LIGO-G W Is there a future for LIGO underground? Fred Raab, LIGO Hanford Observatory

LIGO-G W Raab: Is there a future for LIGO underground?2 Basic Signature of Gravitational Waves for All Detectors

LIGO-G W Raab: Is there a future for LIGO underground?3 Different Frequency Bands of Laser- Based Detectors and Sources There exists a hole in the coverage afforded by currently planned terrestrial and space-based gravitational-wave detectors Audio band spaceterrestrial

LIGO-G W Raab: Is there a future for LIGO underground?4 What Limits Sensitivity of Interferometers? Seismic noise & vibration limit at low frequencies Atomic vibrations (Thermal Noise) inside components limit at mid frequencies Quantum nature of light (Shot Noise) limits at high frequencies Myriad details of the lasers, electronics, etc., can make problems above these levels Limit for a terrestrial surface facility

LIGO-G W Raab: Is there a future for LIGO underground?5 Gravity gradients: low-f limit for terrestrial detectors First estimated by Saulson (1984) prior to LIGO construction »Revisited by Hughes and Thorne (1998) after LIGO sites were selected and seismic backgrounds characterized »Limits detection band of surface terrestrial detectors to f > Hz Lower-f operation a rationale for space-based detectors »LISA is optimized for a much lower band (10 -4 – ) Hz »Seto, Kawamura and Nakamura (2001) introduce idea of DECIGO to target band around 0.1 Hz Campagna, Cella and DeSalvo introduce idea of gravity- gradient mitigation in an underground detector optimized for lower-f operation at an Aspen Workshop (2004)

LIGO-G W Raab: Is there a future for LIGO underground?6 Scientific rationale to push for lower frequency operation Binary neutron star inspirals have longer dwell times at lower frequencies; more opportunity to integrate up signals Black hole binaries merge at lower frequencies as the mass rises Known radio pulsars exist in larger numbers at lower frequencies

LIGO-G W Raab: Is there a future for LIGO underground?7 A “Baseline” Source: Waves From Orbiting Black Holes and Neutron Stars Sketches courtesy of Kip Thorne Exercises most of the frequency range of the detector

LIGO-G W Raab: Is there a future for LIGO underground?8 LIGO (Washington) (4-km and 2km) LIGO (Louisiana) (4-km) North America: Laser Interferometer Gravitational-Wave Observatory Funded by the National Science Foundation; operated by Caltech and MIT; the research focus for ~ 500 LIGO Scientific Collaboration members worldwide.

LIGO-G W Raab: Is there a future for LIGO underground?9 Initial LIGO detectors are working Goal sensitivity A possible design that meets goal sensitivity

LIGO-G W Raab: Is there a future for LIGO underground?10 Binary Neutron Stars: Initial LIGO Target Range Image: R. Powell S2 Range

LIGO-G W Raab: Is there a future for LIGO underground?11 What’s next? Advanced LIGO… Major technological differences between LIGO and Advanced LIGO Initial Interferometers Advanced Interferometers Open up wider band Reshape Noise Advanced interferometry Signal recycling Active vibration isolation systems High power laser (180W) 40kg Quadruple pendulum: Silica optics, welded to silica suspension fibers

LIGO-G W Raab: Is there a future for LIGO underground?12 Binary Neutron Stars: AdLIGO Range Image: R. Powell LIGO Range

LIGO-G W Raab: Is there a future for LIGO underground?13 Future Plans for Terrestrial Detectors LIGO long-term search (~one integrated year) using initial LIGO Virgo has made steady progress in commissioning, hope to begin science searches in near future Increased networking of resonant bars with interferometers Advanced LIGO (AdLIGO), approved by US National Science Board, planning a detector construction start for FY2008: PPARC funding in place in UK; funding being worked in Germany Japan working on a design for a large-scale, underground detector with cryogenic mirrors (LCGT)

LIGO-G W Raab: Is there a future for LIGO underground?14 What would an underground version of LIGO look like Long arms: probably 3-4 km, perhaps longer Equilateral triangle, rather than “L” shaped? Corner and end stations comparable to current surface facilities, with clean-room environments “Shaped” excavations at corners and ends to optimize gravity gradient noise? Thermal noise mitigation: by cryogenics(?), subtraction(?), or use of extremely low-loss materials Quantum noise mitigation: large mirrored test masses, QND or squeezing techniques using relatively low laser power Very-low-frequency seismic isolation systems Large vacuum system with cryogenics to trap contaminants Vibration-free pumping

LIGO-G W Raab: Is there a future for LIGO underground?15 TBD: Requirements and Concept Acquire seismic data from existing and planned sites Model gravity-gradient noise in existing and potential environments Identify constraints from other users of underground facilities; is coexistence feasible? Identify construction and life-cycle costs; is it more economical to build far below Earth’s surface or far above? Experience the next generation of GW detector technology as the push toward lower frequencies continues; develop schemes to reduce the non-terrestrial noise sources At this point a smaller prototype detector may make sense

LIGO-G W Raab: Is there a future for LIGO underground?16 Closing remarks… We are experiencing a rapid advance in the sensitivity of searches for gravitational waves A decade from now, gravitational-wave astronomy should be commonplace, using detectors on Earth’s surface and in space A significant coverage gap will likely be filled eventually, by an underground and/or a space-based detector.

LIGO-G W Raab: Is there a future for LIGO underground?17 …and opening a new channel with a detector in space. Planning underway for space-based detector, LISA, hoping to fly in next decade to open up a lower frequency band

LIGO-G W Raab: Is there a future for LIGO underground?18 Some of the Technical Challenges Typical Strains < at Earth ~ 1 hair’s width at 4 light years Understand displacement fluctuations of 4-km arms at the millifermi level (1/1000 th of a proton diameter) Control arm lengths to meters RMS Detect optical phase changes of ~ radians Hold mirror alignments to radians Engineer structures to mitigate recoil from atomic vibrations in suspended mirrors

LIGO-G W Raab: Is there a future for LIGO underground?19 The International Interferometer Network LIGO Simultaneously detect signal (within msec) detection confidence locate the sources decompose the polarization of gravitational waves GEO Virgo TAMA AIGO

LIGO-G W Raab: Is there a future for LIGO underground?20 Laser-Interferometer or “Free- Mass” Detectors suspended mirrors mark inertial frames antisymmetric port carries GW signal Symmetric port carries common-mode info Intrinsically broad band and size-limited by speed of light.

LIGO-G W Raab: Is there a future for LIGO underground?21 LIGO Science Runs S1: 17 days in Aug-Sep 2002 »3 LIGO interferometers in coincidence with GEO600 and ~2 days with TAMA300 S2: Feb 14 – Apr 14, 2003 »3 LIGO interferometers in coincidence with TAMA300 S3: Oct 31, 2003 – Jan 9, 2004 »3 LIGO interferometers in coincidence with periods of operation of TAMA300, GEO600 and Allegro S4: Feb 22 – Mar 23, 2005 »3 LIGO interferometers in coincidence with GEO600, Allegro, Auriga S5: Nov 4, 2005 – until 1 year of coincidence data collected