Non-full equivalence of the four-dimensional Einstein-Gauss-Bonnet gravity and Horndeksi gravity for Bianchi type I metric

The four-dimensional Einstein-Gauss-Bonnet (4DEGB) gravity is proposed as a singular limit of the higher-dimensional EGB gravity with a rescaled coupling constant. Follow-up work indicates that the 4DEGB gravity may be equivalent to a special Horndeski gravity. However, the full equivalence of the two theories has not been proven. In this paper, we test the possible equivalence for Bianchi type I metric. We consider two symmetry choices of the extra dimensions in the 4DEGB gravity and obtain the explicit cosmological evolution equations of the theories. Our result shows that, for one symmetry choice, there is a correspondence between the 4DEGB gravity and the special Horndeski gravity. However, for the other symmetry choice, there is no correspondence between the two theories. Therefore, the 4DEGB gravity is not equivalent to the special Horndeski gravity for the general case.