Probing Strong-field General Relativity with Gravitational Waves
Abstract
We are on the verge of a new era in astrophysics as a world-wide effort to observe the universe with gravitational waves takes hold---ground based laser interferometers (Hz to kHz), pulsar timing (micro to nano Hz), measurements of polarization of the cosmic microwave background (sub-nano Hz), and the planned NASA/ESA mission LISA (.1 mHz to .1 Hz). This project will study the theoretical nature of gravitational waves (GWs) emitted by two sources in the LISA band, namely supermassive-black-hole (SMBH) binary mergers, and extreme-mass-ratio-inspirals (EMRI's)---the merger of a stellar mass black hole, neutron star, or white dwarf with a SMBH. The primary goal will be to ascertain how well LISA, by observing these sources, could answer the following related questions about the fundamental nature of strong-field gravity: Does Einstein's theory of general relativity (GR) describe the geometry of black holes in the universe? What constraints can GW observations of SMBH mergers and EMRIs place on alternative theories of gravity? If there are deviations from GR, are there statistics that could give indications of a deviation if sources are detected using a search strategy based solely on GR waveforms? The primary reasons for focusing on LISA sources to answer these questions are (a) binary SMBH mergers could be detected by LISA with exquisitely high signal-to- noise, allowing enough parameters of the system to be accurately extracted to perform consistency checks of the underlying theory, (b) EMRIs will spend numerous orbits close to the central black hole, and thus will be quite sensitive to even small near-horizon deviations from GR. One approach to develop the requisite knowledge and tools to answer these questions is to study a concrete, theoretically viable alternative to GR. We will focus on the dynamical variant of Chern-Simons modified gravity (CSMG), which is interesting for several reasons, chief among which are (1) that CSMG generically arises in both string theory and loop quantum gravity, and (2) that although CSMG is consistent with all present day tests of GR, it still allows for significant, near-horizon deformations in the geometry of rotating (Kerr) black holes. Here is a brief list of the steps and research methodology we will employ: (i) Obtain the equivalent of the full Kerr solution in CSMG using numerical methods. (ii) Explore the structure of GWs emitted by EMRIs about the CS rotating black hole solution. Given simulated LISA noise curves, we can then address the questions posed above within the context of CSMG. (iii) Simulate the latter stages of comparable mass SMBH binary mergers in CSMG by numerically solving the full CSMG field equations to learn about highly dynamical, non- linear GR deformations. We can then repeat the analysis of (ii). (iv) Study whether CSMG GWs fit in the recently proposed parameterized post- Einsteinian (ppE) framework, to study generic deviations from GR in a statistical fashion. One can then repeat the analysis of (ii) but within the ppE scheme. We believe this proposed work is of significance and import to both the objectives of this solicitation, and the interests of NASA---knowing the nature of strong-field gravity will be one of the keys to unraveling the origin of the universe, and will tell us how black holes behave and interact with their environs, the details of which are important in understanding the formation and evolution of structure in the universe. Furthermore, these questions are best suited to be answered by LISA, a planned joint NASA-ESA mission. The ultimate success of LISA is very much dependent on (amongst other things) how well the community understands the complete nature of gravitational wave sources.
- Publication:
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NASA ATP Proposal
- Pub Date:
- 2010
- Bibcode:
- 2010atp..prop...96P