7. Electroweak restoration, QCD plasma, and grand unification

Energy is not temperature #

Temperature describes an equilibrated statistical state; a collision energy describes an individual event. A high-energy collision does not automatically create an equilibrated hot universe. When T is quoted in MeV, the shorthand means kBT in energy units. QCD deconfinement and electroweak restoration involve very different scales; neither establishes grand unification.

7.1 High temperature can restore symmetries #

At finite temperature the effective potential of a quantum field changes. In the early universe, thermal corrections can restore symmetries that are broken in the present vacuum. The electroweak crossover occurs around the electroweak scale, roughly $T\sim100\,\mathrm{GeV}$ in energy units. However, a collider hard scattering with center-of-mass energy above $100\,\mathrm{GeV}$ is not automatically the same as creating a macroscopically thermalized electroweak-symmetric universe. Energy scale and thermodynamic temperature are related but not interchangeable concepts.

7.2 QCD deconfinement is experimentally accessible #

The statement that deconfinement requires $10^{15}\,\mathrm{GeV}$ is wrong by many orders of magnitude. Lattice QCD at small baryon chemical potential finds a smooth crossover around

for a commonly used chiral pseudocritical definition. Chiral crossover in QCD at zero and non-zero chemical potentials (2019)

Relativistic heavy-ion experiments at RHIC and the LHC routinely create quark-gluon plasma (QGP), a hot QCD medium in which quarks and gluons are not confined into individual hadrons on the same scales as in cold matter. This does not mean that the strong interaction has become an electromagnetic-like long-range force; screening, collective effects, and non-Abelian interactions remain essential.

7.3 Grand unification is a different idea #

Grand unified theories attempt to embed

into a larger gauge group such as $SU(5)$ or $SO(10)$. The simplest models predict new gauge bosons and often proton decay. The characteristic unification scale is model-dependent and typically discussed around $10^{15}$–$10^{16}\,\mathrm{GeV}$, far above direct accelerator energies. Standard Model gauge couplings evolve with energy but do not meet exactly at one point without additional model assumptions. Grand Unified Theories (2026)

Grand unification therefore has nothing to do with physically “knocking gluons out of protons until the strong force becomes long ranged.” QCD deconfinement, asymptotic freedom, and GUT unification are three separate concepts.

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