3rd International LISA Symposium

Curt Cutler, Max-Planck-Institut fuer Gravitationsphysik, Golm
cutler@aei-potsdam.mpg.de
The 3rd International LISA Symposium was held July 11-14, 2000 at the Max-Planck-Institut fuer Gravitationsphysik in Golm, Germany. LISA Symposia are being held every two years, with venues alternating between Europe and the United States. The first LISA Symposium was held at RAL in July, 1996; the second was held in July, 1998 at Caltech. The main organizing bodies for the 3rd LISA Symposium were the Max-Planck-Institut fuer Gravitationsphysik and the Max-Planck-Institut fuer Quantenoptik. The were about 100 participants. The Symposium proceedings will be published as a special issue of Classical and Quantum Gravity in July, 2001. A detailed (320-page) description of the LISA Mission, the recent LISA STS Report, is available on-line at ftp://ftp.rzg.mpg.de/pub/grav/lisa/sts_1.02.pdf A LISA conference naturally begins with an update on the politics. Within ESA, LISA is already approved as a Horizon 2000+ Cornerstone Mission, but that status has little practical worth, since ``approval'' leaves open the flight sequence, and without NASA cost-sharing the flight probably could not happen before 2017. Practically, LISA's proponents within both NASA and ESA see LISA as a joint NASA/ESA mission to be flown around 2010. There is now considerable enthusiasm for a shared LISA at NASA-so much enthusiasm that, as of this writing, Goddard is competing with JPL over leadership of the project. A cost-shared LISA would be a ``moderate mission'' from NASA's perspective, lying within the SEU Program (Structure and Evolution of the Universe). It was important that LISA did very well in the recent Taylor/McKee decadal review, ``Astronomy and Astrophysics in the New Millennium''- being the secondest-highest ranked ``moderate'' mission. (GLAST was first.) The full report is at http://books.nap.edu/books/0309070317/html/. However the general perception is that, for LISA to get funded, there first needs to be technology demonstration mission, to be launched (one hopes) around 2005-6. Several possible avenues for this are being pursued; as of this writing, the best bet seems to be a NASA ST3 mission, shared with ESA. The demonstrator mission would be a single satellite and would basically test the drag-free system (which cannot be tested on the ground), with the goal of demonstrating test mass isolation to within $3\times 10^{-14} {\rm m s}^{-2} {\rm Hz}^{-1/2}$ between 1 and 5 mHz, i.e., within one order of magnitude of the LISA goal. (Noteworthy: two ``graybeards'' at the Symposium gave strong warnings about the technology demonstrator. Rai Weiss warned repeatedly that it should not be made too ambitious, since 1) you can't risk it failing and 2) you don't want it to absorb all your time/energy. Ben Lange, a pioneer of drag-free flight and a veteran of many, many of successful space missions, advised that he'd ``avoid a demonstrator mission like the plague.'') The Symposium included about 50 talks, which is too many to summarize. I'll confine myself to listing what were, to me, a few highlights, and apologize in advance for the many excellent presentations I won't even mention here, but which you'll be able to read in the Proceedings. Sterl Phinney gave a beautiful and very upbeat summary of LISA sources. New to me were the quite optimistic estimates for the merger rate for massive black hole binaries (MBH's), based on a hierarchical clustering picture of structure formation, where small galaxies form first and merge to form bigger galaxies. This picture leads to estimates of event rates of $\sim 0.1-10$/yr for $10^6 M_\odot$ BH's out to z=2. But, importantly, LISA can see far beyond z=2; Phinney argued that the merger rate for $\sim 10^5 M_\odot$ BH's out to $z \sim 20$ might be $\sim 1/$day. John Armstrong and Massimo Tinto discussed their very important work (done with Frank Estabrook), showing how one can (with some changes in hardware) linearly combine the LISA data streams, with time delays, to form three linearly independent combinations for which the laser phase noise exactly cancels. Two combinations contain information on the two gw polarizations, and the third describes a ``breathing mode'' that doesn't couple to GR. This third mode can be used to help calibrate and eliminate noise sources, and