Chae-min Yun a, Jubin Park a,∗, Myung-Ki Cheoun a, Dukjae Jang b
a. Department of Physics and Origin of Matter and Evolution of Galaxies (OMEG) Institute, Soongsil University, Seoul 06978, Republic of Korea
b. Center for Relativistic Laser Science, Institute for Basic Science, Gwangju 61005, Republic of Korea
a b s t r a c t
We report that the standard evolution of radiation-dominated era (RDE) universe a ∝ t1/2 is a sufficient condition for solving a sixth order gravitational field equation derived from the Lagrangian containing
B Rab Rab + CRR;c c as well as a polynomial f (R) for a spatially flat radiation FLRW universe. By virtue of the similarity between Rab Rab and R2 models up to the background order and of the vanishing property
of R;c c for H = 1/(2t), the analytical solution can be obtained from a special case to general one. This proves that the standard cosmic evolution is valid even within modified gravitational theory involving higher-order terms. An application of this background solution to the tensor-type perturbation reduces the complicated equation to the standard second order equation of gravitational wave. We discuss the possible ways to discriminate the modified gravity model on the observations such as the gravitational wave from the disturbed universe and primordial abundances.
https://doi.org/10.1016/j.physletb.2022.137652