Propagating degrees of freedom in f(R) gravity

Understand

We have computed the number of polarization modes of gravitational waves propagating in the Minkowski background in $f(R)$ gravity.

• This is three of two from transverse-traceless tensor modes and one from a massive trace mode, which confirms the results found in the literature.
• There is no massless breathing mode and the massive trace mode corresponds to the Ricci scalar.
• A newly defined metric tensor in $f(R)$ gravity satisfies the transverse-traceless (TT) condition as well as the TT wave equation.