In astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the redshift of astronomical objects. Using Hubble's law, the redshift can be used to calculate the distance of an object from Earth. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure properties of the large-scale structure of the universe. The Great Wall, a vast conglomeration of galaxies over 500 million light-years wide, provides a dramatic example of a large-scale structure that redshift surveys can detect.
The first redshift survey was the CfA Redshift Survey, started in 1977 with the initial data collection completed in 1982.
The most notable, recent and low-redshift surveys are the 2dF Galaxy Redshift Survey, Sloan Digital Sky Survey and the Galaxy And Mass Assembly survey. At high redshift there exist the DEEP2 Redshift Survey and the VIMOS-VLT Deep Survey (VVDS).
Because of the demands on observing time required to obtain spectroscopic redshifts (i.e., redshifts determined directly from spectral features measured at high precision), a common alternative is to use photometric redshifts based on model fits to the brightnesses and colors of objects. Such "redshifts" can be used in large surveys to estimate the spatial distribution of galaxies and quasars, provided the galaxy types and colors are well understood in a particular redshift range. At present, the errors on photometric redshift measurements are significantly higher than those of spectroscopic redshifts, but future surveys (for example, the LSST) aim to significantly refine the technique.
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