114 Thermal History of the UniverseParticle Physics Symmetries. Above퐸≈ 1015 GeV the Universe was filled with
aplasmaofaGrand Unified fieldof extreme heat and pressure, and occupying an
exceedingly small volume. Neither elementary particles nor their more elementary
constituents were stable under such conditions, only quantum packets of radiative
energy incessantly exchanging energy existed. Only when the Universe had cooled to
below 1TeV the leptons characterized by their electroweak interactions and the quarks
characterized by their color interactions appeared as constituents in the plasma.
In the energy range 10^12 GeV≲퐸≲ 1016 GeV the strengths of the electro-weak force
and the quark color force experienced by photons, leptons and quarks, might have
been unified in a unique symmetry group퐺GUT, with one coupling constant푔.Allthe
leptons and quarks should be components of that same field.
If there is such a symmetry group its mathematical form is not known. Although all
GUTs are designed to answer some of the questions and relate some of the parameters
in the standard model, they still have the drawback of introducing large numbers of
new particles, vector bosons and Higgs scalars, all of which have to be discovered.
The global symmetry group in particle physics may have been of the form
퐺s≡SU( 3 )c⊗SU( 2 )w⊗푈( 1 )퐵−퐿. (6.7)This is referred to as thestandard model, where the subscripts푐stands forcolor,푤for
weak interactionsand퐵−퐿forBaryonandLeptonnumber, respectively. It is consid-
ered to be an exact symmetry in the 1 TeV≲퐸≲ 1011 −^12 TeV energy range. It is lab-
oratory physics that has led us to construct the standard model, which is fairly well
understood although experimental information above 1TeV is lacking. Moreover, the
standard model leaves a number of questions open which one would very much like
to have answered within a GUT model.
There are too many free parameters in the standard model. Why are the electric
charges such as they are? How many Higgs scalars are there, more than the one dis-
covered in 2012? Why do the leptons and the quarks come in three families. The GUT
symmetry group would require only one family, but if nature provides us with more,
why are there precisely three? What is the reason for CP violation? The only hint is
that CP violation seems to require (but not explain) at least three families of quarks.
The big question is what new physics may appear in the enormous range between
1 TeV and the GUT energy 10^14 GeV. The possibility that nothing new appears is called
‘the desert’.
The new physics could be a higher symmetry which would be broken at the lower
end of this energy range. Somewhere there would then be a phase transition between
the exactly symmetric phase and the spontaneously broken phase. Even in the case of
a ‘desert’, one expects a phase transition at 10^14 or 10^15 GeV. One effect which finds its
explanation in processes at about 10^14 GeV is the remarkable absence of antimatter
in the Universe.
If the GUT symmetry breaks down to the standard model through intermediate
steps, the phenomenology could be very rich. For instance, there aresubconstituent
modelsbuilding leptons and quarks out of elementary particles of one level deeper
elementarity. These subconstituents would freeze out at some intermediate energy,