Section 12.5 Observational Tests
Three pieces of observational evidence can serve as a testing ground for the theories of what went on in the early universe:
the discovery of the \(3\Xunits{K}\) microwave background radiation,
the measurement of the ratio of photons to baryons,
the observation of the relative abundances of hydrogen, helium, deuterium, and lithium in the interstellar medium.
Let's look at each in turn.
Penzias and Wilson's discovery of the \(3\Xunits{K}\) microwave radiation in 1965 was a stunning confirmation of the Big Bang model. This radiation seems to be coming from everywhere in the universe. It has a blackbody spectrum corresponding to a body at \(3\Xunits{K}\text{.}\) A simple calculation shows that these photons are the remnants of the Big Bang, the photons that decoupled at \(\sim 10^5\) years and have been cooling ever since!
The present observed ratio of photons to baryons seems to be about \(10^9\text{.}\) Recall our previous claim that these photons are those produced during the annihilation of almost equal numbers of hadronic particles and antiparticles. If Grand Unified Theories (GUTs) are correct in their description of the \(X\)-boson, then the slightly different decay modes for \(\overline X\) and \(X\) should lead to a quark excess (over antiquarks) of about 1 in \(10^9\text{.}\) Various versions of GUTs are now being developed, and each version will presumably lead to an estimate of the \(X\)-\(\overline X\) decay asymmetry. See Problem Exercise 12.6.9. The 1 in \(10^9\) figure thus provides an “experimental” test of various possible grand unified theories. It is known so far that the simplest conceivable scheme for grand unification (so-called minimal coupling) does not get the \(X\) and \(\overline X\) decay modes right, and thus is effectively ruled out as a viable theory on observational grounds.
Finally let's consider the question of relative abundances of the light elements. The more neutrons there were at the end of the time of nuclear synthesis, the higher the ratio of helium to hydrogen would be. (Ignore for the moment all the other elements; these constitute less than 1% of the mass of the universe.) For example, if there were seven times as many protons as neutrons, then in a group of sixteen baryons, fourteen would be protons and two would be neutrons. After combination (see sketch below), there is one helium nucleus (mass 4) and twelve hydrogen nuclei (total mass 12). The universe would then be 25% helium, by weight. See Figure 12.2.
One of the things the exact proton to neutron ratio depends on is the number of different types of neutrinos that exist. The more types there are, the longer the neutrinos could initiate proton-to-neutron conversions (inverse beta decay) in the period just before nuclear synthesis, and the more neutrons there would be. This in turn would lead to a higher helium abundance in the present universe. Using standard nuclear physics, the helium abundances for two, three, or four neutrino types can be calculated and compared to the observational abundance data. It turns out that three neutrino types is the best fit. This means that cosmology can tell us what no theoretical arguments have yet been able to show, that there are just three neutrino types and thus just three generations of quarks and leptons! The predictions have been confirmed recently by measuring the uncertainty in the mass of the \(Z^0\) boson.