Mystery of the Missing Galaxy Clusters

Astronomers have counted up the number of clusters in the cosmos and found a problem.

All is not quite right in the world of cosmology.

Last year, the Planck mission team released its first 15.5 months of temperature observations of the cosmic microwave background (CMB), the leftover radiation from the Big Bang. As I reported at the time, the observations were largely in phenomenal agreement with the status-quo cosmological model.

Planck cosmic microwave background
The oldest light in our universe, the cosmic microwave background suffuses the cosmos. This map created from all nine frequency bands of the Planck spacecraft shows the CMB's details at a precision never before acquired. The lumpiness revealed by the CMB's patterns should match up with the lumpiness of today's universe. But cosmologists are having trouble reconciling the two.
ESA and the Planck Collaboration
But the Planck data don’t entirely tie the universe up in a nice tidy bow. Some oddities arose. One of the lingering mysteries has to do with the number of galaxy clusters.

Galaxy clusters are essentially big lumps in the distribution of matter in the universe. Although clusters are relatively recent developments in the cosmos, the lumpiness they reveal should correspond to matter’s lumpiness in the primordial universe. The minute temperature fluctuations in the CMB reveal this primordial lumpiness, so with the right theoretical model, astronomers should be able to match up the two sets of observations, explains David Spergel (Princeton).

But as researchers reported at the winter American Astronomical Society meeting, it’s not proving that easy. James Bartlett (JPL and APC Université Paris Diderot-Paris 7, France) presented a map of 189 galaxy clusters based on Planck observations of what’s called the Sunyaev-Zeldovich (SZ) effect. The SZ effect happens when CMB photons steal energy from hot gas as they pass through it. This effect means that galaxy clusters (which are full of hot gas) basically leave “shadows” in the CMB. By looking for clusters’ fingerprints in microwave observations, astronomers can locate distant galaxy clusters and count how many existed at different times.

The number of galaxy clusters found using the SZ effect’s shadows match those tallied using X-ray and optical surveys. But it doesn’t match what researchers predict based on the clumpiness in the CMB. In fact, as Brad Benson (Fermilab/University of Chicago) explained in a subsequent talk, the CMB as seen by Planck suggests that the universe should have 2.5 times more galaxy clusters than astronomers actually observe. The problem isn’t going away, either: his team’s preliminary analysis of about 300 clusters, using the clusters’ SZ fingerprints in data from the South Pole Telescope, also show conflict with Planck’s CMB-based cosmology.

massive cluster
The young, massive cluster ICDS J1426.5+3508 contains the mass of as much as 500 trillion Suns. The cluster existed when the universe was less than 4 billion years old, making this the most massive cluster known for its age.
NASA, ESA, A. Gonzalez (Univ. of Florida), A. Stanford (UC Davis and Lawrence Livermore National Laboratory), and M. Brodwin (Univ. of Missouri and Harvard-Smithsonian Center for Astrophysics)
There are several potential explanations. One, the tension might not actually be there. Astronomers can calculate a cluster’s mass by extrapolating from X-ray observations, or by detecting its weak gravitational lensing effect when the cluster’s gravity slightly bends light that is passing by it en route to us. The assumptions that go into relating a cluster’s X-ray luminosity and its mass might be off. If all cluster masses are low by a factor of about 1.4, that could resolve the problem.

That might sound like an easy fix, but it’s not. Masses calculated using X-ray observations and weak lensing are consistent within about 10 to 15% of each other, Benson said. The chances of the measurements being off enough to match CMB predictions are less than 1 in 300 at best.

Neutrinos might also be to blame. Neutrinos are the second most common particle in the universe, after photons, and the majority was created during the Big Bang, Benson explains. Scientists know the particles should have some tiny mass (still undetermined). If neutrinos are 4 to 5 times heavier than the lower limit calculated from experiments, these relativistic particles could be the answer.

