Astroparticle Physics

1919 The Early Universe“Who cares about half a second after theBig Bang; what about the half second before?”Fay Weldon quoted by Paul DaviesIn this chapter the history of the universe through the firstten microseconds of its existence will be described. First,in Sect. 9.1 the Planck scale, where quantum-mechanicaland gravitational effects both become important, will be defined.This sets the starting point for the theory to be described.In Sect. 9.2 some formulae from statistical and thermalphysics will be assembled which are needed to describethe hot dense phase out of which the universe then evolved.Then, in Sects. 9.3 and 9.4 these formulae will be used tosolve the Friedmann equation and to investigate the propertiesof the universe at very early times. Finally, in Sect. 9.5one of the outstanding puzzles of the Hot Big Bang modelwill be presented, namely, why the universe appears to consistalmost entirely of matter rather than a mixture of matterand antimatter.Planck scalematter-dominated universe9.1 The Planck Scale“A new scientific truth does not triumphby convincing its opponents and makingthem see the light, but rather becauseits opponents eventually die, anda new generation grows up that is familiarwith it.”Max PlanckThe earliest time about which one can meaningfully speculatewith our current theories is called the Planck era, around10 −43 seconds after the Big Bang. Before then, the quantummechanicalaspects of gravity are expected to be important,so one would need a quantum theory of gravity to describethis period. Although superstrings could perhaps providesuch a theory, it is not yet in such a shape that one can use itto make specific predictions.Planck timequantum theory of gravity