Fingers of God
Encyclopedia
Fingers of God is an effect in observational cosmology
that causes clusters of galaxies
to be elongated in redshift
space, with an axis of elongation pointed toward the observer. It is caused by a Doppler shift associated with the peculiar velocities
of galaxies in a cluster. 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.
A closely related effect is the Kaiser effect. It is caused, again, by peculiar velocities lending an additional Doppler shift to the cosmological redshift, and it leads also to a kind of line-of-sight distortion. It is not caused, however, by the random internal motions of the cluster predicted by the virial theorem; rather, it arises from coherent motions as the galaxies 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.
Observational cosmology
Observational cosmology is the study of the structure, the evolution and the origin of the universe through observation, using instruments such as telescopes and cosmic ray detectors.-Early observations:...
that causes clusters of galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
to be elongated in redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
space, with an axis of elongation pointed toward the observer. It is caused by a Doppler shift associated with the peculiar velocities
Peculiar velocity
Peculiar motion or peculiar velocity refers to the true velocity of an object, relative to a rest frame.-Galactic astronomy:In galactic astronomy, the term peculiar motion refers to the motion of an object through space.Local objects are usually related in terms of proper motion and radial...
of galaxies in a cluster. 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
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
relationship between distance and redshift is altered, and this leads to inaccurate distance measurements.
A closely related effect is the Kaiser effect. It is caused, again, by peculiar velocities lending an additional Doppler shift to the cosmological redshift, and it leads also to a kind of line-of-sight distortion. It is not caused, however, by the random internal motions of the cluster predicted by the virial theorem; rather, it arises from coherent motions as the galaxies 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.