Anisotropic particle creation from $T-$vacuum in the radiation dominated universe

We further investigate novel features of the $T-$vacuum state, originally defined in the context of quantum field theory in a (1+1) dimensional radiation dominated universe [Modak, JHEP 12, 031 (2020)]. Here we extend the previous work to a realistic (3+1) dimensional set up and show that $T-$vacuum gives rise to an anisotropic particle creation phenomena in the radiation dominated early universe. Unlike the Hawking or Unruh effect, where the particle content is thermal and asymptotically defined, here it is non-thermal and instantaneous. This novel example of particle creation is interesting because these particles are detected in the frame of physical/cosmological observers, who envision $T-$vacuum as a particle excited state. Such results comes with a potential to be eventually compared with the observed anisotropies from the early universe and may provide new insights on cosmological particle creation.