LISA will also be able to test the nature of the gravitational waves and test specific alternative theories to general relativity. Using massive-black-hole inspiral data, LISA will be able to measure any hypothetical difference in the speeds of gravitational waves and of light with a precision of parts in 10¹⁷ and test whether or not the “graviton,” the putative quantum particle of gravity, has a mass.⁶⁶ Because the LISA spacecraft orbit the Sun, they will be sensitive to different mixtures of the polarization modes in the waves from a sufficiently longlasting source and may be able to test whether the general relativistic prediction of only two transverse quadrupolar modes is correct. These would constitute tests of Einstein’s theory in an entirely new regime.

Because binary black hole inspirals are controlled by a relatively small number of parameters, such as mass, spin, and orbital eccentricity, they are good candidates for standard candles.⁶⁷ They are good candidates because the frequency and frequency evolution of the waves are determined only by the system’s parameters, while the wave amplitude depends on those same parameters and on the luminosity distance to the source. No complex calibrations are needed. Matched filtering analyses have shown that, for nearly circular inspirals, LISA could measure luminosity distances to a few percent at redshift 2 and to tens of percent at z = 10. At the same time, because of the changing orientation of the LISA array with respect to the source, it can also determine the orientation, with precision of better than a degree for massive inspirals at z = 1. If this angular and distance resolution were enough to link a LISA event with a corresponding electromagnetic event in a host galaxy or quasar and thereby to yield a redshift, LISA would contribute a direct, absolute calibration of the cosmic distance scale (Hubble diagram) that relies only on fundamental physics rather than on the complex chain of largely empirical distance ladders on which researchers rely at present. A 2 percent measurement of distance combined with a redshift at z = 1 would give a 2 percent measurement of the dark energy parameter w. The combination of several such measurements could give a dark energy bound that begins to be competitive with JDEM. The main challenge will be in using LISA’s angular resolution to identify the host galaxy.

Because of the apparent close connection between galactic center black holes and the structure of their host galaxies, information on the formation and growth of massive black holes over cosmic time will feed into models of galactic formation and evolution. The study of EMRIs using coordinated gravitational-wave and electromagnetic observations will improve the understanding of the stars and gas in the close vicinity of galactic black holes. Within our own Galaxy, LISA will measure the orbits and determine the locations of up to 10,000 close binary systems consisting mainly of white dwarfs; as such systems are the precursors of Type Ia supernovas and millisecond pulsars, such a census will aid in understanding the evolution of such systems (see Table 2.24 for a summary).

Front Matter (R1-R12)

Executive Summary (1-8)

1 Introduction (9-14)

2 Science Impact (15-65)

3 Mission Readiness and Cost Assessment (66-114)

4 Policy Issues (115-117)

5 Findings and Recommendations (118-132)

Appendixes (133-134)

Appendix A: Letter of Request (135-136)

Appendix B: Background and Statement of Task (137-138)

Appendix C: Input from the Broader Astronomy and Astrophysics Community (139-142)

Appendix D: List of Briefings to the Committee (143-145)

Appendix E: Request for Information to Mission Teams (146-151)

Appendix F: Mission Teams' Technology Funding Inputs to the Committee (152-153)

Appendix G: Acronyms (154-157)

Appendix H: Glossary (158-167)

Appendix I: Biographies of Committee Members and Staff (168-174)