Dark Energy Task Force Report

By Risa Wechsler | May 18, 2006 12:12 pm

Andy Albrecht gave a very nice colloquium at Chicago last Wednesday about the Dark Energy Task Force report, the final draft of which was supposed to be available on Friday — such a nice colloquium that I actually think I caught a good fraction of what he talked about and felt like I should pass it on. The basic charge of the DETF was to report back to NSF, NASA & DOE with a summary of the proposed approaches for studying dark energy, characterized their relative merits, identify steps required to get there and evaluate how well proposed projects and approaches will do in sorting this mess out.

Here’s a summary of the report based on what I gleaned from Andy’s talk — but you should all go read the thing yourselves if you’re interested in such things, I make no promise of accuracy or completeness. There’s been lots of talk before on this blog about dark energy and why it’s interesting and why it’s perplexing and what it might teach us about fundamental physics, so I won’t get into that here. But I think cosmologists and particle physicists generally agree that the fact that we have so little understanding of the primary constituent of the energy density of the Universe is one of the most important questions in all of science.

The panel, chaired by Rocky Kolb, was composed of a pretty distinguished crew, both theorists and experiementalists and observers that specialize in the various techniques that have been proposed to measure something about the equation of state of the Universe (Albrecht, Gary Bernstein, Bob Cahn, Wendy Freedman, Jackie Hewitt, Wayne Hu, John Huth, Lloyd Knox, Mark Kamionkowsky, John Mathur, Suzanne Staggs, Nick Suntzeff). They clearly took their job pretty seriously. Over a period of many months, they had weekly phonecons, several meetings, and seem to have actually done lots of calculations. Now, I’m young enough that I don’t know the varied history of task force reports, but I was certainly impressed to see that in addition to ruminating on various things, they clearly actually wrote some code, and produced a lot of interesting numbers from it 😉

As Andy stated it (and I think this is pretty much right), the state of the field was that it was full of conflicting claims about what various projects could do and about the relative merits of various approaches and lacked any way to compare these approaches with any kind of standard. This really made it hard to see clearly or to evaluate what the best way forward was. So the first thing they tried to do was remedy this.

Here’s how the panel framed the issues:

The eventual goal should be to understand the nature of dark energy, but of course that goal is likely a long way away. In the meantime, we can hope to make progress in a few stages. The first thing to note is that the the effect of dark energy is characterized by an equation of state for the Universe, w(a) = P(a)/rho(a) (“a” refers to the scale factor of the Universe, P to the presure, and rho to the energy density). There is now pretty conclusive evidence that the Universe is accelerating, which is true for w < -1/3. In this framework, one can then ask:

  • is w(a) constant?
  • what are the equation of state parameters w_0 and w_a,
    as specified by w = w_0 + w_a(1-a)

  • is there evidence for changes in the nature of general relativity?

They then evaluated what we might learn about these issues in 4 stages:
1) what is known now
2) what will be known upon completion of existing projects
3) medium term projects, on a 5 year time scale, costing ~10’s of millions
eg. the Dark Energy Survey
4) long term projects, ~10 year, costing ~0.3-1 billion
(e.g., LST, JDEM, SKA)

They then chose one figure of merit for evaluating the worth of a given project or method, the inverse area of the error ellipse enclosing 95% confidence in the w0-wa plane. This particular choice of this certainly doesn’t seem unambiguous, but it does seem like one of the most natural way to evaluate it.

The primary observables in the Universe that can say something about the equation of state are the distance-redshift relation D(z), which has a one-to-one relation with w(a), and the growth factor, g(z), which has a one-to-one relation with D(z), if GR is correct. So if one can measure both of them independently it not only provides some independent measures of w(a), but it allows a test of GR and our standard cosmological model on very large scales. Note that by “large”, I mean really large, roughly half the “length” of the Universe; the base line over which most of these methods operate is about z~0 to z~1, which is roughly half the age of the Universe.

