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Gravitational Lensing Probes Dark Energy

Abell 1689 is one of the most massive clusters of galaxies known, making it a superb venue for the study of dark matter. That’s because the cluster, some 2.2 billion light years away, creates gravitational lensing that magnifies and distorts the light from galaxies far beyond it. Astronomers used Abell 1689 in 2008 to identify one of the youngest and brightest galaxies ever seen, a galaxy in existence a mere 700 million years after the beginning of the universe. That find, A1689-zD1, turned out to be ablaze with star formation in an era when stars were only beginning to emerge.

New Hubble studies have now used Abell 1689 yet again to make some of the most detailed maps yet of dark matter. The idea is this: The cluster’s gravitational lensing bends and amplifies the light of objects beyond it. The researchers, led by JPL’s Dan Coe, go to work on the distorted images that result, figuring out the mass it would take to produce them. If the galaxies we see in the cluster were the sole source of gravity, the distortions would be much weaker. To straighten out the images, then, requires a great deal of dark matter within the cluster.

Image: Compass and Scale Image for Abell 1689 Dark Matter Map. Credit: NASA, ESA, D. Coe (NASA, Jet Propulsion Laboratory/California Institute of Technology, and Space Telescope Science Institute), N. Benitez (Institute of Astrophysics of Andulusia, Spain), T. Broadhurst (University of the Basque Country, Spain), and H. Ford (Johns Hopkins University).

The lensing effect is powerful, with the Coe team finding 135 multiple images of 42 background galaxies at distances ranging from 7 to 12 billion light years. The map of dark matter distribution that results from this work, if verified, would represent the highest resolution depiction of a galactic cluster’s dark matter distribution yet produced. It’s a particularly interesting result because the effects of dark energy, pushing against the gravitational pull of dark matter, should have had a disruptive effect on the growth of the cluster. The results parallel studies of other galactic clusters with dense cores, leading Coe to this conclusion:

“Galaxy clusters, therefore, would had to have started forming billions of years earlier in order to build up to the numbers we see today. At earlier times, the universe was smaller and more densely packed with dark matter. Abell 1689 appears to have been well fed at birth by the dense matter surrounding it in the early universe. The cluster has carried this bulk with it through its adult life to appear as we observe it today.”

Galaxy clusters, in other words, probably formed earlier than previously thought, before dark energy could go to work to inhibit their growth. Coe’s work with mathematician Edward Fuselier has produced new techniques for calculating the dark energy map, a feat the scientist likens to ‘cracking the code’ of gravitational lensing. Adds Coe:

“Other methods are based on making a series of guesses as to what the mass map is, and then astronomers find the one that best fits the data. Using our method, we can obtain, directly from the data, a mass map that gives a perfect fit.”

The analysis method in play is called LensPerfect, described this way in the paper on this work:

LensPerfect is a novel approach to gravitational lens mass map reconstruction. The 100+ SL features produced by A1689 present us with a large puzzle. We must produce a mass model of A1689 with the correct amounts of mass in all the right places to deflect light from 30+ background galaxies into multiple paths such that they arrive at the 100+ positions observed.

Most SL [strong gravitational lensing] analysis methods construct many possible models and then iterate to find that which best matches the data. LensPerfect instead uses direct matrix inversion to find perfect solutions to the input data. Using LensPerfect, we may, for the first time, obtain a mass map solution which perfectly12 reproduces the input positions of all 100+ multiple images observed in A1689.

Gravitational lens work, then, involves reconstructing the actual mass distribution based on the highly magnified and distorted images produced by the lensing. It’s no small feat, but dark matter and dark energy are among the highest priority targets for modern science. Sharpening our tools for understanding what lensing is telling us is a step toward understanding both. This work studies dark energy through matter which, though dark, is increasingly within the grasp of study because of its profound effects on spacetime at the galactic cluster scale.

More clusters are to be studied in the same way through the Cluster Lensing and Supernova survey with Hubble (CLASH) program, which will examine 25 clusters over the course of the next three years. Conclusive evidence of early cluster formation may help us put some boundaries on dark energy in the early universe. Let’s hope so, for a universe of which we see and understand a mere four percent (the rest being dark matter and dark energy) is a challenge that energizes the very heart of physics.

