Astronomy Without A Telescope – Through A Lens Darkly


Massive galactic clusters – which are roughly orientated in a plane that is roughly face-on to Earth – can generate strong gravitational lensing. However, several surveys of such clusters have reached the conclusion that these clusters have a tendency towards lensing too much – at least more than is predicted based on their expected mass.

Known (to some researchers working in the area) as the ‘over-concentration problem’, it does seem to be a prima facie case of missing mass. But rather than just playing the dark matter card, researchers are pursuing more detailed observations – if only to eliminate other possible causes.

The Sunyaev-Zel’dovich (SZ) effect is a novel way of scanning the sky for massive objects like galactic clusters – which distort the Cosmic Microwave Background (CMB) via inverse Compton scattering – where photons (in this case, CMB photons) interact with very energized electrons which impart energy to the photons during a collision, shifting the protons to a shorter wavelength frequency.

The SZ effect is largely independent of red-shift – since you start with the most consistently red-shifted light in the universe and are looking for a one-off event that will have the same effect on that light whether it happens close by or far away. So, with equipment sensitive to CMB wavelengths, you can scan the whole sky – detecting both close objects, which might be directly observable in optical, as well as very distant objects which may have been red-shifted into the radio spectrum.

The SZ effect causes CMB distortions in the order of one thousandth of a Kelvin and the effect does require really massive structures – a single galaxy is not sufficient to generate the SZ effect on its own. But, when it works – the SZ effect offers a method to measure the mass of a galactic cluster – and does it in a way that is quite different to gravitational lensing.

The SZ effect is thought to be mediated by electrons in the inter-cluster medium. This means that the SZ effect is solely the result of baryonic matter, since it is a consequence of the inverse Compton effect. However, gravitational lensing is the result of the warping of space-time – which is partly due to the presence of baryonic matter, but also of dark (i.e. non-baryonic) matter.

Gralla et al used the Sunyaev-Zel’dovich Array, an array of eight 3.5 meter radio telescopes in California, to survey 10 strongly lensing galactic clusters. They found a consistent tendency for the Einstein radius of each gravitational lens to be around twice the value expected for the mass, determined from the SZ effect, of each cluster.

A distant, actually double, Einstein ring captured by the Hubble Space Telescope. Many more Einstein rings are visible from distant galactic clusters - although they are generally only 'visible' in radio wavelengths.

The Einstein radius is a measure of the size of the Einstein ring that would be formed if a cluster was exactly orientated in a plane that was exactly face-on to Earth – and where you, the lens and the distant light source being magnified, are all in a straight line of sight. Strongly lensing galaxies are generally only in close approximation to this geometry, but their Einstein ring and radius (and hence their mass) can be inferred easily enough.

Gralla et al note that this is work in progress, for now just confirming the over-concentration problem found in other surveys. They suggest one possibility is that the amount of inter-cluster medium may be less than expected – meaning that the SZ effect is underestimating the real mass of the cluster.

If, alternatively, it is a dark matter effect, there would be more dark matter in these clusters than the current ‘standard model’ for cosmology (Lambda-Cold Dark Matter) predicts. The researchers seem intent on undertaking further observations before they go there.

Further reading: Gralla et al. Sunyaev Zel’dovich Effect Observations of Strong Lensing Galaxy Clusters: Probing the Over-Concentration Problem.

And just for interest, Einstein’s letter on lensing and rings: Einstein, A (1936) Lens-like Action of a Star by the Deviation of Light in the Gravitational Field. Science 84 (2188): 506–507.

19 Replies to “Astronomy Without A Telescope – Through A Lens Darkly”

  1. One thing which I find a bit strange in its wording is:

    “The SZ effect is thought to be mediated by electrons in the inter-cluster medium. This means that the SZ effect is solely the result of baryonic matter, since it is a consequence of the inverse Compton effect.”

    The SZ effect is then not due to baryons, but electrons (leptons). The SZ effect does allow for an accounting of baryonic matter in a galaxy cluster since the number of protons equals the number of electrons accounted for by the inverse Compton scattering.

