The Power of Cosmic Flexion in Testing Modified Matter and Gravity

Flexion is the weak lensing effect responsible for the weakly skewed and arc-like appearance of lensed galaxies. The flexion signal-to-noise ratio can be an order of magnitude larger than that of shear. For the first time, we show how this makes flexion an invaluable tool for discriminating among alternative cosmological models. We analyse a scalar field model of unified dark matter and dark energy, a brane-world cosmology and two f(R) modified-action theories. We show that these models can be distinguished from LCDM at several standard deviations by measuring the power spectrum of cosmic flexion.