Cosmology is in pretty bad shape; we don’t know what makes up 95% of the Universe. Galaxies spin too fast and the expansion of the Universe is unexpectedly accelerating. Cosmologists deal with this by inferring the existence of dark matter and dark energy. These form the backbone of the Lambda Cold Dark Matter (LCDM) model. The problem is that we have no physical explanation for the existence of these two components.
To learn more about the nature of dark matter and dark energy, we take advantage of gravitational lensing, an effect predicted by Einstein’s general theory of relativity. Gravitational lensing simply refers to the bending of light rays around massive bodies. This causes small changes to the observed ellipticity of galaxies, which is referred to as weak gravitational lensing — and on the largest scales this is called cosmic shear. By examining the shape distortions over millions, or even billions of galaxies, we can distinguish between theories of dark energy and dark matter.
All cosmic shear studies to date have assumed a cosmological model and then measured a few free model parameters. This allows us to measure the model parameters which describe the structure of the Universe on the largest scales to very high precision— but we can not test the model itself. In our recent paper (https://arxiv.org/abs/1810.10552) we take a different approach.
We measure the expansion of the Universe and the growth of structure directly using cosmic shear data from the Canada-France Hawaii Telescope Lensing Survey (CFHTLenS)— without assuming a cosmological model. We find evidence that locally the Universe is expanding slightly faster than expected by LCDM. This is shown in the image above where we plot our constraints in red and those predicted by LCDM in black. This could be the first signs of exciting new physics, bridging the gap between local and distant measures of the Universe’s expansion rate (https://www.scientificamerican.com/article/cosmic-conflict-diverging-data-on-universes-expansion-polarizes-scientists1/). It could also be due to residual systematics in the data. To find out, we are currently working on applying our technique to next generation cosmic shear data sets.