To simplify the expression \( t^{400} t^{100} \) using the properties of exponents, we can add the exponents, since the bases are the same.
According to the property of exponents:
\[ a^m \cdot a^n = a^{m+n} \]
Here, we have:
\[ t^{400} \cdot t^{100} = t^{400 + 100} = t^{500} \]
So, the simplified expression is:
\[ t^{500} \]