Trouble for Dark Energy Space Mission?

By Sean Carroll | January 3, 2011 5:27 pm

NASA doesn’t have nearly enough money to do what it wants to do. Well, nothing unusual about that. We’ve talked recently about the constraints that budgetary realities are putting on astronomers’ ambitions — here, here, here. Now it’s chickens-coming-home-to-roost time, apparently. Dennis Overbye has an article in the Times (via Brian Schmidt) about how cost overruns on the James Webb Space Telescope — the giant multipurpose infrared satellite into which basket NASA is putting many of its eggs — are forcing dark energy onto the back burner.

The current NASA vision for dark energy is a mission called WFIRST (Wide-Field Infrared Survey Telescope), which grew out of JDEM (the Joint Dark Energy Mission), which was in turn descended from SNAP (Supernova Acceleration Probe). WFIRST would try to use three different techniques to constrain dark energy parameters — weak lensing, baryon acoustic oscillations, and supernovae. It would also be able to search for exoplanets using microlensing, just as a bonus. But cost overruns on JWST have left NASA with very little money to do ambitious (or even not-very-ambitious) new missions, so WFIRST is now up in the air, despite being judged the highest priority by the National Academy Decadal Survey.

It looks like the U.S. might try to stay in the dark-energy game by funding a 20% share in Euclid, a planned mission by the European Space Agency. Meanwhile, techniques that try to measure parameters of dark energy without leaving the ground are continuing to improve. So maybe it will end up not being a big deal, and we’ll learn what we need to know anyway. Or maybe we’ll miss out on the opportunity for a transformative discovery. The only thing we know for certain is that it’s not easy to make these tough choices when it comes to planning missions over the course of decades.

  • Phillip Helbig

    Sean, you had the great suggestion of using “smooth tension” instead of “dark energy”. Whatever happened to that? (“Everything has energy, and lots of things are dark”. Smooth tension emphasises the two main aspects of what I still refer to as the cosmological constant (at least until someone shows that smooth tension is not a pure cosmological constant).)

  • Torbjörn Larsson, OM

    Speaking of exoplanets, IIRC this was supposed to be *the only* exoplanet effort in the NADS, wasn’t it? From goNADS to gone NADS, perhaps.

    So this may threat loosing years or decades in two interesting areas, I take it? Or is there now a larger window for international cooperation?

  • HelderVelez

    There is no Dark Energy.

    There exists a model that fits the ‘observed’ data, both cosmic and local (*),
    and does not need any darkness.
    Only uses one parameter: the Hubble constant.

    Read it here, at no cost 😉
    A relativistic time variation of matter/space fits both local and cosmic data

    (*) The Standard Model does not fit the local data.
    SM fits only 4% of the observed data.
    Stubbornly we are collectively refusing to think/read ,even to comment, in other directions.

  • Joseph Smidt

    Well, the cosmological constant fits dark energy well enough that I am not losing sleep over worrying about we may not get more precision data on the matter. But, I will admit it would be nice.

    JWST better be freekin awesome with a tremendous science output!!! Just sayin.

  • Sam Gralla

    I’m not a dark energy person, but it does seem to me that Euclid does much of what WFIRST does–and why compete in a world with limited funding and lots of science needing it? Of course I also seem to remember that WFIRST does a lot of planets stuff… which would be a loss. But really this doesn’t seem so bad if money is spent on planets elsewhere… and as long as NASA saves its $$$ for my favorite mission (LISA) of course.

  • Kurt

    What advantage is it to have a spacecraft to study Dark Energy? What advantage does it serve compared to ground based missions?

  • King Cynic

    So glad I left the US for greener pastures years ago. Looks like US science funding is in for a few very glum years.

  • g

    so WFIRST is now up in the air

    Or, um, not.

  • Christopher Hirata


    The main advantages of a space mission for dark energy are:

    [1] You get above the glow of the Earth’s upper atmosphere (and in some bands water absorption in the troposphere) in the near-infrared. This is needed for some part of all 3 of the dark energy probes mentioned at high redshifts, where you need high-quality data on spectral features that are in the visible when emitted from a supernova or galaxy but move into the infrared when seen by us. “Features” would include the emission lines and broadband spectral features of galaxies, as well as a number of broad absorption lines in supernovae. The “high redshifts” where you need a space mission are above z~0.8 for the supernovae and ~1.3 for the weak lensing (WL) and baryon-acoustic oscillations (BAO) — although see below.

    [2] The ability to do high resolution, wide field imaging with a stable point spread function, critical for the WL, where you need to measure small distortions in the apparent shapes of distant galaxies.