Or perhaps scientists’ analysis of the CMB temperature map is off, and the primordial matter fluctuations aren’t as strong as the Planck team proposes. An independent analysis by Spergel’s team suggests such is the case, which would ease the apparent discrepancy.

“This is, I think, a wonderful situation,” Spergel said at the meeting. In the next year or two, astronomers will either have evidence for new physics or have done away with the tension, he explained. “It’s an exciting moment.”

Part of the solution could come from the 5-year Dark Energy Survey (based in Chile), which has begun its study of the universe’s expansion and structure growth history. The effort includes studying how the number of galaxy clusters in a given volume has changed with time. At the moment, astronomers have observed about 1,300 clusters with the SZ effect; DES is expected to detect 100,000 (about one-hundredth the cosmos’s expected total), spread out over the latter half of the universe’s existence.

DES’s field of view overlaps that of the South Pole Telescope, which observes at the other end of the spectrum. If both sets of observations uphold the discrepancy, then it could mean cosmologists are indeed missing something.


AAS presentations (click here for meeting program):
J. G. Bartlett et al. "Planck Cluster Cosmology Results." Abstract #135.01
B. Benson et al. "The South Pole Telescope Cluster Survey." Abstract #135.02

Planck Collaboration. "Planck 2013 results. XX. Cosmology from Sunyaev-Zeldovich cluster counts."
Planck Collaboration. "Planck 2013 results. XXIX. Planck catalogue of Sunyaev-Zeldovich sources."

14 thoughts on “Mystery of the Missing Galaxy Clusters

  1. Robert L. Oldershaw

    Camille comments: that the recent Planck results were "in phenomenal agreement with the status-quo cosmological model".


    I am looking at the Planck mission paper posted to by the Planck Collaboration as I write this.

    Deviations from isotropy have been found and confirmed, so has the surprising quadrupole-octopole alignment and a persistence of the dipolar power asymmetry to much smaller angular scales.

    If I remember correctly the Planck mission also discovered a significantly slower expansion rate than was predicted.

    The Planck Collaboration’s abstract ends with the comment: "a satisfactory explanation based on physically motivated models is still lacking."

    Camille, please read that last sentence again and be a bit more careful in how you characterize the current status of cosmological research.

    We are still groping in the dark, and using questionable over-idealized models that are being observationally contradicted.

    Robert L. Oldershaw
    Discrete Scale Relativity/Fractal Cosmology

  2. Robert L. Oldershaw

    One possible explanation for the newly verified dipole anisotropy (inherent directionality) in the CMB (see latest Planck results) is that the structure of the cosmos has a fractal geometry and that nature’s hierarchy extends far beyond the observable universe.

    Unlike the dubiously extravagant idea of a multiverse containing 10^500 different universes with random properties, the discrete fractal paradigm proposes one unified physics for the entire cosmos. It is a new paradigm that is based on enlarging the symmetry properties of nature (to include global discrete conformal symmetry), rather than invoking ad hoc and effectively untestable speculations.

    Robert L. Oldershaw
    Discrete Scale Relativity/Fractal Cosmology

  3. Camille M. CarlisleCamille Carlisle

    Nevertheless, Robert, I stand by what I said. The agreement between Planck’s initial data release and the current cosmological model is impressive: check out the graph in my blog (, which shows the tight fit. That result *is* phenomenal. But as I noted above, not *everything* is solved. (You’ll note how I said “*largely* in phenomenal agreement.” Key word!) There are certainly enduring questions, and this galaxy cluster mystery is one of them.

  4. Robert L. Oldershaw

    Ok, it’s sort of the half full, half empty situation.

    One can be impressed with what the current cosmological paradigm gets right.

    Or one can be impressed by what the current cosmological paradigm never predicted and/or cannot explain, such as dark matter, galaxy formation, the cosmic filamentary web, accelerated expansion (albeit slower than originally thought), why the BIG (Little?) BANG happened at all, and so on.

    Clearly we have a very long way to go before we can say we have a good understanding of the cosmos.


