Dark Energy
Dark energy affects the cosmic history of the Hubble expansion H(z) as well as the cosmic history of mass clustering. If combined, different types of probes of the expansion history and structure history can lead to percent level precision in dark energy parameters. This is because each probe depends on the other cosmological parameters or errors in different weak lensing ways. These probes range from cosmic shear, baryon acoustic oscillations, supernovae, and cluster counting -- all as a function of redshift. Using the CMB as normalization, the combination of these probes will yield the needed precision to distinguish between models of dark energy. What is required is a facility which can undertake all of these probes with deep data over wide area with cross checks to control systematic error.
Due to its unique high etendue, the LSST survey will produce all of these complementary probes of dark energy from the same survey data. Weak gravitational lensing is sensitive to angular diameter distance as a function of redshift and to trends in mass clustering with redshift. Several different probes -- each with its own different sensitivity to these effects - may be undertaken with the same deep weak lens data. These same imaging data (with precision photometric redshifts) will track the baryon acoustic oscillations over cosmic time. Counts of giant clusters of dark matter from these data provide another complementary probe. Finally, the tens of thousands of supernovae out to z=1 provide yet another cross check. When combined with the cosmic microwave background anisotropy data these tests form interlocking checks on cosmological models and the physics of dark energy. These tests are described in the above subpages, along with the program for precision photometric redshifts. Below we describe the resulting precision in 2-parameter dark energy model space enabled by the LSST survey.
More information can be found from the Dark Energy Task Force Whitepaper (PDF). Or click here for an html version.
LSST precision on Dark Enery Parameters based on recent results with WMAP3 normalization:
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Error ellipsoid area σ(wa) X σ(wp) for three probes of dark energy (SN, BAO, and Weak lensing) as a function of the time-throughput (etendue years) product A Ω t. The combined result includes an estimate of the improvement with shear 3-point correlations. A fifth probe (galaxy cluster counts) is not shown, but is of comparable power. The precision of the constraints on the physics of dark energy go inversely as this error ellipsoid area, reaching percent level precision in each of the two dark energy parameters after ten years of LSST operations. Major advances are made just in the first two years of the survey. Dark energy precision achievable with other facilities with a different etendue may be plotted on this plot by inserting their A Ω t, assuming equal control of systematic errors. The thickness of the bands for each probe reflect a range (pessimistic to optimistic) of assumed precision in photometric redshift after calibration, depth, source galaxy redshift distribution, weak lens shear systematics, and galaxy bias. These estimates are conservative in the sense that the analysis is confined to the more linear regions of lower peak mass density and larger scales. Once good n-body simulations are available on small scales for various cosmologies, more of the data can be reliably used, leading to higher precision. The assumptions, based on existing tests in small areas of the sky and data to the LSST depth and image quality, can be found here.
Left panel: Forecasts of the errors on the dark energy equation-of-state parameters w0 and wa for BAO (dotted line), WL (solid line), and the two combined (shaded area). The constraints are marginalized over 9 other cosmological parameters including the curvature and over 120 parameters that model the linear galaxy clustering bias, photometric redshift bias, and rms photometric redshift error. We take priors σP(δ z) = σP(σz)/sqrt{2} = 0.05 σz with σz = 0.05(1+z), which correspond to a calibration requirement of 400 spectra fairly sampled at 0.1 redshift intervals if the photo-z errors are Gaussian. The galaxy bias b is assumed to be known to 15%. We have included a redshift-independent additive systematic noise (power) of 10-8 to BAO angular power spectra and shear noise (power) of 10-8 to WL shear power spectra. The dark energy constraints from LSST cluster counting (dN/dz) and cluster power spectrum are also included as the dashed contour (Wang et al 2004).
Right panel: Same as the left panel, but for the matter density &omegam and curvature term ΩK. Note that Planck alone constrains ωm to 1%, but it does poorly on the curvature.