We describe the effect of the expansion of space on the wavelength of the light beam in a Fabry-Pérot interferometer. For an instrument such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has high sensitivity and a long period of light storage, the wavelength ${\lambda }_L$ of laser photons are redshifted due to the expansion of space in each cavity by an amount $\delta \lambda$ given by $\delta \lambda /{\lambda }_L=H_0{\tau }_s\approx 8.8\times {10}^{-21}$ , where $H_0\approx 2.2\times {10}^{-18}{\mathrm{s}}^{-1}$ is the Hubble constant and ${\tau }_s\approx 4\mathrm{ms}$ is the light storage time for the cavity. Since ${\tau }_s$ is based on the cavity finesse $\mathcal{F}$ which depends on the laser beam full width at half maximum (FWHM) $\delta \omega$ of each cavity, we show that a difference in finesses between the LIGO arm cavities produces a signal $h_H\left(t\right)$ at the anti-symmetric output port given by $h_H\left(t\right)=2a_1{H}_0\left(\frac{1}{\delta {\omega }_X\left(t\right)}-\frac{1}{\delta {\omega }_Y\left(t\right)}\right),$ where $\delta {\omega }_X\left(t\right)$ and $\delta {\omega }_Y\left(t\right)$ are the beam FWHM at time t, respectively, for the X and Y arm cavities and $a_1$ is a beam proportionality constant to be determined expermentally. Assuming $a_1\approx 1$ , then for cavity beams FWHM of $\delta \omega \left(t\right)\approx (523.2\pm 31)\mathrm{rad}.{\mathrm{s}}^{-1}$ the output signal has the range $\mid h_H\left(t\right)\mid \le 1\times {10}^{-21}$ , which is detectable by advanced LIGO.