    Discrete Scale Relativity/Fractal Cosmology

  5. Robert L. Oldershaw

    At the risk of coming on way too strong here, and with apologies to those who think so, here is Discrete Scale Relativity’s reinterpretation of the "Big Bang".

    Consider this simple and natural paradigm for understanding the Big Bang, the global expansion, the large peculiar velocities of galaxies, and a low-entropy start to the Big Bang without it being the acausal "birth" of the entire Universe, etc.

    The discrete self-similar (fractal) paradigm proposes that the entire observable universe constitutes a nearly infinitesimal region of one Metagalactic Scale object. The galaxies within this region are crammed together
    at very high densities. Galactic Scale objects are also chaotically moving at high "peculiar" velocities and this indicates an extremely high ambient temperature. The combination of very high temperature and density
    produces frequent galactic interactions and mergers. The mass spectrum of galaxies is relatively flat, unlike the abundance trends on the Atomic and Stellar Scales, and therefore significant numbers of moderately massive and very massive systems are present. The evidence for an extremely high-energy environment, the presence of substantial numbers of massive galactic systems, and the strong evidence for global expansion, all suggest a reasonably unique analogy to the interior of a supernova shortly after detonation. In this analogy, galactic systems play the role of fully ionized Atomic Scale particles and nuclei under very-high-energy plasma conditions. The discrete fractal paradigm leads inevitably to this radical reinterpretation of the standard Big Bang model for the “origin” of the “Universe”.

    Robert L. Oldershaw

    Discrete Scale Relativity/Fractal Cosmology

  6. Peter WilsonPeter

    Dark matter…dark energy…dark clusters? Cosmologists have painted themselves into a corner, so to speak, where nobody can follow them. Observations of the CMB are in phenomenal agreement with the model, but most of that agreement is “generic.” I can predict that “height” in a population will follow a Gaussian distribution, or that earthquake-frequency vs. magnitude will follow a power-law distribution. My predictions will be in “phenomenal agreement” with the data, but of limited use. We only get information in such situations where the prediction fails. At the high-end, the “missing earthquakes” tell us about the inherent strength (or weakness) of Earth’s crust, which can only take so-much stress; at the low-end, the “excess earthquakes” give us information about wind, waves, passing trucks, etc. And some of us are from Missouri. We’re still waiting to see a map of the CMB side-by-side with an optical map of galaxy clusters. Robert may have a legitimate gripe; he may not. Is anybody not confused by this latest missing piece of the puzzle?

  7. Robert L. Oldershaw

    If new ideas, especially a comprehensive new paradigm for all of nature, cause you unacceptable levels of anxiety, you might avert your eyes.

    Or do what the proverbial ostrich does.

  8. Mike W. Herberich

    Freedom of speech, though granted, is -like any freedom or liberty- not an absolute one. Easy example: almost all sensible constitutional states (republics, democracies) do also know a statute or rule prohibiting insult or defamation. Therefore, it is Roberts right to indicate his issue, as well as it is your right, Anton, to express your annoyance with it, AS LONG AS both comply with -among other things- with said law of insult. The word "infest", though, next to the general tone of voice in your few lines, Anton, seems to be -by a hair’s breadth- extremely close to that borderline; whether it is beyond already or still within this side of that line would probably have to be decided by a judge and his or her jury, should need arise. Also, the "alleged brilliance", as I see it, is an allegation and an innuendo in itself. In any case, I can not discern anything even close to an insult in Robert’s statement or the frequency of his postings. Is what I’m saying to be contested?

  9. Bruce Mayfield

    Mike, you make an excellent case for freedom of expression and speech, which I too enjoy exercising quite frequently. So you have served Mr. Oldershaw well as his advocate. (Please note that I didn’t put anything derogatory before the word “advocate.”) While I concede that there are aspects of Mr. Oldershaw’s fractal universe that are interesting and that his radical new paradigm has demonstrated some predictive value, I was pushed away from his notions by the suggestion that, just like atoms, stellar systems come in discrete mass units, when stellar systems are observed shedding mass almost throughout their entire lifetimes. I’m also repelled by the notion of there being no beginning, that everything has always been expanding for infinite time and on infinite levels. Therefore I understand Anton’s point and his frustration too.