The paper is Coe et al., “A High-Resolution Mass Map of Galaxy Cluster Substructure: LensPerfect Analysis of A1689,” The Astrophysical Journal 722 (2010), pp. 1-25 (abstract).


Comments on this entry are closed.

  • spaceman November 12, 2010, 17:35

    This is a beautiful picture!

    There is considerable indirect evidence for dark matter, including the work mentioned in this thread. Will this be the “decade of dark matter” as cosmologist Michael Turner put it? I think Turner means that the dark matter problem will be solved sometime between now and 2020. Correct me if I am wrong, but didn’t quite a few cosmologists expect the 2000-2010 decade to be the decade during which dark matter particles would be found? It is amazing that all of the searches continue to come up dry; after all, our efforts to find neutrinos, the Universe’s hot dark matter, paid off. The lack of direct evidence for dark matter makes me skeptical of declarations that such and such a time will be “the decade of dark matter.” How many of you think this will be the decade of dark matter… how long can this problem continue to elude billions of research dollars and the clever efforts of many very smart folks?

  • William Truderung November 12, 2010, 21:42

    I doubt that 2011-2020 will be “the decade of dark matter”, because I very much doubt that dark matter actually exists. The anomalous effects that dark matter was invented to explain, can just as easily be explained by some form of modified gravity theory. (Actually, more easily, since the observed effects fit certain modified gravity theories without the need for additional hand-set parameters, which all dark matter theories seem to require.) A particularly strong candidate at the moment is Verlinde’s “entropic gravity”, which has been discovered to automatically produce effects very similar to those of both “dark matter” and “dark energy” without the need for additional new physics.

  • Ron S November 13, 2010, 12:22

    Modified gravity theories “explain” the data because they are no more than manufactured curves to fit the data. Actually, several curves are needed, that are then stitched together in an arbitrary fashion. Every MOND or similar theory that I’ve looked at is just so.

    To be blunt, I find the occasional animosity toward “dark matter” inexplicable. Do you really prefer an arbitrary curve over the fact that gathering data about gravitating matter over cosmic scales is difficult and incomplete? Especially since, in all cases where the data are robust, our existing theories, which are principle based and not merely fitted curves, are extraordinary accurate at all scales.

  • Bob Steinke November 13, 2010, 16:05

    I do wonder whether, like the search for aether a century ago, the explanation to dark matter/energy will be something much wierder than just “there’s a new type of substance that we never detected before.” In any case, gathering data and trying to explain it is the right way to go.

  • Jay Lazor November 13, 2010, 17:23

    Dark matter is perplexing because it could be one of two possibilities; “traditional” matter with mass and gravity that cannot be detected through other means (such as WIMPs), or a consequence of laws of physics that we do not fully understand. I don’t think we should discount either possibility. Modified theories of gravity may not be correct, but may provide a new way of thinking that leads to future breakthroughs.

  • John Q November 13, 2010, 23:29

    Though I am a dark matter (DM) denier, this paper is a remarkable document both for the image and the associated mathematics. I will never discount the value of either. But the failure of many experiments, admittedly complex, to detect what might be particles of DM is not a minor development. Finding stuff in space (e.g. helium) prior to its being detected on earth has precedent. “Finding” stuff in space and year after year not being able to find its correlative in earthly laboratories is very troubling. I believe ultimately is will be found that a true quantum theory of gravity (one which will modify both QM and GR) will be able to explain the image and all the other phenomena we associate with DM, and DM itself will fade away into the void.

    In science, you cannot see fully without a good theory. The better the theory, the better the seeing. Right now we do not have a good theory so we are not seeing well. There are plenty of intriguing ideas but that is it. Nothing would please me more than if tomorrow some laboratory were to announce, following the elimination of all other possibilities, a solid candidate for DM has been identified, but I seriously doubt it is going to happen.

