As shown recently, the common KSB method for measuring weak gravitational shear creates a non-linear relation between the measured and the true ellipticity of objects. We investigate here what effect such a non-linear calibration relation may have on cosmological parameter estimates from weak lensing if a simpler, linear calibration relation is assumed. We show that the non-linear relation introduces a bias in the shear-correlation amplitude and thus a bias in the cosmological parameters Ω m0 and σ 8. Its direction and magnitude depends on whether the point-spread function is narrow or wide compared to the galaxy images from which the shear is estimated. Substantial over-or underestimates of the cosmological parameters are equally possible, depending also on the variant of the KSB method. Our results show that for trustable cosmological-parameter estimates from measurements of weak lensing, one must verify that the method employed is free from ellipticity-dependent biases or monitor that the calibration relation inferred from simulations is applicable to the survey at hand.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
- Cosmological parameters
- Cosmology: observations
- Dark matter
- Large-scale structure of Universe