National Academy: Dark Energy First, Maybe LISA Second

By Sean Carroll | September 5, 2007 4:21 pm

The National Academy of Sciences panel charged with evaluating the Beyond Einstein program has come out with its recommendations. Briefly: the first priority should be the Joint Dark Energy Mission (where “joint” means “with the Department of Energy”), but we should keep up some amount of work on LISA, the Laser Interferometer Space Antenna. Steinn has the lowdown, so you should go there for details.

I am happy to know that JDEM will go forward (if NASA listens to the panel, about which I’m less sure than Steinn seems to be); very happy that LISA gets at least some support, although if I were the European Space Agency I’d certainly be shopping around for more reliable partners; slightly bemused that little effort seemed to go into pushing a CMB probe; and very sad to see X-ray astronomy get the shaft, as Constellation-X and EXIST seem right out of the picture. We can only hope for happier times ahead.

CATEGORIZED UNDER: Science
  • Jeff

    heh! you beat me to it! I just sent you an email at your cosmicvariance account “requesting” a post on the BEPAC announcement!

  • Brian

    Too bad! I was rooting for LISA. I’m very disappointed that this stupid moondoggle has crippled the science program.

  • Ellipsis

    Indeed it is a little bit harsh on x-rays, but it was generally the right result. They will all happen in some form eventually, just JDEM first. Basically the right thing to do.

  • Brad Holden

    The reason why Steinn thinks NASA will listen to the panels recommendations is that NASA asked for this evaluation. The current administrator and the head of the Science Mission Directorate are also the types who appreciate input from the community.

    That said, it was always weird to me that these 5 kinds of missions were lumped together. Some, like Con-X, are not really Beyond Einstein missions, and others, like some of the CMB experiments, were in the bare planning stages. In contrast, JDEM is a Beyond Einstein mission with people making actual prototype hardware.

  • graviton383

    ….how about $45 to read the full report? Is this how they’ll pay for short-falls??

  • Aaron Bergman

    That’s just for the paper copy, I think. You can read online for free.

  • chemicalscum

    5. graviton383 on Sep 5th, 2007 at 6:51 pm

    ….how about $45 to read the full report? Is this how they’ll pay for short-falls??

    6. Aaron Bergman on Sep 5th, 2007 at 6:54 pm

    That’s just for the paper copy, I think. You can read online for free.

    .

    http://books.nap.edu/openbook.php?isbn=0309111625

  • Sam Gralla

    A reasonable decision. LISA is the obvious winner in terms of potential scientific payoff, but very risky–it might just not work. So, do an easy, low-payoff but gauranteed-to-succeed mission first, and then launch the big guns.

  • http://evolutionarydesign.blogspot.com/ island

    The dark energy mission will determine how expansion has changed over time, and from this we’re supposed to be able to determine whether the energy is a property of empty space as Einstein proposed, or whether it is a “dynamic force field that evolves”.

    But Einstein’s dark energy was counterbalancing property, (antigravitational *effect*), of the space of a finite closed spherical universe, which Einstein abandoned after Hubble showed that the universe was expanding, because he thought this would result in cumulatively runaway expansion.

    So if the mission confirms that the expansion of the universe is accelerating exponentially, then we’re back to Einstein’s finite deterministic model?

    I’ll buy that.

  • http://cosmiczoo.blogspot.com Matt

    Apparently these recommendations will pretty much match the timescale that ESA was planning for LISA anyway, so hopefully they wont cause any additional delays for it. Hopefully the launch of LISA Pathfinder in a couple of years time will prove that we can pull off the real thing, as LISA has the potentially biggest science payoff of all the Beyond Einstein missions.

  • Ellipsis

    Just to be a bit of a devil’s advocate on LISA here for a moment (I don’t take this view — I think LISA is an _incredibly_ worthwhile and fantastic project — but I think some people outside of the gravitational wave field may occasionally have a few misconceptions): LISA is undoubtedly the most technologically exciting of the Beyond Einstein projects. But in terms of actually seeing physics beyond Einstein, i.e. gravitational physics beyond GR, it is _LIGO_, not LISA, which is sensitive to the range of gravitational wave frequencies that is probably (of course “probably” is as sure as one can get in these situations) more sensitive to the high-energy effects (from black hole – black hole and black hole – neutron star collisions) that could modify GR. LISA is more sensitive to the non-impulse gravitational wave radiation that has already been previously probed in, e.g., the Hulse-Taylor binary system, and could directly detect such waves, but these are not primarily the violent collision-induced waves that LIGO probes, and that potentially have more reach for seeing beyond GR. LIGO exists and is being upgraded — and everyone is _very_ greatly looking forward to the results from LIGO, and hopes that a gravitational wave discovery will be made soon. JDEM, an utterly different type of device and project, is technologically very simple compared with LISA (even perhaps trivial when compared with such a dramatic undertaking), but that hardly means it is any sort of “low payoff” version of it or anything even remotely of the sort. In fact its reach for discoving effects that are actually truly of the “Beyond Einstein” variety could even potentially be a lot greater, as dark energy is a poorly-explored and very sensitive probe, and is in the regime where GR and quantum mechanics may actually meet.

