page_title: Dark Energy Survey
section_title: BAO observations in DES
hierarchical_section_title: Dark Energy Survey / Baryon Acoustic Oscillations (BAO) / BAO observations in DES
caption_attribution_description: English: Source: DES proposal to NSF and DOE in December 2006 Description: Forecast for the precision goal of the Dark Energy Survey
context_page_description: The Dark Energy Survey is a visible and near-infrared survey that aims to probe the dynamics of the expansion of the Universe and the growth of large-scale structure. The collaboration is composed of research institutions and universities from the United States, Australia, Brazil, the United Kingdom, Germany, Spain, and Switzerland.
  The survey uses the 4-meter Victor M. Blanco Telescope located at Cerro Tololo Inter-American Observatory in Chile, outfitted with the Dark Energy Camera. This camera allows for more sensitive images in the red part of the visible spectrum and in the near infrared, in comparison to previous instruments.
  DECam has one of the widest fields of view available for ground-based optical and infrared imaging. The survey has imaged 5,000 square degrees of the southern sky in a footprint that overlaps with the South Pole Telescope and Stripe 82. The survey took 758 observing nights spread over six years to complete, covering the survey footprint ten times in five photometric bands. DES officially began in August 2013 and completed its last observing session on the 9th of January 2019.
context_section_description: The sample of 300 million galaxies with accurate photometric redshifts,  ~ 0.08, up to z ~ 1.4 provided by DES is well suited for measurement of BAO to study dark energy. The survey volume is 20 times that of SDSS photometric LRGs, enabling much higher precision measurements over a much wider redshift range. DES will measure the clustering on the sky of hundreds of millions of galaxies at different distances from us. These measurements will determine the angular scale of the sound horizon for galaxies at different redshifts. Combining those measurements together will provide information on the history of the cosmic expansion rate that complements the Type Ia supernova measurements. Defining a pivot epoch, , at which the uncertainty in the dark energy equation of state  is minimized for a given measurement, the figure of merit (FoM), which is proportional to the reciprocal of the area in the  plane that encloses the 95% CL region, is . Based on the fiducial cosmological model and some assumptions, the constrains of parameters can be calculated and confined.
  The main theoretical uncertainties in the interpretation of BAO measurements are the effects of nonlinear gravitational evolution and of scale-dependent bias between galaxies and dark matter. Non-linear evolution erases the acoustic oscillations on small scales, while both non-linear mode coupling and scale-dependent bias can shift the positions of the BAO features. Scale-dependent bias on large scales is potentially more of a concern for interpreting the broad-band power spectrum shape than for the BAO signal. Simulations that will become available over the next few years, should enable us to calculate the corrections due to non-linearity and scale-dependent bias to sufficient accuracy that the remaining systematic uncertainty will be small compared to the DES statistical errors on large scales. In addition, since the bispectrum(The Fourier transform of the second-order cumulant used to search for nonlinear interactions) responds to non-linearity and bias differently from the power spectrum, measurement of the angular bispectrum shape in DES will provide a cross-check on these effects, constrain this scale-dependence and determine the efficacy and robustness of including the broad-band shape in constraining dark energy.
  BAO are in principle sensitive to the uncertainty in the variance, , and in the bias, , of the photo-z estimates in redshift bins. To ensure that the dark energy parameter
  constraints aren't degraded (i.e., the errors increased) by no more than 10%, these uncertainties in z and 
  per redshift bin of 0.1 must be kept below ~ 0.01 (for ) and ~ 0.005 (for ). These performance levels should be conserved in DES, and the BAO constraints are therefore expected to be quite insensitive to uncertainties in photo-z parameters.
  Photometric zero-point drift affects the number density of galaxies in each redshift bin that lie above the detection threshold. The DES survey strategy, with its multiple overlapping tiles, is designed to minimize photometric drifts and to make them negligible on the scale of the DECam field of view. In addition to direct control from the tiling strategy, DES has a number of internal cross-checks on such drifts, including the redshift evolution of the BAO and matter-radiation scales, cross-correlations between different photo-z bins, consistency with the angular bispectrum, and comparison of the power spectrum shape and features for different galaxy-type subsamples.
caption: 68% CL forecast DES constraints in the 
    
      
        
          
            w
            
              0
            
          
          −
          
            w
            
              a
            
          
        
      
      {\displaystyle w_{0}-w_{a}}
    
   plane from the four probes: BAO, clusters, weak lensing and SNe, each combined with the Planck CMB prior; the filled red region shows the constraints from combining the four methods.
exif: {}