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Observational cosmology
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===Telescope observations=== ====Radio==== The brightest sources of low-frequency radio emission (10 MHz and 100 GHz) are [[radio galaxy|radio galaxies]] which can be observed out to extremely high redshifts. These are subsets of the [[active galaxy|active galaxies]] that have extended features known as lobes and jets which extend away from the [[galactic nucleus]] distances on the order of [[megaparsec]]s. Because radio galaxies are so bright, astronomers have used them to probe extreme distances and early times in the evolution of the universe. ====Infrared==== Far [[infrared]] observations including [[submillimeter astronomy]] have revealed a number of sources at cosmological distances. With the exception of a few [[infrared window|atmospheric window]]s, most of infrared light is blocked by the atmosphere, so the observations generally take place from balloon or space-based instruments. Current observational experiments in the infrared include [[NICMOS]], the [[Cosmic Origins Spectrograph]], the [[Spitzer Space Telescope]], the [[Keck Interferometer]], the [[Stratospheric Observatory For Infrared Astronomy]], and the [[Herschel Space Observatory]]. The next large space telescope planned by NASA, the [[James Webb Space Telescope]] will also explore in the infrared. An additional infrared survey, the [[Two-Micron All Sky Survey]], has also been very useful in revealing the distribution of galaxies, similar to other optical surveys described below. ====Optical rays (visible to human eyes)==== Optical light is still the primary means by which astronomy occurs, and in the context of cosmology, this means observing distant galaxies and galaxy clusters in order to learn about the [[Large-scale structure of the universe|large scale structure]] of the Universe as well as [[galaxy evolution]]. [[Redshift survey]]s have been a common means by which this has been accomplished with some of the most famous including the [[2dF Galaxy Redshift Survey]], the [[Sloan Digital Sky Survey]], and the upcoming [[Large Synoptic Survey Telescope]]. These optical observations generally use either [[photometry (astronomy)|photometry]] or [[spectroscopy]] to measure the [[redshift]] of a galaxy and then, via [[Hubble's law]], determine its distance modulo redshift distortions due to [[peculiar velocities]]. Additionally, the position of the galaxies as seen on the sky in [[celestial coordinates]] can be used to gain information about the other two spatial dimensions. Very deep observations (which is to say sensitive to dim sources) are also useful tools in cosmology. The [[Hubble Deep Field]], [[Hubble Ultra Deep Field]], [[Hubble Extreme Deep Field]], and [[Hubble Deep Field South]] are all examples of this. ====Ultraviolet==== See [[Ultraviolet astronomy]]. ====X-rays==== See [[X-ray astronomy]]. ====Gamma-rays==== See [[Gamma-ray astronomy]].
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