LSST will answer key questions about the nature of dark energy
The LSST telescope will be able to measure tiny perturbations on galaxy shapes caused by gravitational lensing. These arise from the deviation of light from its path due to the gravity of matter in the Universe. Lensing is expected to serve as a powerful probe of the nature of the mysterious dark energy that is sourcing the accelerated expansion of the Universe. On the other hand, it is thought that tides from the distribution of matter in the Universe play a role in determining galaxy shapes and orientations, an effect known as 'intrinsic alignments’. These alignments can act as contaminants to gravitational lensing. In preparation for LSST, we are using supercomputer simulations of the Universe to measure alignments of different types of galaxies and to build better models to describe them.
Figure 1: This figure represents a slice through the Horizon-AGN simulation box, a simulation of the Universe of 100 co-moving Mpc/h on each side, at redshift z=0.5. The slice shown here covers the full width and height of the box, and it is projected over 20% of its depth. The different colours indicate the gas density (in green), the gas metallicity (in blue) and the stars in the simulation (in red). Galaxies are identified from agglomerations of stars and their projected shapes are measured. We use these shapes to investigate intrinsic ellipticity correlations that can contaminate weak gravitational lensing. These intrinsic correlations are thought to arise due to the interaction of the galaxies with the tidal field of the large-scale structure. Learning about these "intrinsic alignments" from the simulations allows us to make predictions of how to account for them in cosmological analysis of future LSST measurements.
Figure 2: This is a zoom in of the bottom left of the previous figure over a region of 1/10 of the size of the Horizon-AGN simulation box. The projection depth is 1/5 of the simulation box. Stars are indicated in red; gas metallicity, in blue; and gas density, in green. Pristine gas is flowing along filaments of the cosmic web into its knots, where the feedback from supernovae and active galactic nuclei in galaxies eject metals into the intergalactic medium. The shapes of galaxies are correlated with each other, giving rise to potential "intrinsic alignment" contamination to weak gravitational lensing that is taken into account when extracting constraints on dark energy from the observations.
Credit: Julien Devriendt, Elisa Chisari, Adrianne Slyz, Lance Miller (University of Oxford) + Hz-AGN team