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We analyse the functional derivative of the cosmic-shear power spectrum$C_\ell^\gamma$ with respect to the cosmic expansion function. Our interest indoing so is two-fold: (i) In view of attempts to detect minor changes of thecosmic expansion function which may be due to a possibly time-dependentdark-energy density, we wish to know how sensitive the weak-lensing powerspectrum is to changes in the expansion function. (ii) In view of recentempirical determinations of the cosmic expansion function from distancemeasurements, independent of specific cosmological models, we wish to find outhow uncertainties in the expansion function translate to uncertainties in thecosmic-shear power spectrum. We find the following answers: Relative changes ofthe expansion function are amplified by the cosmic-shear power spectrum by afactor $\approx 2-6$, weakly depending on the scale factor where the change isapplied, and the current uncertainty of one example for an empiricallydetermined expansion function translates to a relative uncertainty of thecosmic-shear power spectrum of $\approx10\,\%$.

On the sensitivity of weak gravitational lensing to the cosmic expansion function