- We show that the axion gauge field coupling leads to tachyonic mass generation for the gauge fields when the axion field has a homogeneous and isotropic time dependent (i.e. cosmological) expectation value. In the context of the standard SU (3) × SU (2) × U (1) gauge group, it is possible to eliminate this instability in a thermal equilibrium environment, provided a finite chemical potential is introduced for B + L . This finite density environment that stabilizes the system, can be generated through B + L violating processes that take place with a considerable rate in the standard model at TeV temperatures. In this way, through a mechanism where CPT is spontaneously violated, baryogenesis can take place in a thermal equilibrium environment at TeV temperatures.