  • Christopher Hirata

    Ellipsis: LISA is also capable of seeing strong gravity effects, except that because of the frequency band they would involve at least one supermassive black hole (instead of stellar mass BH as in LIGO). The impression I had from talking to people who do these calculations is that in terms of testing strong-field GR the comparison between LISA and LIGO is hands-down in favor of LISA. Someone please correct me if I’m wrong!

    Sam: I disagree on JDEM being “low payoff”. (Disclaimer: I’m on one of the JDEMs.) I cannot apply the phrase “low payoff” to completing the pie chart of what makes up the universe and measuring the basic phenomenological properties of each slice. I know some people are already convinced that it is a cosmological constant to whatever precision is plausibly achievable, the past suggests that this type of prejudice can be a mistake (remember Omega_m=1?).

    Let me conclude by saying I don’t envy anyone on the panel, seeing as they were bound to be on the receiving end of someone’s anger …

  • http://web.mit.edu/sahughes/www/ Scott H.

    Ellipsis: LISA is also capable of seeing strong gravity effects, except that because of the frequency band they would involve at least one supermassive black hole (instead of stellar mass BH as in LIGO). The impression I had from talking to people who do these calculations is that in terms of testing strong-field GR the comparison between LISA and LIGO is hands-down in favor of LISA. Someone please correct me if I’m wrong!

    No, this is exactly right. Ellipsis’s point, roughly, is that “strong field” really means “strong curvature”, ie “strong tidal forces”, and that LIGO, by testing black hole systems in a lower mass regime, actually looks for stronger tidal forces than the events that LISA should be able to measure (via tide ~ M/r^3, and r ~ M). There’s merit to this, though I personally think that the difference between 10 solar masses and 100000 solar masses is negligible as far as likely deviations from GR are concerned — both are starkly classical systems.

    The reason that LISA is “hands-down” favored for these tests is simply one of precision. The signals LISA can measure are a few orders of magnitude stronger (standing clearly above projected noise budgets, even without fancy filtering) and so you can measure/test/etc much more cleanly and precisely. LIGO is probably going to get the discovery (though not on a timescale for me to win my bet with Holz, sadly) and first, somewhat crude, cut on the fun science; LISA should be able to clean up on that science.

  • http://web.mit.edu/sahughes/www/ Scott H.

    So, do an easy, low-payoff but gauranteed-to-succeed mission first, and then launch the big guns.

    Or, to put this more accurately: A challenging (but based on very well-tested and flight-qualified technology), high-payoff mission that has a good probability to succeed. I don’t think ANYTHING stuck on the end of a rocket is “guaranteed” to succeed; if it were “low-payoff”, it wouldn’t even be considered; and if it were “easy”, we wouldn’t have needed the BEPAC committee in the first place …

  • Ellipsis

    thanks! I stand corrected.

  • http://blogs.discovermagazine.com/cosmicvariance/julianne Julianne

    I know some people are already convinced that it is a cosmological constant to whatever precision is plausibly achievable, the past suggests that this type of prejudice can be a mistake (remember Omega_m=1?)

    Having lived through the Omega_m=1 years, I would say that the current lamda measurements are fundamentally different. Back then, just about every observational test except flow measurements (which were by far the hardest) was getting Omega_m=0.4ish, and theorists kept arguing that the observers had to be wrong because Omega=1, and that sooner or later we’d measure it properly. There was tons of evidence that there was a massive disconnect between theoretical expectations and reality. With w however, different measurement techniques all converge at w=-1, and there is no strong theoretical prejudice that nature should favor some other value. Observationally, there is little evidence that the dark energy is anything but a cosmological constant. If JDEM conclusively shows that w=-0.97, that will be interesting, but I’m not holding my breath.

  • http://andyxl.wordpress.com/ Andy Lawrence

    This seems a sad confirmation of what Simon White was recently warning of – particle cosmology taking over astronomy. Dark Energy is probably the single key problem of our age.. but there is SO much more science to do with a proper astrophysics mission like Constellation X.

  • Ellipsis

    ah, how times have changed… there was a time long ago when some astronomers derided x-ray astronomers as a bunch of particle physicists!…

  • http://andyxl.wordpress.com/ Andy Lawrence

    err… really ? I began life as an X-ray astronomer in 1976 and never really detected people thinking we were particle physicists … weird aliens yes, but not particle physicists.

