How to compute the first eigenvalue of $M = R{ \times _{\cosh t}}{S^2}\left( {\frac{1}{2}} \right)$

$$M = R \times N$$with the warped product metric$$d{s^2} = d{t^2} + {\cosh ^2}\left( t \right)ds_N^2$$$$N = {S^{2}}\left( {\frac{1}{2}} \right)$$2-dim sphere with radius 1/2

-