, the matter vacuum in an expanding universe has fluctuations: the number of all particles is zero on average—we are, after all, in a vacuum state—but not precisely so. This fluctuating behavior is analogous to the fact that the position of an electron’s wave function is precisely determined only on average, while concretely it fluctuates within the spread of the wave function; the electron can be encountered at different places. Similarly, the matter wave function, in which the number of individual particles plays a role comparable to that of position in quantum mechanics, is spread out and does not have a precise particle number. If the universe did not expand, there would be no further consequences; the particle number would be imprecise but zero on average and remain so forever. But the expansion of the universe implies that, as in the case of effective forces in quantum mechanics, fluctuations influence the particle number as determined by the wave function of matter. In the course of time, new excitations arise, making the particle number nonzero even on average.

The concept of particle pairs is useful here. As is the particle number in a vacuum, energy is also imprecise, allowing the creation of pairs of matter and antimatter particles—such as an electron and a positron—for brief periods of time. Under normal circumstances, the partners in such a pair would immediately annihilate each other into radiation, but this can be prevented if the two particles are rapidly separated far enough. Handling individual particles directly and keeping them apart is certainly impossible, but when the particles are electrically charged, with the antiparticle always having the opposite charge from the particle, it could be achieved by subjecting the pair to a strong electric field. While such an experiment could in principle be performed in the laboratory, it would not explain the creation of particles in the early universe. At those times, no strong electric fields that could have achieved large-scale matter separations were active; otherwise the universe, and the distribution of its galaxies or background radiation, would be much more anisotropic as a result of the direction of the electric field.

Alternatively, one can separate the particles by, as it were, pulling apart the ground beneath them. This is exactly what happens in a universe expanding with acceleration: Space-time itself so rapidly pushes outward that particles and antiparticles emerging from the vacuum no longer annihilate each other. They simply have no chance of getting close, against the burst of space-time. Instead they remain as long-lived particles created from the vacuum. According to these inflationary ideas, the universe brought about matter, and consequently all structures now visible, in its initial nearly homogeneous distribution. In this picture, the entire matter content of the universe is a random event of quantum fluctuations, enhanced by gravity and frozen into real existing matter.7 All that is necessary is a phase of accelerated expansion, or a matter form with negative pressure.

Here theory has to fight hard to provide a concrete form of such suppressed matter. Very special arrangements must again be made to achieve negative pressure for sufficiently long times. In the simplest and most elegant models of inflation, the negative pressure has to reign so long that the universe expands by a factor with twenty digits; otherwise no sufficiently homogeneous matter distribution in agreement with the cosmic background observations would result.

One can mathematically tailor such a matter form, but it requires very special interactions as well as initial values for that matter forced to be under negative pressure. This is similar to the production of negative pressure on Earth, which can be achieved, for instance, by supercooling an amount of matter. As matter usually expands when it gets hotter, it contracts when cooling