Daniel Shaddock - Space-based gravitational-wave detection with LISA

- affiliation: JPL

- LISA overview: mission, arms, measurement, sensitivity
- why space?
	- ground can only do > 5 Hz (seismic) and L < 10 km

- science objectives: formation of MBH, MBH growth/merger history, galactic nuclei populations, survey compact stellar-mass binaries, confront GR with observations, probe new physics and cosmology, search for unforeseen sources

- no-hair theorem (Kip!)

- LISA constellation
	- 3 arms/2-3 Michelsons yield two-polarization sensitivity
		- vacuum is cheap in space, once you build two arms might as well do another one
	- two ways to measure position: amplitude modulation, Doppler
	- unsteered orbits
	- constant solar illumination

- how LISA works
	- interferometry (not very hard compared to Earth) with telescopes
		- heterodyne interferometry (two optical fields generate beat note, follow phase)
		- interspacecraft: 40cm telescope, 1 W transmitted, 10^-10 W received, 10 pm/rtHz shot noise pointing fluctuations must be kept below to 10 nrad/rtHz (heterodyne wavefront sensing)
		- armlength variation: various corrections, point-ahead between reception and emission direction
	- proof mass: capacitive sensors and actuators, optical fiber UV-illuminate for discharging
	- micro-Newton thrusters: push back against solar wind
	- laser frequency noise: need to beat down 12 orders of magnitude
		- light is ruler to measure distance fluctuations; wavelength changes indistinguishable from distance change
		- interferometry helps because wavelength changes are common along two arms, and subtract; but arms need to be the same!
		- three-step approach: cavity stabilization, arm locking, TDI
			- cavity: ultra-low expansion glass inside 5 layers of passive thermal shield; has demonstrated target stability of 30 Hz/rtHz
			- arm-locking: arm is the most stable length reference; high bandwidth feedback despite 33 s delay
				- improvements: want to get single impulse; common arm better than single arm, differential better, integrated difference can be combined with common arm to get direct impulse
				- need variable offset-frequency cavity stabilization
			- TDI: synthesizing interferometers that look like they have equal arms
				- nice animations in loops
				- combination in post-processing: requires large dynamical range which works because phase measurement is fundamentally different than a power measurement
				- experimental proof that recombined correlated signals really cancel out
			- three steps demonstrated with margin in each

- LISA pathfinder: demonstrate critical LISA technologies in a space environment
	- test GRS, drag-free attitude control, thrusters, interferometry
	- scheduled for 2010 launch, 2 months shake-out, 6 months operations
	- shrink a LISA arm inside a single spacecraft
	- demonstrated performance would scale to LISA, without changes, in the right environment

- LISA architecture: optical bench x 2 x 3

- status
	- ESA mission formulation study began Jan 2005
		- moved into Cosmic Vision program (2015-2025), with stronger funding
	- National Academies' Beyond Einstein program review
		- determine which will launch first

- summary
	- spectacular science: many sources, high SNR
	- mature design, advanced technology
	- eagerly awaiting start of major funding