  • Eniac November 15, 2010, 17:12

    John Q: Troubling as it may be, I don’t think there is any rule requiring things that exist to have a correlative in Earthly laboratories. Neutrinos are near this limit, and so is the Higgs boson. Just a few orders of magnitude beyond (more energy for the Higgs, less cross section for the neutrino), any chance of such Earthly detectability all but vanishes. That does not mean there is nothing there, but it certainly is a dilemma for particle physicists trying to discover the thing.

  • kzb November 16, 2010, 9:02

    Let’s remember the reason why non-baryonic DM got hypothesised in the first place was the apparently non-Keplerian galactic rotation curve. Gallo, in the paper linked below, says that the reason for this apparent difficulty is the assumption of a constant mass-to-light ratio for baryonic matter across the galactic disc.

    If you let this assumption go, it is possible to model galactic rotation without recourse to either non-Newtonian gravity or new forms of indetectable matter. If you postulate instead that there is a step-change in the baryonic M/L ratio at some galactocentric radius, it all becomes explicable. The best of it is, this is being supported by recent observations of the outskirts of galaxies: where it has become possible to resolve individual stars at the “edge” of discs, it is clear the stellar number count is greater than the luminosity scale length would have you believe.


  • Mike Prather November 16, 2010, 21:30

    Although there’s currently no way to investigate it; what if dark matter and energy don’t actually exist inside our universe proper? If our universe is an expanding bubble of spacetime within hyperspace, perhaps what’s driving that is whatever passes for matter and energy in the hyperspace realm. Since we’re “riding the back” of another dimension is there anything which would prevent elements from that dimension from influencing matter in our ours? The distribution of matter in our universe might be directly related to the distribution of that other mysterious “hypermatter”. It might make sense that gravity is the common glue. We’re probably looking in the right place, but not for the right thing. Just some wild musings…

  • ProtoAvatar November 17, 2010, 4:52

    “Troubling as it may be, I don’t think there is any rule requiring things that exist to have a correlative in Earthly laboratories.”

    If you can’t test the dark matter theory – in laboratories/observatories/etc -, proving it to be either true or false, then the theory is one of those “not even false” – a position, apparently, shared by string theory (quantum mechanics, much like the movement of galaxies, is quite real).

  • Eniac November 17, 2010, 21:33

    ProtoAvatar: The point was that dark matter has indeed been seen, just not in Earthly laboratories (which troubles John Q). Like black holes, which have never been observed in a laboratory, but are nevertheless widely believed to be real.

    I am quite sure there are multiple lines of evidence leading to dark matter, and they all agree on its amount and nature. To say that dark matter is not real because galactic light curves can be explained differently (as William Trudering does), is a little bit like saying the Earth is flat because Eratosthenes’ observation could be explained by the sun being closer than he thought. Yes, perhaps Eratosthenes did not prove the Earth is round, but that does not mean it is flat.

  • kzb November 18, 2010, 8:49

    Eniac, the problem I have with DM is that simpler explanations of the data have been stepped over. See the Gallo paper on galactic rotation I linked above for one example.

    For another example, see the link below. I’ve got to admit it’s a bit left field, but then the authors are bona-fide academics. What’s more these are testable hypotheses:


  • ProtoAvatar November 19, 2010, 5:19


    “The point was that dark matter has indeed been seen, just not in Earthly laboratories (which troubles John Q).”
    No, Eniac.
    Galaxies behaving inconsistent with GR have been ‘seen’. Much like quantum mechanics has been ‘seen’.
    That does not mean the ‘dark matter’ explanation for the galaxies or the ‘string theory’ explanation for quantum mechanics have been ‘seen’ – or are proven in any way. Especially considering they are not the only conjectures attempting to explain what was ‘seen’.

    “Like black holes, which have never been observed in a laboratory, but are nevertheless widely believed to be real.”
    Black holes have been thoeretically detected and then, yes, they have been ‘sees’ (X rays, gravitational lensing, etc) – I’m using ‘seen’ as meaning ‘detected by scientific instruments’.

    “I am quite sure there are multiple lines of evidence leading to dark matter, and they all agree on its amount and nature.”
    All agree on its amount? Yes, that would be the amount needed to account for the missing mass of the galaxies.

    All agree on its nature? Not even close – there are half a dozen contradictory theories about what exactly is dark matter – and all present some really BIG assumptions.