There are four main probes of DE that the panel considered, and on which most of the large project work towards constraining DE is focused:

  • Type Ia Supernovae, which provide a measure of D(z).
  • Baryon acoustic oscillations, a standing wave pattern on ~ 140 Mpc scales, which depends only on timescales in the early Universe and the speed of sound in the baryon-photon plasma, and provides a standard ruler and thus a measure of D(z).
  • The abundances of galaxy clusters, which probes the mass function of dark matter halos. The evolution of this abundance probes both g(z) and D(z), but is sensitive to uncertainties in the mass-observable relation
  • Weak lensing, which uses measurements of the distortion of galaxy shapes to probe the mass distribution.

The panel assesed these various methods in light of the four stages previously discussed. A few of their basic conclusions about the relative merits and how well one will be able to do with various methods:

  • Measuring D(z) with SN is a relatively mature method, limited primarily by the accuracy of photometric redshifts.
  • The baryon oscillation method is relatively young, but is less affected by systematics than the other methods — because the length scale of the wiggles is not much affected by the bias of the galaxy population that its measured with.
  • The panel concluded that cluster abundances had in principle higher potential than the previous two, but the primary uncertainty here is in understanding the relation between observables and halo mass, and it is not yet clear how well this will be able to be done. (This is one of the main things I’ve been working on with the Sloan Digital Sky Survey and the Dark Energy Survey, a “stage 3” project in the terms of the DETF, and I can certainly say that most of the assumptions in the literature and probably in various white papers that were contributed to the DETF are still pretty naive on this point).
  • Weak lensing is a relatively new method, and has suffered from a lack of standardization that has made different measurements difficult to compare. The biggest noise here is in the intrinsic shape of galaxies. If the systematics can be controlled and understood, this method has the most potential standing on its own.
  • From stage 3 projects (~5 year timescale), one can hope to get a factor of ~2 improvement over data from current surveys from individual methods, and a factor of 3-5 improvement if all four methods combined.
  • From stage 4 projects, one can hope to get an order of magnitude improvement over data from current surveys using all of the methods.

An essential conclusion that the task force came to was that combining all four methods gets you a lot that none of the methods can do on their own. This is true partially because of the added intrinsic value and the differing systematics of the various methods, but also because the combination of growth and distance tests can provide a fundamental test of general relativity. A second important conclusion of the task force was that stage 3 projects should really target improved and improved understanding of systematics. Because most of the stage 3 and 4 projects depend on large photometric surveys without full spectroscopic followup, understanding and minimizing systematic uncertainties in the photo-zs is one of the most essential for dark energy constraints. I’m currently in Barcelona at a Dark Energy Survey collaboration meeting, and I can tell you that how to do this best is one of the primary things being discussed.

My understanding is that the final version of the report came out last Friday, but I’m not sure where “out” means, and I haven’t had a chance to look. I’m sure one of our commenters can point us to a copy.

  • Michael

    Andy Albrecht has posted his DETF talk from the APS meeting on his web page:

    April APS slides

    And, of course the dark energy parameterization is w(a) = w_0 + w_a(1-a) (not (1+a)).

  • Cynthia

    I stumbled across a webcast talk by Rocky Kolb entitled “Dark Matter and Dark Energy” at the Space Telescope Science Institute (2/2006). Speaking as a confirmed layperson, I – oddly enough – find his explanation for the accelerating universe quite compelling. Evidently, he and his collaborators have devised a model in which cosmic acceleration is due to a back reaction emerging from the inhomogeneities of the universe. There are two appealing components to this explanation: 1) no need to invoke dark energy and 2) no need to go beyond general relativity. Perhaps this model is already on the chopping block or has been completely shot down. Regardless, I would still rank Rocky Kolb as one of the most dynamic/charismatic speakers on the video-streaming-circuit of physics colloquiums.

  • Aaron Bergman

    There are two appealing components to this explanation: 1) no need to invoke dark energy and 2) no need to go beyond general relativity. Perhaps this model is already on the chopping block or has been completely shot down.

    As you say, it would be nice if it were true. Unfortunately, there are very general theorems (based on the Raychaudhuri equation) that it can’t be true. Why Kolb et al don’t accept that is beyond me.

  • Rick

    The only problem I have with dark energy is that nothing remotely like it is ONCE mentioned in the Bible. So I have my doubts about whether it is the correct path to take. Therefore, I think modifying general relativity is the way to go.

