Introduction to anomalous redshifts

It is very difficult to determine the exact meaning of the term “redshift anomaly”. It can be lot of things. It can be said that redshift is anomalous if it doesn’t somehow fit in our current descriptions of the things in the universe. Basically any strange thing studied by redshifts is a redshift anomaly. Redshift anomalies could be found within one object, or within a group of objects, or then a group of objects could exhibit a redshift anomaly as a whole.

In traditional view, redshift of an object is composed of three components; kinematical, gravitational, and cosmological. Local and nearby objects don’t have any cosmological redshift, and gravitational redshift is so small that it can usually be ignored (although in the context of redshift anomalies, gravitational redshift is often mentioned, and sometimes with a good reason), so in those objects we are generally only interested in kinematical redshift, which is caused by peculiar velocity differences between the source of light and the receiving end. On the other hand, objects that are far away have so large cosmological redshift that all other redshift components can be ignored. Generally an extragalactic object has cosmological redshift component that is proportional to its distance from us, and kinematical redshift component which usually is considered to be smaller than cz ~ 1000 km/s (although in some extreme cases even 3000 km/s has been accepted, NGC 1275 system is an example of that). In some cases there are also other redshift components from scattering processes (Compton scattering for example), but they are not usually considered in average extragalactic objects. If there are some redshift observations that don’t fit to this traditional view, then those redshifts are anomalous.

Through the history of spectroscopy there has been redshift anomaly candidates. There is an apparent solar limb effect, where the redshifts at the limb of the Sun don’t quite match the expected values (Mikhail et al., 2002, and references therein). Within Milky Way, there is so called K-effect, where certain types of stars have a small excess redshift component (Arp, 1992). Outside Milky Way lot of redshift anomaly candidates have been found. Companion galaxies might have an excess redshift component (Arp, 1994). Some higher redshift objects might be associated with lower redshift objects, as suggested by their apparent nearness, or by their geometrical alignment, or by apparent connecting bridges (Lopez-Corredoira, 2009). Supporting these, some strange coincidences in redshift values have been cited, such as redshifts of higher redshift objects decreasing when their location being further out from the lower redshift object (Arp, 1999), or redshift values clustering around certain values (Tifft, 1995). Redshift anomalies relating to associations of objects with differing redshifts are usually called discordant redshifts.

Other type of redshift anomaly candidates are already mentioned redshift clustering around certain values (which is known as redshift quantization or periodic redshifts), excess redshift in certain galaxy types, unexpected redshift behaviour across galaxy disks (Jaakkola et al., 1975), and multitude of other suggested anomalous redshift issues (Pioneer anomalies, blueshifted quasars, etc.).

Solutions for redshift anomalies are usually sought from observational problems and from traditional science. Often it is the case that traditional physics are performing some unexpected tricks, and we see them as redshift anomalies. Those occasions give us an opportunity to extend our knowledge and polish our theories. Such seems to be the case with the solar limb effect, which has been a problem for a long time, and now seems to be getting solved mainly by gravitational redshift component revision (Mikhail et al., 2002). Another example of redshift anomaly that got solved, and is now part of mainstream science is the redshift anomaly found in 1920’s that most galaxies seemed to have “velocity shift” towards the red color in the spectrum. This was solved by Hubble (1929) by showing that there is a redshift-luminosity relation in galaxies which was interpreted as redshift-distance relation.

One interesting aspect of anomalous redshifts is the possibility to find something remarkably unexpected, perhaps something that doesn’t fit to our current theories at all. A whole new redshift component, perhaps intrinsic to the object itself (as has been suggested in many papers dealing with some redshift anomaly candidate)? We’ll see…


Arp, 1992, MNRAS, 258, 800, “Redshifts of high-luminosity stars – The K effect, the Trumpler effect and mass-loss corrections”

Arp, 1994, ApJ, 430, 74, “Companion galaxies: A test of the assumption that velocities can be inferred from redshifts”

Arp, 1999, A&A, 341L, 5, “A QSO 2.4 arcsec from a dwarf galaxy – the rest of the story”

Hubble, 1929, PNAS, 15, 168, “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae”

Jaakkola et al., 1975, A&A, 40, 257, “On possible systematic redshifts across the disks of galaxies”

Lopez-Corredoira, 2009, arXiv, 0901.4534, “Apparent discordant redshift QSO-galaxy associations”

Mikhail et al., 2002, Ap&SS, 280, 223, “Application of Theorems on Null-Geodesics on The Solar Limb Effect”

Russell, 2005, Ap&SS, 298, 577, “Evidence for Intrinsic Redshifts in Normal Spiral Galaxies”

Tifft, 1995, Ap&SS, 227, 25, “Redshift Quantization – A Review”

3 Responses

  1. What is the maximum percentage of redshift at the limit of the solar limb? Thanks!

  2. I should clarify that last question to be asking for the amount of the anomalous part of the total redshift at the limb. Like Z = Z_tot – Z_o,, where Z_o is the regular Einstein gravitational redshift at the center of the solar disc.

  3. I don’t know that, sorry.

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