Astroparticle Physics

9.5 The Baryon Asymmetry of the Universe 205confined into colour-neutral hadrons: protons, neutrons, andtheir antiparticles. This process, called hadronization, occursaround t ≈ 10 −5 s, where to obtain this time from(9.39) one should use a value of g ∗ somewhat smaller thanbefore, since not all of the particles are relativistic. If onetakes photons, gluons, u, d, s, e, µ, and all families of νas relativistic just before hadronization then one would getg ∗ = 61.75.It is interesting to convert the energy density at the QCDscale to normal units, which gives about 1 GeV/fm 3 .Thisis about seven times the density of ordinary nuclear matter.Experiments at the Relativistic Heavy Ion Collider (RHIC)at the Brookhaven National Laboratory near New York arecurrently underway to recreate these conditions by collidingtogether heavy ions at very high energies [12]. This will allowmore detailed studies of the ‘quark–gluon plasma’ andits transition to colour-neutral hadrons.This short sketch of the early universe has ignored atleast two major issues. First, it has not yet been explainedwhy the universe appears to be composed of matter, ratherthan a mixture of matter and antimatter. There is no definitiveanswer to this question but there are plausible scenarioswhereby the so-called baryon asymmetry of the universecould have arisen at very early times, perhaps at the GUTscale or later at the electroweak scale. This question will belooked at in greater detail in the next section.Second, one will see that the model that has been developedthus far fails to explain several observational facts,the most important of which are related to the cosmic microwavebackground radiation. A possible remedy to theseproblems will be to suppose that the energy density at somevery early time was dominated by vacuum energy. In thiscase one will not find R ∼ t 1/2 but rather an exponentialincrease, known as inflation.hadronizationenergy densityquark–gluon plasmabaryon asymmetryvacuum energyinflation9.5 The Baryon Asymmetry of the Universe“Astronomy, the oldest and one of themost juvenile of the sciences, may stillhave some surprises in store. May antimatterbe commended to its case.”Arthur SchusterFor several decades after the discovery of the positron, it appearedthat the laws of nature were completely symmetric