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As expected in a bosonic string theory, the ground eigenstate of the theory is tachyonic, with mass squared \[ m^2 = - 2 / L^2 \ . \] The ground state has the energy of the string scale \(L\), which is thought to be that of grand unification. The tachyonic ground state is generally considered reason to discard bosonic string theory as unphysical.
However, the Spin(11,1) grand Higgs field \(\langle \boldsymbol{T} \rangle\) matches the properties of the tachyonic ground state: it is a spacetime scalar, it has zero standard-model charge, and it is unstable by the Higgs mechanism. \(\langle \boldsymbol{T} \rangle\) does have \(\color{gold}{t}\) charge, which allows it to generate a Majorana mass for the right-handed neutrino, but it has zero \(\color{silver}{y}\color{bronze}{z}\color{red}{r}\color{dgreen}{g}\color{blue}{b}\) standard-model charge.
In the standard model of cosmology, vacuum energy has decayed through successive Higgs-mediated symmetry breakings to the present time, where the vacuum, the dark energy, has a tiny but nonzero positive density. The dark energy is apparently stable over cosmological timescales, but it is not known whether it is stable in an absolute sense.
13/15. The weakness of string theory revisited