The Laser Interferometer Gravitational-wave Observatory and the search for the elusive wave Nergis Mavalvala Visiting committee, October 2008

Gravitational waves (GWs) Prediction of Einstein’s General Relativity (1916) Indirect detection led to Nobel prize in 1993 Ripples of the space-time fabric GWs stretch and squeeze the space transverse to direction of propagation Emitted by accelerating massive objects Cosmic explosions Compact stars orbiting each other Stars gobbling up stars “Mountains” on stellar crusts 

GW detector at a glance Mirrors hang as pendulums Quasi-free particles Respond to passing GW 4 km 20 kW Optical cavities Mirrors facing each other Builds up light power Lots of laser power P Signal  P Noise  10 W

Global network of detectors GEO VIRGO LIGO TAMA AIGO LIGO Detection confidence Source polarization Sky location LISA

Initial LIGO Sensitivity

Gravitational-wave searches

Science Run 5 (2005 – 2007) S5

The mystery of GAMMA RAY BURSTs Is it a supernova explosion or is it a compact binary merger?

GRB070201 Intense GRB located in an error box that includes the spiral arms of the nearby Andromeda galaxy (M31) About 2.5 million l.y. away Detected by Konus-Wind, Integral, Swift and Messenger LIGO detectors were on the air at the time

What did LIGO find? LIGO did not detect a signal DM31 25% 50% 75% 90% LIGO did not detect a signal Consequence: we can say with >99% confidence that GRB070201 was NOT caused by a binary star merger in M31 35 observational papers since 2003 Astrophys. J 681 (2008) 1419

Coming soon… to an interferometer near you Enhanced LIGO Advanced LIGO

Ultimate limits ? Seismic gravity gradient When ambient seismic waves pass near and under an interferometric gravitational-wave detector, they induce density perturbations in the Earth, which in turn produce fluctuating gravitational forces on the interferometer’s test masses. Human gravity gradient The beginning and end of weight transfer from one foot to the other during walking produces the strongest human-made gravity-gradient noise in interferometric gravitational-wave detectors (e.g. LIGO). The beginning and end of weight transfer entail sharp changes (time scale τ∼20 msec) in the horizontal jerk (first time derivative of acceleration) of a person’s center of mass.

Improved sensitivity Initial LIGO Enhanced LIGO Advanced LIGO

Improved reach Initial LIGO Enhanced LIGO Advanced LIGO EnLIGO Enhanced LIGO Initial LIGO can “see” ~ 1000 galaxies Enhanced LIGO (2009) ~ 10000 galaxies Advanced LIGO (2015) ~ million galaxies Important for catching distant and/or rare events Advanced LIGO

The LIGO Group at MIT

LIGO Laboratory Organization

LIGO Laboratory Joint Caltech and MIT project Initial LIGO was ~ $400M to construct and ~$30M/yr to operate Advanced LIGO ~$200M Largest project at the NSF Laboratory personnel Caltech ~ 100 (6 faculty, 30 senior research staff) MIT ~ 35 (4 faculty, 5 senior research staff) Hanford Observatory ~ 30 Livingston Observatory ~ 30 LIGO directorate and business office at Caltech Much leadership and intellectual influence at MIT

MIT LIGO group Faculty Senior research staff Founding father, problem solver at large Lead on searches for burst (transient) sources, computing Leader of the quantum measurement group GR astrophysics and GW sources Faculty Rai Weiss Erik Katsavounidis Nergis Mavalvala (Scott Hughes) Senior research staff David Shoemaker Peter Fritschel Mike Zucker Gregg Harry Rich Mittleman 6 Ph.D. and 4 Masters since 2003 Leader of AdLIGO, Director of MIT LIGO Lab Systems scientist, leader of major subsystem for AdLIGO, initial proposer and designer of EnLIGO Former director of LIGO Livingston Observatory, presently co-leading Enhanced LIGO implementation Lead scientist on optical coatings for Advanced LIGO Head of the prototyping lab at MIT, lead on AdLIGO mechanical isolation systems

Closing remarks

When the elusive wave is captured… Tests of general relativity Waves  direct evidence for time-dependent metric Black hole signatures  test of strong field gravity Polarization of the waves  spin of graviton Propagation velocity  mass of graviton Astrophysics Predicted and unexpected sources Inner dynamics of processes hidden from EM astronomy Dynamics of neutron stars  large scale nuclear matter The earliest moments of the Big Bang  Planck epoch Precision measurement below the quantum limit

In closing... Astrophysical searches from early science data runs completed The most sensitive search yet (S5) completed with 1 year of data at initial LIGO sensitivity Joint searches with partner observatories Enhanced LIGO implementation that should give 2 to 3x improvement in sensitivity underway Advanced LIGO construction begun (funded as of FY2008) Promising prospects for direct GW detection in coming years

The End