  • http://countiblis.blogspot.com Count Iblis

    Rick, dark energy is mentioned in the Bible. It is the Devil. If W

  • http://countiblis.blogspot.com Count Iblis

    I see, we can’t use the symbol for ”less” here. Anyway, the universe can end in a Big Rip. Surely that is the ultimate force of destruction and that can only be a manifestation of the Devil.

  • http://blogs.discovermagazine.com/cosmicvariance/joanne/ JoAnne

    Nice summary! I think HEPAP has to bless the report at our next meeting.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Risa, thanks for the summary. I’ve seen Andy give this talk elsewhere, and it is true that the committee obviously did a lot of independent work.

    Aaron, I don’t think that the Raychaudhuri arguments rule out the latest Kolb et al. scenario; in fact there are exact solutions in which you would infer acceleration from a redshift vs. distance plot even without dark energy, so I certainly hope that there aren’t any theorems against it. What the theorems do rule out is that *super-Hubble-radius* perturbations can mimic dark energy all by themselves; that was the original idea, and it definitely doesn’t work. The new idea is to use back-reaction from the second-order terms in the cosmological perturbation expansion (see here). In my judgment that’s also unlikely to work, but I don’t think that it’s ruled out by any theorems.

  • Aaron Bergman

    in fact there are exact solutions in which you would infer acceleration from a redshift vs. distance plot even without dark energy,

    Can you give an example? I don’t think inside or outside the horizon is really the issue — even if the wavelength of the fluctuation is larger than the horizon, it still varies inside the horizon. My recollection was that you need to violate an energy condition.

  • Shantanu

    Thanks for pointing to the report. IMO one point (which is hardly mentioned in any reports) is that in order to attack the dark energy problem from all points of view , aggressive efforts should also be invested in testing gravity in the lab: for example more precision tests of equivalence principle,
    tests for violaton of strong equivalence principle, search for preferred frames of frames, deviations
    from Newton’s law at small distances, etc. . If teh dark energy problem is really because of “modified gravity” then we should spend some efforts in designing
    laboratory based experiments to look for signatures of deviation from newton/einsteinian gravity.
    and this is equally important as compared to mapping out equation of state using astrophysical
    observations. for example I would love to see this (and other novel ideas for probing gravity) experiment fully funded.

  • http://www.amara.com/ Amara

    Shantanu: A long time ago (later 80s), I worked on part of that problem for my Master’s degree [1]. My project simulated the cold trap part of an experiment of Einstein’s Equivalence Principle that measured the gravitational force on charged particles — antiprotons, where the experimenters were interested to know how the rotational and kinetic energy of the charged particles was transferred among n particles because slight inhomogeneities in the Bfield made the experiment unworkable. [2,3,4]. It was an elegant experiment, but it ran out of funding and sits gathering dust in a lab room at Los Alamos. However I see from a web search [5,6] that others in Europe are working to continue it. Maybe antiprotons have become affordable now.

    [1] http://www.amara.com/ftpstuff/MSThesis.pdf
    [2] “The fall of charged particles under gravity: A study of experimental problems”, T. W. Darling, F. Rossi, G. I. Opat, and G. F. Moorhead Rev. Mod. Phys. 64, 237—257 (1992) http://prola.aps.org/abstract/RMP/v64/i1/p237_1
    [3] “Thermal Fluctuations and Experiments on the Free Fall of Electrons”, Humphrey J. Maris, Phys. Rev. Lett. 33, 1177—1180 (1974)
    [4] F.C. Witteborn and W.M. Fairbank, Nature 220 (1968) 436, and “Experimental Comparison of the Gravitational Force on Freely Falling Electrons and Metallic Electrons”, F.C. Witteborn and W.M. Fairbank, Phys. Rev. Lett. (1967) vol. 19, Issue 18, pp. 1049-1052.
    [5]”Testing the Universality of Free Fall for Charged Particles in Space”, (2004) Hansjorg Dittus, Claus Lammerzahl, and Hanns Selig, General Relativity and Gravitation,Vol.36, No. 3, March 2004 http://www.zarm.uni-bremen.de/2forschung/gravi/publications/papers/2003DittusLaemmerzahlSelig.pdf.
    [6] A new path toward gravity experiments with anti-hydrogen”, P. Perez, A. Rosowsky, Nucl.Instrum.Meth. A545 (2005) 20-30. http://arxiv.org/abs/hep-ex/0411077/

  • Doug

    I guess the point is, since we don’t understand “light matter” or “light energy,” why the big fuss over dark matter or dark energy? Where’s the LETF? Or is thinking that “light energy” is the cause of gravity inconsistent somehow?

