  A tachyon field having a negative squared-mass $-m_t^2$ can be described in
terms of massless fields degenerating infinitely with respect to helicities.
This picture for tachyons does not contradict causality. It is seen that the
tachyon vector field can be quenched from the interactions with matter fields,
and the effects can be represented by a phase factor. The accelerated expansion
of the universe and the dark energies are interpreted in terms of the phase
factor.
