Universe, Evolving

The general theory of relativity by Albert Einstein (1879–1955) forms the basis of any discussion about the fate and the evolution of the universe. The theory's principal feature is a system of differential equations that connects the cosmic matter content with the intrinsic geometry of spacetime: A massive particle curves the space in its vicinity, and the deformed space exerts influence on the motion of another particle. The Einstein system of equations reduces to only two independent equations if applied to the universe as a whole. This is due to the assumption that the universe is homogenous and isotropic on very large scales (e.g., above a 100 million light-years). The remaining two equations describe the course of cosmic expansion via three free parameters (the expansion rate H0, the mean energy density ω, and the cosmological constant ω). The time dependence of the expansion also determines the behavior of the global temperature, which (together with matter and radiation density) again defines the conditions for any physical process in the early universe.

Today it is widely accepted that the universe was brought into existence 13.7 billion years ago in a very hot and dense state. The current temperature of the universe is 2.7 Kelvin, or 2.7° Celsius above absolute zero. Right after the big bang (BB) the temperature is believed to have been some 1032 K. It is assumed that under such conditions the four fundamental forces (gravitation, weak, strong, and electromagnetic interaction) were unified as a single force, described by a hypothetical theory of everything (TOE), which has not yet been put forth. In the course of the (initially very rapid) cooling, the fundamental forces we know today separated from the unified interaction one after another. Coevally the first elementary particles could freeze out of the high energetic matter/radiation mixture: Following quantum field theory, particle/antiparticle (P/AP) pairs can form from radiation and decay instantaneously again, while the Einstein relation E = mc2 is preserved. Here E is the energy of the radiation quantum, that is, the photon; m the P/AP mass; and c the speed of light. Due to a not completely understood asymmetry during the grand unification (of strong, weak, and electromagnetic force) epoch, there was a tiny excess of matter compared to antimatter (1,000,000,001: 1,000,000,000). So, after the decay of all P/AP pairs there was some extremely small amount of matter left over, which provides the substantial content of the universe today.

Quarks and antiquarks were the first particles to freeze out 10“33 seconds after the BB. Their high energies prevented them from forming bound states with each other. They instead constitute a so-called quark-gluon-plasma consisting of free (anti) quarks and gluons, the mediator particles of the strong interaction. Via permanent crashes most quark-antiquark pairs annihilated each other instantaneously, producing radiation until the one-billionth-excess was finally left over. After a millionth second, the temperature had dropped enough to allow for bound states of various quark species (flavors), the best known of which we know as protons and neutrons. After about one second all electron-positron (= anti-electron) pairs also annihilated except for the above-quoted excess, so that the substantial basis of the” normal' matter we know today (in contrast to dark matter) was provided.

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