    That said, not everything in the dark energy business needs to be done from space. The shapes of larger galaxies can be accurately measured from the ground (yes, I am one of the people doing this). The low-redshift supernova work has also been very successful from the ground, and even at z~1 a ground-based project could reach the cosmic variance limit by measuring galaxy redshifts from the O+ line. In their domains of applicability, they even have advantages over space (other than being cheaper) — for example, covering the *entire* sky with WFIRST is not going to be practical (the telescope would spend too much time repointing and wouldn’t have time to read out its detectors with adequate signal-to-noise ratio), and for projects like the z<1 BAO that demand area over depth/stability, the ground is the best option. These projects will and should go ahead. But they complement a space mission — they are not substitutes (nor are they substitutes when combined with wishful thinking :).


    As for exoplanets: both Euclid and WFIRST have the technical capability to do the microlensing project. For both missions, the limiting factor is observing time allocation: there are more good projects to do than there is promised lifetime of the satellite. (A "5 year" mission may still be functioning after 10 years … but you can't count on it.) That said, the microlensing is (i) not driving hardware requirements; (ii) has been declared a priority; and (iii) can use some portions of the year that dark energy is not using effectively. So I'm sure that whichever satellite we get will do at least some microlensing.

    As for your headline question: Yes, Euclid and WFIRST (in its JDEM-Omega version) are very similar missions. I don't see how we would possibly end up doing both. Nor can one side of the Atlantic press forward without the involvement of the other (Europe needs US infrared detectors; US is near bankrupt; and both sides need to tap into the other's scientific/technical expertise). Somehow the projects will have to be merged, hopefully sooner rather than later. I could say more on this subject but it would drift into unconstrained speculation …

  • Brian137

    Christopher Hirata,
    Thank you for your informative post.

  • Cosmonut

    In the Denis Overbye article referred here, I read:
    ” The news has dismayed many American astronomers, who worry they will wind up playing second fiddle to their European counterparts in what they say is the deepest mystery in the universe.”

    I find this kind of statement- about “American science vs European science” rather annoying.
    Science is not a football tournament between nations.
    Its a human quest to understand the universe.
    How does it matter whether the crucial observation is made by an American spacecraft or an European or African one ?

  • réalta fuar

    Can’t say I’m terribly upset by this as I’ve always thought WFIRST has been quite oversold.
    But it’s certainly not true that the U.S. is “bankrupt” as mentioned by a previous poster as their economy seems to be more stable than the EU’s. Personally, I’d like to see a
    “Kepler” class external occulter with about a 2 meter aperture for the imager, but that makes too much sense for NASA to actually do. Cost would be about 40% that of WFIRST, it is no harder to do, and it would produce science that definitely cannot be done from the ground.

  • Jane Rigby

    #11: I agree that that part of the article was silly, and poorly describes the international character of astronomy.

    Most US astronomers have international collaborators. The US, European, and Japanese space astronomy long-term plans all know about each other. Missions are frequently international collaborations — for example, Hubble, Herschel, Suzaku, JWST.

    As an example: a few years back, the US far-IR community sat down to plan — what did they want to ask for in the Decadal Survey? Rather than try to build a competitor to the Japanese SPICA mission, they agreed to ask for the US to build an instrument for SPICA. Cheaper, less duplicated effort, and good synergy.

  • J.

    I think it is a bit ridiculous to blame JWST for NASA’s problems with following through on a dark energy mission. Yes, JWST is over budget, and therefore an easy scapegoat. But JWST is not to blame for NASA’s lack of vision/leadership during the whole SNAP/JDEM debaucle, and sudden realization that EUCLID is going to beat them to the punch. How much money was poured in SNAP and then JDEM over the past 10 years that is now simply gone?

  • Martin E.

    The whole Decadal process was pretty roundly trashed by the $1.2-16B overrun on JWST. One could ask why the decadal committees were not told of the possibility of large overruns during their final round of meetings; it must have been at least looming large as a worst case scenario.
    But, given that the #1 priority (WFIRST) can’t be done, the #2 Decadal priority is something we can get started on at once. That’s because it isn’t a single big mission, but the Explorer Program, which doesn’t have the same all or nothing quality, but is scalable to whatever funding is available.
    Explorers may well include excellent exoplanet missions. Coupled with 20% of Euclid, we’d still be doing the science areas the decadal favored, maybe even doing something of priority #4 with some kind of streamlined X-ray observatory. Not bad in the circumstances.

  • James Ph. Kotsybar

    — James Ph. Kotsybar

    The universe is mostly abnormal,
    if we accept that physicists aren’t wrong
    and Newton’s gravity’s uniformal,
    otherwise galaxies couldn’t last long.

    They’d spin themselves apart, unless, unseen,
    missing mass resolves the disparity.
    Dark Matter is needed to intervene.
    Though not found, it can’t be a rarity.

    “Shining stars are like icebergs,” they patter,
    “if the mathematics are to be served.
    There’s as much as five times normal matter
    needed to resolve dynamics observed.”

    Though they’ll say science is observation,
    that can tweak, if it fits the equation.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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