Astroparticle Physics

1718 Cosmology“As far as the laws of mathematics refer toreality, they are not certain; as far as theyare certain, they do not refer to reality.”Albert EinsteinIn the following chapters the application of our knowledgeof particle physics to the very early universe in the context ofthe Hot Big Bang model of cosmology will be explored. Thebasic picture is that the universe emerged from an extremelyhot, dense phase about 14 billion years ago. The earliest timeabout which one can meaningfully speculate is about 10 −43seconds after the Big Bang (the Planck time). To go earlierrequires a quantum-mechanical theory of gravity, and this isnot yet available.At early times the particle densities and typical energieswere extremely high, and particles of all types were continuallybeing created and destroyed. For the first 10 −38 secondsor so, it appears that all of the particle interactions couldhave been ‘unified’ in a theory containing only a single couplingstrength. It was not until after this, when typical particleenergies dropped below around 10 16 GeV, that the strongand electroweak interactions became distinct. At this time,from perhaps 10 −38 to 10 −36 seconds after the Big Bang,the universe may have undergone a period of inflation, atremendous expansion where the distances between any twoelements of the primordial plasma increased by a factor ofperhaps e 100 . When the temperature of the universe droppedbelow 100 GeV, the electroweak unification broke apart intoseparate electromagnetic and weak interactions.Until around 1 microsecond after the Big Bang, quarksand gluons could exist as essentially free particles. After thispoint, energies dropped below around 1 GeV and the partonsbecame bound into hadrons, namely, protons, neutrons,and their antiparticles. Had the universe contained at thispoint equal amounts of matter and antimatter, almost all ofit would have annihilated, leaving us with photons, neutrinos,and little else. For whatever reason, nature apparentlymade one a bit more abundant than the other, so there wassome matter left over after the annihilation phase to makeHot Big Bangquantum gravityinflationannihilationof matter and antimatterannihilation phase