  10. Mike W. Herberich

    I more than merely understand what you are saying, Bruce. I even second your two main points of doubt, even if possibly not out of the exact same motifs. I’ve not even come so far as to having read what Robert’s theories do exactly posit and therefore am not in a position to attack (or second) this or that particular argument. — My main issue, if possibly expressed badly, was the actual manner in which one expresses oneself. Sure, it is not always easy to control one’s words at every instance (whereas you yourself are a perfect example for the opposite!). Yet, and all the more in rather scientifically inclined forums, one should strive not to use derogatory or outright insulting words or pretty clear implications thereof. Not the least reason for that, I think, being that usually such utterances do not only NOT contribute to the issue at hand (or actually ANY factual issue altogether!) but rather do incite unpleasant (and unnecessary!), even bad feelings. In other words, they artificially construct and further conflict of the unproductive, angry kind where there is no real (factual) reason to support such a thing. — Freedom of speech (plus all other laws, be they actual or moral and ethical) do absolutely allow for Anton to state: "I feel terribly frustrated about Robert’s theories and the frequency of his postings. These are my points: etc., etc.". More or less the way you stated it yourself, Bruce. But it is another thing to say: "this guy is getting on my nerves: he’s infesting forums with theories that only he thinks are brilliant. Etc., etc.". Am I seeing it wrong?

  11. Bruce Mayfield

    Mike, those last two comments of yours are IMO the best expressed that you have ever posted in this forum. In no way whatsoever were they expressed badly. I wholeheartedly agree with your call for civility too. My point was just that I understood Anton’s comment, but you were and are right to object to what was an attempt to bully Robert L. Oldershaw off this forum. No, you weren’t seeing it wrong at all. And to the publishers let me say thanks again for allowing diverse ideas to be expressed here while maintaining rules that promote polite discussion.

  12. Mike W. Herberich

    I’m happy we seem to agree witch each other on this, Bruce. I dare interpret the remaining silent of Robert and Anton (and others) as implied accordance, too. Still, there is, I think, on more point to be made, or differentiated: it is not as if Robert had come on and stated: "Folks! You all don’t get it: the Earth is neither flat, nor is it a sphere; it is rather a cube, swimming in the essence of four-dimensional glue … bla-bla". Again, I haven’t read more than maybe three or four pages or items of Robert’s (by the way, pretty neatly) presented webpage. I am utterly unable to give any sensible judgement of the entire construct of Robert’s as for factuality, faults thereof, correctness, other than doubts. For the exact same reason I may not decry, let alone disavow a thing that -at first sight- looks rather comprehensive, is accessible completely freely, for everyone to judge and does not seem to reveal open or easy basic faults to be demonstrated clearly and easily.

  13. Mike W. Herberich

    … This all rather sounds more pretty close to what one would expect "real" science to comply with. In other words, there are, if pretty hard to pin down, different levels or ways of statement, assertion and their way of presenting them to the public. To start with and to help discern such levels, it is always a good thing to present a theory or claim in an unagitated, unexcited, rather clear and neat way and grant anyone access to its premises to judge for themselves, to even object, discuss and evaluate. Usually, the "bad" or outright "crazy" kind of "oddballs" fall short of even these primary requisites. Completely openly accessible, democratically free proceedings usually hint at people of the same kind and, many times, at issues of the same kind … which NEVER, I repeat: NEVER are equivalent to absolutely faultless constructs, be it in theory, experiment and practice or modelling. Such a thing simply does NOT exist. Or else, it were not science, to begin with. Also, it has been shown that, utterly normal and sane people may turn into crackheads … and, sometimes even vice versa, with time. So, we’re all not safe of committing mistakes. We’re all human … or are we?

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