Sunday, June 16, 2013

Discordant Redshifts: A Post-Mortem

I've waited many years to accumulate the information I needed to write this series of articles dealing with the claims of discordant redshifts (DZ) so it's only appropriate to create a one-page quick reference of what we've learned. 

I have contacted some of the amateur sites advocating discordant redshifts, so I may be hearing from some of them which may add to a final project list.   I doubt this will be the last I write on this topic as I still have a couple of demonstrations I'd like to complete.  Right now that list includes a demonstration of how extended objects in a field of view can have apparent 'connections' and how it is we can see through galaxies to what is beyond.  

Their 'prime' piece of evidence was that the connections and associations were low probability.  Back at the start of the claims about discordant redshifts, Halton Arp and others claimed that chance alignments were very low probability in the standard cosmological model.

I've found no evidence that anyone amongst that community bothered to investigate, or even ask, if that statement were really true!

As we see in sections (Discord for Discordant Redshifts. I., Discord for Discordant Redshifts. II.), in a universe full of galaxies roughly uniform in space, you will see many more high-redshift objects in a given area of sky, the number increasing as the square of redshift.  The 'low probability of chance alignment' fails based on simple 3-D geometry and projection effects.  

Once that fails, all the other associations and connections become chance alignments and irrevelant (Discordant Redshift Excuses: But the Galaxies Show Connections!).  

Even worse is that this fact was recognized and documented back in 1975 (Reexamination of the correlation of galaxies and QSO's).  With the small catalogs of galaxies and quasars (a few thousand entries) of the 1970s, Noerdlinger's conclusions become pretty definitive with todays catalogs containing on the order of a million of galaxies.  It is no longer statistics of small numbers creating a bottleneck.

To be fair, for researchers who learned cosmology in the 1950s, when values of the Hubble constant were thought to be about five times larger than today (see CfA: The Hubble Constant), and the Universe was consequently much smaller, chance alignments would have had a lower probability.  In that picture of the universe, investigation of discordant redshift pairs made a lot more sense.  But as the value of the Hubble constant was refined, and smaller, the universe got larger, and full of galaxies, and chance alignments became unavoidable.

The only way to rescue discordant redshift claims from the simple geometrical effect is for them to retreat to a small, geocentric universe (which decreases the probability of chance alignments at high-redshift).

There are still a number of motivated amateurs identifying 'discordant' objects in some of the large galaxy surveys.  Of course, as we've seen in this series, it's a rather easy task with the new deep surveys such as SDSS and others.  Not that those results would change any of the facts, and conclusions, of this series.

Although I have heard some stories of advocates of discordant redshifts actually realizing their error, it's sad that a lot of professionals have wasted a large part of their career in pursuit of the claims.  I suspect when the current generation of professional astronomers advocating discordant redshifts are gone, there will be no more, and this notion will move completely into the realm of crank science.


Discordant Redshifts: RIP

Some Amateur Sites Advocating Discordant Redshifts
Update, June 18, 2013:  After some correspondence with the operator of the site Anomalous Redshift Investigator (see correspondence in the comments), I have concluded (preliminary and subject to change) that what they are calling 'discordant' is not the extreme cases pushed by Arp and similar supporters.


Ari Jokimäki said...

Just for the record: I'm not advocating discordant redshifts, just studying the issues relating to them.

W.T."Tom" Bridgman said...

To Ari,

If they are just chance alignments, I can't help but wonder what those issues might be, beyond a certain aesthetic of the imagery.


Ari Jokimäki said...

Discordant redshifts are often present in basic extragalactic astronomy. An example of that is our paper (Jokimäki et al. 2008). Imagine our surprise when we found out that 40 % of our sample belongs into the realm of crank science.

W.T."Tom" Bridgman said...

To Ari,

Are you really unfamiliar with how Arp and supporters have poisoned the terminology in this topic? Another researcher who was examining planetary orbit resonances used the term 'quantization', apparently unfamiliar with how loaded that term had become (see comments in Geocentrism's "Quantized Planetary Orbits").
I've perused your paper linked above.

It appears you are making a legitimate investigation of possibly interacting galaxies.

You still resort to Arp's probability computation which treats the universe as a 2-D projection on a spherical surface. I explore some of the problems with that with the 3-D examples in the earlier posts. It is explored in more mathematical detail by Noerdlinger (1975).

You note that 17 of the 41 pairs with published redshifts have a velocity difference > = 1000 km/s = delta_z of 0.00333.
a) Most of your pairs are certainly not that 'discordant' in context usually meant in the topic (redshifts differing by factors of 2-10 and larger) and are probably legitimate interactions.
b) a 1000 km/s difference is within the velocity dispersion (Wikipedia) of rich clusters of galaxies.

Other possible analyses you should consider:
1) You could use the implied velocities and positions (in 3-D space) to compute a possible time of the interaction as a test of a past (or future) interaction.

2) Is the 'strength' of the interaction between the galaxies dependent on the apparent distance of separation (including the 3rd dimension implied by redshift)?

You might also enjoy a warmer reception by the community if you don't use the term 'discordant' when describing even small differences in redshift.


Anonymous said...

@Ari: Have you thought about doing follow-on work, looking for M51-like galaxies in the Galaxy Zoo overlaps catalog (see, or searching the SDSS DR8 (or DR9) data (using CasJobs, for example)? The role of small companions in creating and sustaining 'grand design' spiral arm systems is an interesting one, and a larger catalog could be useful.

Jean Tate

Ari Jokimäki said...

Jean, we started doing volume 2 for this catalog but all the guys got busy with something else and project then drifted to the background and died.