  • http://whenindoubtdo.blogspot.com/ Eugene

    Awww. I was rooting for LISA too. But I guess they went for ezmode.

  • Shantanu

    Andy,
    In LIGO also, lot of people are from HEP background.

  • Ellipsis

    Andy: Back another 5-10 years. (Note that the term “particle physics” didn’t really exist then, it was nuclear physics, but completely analogous given the time period.) Giacconi got his education building cloud chambers. Almost all of his colleagues were from a nuclear physics background rather than an astronomy one. The tiny bit of funding they started getting (in the US) was from the Atomic Energy Commission. And they were clearly viewed as such.

  • http://thecrossedpond.com adam

    Glad they picked JDEM. LISA as second is fine (although I don’t care so much about it). As others (and the panel) have pointed out, Constellation-X wasn’t really a Beyond Einstein deal anyhow.

    Go go JDEM.

  • zankaon

    DO WE ALREADY HAVE EVIDENCE OF DARK ENERGY – OUR MANIFOLD?

    If LIGO I with it’s 10^-21 sensitivity, VIRGO etc. don’t detect gravity waves, might this then be interpreted as indicating that C_R spacetime manifold’s stiffness is not INsignificant; rather than the assumption that g.w.s propagate long distance, and that it just requires a more sensitive detector? Statistically LIGO I seems to have a large enough volume and sample size for inclusion of compact objects in NS and BH binary systems in tight orbits at least, even if not catching any coalescing events. However even for binary coalescence of BHs, might generated {g.w.} decay very rapidly? So resistance to deformation (normal stress: extension and compression, and even any shear stress) might not be insignificant. Might such stiffness (resistance to deformation/distortion) be considered as like inertia of C_R manifold? That is, {g.w.} have non-localized energy, but such energy is associated with deformation of manifold. Hence such {g.w.} energy might be considered as trying to overcome resistance to deformation (i.e. stiffness) of C_R manifold. Hence such inertia of manifold (resistance to deformation) would seem to represent a contribution to stress energy momentum tensor and it’s matrix representation; thus contributing not insignificantly to overall curvature? So if long range g.w.s are not detected, then might LIGO I actually be exploring a qualitative assessment (not limits) as to stiffness of C_R manifold? Thus might C_R manifold be quite robust to perturbation? Any such robustness would seem consistent with such manifold not breaking up (i.e. so no ‘foam’?) for near to, and at C_p Planck scale; hence also consistent with no quantization of manifold C_R? Also then less likely to have leakage of g.w.s propagating out of a manifold into another dimension i.e. brane? Also wouldn’t any such significant stiffness of C_R manifold be less consistent with deformations associated with superstrings? Also if the concept of inertia of manifold is descriptive, then any entertained recent new acceleration (i.e. resulting then in a strain or elasticity of manifold) of such C_R manifold would seem less likely. Might energy associated with resistance to deformation of manifold represent a significant portion of energy required to approach flatness? That is, rather than a quest for so-called ‘DARK ENERGY’, perhaps an additional significant contribution is right before us, in the form of ENERGY of manifold C_R; such stiffness of C_R manifold contributing to stress energy momentum tensor, and hence to curvature. How would one further explore such latter conjecture, other than any qualitative finding of no long range g.w. propagation? Perhaps one could consider all alternative possibilities of sources of energy sufficient for approach to flatness. Then to the extent that they can be found to be less probable and/or no supportive evidence, then the last standing definitive contributing source of such energy (i.e. energy of C_R manifold) might have to be in part (or in full) accepted. So have LIGO I, VIRGO already made a GREAT DISCOVERY – that is, the inertia of C_R manifold? So C_R manifold seems to have significant stiffness, and hence contributes a significant amount of energy to Tuv, and thus contributes significantly to curvature. SRM.

  • Shantanu

    Sean, Mark or anyone else: can someone tell me the latest status of the LATOR experiment?
    when is it is going to be launched/whether it is fully funded or going through some technology
    review(similar to the beyond einstein missions, etc.)
    Thanks

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

    Don’t know anything about it, sorry. There is unfortunately no clearinghouse at which one can look up the status of all these different missions.

  • Ellipsis

    I think gr-qc/0701102 is the latest from the LATOR folks. E-mail Turyshev at JPL or his colleagues if you want something more recent. These projects take awhile.

  • Jeff

    LATOR was submitted with Turyshev and ESA collaborators to the Cosmic Visions call from ESA, and will probably be fed into the NRC Decadal Survey too, so it’s still a concept looking for a funding source mostly.
    List of all Cosmic Vision proposals:
    http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=40902

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Cosmic Variance

Random samplings from a universe of ideas.

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] cosmicvariance.com .

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