    But you would think that since it’s thought that the energy of light matter is the engine of the observed gravity, then it would follow that the energy of dark matter is the engine of the observed antigravity, and if this is so then the real question is still, “What is matter, dark or light?” Are we getting the cart before the horse here?

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    I don’t quite get what you mean Doug. We have an exquisite understanding of “light matter”. QED is one of the best-tested theories in all of science.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Aaron, you’ll have to look in the papers for the examples; they are basically Swiss-cheese type models. I think that you have to violate an energy condition to get a locally negative deceleration parameter, but that’s not the same as inferring a negative deceleration parameter from a plot of luminosity distance vs. redshift.

    Again, I don’t think it works, but I also don’t think that there is a theorem ruling it out.

  • Aaron Bergman

    Can you maybe be a little more specific? I don’t know which papers you’re referring to. The Kolb papers? The last time I was looking at those, they had some weird coarse graining procedure that I couldn’t decipher the physical relevance of. What I was thinking of is essentially the content of Hirata and Selkac, astro-ph/0503582.

  • Aaron Bergman

    Selkac -> Seljak. Not sure how that happened.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    References in the most recent Kolb papers. The Hirata and Seljak argument refers to the earlier incarnation, which everyone agrees is dead.

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  • Doug

    Mark wrote:

    I don’t quite get what you mean Doug. We have an exquisite understanding of “light matter”. QED is one of the best-tested theories in all of science.

    Mark, where would QED, or the standard model be, if we couldn’t input mass as a free parameter? The only difference between our knowledge of the dark and light matter/energy is that we can measure the properties of the latter more directly, but we can’t explain why the light side has the properties we measure in its effects, such as mass, charge, spin, etc, any more than we can explain why the dark side has the properties we measure in its effects, or the effects that we are currently attributing to it.

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    It is quite right to say that there are things we don’t understand in the SM. However, it is incorrect to describe it as merely a parameterization of the data, as you seem to be doing, and as we in essence treat dark matter and dark energy as. The SM, once one chooses those few parameters, makes all kind of detailed predictions qwhich have then been spectacularly confirmed by experiments, both laboratory and cosmological. It is just not right to make a comparison between this and the dark components.

  • http://home.earthlink.net/~dolascetta/PhysicsFrameSet.html Dick

    Dear Physicists,

    After you have measured the equation of state of the dark energy, what will you know then? Let’s say it turns out to be w = -1. “Aha!,” you will say, “It is the cosmological constant.” But what will that tell you? What is the cosmological constant? Why is it there? You won’t know the answers as long as you have no idea what spacetime is. You really need to put the abstract mathematics aside for a while and get back to doing physics. Quantum field theory and general relativity are inadequate to the task, being only effective theories. String theory and loop quantum gravity are impressive mathematically, but as physics theories, they aren’t even wrong. Quantum mechanics offers all the tools you need. You will discover that spacetime inflates, expands, and accelerates because that is its nature; it can’t do anything else. But perhaps you prefer stumbling around in a pitch-black room, looking for a pitch-black cat that you aren’t even sure is there. (It is.)

  • Aaron Bergman

    Sean, those papers seem to do this weird spatial averaging procedure to get “acceleration”. As far as I understand it, we measure the distances to distant supernovae. I can’t see why that has anything to do with the quantity that Kolb et al define. All that matters is what happens along the geodesic from the supernova to us, so I still think Hirata and Seljak (or arguments along those lines) ought to apply.

  • Doug


    Remember, the SM is the SM of fundamental interactions of light matter/energy; that is, it is a model of how matter/energy behaves. If we had a comparable SM of fundamental interactions of dark matter/energy, we still would not know what matter/energy is, which is the ultimate answer we seek, if we are to understand the physical structure of the universe.

    That’s the point that leads to the question, “where is the LETF?”

  • Mike

    Mark, you’re a poo-poo head.

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    Thanks Mike.

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