to discriminate between non-Gaussian noise bursts and real gw bursts. There were several very interesting talks on solar-mass compact objects spiraling into MBH's. Scott Hughes showed that, when the MBH is near-extreme Kerr, the inspiral is strongly affected by superradiant scattering of gw's from the BH horizon. Gw's that scatter off the horizon tend to ``hold up'' the test-body and increase the inspiral time by $\sim 3\%$ (which is a lot of cycles). Wolfgang Tichy discussed work-in-progress with E. Flanagan, claiming that, because the background Kerr metric is stationary, it actually is possible to determine how the Carter constant evolves from fluxes at infinity (and at the horizon). And Bernard Schutz discussed his worry (aroused, I assume, by recent work by Janna Levin) that because the orbits of spinning bodies in Kerr are chaotic, the number of matched filters will grow exponentially with the integration time, and may be vastly greater than previously anticipated-effectively obliterating the usual gains from matched filtering. This was a warning, not result, and somebody needs to look more carefully at this issue. Large extra dimensions (perhaps as large as 0.3 mm) are now much-discussed in string theory, and Craig Hogan showed how these might lead to a gw background observable by LISA. He argued that the early universe should produce copious gw's with wavelength comparable to the size of the large extra dimension, which would be redshifted into the LISA band today. Since the gw spectrum so-produced would be highly peaked rather than flat, it is not constrained by bounds at much lower frequencies coming from millisecond pulsar timing and COBE. Ben Lange, who attended the whole meeting and then gave us his outsider's perspective, made several recommendations, especially emphasizing advantages of spherical test masses, instead of cubes as in the current plan. And he gave a delightful, short summary of how in practice one can use a felt pen and the classical mechanics of precession to find the principal axes of an almost-perfect sphere. Something new for a LISA Symposium: there were several talks describing laboratory prototypes for LISA systems. (Oliver Jennrich: ``LISA is now more than just ink on paper.'') Harry Ward discussed his work on developing an interferometric read-out system. He also reported on tests of how well hydrogen catalysis bonding of optical elements would survive the rigors of launch and space-and found the bonding held quite well under shaking and thermal cycling. Oliver Jennrich described his experiment showing the feasibility of the LISA phase measurement scheme, using the same amount of light as will be available for LISA. Manuel Rodrigues described a laboratory prototype for the inertial sensor, and Stefano Vitale described a torsion pendulum test bench he is building to testing the performance of the inertial sensor on the ground to some $5\times 10^{-13} {\rm Newton}/\sqrt{\rm {Hz}}$. And Michael Petersheim discussed a a prototype for the highly stable laser required by LISA ( $\Delta P/P < 4\times 10^{-4}/\sqrt{{\rm Hz}}$). Lastly, there was very interesting discussion both of possible variations in the LISA mission and possible follow-on missions (the latter to be flown around 2020-2025, so fancy was free). Bernard Schutz pointed out the possible advantages of LISA starting out as short-arm interferometer, before moving the satellites to the current baseline separation of $5\times 10^6$ km. He also suggested the addition of a 4th spacecraft, to fly at the midpoint of one of the three arms. NASA has strongly encouraged LISA scientists to think about possible follow-on missions to LISA, and is especially interested in missions that might detect a primordial background of gw's. There is little chance that LISA itself can detect a primordial background, since in the LISA band it will be swamped by the background from galactic and extra-galactic binaries. The binary background falls off at high frequencies, which leads to a next-generation LISA concept featuring 3 constellations of mini-LISA's, with the constellations forming a equilateral triangle around the Sun at 1 AU, and each mini-LISA having short ( $\sim
20,000$-km) arms to push the sensitivity band up to 1-10 Hz. All of this seems technologically feasible even in the near term-it's ``just'' a matter of money.