Step 1: Analyze the function \( x^3 \).
The function \( x^3 \) is an odd function, meaning \( f(-x) = -f(x) \). The Fourier series expansion of an odd function contains only sine terms, as sine functions are odd.
Step 2: Conclusion.
Thus, the Fourier series expansion of \( x^3 \) will have only sine terms.
Final Answer:
\[
\boxed{\text{Only sine terms}}
\]