Evolution of Matter in the Universe
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Matter and energy content of the Universe control its geometry and expansion.In the early Universe, the density was dominated by relativistic particles. At acosmic time of a second these were photons, neutrinos and electron pairs. Theirinfluence on the expansion has become negligible in the present epoch, and nowbaryons, non-baryonic “dark matter” and “dark energy” dominate the large-scaledynamics and geometry of the Universe (Fig. 1). Baryons are the well-knownconstituents of ordinary matter. For the existence of the other two components,we have only indirect, but increasingly compelling evidence.Although the influence of baryons on the overall dynamics and geometry of thepresent Universe is relatively minor, their physical properties are unique.Among the major forms of matter and energy that populate the present Universe,only baryonic matter participates in all the physical forces known to us: thestrong forces (transmitted by gluons), the electromagnetic forces (transmitted byphotons), the weak forces (transmitted by the W
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