The Rubin Observatory Legacy Survey of Space and Time (LSST)

Vera C. Rubin Observatory will perform the Rubin Observatory Legacy Survey of Space and Time (LSST) using the Simonyi Survey Telescope, which is designed to image a substantial fraction of the sky in six optical bands every few nights. LSST is planned to run for a decade, allowing the stacked images to detect galaxies to redshifts well beyond unity. Rubin Observatory, the Simonyi Survey Telescope, the Rubin Observatory LSST Camera, and survey itself (LSST) are designed to meet the requirements of a broad range of science goals in astronomy, astrophysics and cosmology, including the study of dark energy. LSST was the top-ranked large ground-based initiative in the 2010 National Academy of Sciences decadal survey in astronomy and astrophysics. The dark energy science goals of LSST are:

The observatory will be located on Cerro Pachon in northern Chile (near the Gemini South and SOAR telescopes), with first light expected around 2021 – see the baseline schedule here. The survey will yield contiguous overlapping imaging of over half the sky in six optical bands (ugrizy, covering the wavelength range 320-1050 nm). The Rubin Observatory LSST Camera provides a 3.2 Gigapixel focal plane array, tiled by 189 4kx4k CCD science sensors with 10µm pixels. This pixel count is a direct consequence of sampling the 9.6 deg2 field-of-view (0.64m diameter) with 0.2x0.2 arcsec2 pixels (Nyquist sampling in the best expected seeing of ∼0.4 arcsec). The observing strategy for the main survey will be optimized for homogeneity of depth and number of visits. The current baseline design will allow about 20,000 deg2 of sky to be covered using pairs of 15-second exposures in two photometric bands every three nights on average, with typical 5σ depth for point sources of r ~24.5. The system will yield high image quality as well as superb astrometric and photometric accuracy for a ground- based survey. The coadded data within the main survey footprint will have a depth of r ~27.5. 10% of observing time will be used to obtain improved coverage of parameter space, such as very deep (r ∼ 26) observations taken over the course of an hour, optimized for detection of faint SNe.

The dark energy constraining power of LSST could be several orders of magnitude greater than that of a Stage III survey. LSST will go much further than any of its predecessors in its ability to measure growth of structure, and will provide a stringent test of theories of modified-gravity. While these projections for LSST statistical significance are compelling, they probably do not capture the true nature of the revolution that LSST will enable. The sheer statistical power of the LSST dataset will allow for an all-out attack on systematics, using a combination of null tests and hundreds of nuisance parameters and by combining probes. The cosmic frontier has progressed in step with the size of our astronomical surveys, and in this respect, LSST promises to be a major advance: its survey coverage will be approximately ten times greater than that of the Stage III Dark Energy Survey.

Beyond tests of systematics, there is a growing sense in the community that the old, neatly separated categories of dark energy probes will not be appropriate for next generation surveys. For example, instead of obtaining constraints on dark energy from cluster counts and cosmic shear separately, LSST scientists may use clusters and galaxy-galaxy lensing simultaneously to mitigate the twin systematics of photometric redshift error and mass calibration. A homogeneous and carefully calibrated dataset such as LSST’s will be essential for such joint analyses.

More information on LSST can be found on the LSST website, in the following LSST overview papers and the LSST and the LSST DESC Science Requirements Documents: