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Redshift-space distortions are an effect in observational cosmology where the spatial distribution of galaxies appears squashed and distorted when their positions are plotted in redshift-space (i.e. as a function of their redshift) rather than in real-space (as a function of their actual distance). The effect is due to the peculiar velocities of the galaxies causing a Doppler shift in addition to the redshift caused by the cosmological expansion. Redshift-space distortions (RSDs) manifest in two particular ways. The Fingers of God effect is where the galaxy distribution is elongated in redshift space, with an axis of elongation pointed toward the observer.〔Jackson, J.C. (1972). "A critique of Rees's theory of primordial gravitational radiation". ''Monthly Notices of the Royal Astronomical Society'', 156, 1P-6P.〕 It is caused by a Doppler shift associated with the random peculiar velocities of galaxies bound in structures such as clusters. The large velocities that lead to this effect are associated with the gravity of the cluster by means of the virial theorem; they change the observed redshifts of the galaxies in the cluster. The deviation from the Hubble's law relationship between distance and redshift is altered, and this leads to inaccurate distance measurements. Another, closely related effect is the Kaiser effect.〔Kaiser, N. (1987). "Clustering in real space and in redshift space". ''Monthly Notices of the Royal Astronomical Society'', 227, 1-21.〕 This time, the distortion is caused by the coherent motions of galaxies as they fall inwards towards the cluster center as the cluster assembles. Depending on the particular dynamics of the situation, the Kaiser effect usually leads not to an elongation, but an apparent flattening ("pancakes of God"), of the structure. It is a much smaller effect than the fingers of God, and can be distinguished by the fact that it occurs on larger scales.〔(Cosmology Presentation )〕 The RSDs measured in galaxy redshift surveys can be used as a cosmological probe in their own right, providing information on how structure formed in the Universe, and how gravity behaves on large scales. ==References== Specific citations: General references: * 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Redshift-space distortions」の詳細全文を読む スポンサード リンク
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