Click here to view the January 2006 AAS poster "LSST Supernova Cosmology". [PDF]
Because of its rapid cadence, large aperture, and flexible scheduling, LSST will be an excellent tool for studying supernovae and using them as cosmological probes. It will enable myriad new SN-based cosmological experiments that are impossible to perform with existing systems.
LSST will find supernovae in two ways. The first is as a result of its normal operating mode providing frequent, all-sky coverage. The baseline observation strategy for the LSST survey will discover roughly 250,000 Type Ia SNe per year. This SNe Ia sample will have a mean redshift of about 0.45 and extend to 0.7. The lightcurves will typically have a sampling of approximately an observation every 5 days in the main search filter (nominally r) and 2 observations each month in other filters (e.g., g, b, and i). This sample, combined with priors from other experiments such as Planck, will be able to constrain w in the nearby universe to better than 1%, assuming we can limit both the systematics and uncertainties due to lack of spectral followup at some reasonable level (an area of intensive current research). One important component of the analysis, the compensation for having observed different SN spectral regions at different redshifts (so-called "K-corrections"), can be refined by "bootstrapping" using photometric redshifts of the host galaxies derived from the underlying LSST survey dataset. Such an enormous sample of SNe, and the attendant color redshifts and morphology of the host galaxies, will provide an unprecedented opportunity to search for hints to the nature of the type Ia progenitors and to search for "third parameters," especially by correlating with environmental properties. The all-sky nature of this sample will allow extending supernova cosmology to look for angular signals in dark matter and dark energy, providing complementarity to similar searches using weak lensing. Indeed, the number of supernovae surrounding individual clusters is sufficient that they will provide their own weak lensing signal - magnification at many points in the sky surrounding mass concentrations. The sample of SNe built up in standard LSST operations will also allow us to measure w as a function of angle across the sky. This will constrain the spatial uniformity of any rolling scalar fields.
About one in a thousand high redshift supernovae will strongly lensed by an intervening mass. For type Ia events alone this will mean well over 100 supernovae with multiple images and accurately measured time delays. These lensing events have a big advantage over strong lensing of QSOs since the intrinsic brightness of the source is well known, thus allowing much stronger constraints on the lensing geometry. Well-modeled strong lenses provide important constraints on the distribution of dark matter in the lensing galaxy. Furthermore, these systems tightly constrain the geometry of the lensing system, and are therefore sensitive to the underlying cosmic geometry.
The second supernova sample will come from a "staring mode" search of a more limited area of sky. After ten years, ten minutes per night spent staring at a single field will yield (with no rate evolution) 14,000 supernovae with lightcurves of unprecedented detail, typically with more than 100 photometric points per supernova in five bands. This sample will have a mean redshift of 0.75 and extend to beyond z=1.4. Such detailed lightcurves will allow fitting for photometric redshifts from the supernovae themselves; simulations show that such SN photo-zs have a typical error of 0.01 in z. These can in turn be combined with host-galaxy photo-zs, and since the area on the sky is limited, it is reasonable to assume that multi-object spectroscopy can be obtained for many of these objects. From SN photo-zs alone, the first two years of such a survey will make an independent measurement of ΩM to 9% and ΩΛ to 6%. These lightcurves will be followed for many years as the supernova ejecta expand to reveal the inner workings of the explosion, leading to new understanding of the SN Ia mechanism through the direct determination of nucleosynthetic yields and signals from the interaction with their circumstellar environments.
Figure 1 w0 & wa error forecasts. One sigma error contours in the w0-wa plane for LSST shear survey, 2000 SNe, and the combination (as labeled) where w(z) = w0+wa(a-I). The dashed curve is for LSST with the source density uniformly decreased by a factor of 2. With the 50,000 SNe which will be followed by LSST in the first few years, the size of the joint error is competitive with the 2+3 point shear tomography limits shown in Figure 2, but with uncorrelated systematics.
Very late-time lightcurves can also provide additional constraints on host-galaxy reddening using the color constancy of their pure-Fe recombination spectra. The thousands of type II supernovae in this sample can be followed with multi-object spectroscopy to provide yet another independent set of distance measurements, to lower redshifts, using the expanding photosphere method.
Figure 2: 2+3 point shear tomography limits on the dark energy parameters for the LSST survey. Combining this with the LSST supernovae will result in percent level constraints on the dark energy equation of state.
