Redshift quantization is the hypothesis that the redshifts of cosmologically distant objects (in particular galaxies) tend to cluster around multiples of some particular value. Since there is a correlation of distance and redshift as expressed in Hubble's Law, redshift quantization would either indicate a quantization of the distances of galaxies from the Earth or a problem with the redshift-distance correlation, either of which would have serious implications for cosmology. Many scientists who oppose the Big Bang theory, including Halton Arp, have referred to observations claimed to be in favor of redshift quantization as reason to reject the standard account of the origin and evolution of the universe.
In 1973, astronomer William G. Tifft was the first to report evidence of such clustering (before that see György Paál). Recent redshift surveys of quasars (QSOs) have produced no evidence of quantization in excess of what is expected due to galaxy clustering,  and consequently most cosmologists dispute the existence of redshift quantization beyond a minimal trace due to the distribution of galaxies across voids and filaments.
Properly speaking, Redshift quantization refers to any of the theories in which the quantum of action appears in equations describing Hubble Red Shift's origin and magnitude of redshift with distance.
Original investigation by William G. Tifft
- "Using more than 200 redshifts in Coma, Perseus, and A2199, the presence of a distinct band-related periodicity in redshifts is indicated. Finally, a new sample of accurate redshifts of bright Coma galaxies on a single band is presented, which shows a strong redshift periodicity of 220 km s−1. An upper limit of 20 km s−1 is placed on the internal Doppler redshift component of motion in the Coma cluster".
Tifft, now Professor Emeritus at the University of Arizona, suggested that this observation conflicted with standard cosmological scenarios. He states in summary:
- "Throughout the development of the program it has seemed increasingly clear that the redshift has properties inconsistent with a simple velocity and/or cosmic scale change interpretation. Various implications have been pointed out from time to time, but basically the work is observationally driven."
Subsequent work by other researchers
In the late 1980s and early 1990s, four studies on redshift quantization were performed:
- In 1989, Martin R. Croasdale reported finding a quantization of redshifts using a different sample of galaxies in increments of 72 km/s (Δz = 2.4×10−4).
- In 1990, Bruce Guthrie and William Napier reported finding a "possible periodicity" of the same magnitude for a slightly larger data set limited to bright spiral galaxies and excluding other types
- In 1992, Guthrie and Napier proposed the observation of a different periodicity in increments of Δz = 1.24×10−4 in a sample of 89 galaxies
- In 1992, G. Paal, et al.  and A. Holba, et al.  reanalyzed the redshift data from a fairly large sample of galaxies and concluded that there was an unexplained periodicity of redshifts.
- In 1994, A. Holba, et al.  also reanalyzed the redshift data of quasars and concluded that there was unexplained periodicity of redshifts in this sample, too.
- In 1997, W. Μ. Napier and B. N. G. Guthrie concluded the same: "So far the redshifts of over 250 galaxies with high-precision HI profiles have been used in the study. In consistently selected sub-samples of the datasets of sufficient precision examined so far, the redshift distribution has been found to be strongly quantized in the galactocentric frame of reference. ... The formal confidence levels associated with these results are extremely high."
All of these studies were performed before the tremendous advances in redshift cataloging that would be made at the end of the 1990s. Since that time, the number of galaxies for which astronomers have measured redshifts has increased by several orders of magnitude.
Evaluation and criticism
After Tifft made his proposal, discussion of it was generally confined to detractors of standard cosmology. Nevertheless, it was nearly 20 years before other researchers tried to corroborate his findings. After a brief flurry of interest, the consensus in the astronomical community became that any quantization was either coincidental or due to so-called geometrical effects. Current observations and models of large-scale structure models trace filamentary superclusters and voids that cause most galaxies in a rough statistical sense to have correlated positions, but such groupings would not allow for a strength of periodicity required if it were a hallmark characteristic of the redshifts of galaxies. As such with exceedingly few exceptions, modern cosmology researchers have suggested that redshift quantizations are manifestations of well-understood phenomena, or not present at all.
In 1987, E. Sepulveda suggested that a geometric paradigm based on the polytrope theory could account for all redshift periodicities, and that:
- "The smallest periodicities (Δz = 72, 144 km/s) are due to parallel line segments of galactic clustering. The largest (Δz = 0.15) are due to circumferential circuits around the universe. Intermediate periodicities are due to other geometric irregularities. These periodicities or apparent quantizations are relics or faithful fossils of a real quantization that occurred in the primordial atom."
In 2002, Hawkins et al. found no evidence for a redshift quantization in the 2dF survey and found using Napier's own guidelines for testing redshift periodicity that none, in fact, could be detected in the sample:
- Given that there are almost eight times as many data points in this sample as in the previous analysis by Burbidge & Napier (2001), we must conclude that the previous detection of a periodic signal arose from the combination of noise and the effects of the window function.
In 2005, Tang and Zhang:
- ".. used the publicly available data from the Sloan Digital Sky Survey and 2dF QSO redshift survey to test the hypothesis that QSOs are ejected from active galaxies with periodic noncosmological redshifts. For two different intrinsic redshift models, [..] and find there is no evidence for a periodicity at the predicted frequency in log(1+z), or at any other frequency. "
A 2006 historical review of study of the redshift periodicity of galaxies by Bajan, et al., concludes that "in our opinion the existence of redshift periodicity among galaxies is not well established."
In 2006, M. B. Bell and D. McDiarmid, reported: "Six Peaks Visible in the Redshift Distribution of 46,400 SDSS Quasars Agree with the Preferred Redshifts Predicted by the Decreasing Intrinsic Redshift Model". The pair acknowledged that selection effects were already reported to cause the most prominent of the peaks. Nevertheless, these peaks were included in their analysis anyway with Bell and McDiarmid questioning whether selection effects could account for the periodicity, but not including any analysis of this beyond cursory cross-survey comparisons in the discussion section of their paper. There is a brief response to this paper in a comment in section 5 of Schneider et al. (2007)  where they note that all "periodic" structure disappears after the previously known selection effects are accounted for.
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