    What seems odd is that the matter accounted for by Einstein lensing seems low. I would expect a 3 to 4 times the amount of matter accounted for by lensing over the SZ effect.


  2. @ Lawrence B. Crowell

    The SZ effect is then not due to baryons, but electrons (leptons).

    As true as this obviously is, but leptons are also part of matter that is normally referred to as “baryonic matter”. One could also say “normal matter”, “matter we are made of”, “electromagnetic interacting matter”, “luminous matter”, etc etc, of course. But baryonic matter is part of normal scientific language, when one refers to both baryons and leptons. 😉

  3. Thanks for a really thorough article! I think I understood the problem.

    Which leaves me with a question on the result instead: *if* the DM is more than CDM predicts, where does it fit in? With ‘warm’ or ‘hot’ DM, or not at all?

    The reasonm I ask is that “warm DM” is becoming “warm” as I understand it. IIRC it seems to fit the pesky galaxy modeling better besides the problem of CDM making an inconsistent result (AFAIU), and since some DM observatories data may fit WDM rather than CDM.

    So it’s an engaging alternative, which doesn’t even have to replace our dear supersymmetric WIMPs at all AFAIK but rather augment the physics. One article on wild LHC results have warm DM (as I understand it) _also_ explaining matter/antimatter asymmetry by a funny symmetry breaking mechanism. Occam can be compelling at times…

  4. The primordial anisotropies of the cosmic microwave background are linearly polarized via Compton-scattering.

    Zitrin et al. (2010) detected that 8 out of the 12 clusters are not having any spectroscopically measured redshifts for any of the lensed sources.

    As an alternative to Einstein’s gravitational lensing is there a chance that the observed rings are only the result of polarized photons passing through hot magnetized plasma, as present in dense galaxy’s?

  5. Check – electrons are leptons.I’ve gotten used to talking about baryonic (non-dark) and non-baryonic (dark) matter. Any suggestions on alternate wording?

  6. To be more precise, the CMB photons detected by WMAP are the ones with low energy. The observed photons are the “untouched” ones [by baryonic matter]. The higher energy photons are out of WMAP’s range, too short wavelength.

    In case of lensing the observed photons are on the other hand the energized photons. They still show their original properties after being absorbed and re-emitted. Showing a picture of the early universe and because they are polarized also they will show it in a circular pattern.

  7. While I do not understand some of the acronyms (okay, several), I enjoy reading this and commend the researchers for doing the research and not playing the dark matter card. I wish the ‘scientists’ in some other fields would follow this lead, instead of pushing their dogma. Thank you.

  8. I’ll add to my previous comments, that at posting them I realized that the excess of DM would fit in with WDM, in effect I was answering myself. Too busy to change the content then, unfortunately.

    @ postman1: Well, the acronyms _are_ linked. More seriously you don’t seem to grasp the content of the article; the successful intent of the researchers was to check up on DM, baryonic matter is unlikely to make up the difference which is how DM was discovered in the first place.

    But we already knew DM was an observable fact from other observations, not a “dogma”. Which is an ironic term in this instance, since the standard cosmology is but 6 years counting from WMAP as test; hardly a dogma by any reasonable measure. (Ah, time flies when you are that young, I remember referring to SC as “merely 5 years”. Now it is suddenly ~ 20 % older.)

    The science today is not to be found in the existence of DM, been there done that, but what it is. CDM according to SC, but there is some leeway as I understand it. This result fits into that frame.

  9. Steve,

    One problem of course is that I am not sure what nomenclature is used with these studies. Maybe the SZ effect is called “baryonic” because it does make an accounting of baryons, which are 98% protons that are matched by an equal number of leptons (electrons). It is the electrons which undergo the inverse Compton scattering.

    As for “dogma,” the use of the term of late has become an irritant to me. It is often used with the same context as saying the Earth orbits the sun is a “dogma.”


  10. I guess I wasn’t clear. I was not implying that DM was any sort of dogma. I meant that this area of research is based on scientific results, not preset hypotheses. Didn’t mean to insult anyone, just interested in the conversation.

    1. Not dogma, check.

      I meant that this area of research is based on scientific results, not preset hypotheses.

      But here you say something that doesn’t pass the smell test. Hypotheses, whether “preset” (axioms?) or not are part and parcel of science. But you do need to test them before accepting them.

      Science (and actually the largest part of math and CS) isn’t axiomatic. But even so, axiomatics isn’t a problem because hypotheses are born out of the same data that they predict and are tested against. Self consistency is sufficient to not reject a hypotheses. Axiomatic methods fits well into such a process.

      Of course to eliminate all competitive (non-rejected) hypotheses, you may need more data and more tests. It is but at higher energies (speeds) that relativity differ from classical mechanics in most cases for example. (Re magnetism, say, which is a low energy relativistic effect.) But it isn’t a necessary requirement.

  11. My initial reaction to the discrepancies in mass derived from SZ observations mentioned here was that this might be related to assumptions made as to the properties of the Intra Cluster Medium, namely the effect of cluster mergers (both major mergers and smaller galaxy groups still being accreted onto the main cluster). I came across a paper today that examined this very issue.

    It seems that assumptions made about the physical state of the ICM (Intra Cluster Gas, in the paper), can indeed cause mass estimates derived solely from SZE observations to be in error. Not only mergers but cooling core mechanisms can lead to bias in SZE-derived masses. Eight known clusters (including Abell 1689) are examined and masses derived using simplistic assumptions of the state of the ICM. The authors caution that using simplistic models of the ICM to derive cluster masses from galaxy clusters only detectable by SZE observations (ACT, SPT & Planck are mentioned) can lead to significant errors in mass.

    The paper “Biased total mass of cool core galaxy clusters by Sunyaev-Zel’dovich Effect measurements” is available here:

  12. Off Topic but may be of interest –

    An independent analysis of the 7-year WMAP data has found no evidence for the CCC model as recently proposed by Gurzadyan & Penrose. From the abstract:

    “We do reproduce the claimed ring structures observed in the WMAP data as presented by Gurzadyan and Penrose, thereby verifying their computational procedures. However, the results from our simulations do not agree with those presented by Gurzadyan and Penrose. On the contrary we obtain a substantially larger variance in our simulations, to the extent that the observed WMAP sky maps are fully consistent with the LCDM model as measured by these statistics.”

    The paper “A search for concentric circles in the 7-year WMAP temperature sky maps” is available here:

    1. I’m please that both groups easily sees circles (and other patterns) in the variance data, as I predicted and now got tested. Also, the .1268 paper uses an actual test statistic instead of G&P pattern search, and they get no deviation from what would be expected; another of my grievances.

      Is this a test that Penrose is now dotty and ditsy, as some in the other thread claimed? At least it means that I can’t take anything claimed on him seriously. Reminds me of Eddington & Pauling: “Like Linus Pauling, Eddington suffered from “great old man disease”.” Maybe so!?

      [I realize this isn’t a subject one should speculate and comment further on. Just stating my now well tested suspicions.]

  13. A reply to both papers by G & P has been posted:

    From the abstract:

    “….the circles we saw are a real structure of the CMB sky and they are not of a random Gaussian nature. Although the structures studied certainly cannot contradict the power spectrum, which is well fitted by LCDM model, we particularly emphasize that the low variance circles occur in concentric families, and this key fact cannot be explained as a purely random effect. It is, however a clear prediction of conformal cyclic cosmology.”

    I suspect the “clear prediction” may be a point of contention.

  14. If you look at the CMB curve there is a wavey curve that gives the intensity with respect to the Legendre polynomial expansion. There is at the L = 40 expansion a little blip, where the data is above the averaged curve. These concentric circles are a funny interpretation of this data point. The error bars are small enough to say this is real, and there may be some physics there. However, the interpretation of this as some CCC pre-big bang physics is incorrect.


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