Transcript LIGOundeground

Is there a future for LIGO
underground?
Fred Raab,
LIGO Hanford Observatory
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Basic Signature of Gravitational
Waves for All Detectors
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Different Frequency Bands of LaserBased Detectors and Sources
There exists a hole in
the coverage
afforded by currently
planned terrestrial
and space-based
gravitational-wave
detectors
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space
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terrestrial
Audio band
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What Limits Sensitivity
of Interferometers?
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Seismic noise & vibration
limit at low frequencies
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Atomic vibrations (Thermal
Noise) inside components
limit at mid frequencies
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Quantum nature of light
(Shot Noise) limits at high
frequencies
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Myriad details of the lasers,
electronics, etc., can make
problems above these levels
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Limit for a terrestrial
surface facility
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Gravity gradients: low-f limit for
terrestrial detectors
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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 > 10-20 Hz
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Lower-f operation a rationale for space-based detectors
» LISA is optimized for a much lower band (10-4 – 10-2 ) Hz
» Seto, Kawamura and Nakamura (2001) introduce idea of DECIGO to target
band around 0.1 Hz
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Campagna, Cella and DeSalvo introduce idea of gravitygradient mitigation in an underground detector optimized for
lower-f operation at an Aspen Workshop (2004)
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Scientific rationale to push for
lower frequency operation
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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
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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
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North America: Laser Interferometer
Gravitational-Wave Observatory
LIGO (Washington)
(4-km and 2km)
LIGO (Louisiana)
(4-km)
Funded by the National Science Foundation; operated by Caltech and MIT; the
research focus for ~ 500 LIGO Scientific Collaboration members worldwide.
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Initial LIGO detectors are working
A possible design that
meets goal sensitivity
Goal
sensitivity
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Binary Neutron Stars:
Initial LIGO Target Range
S2 Range
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Image: R. Powell
What’s next? Advanced LIGO…
Major technological differences between LIGO and Advanced LIGO
40kg
Quadruple pendulum:
Silica optics, welded to
silica suspension fibers
Initial Interferometers
Active vibration
isolation systems
Open up wider band
Reshape
Noise
Advanced Interferometers
High power laser
(180W)
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Advanced interferometry
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Signal recycling
Binary Neutron Stars:
AdLIGO Range
LIGO Range
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Image: R. Powell
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Future Plans for Terrestrial
Detectors
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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)
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What would an underground
version of LIGO look like
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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
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TBD: Requirements and Concept
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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
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Closing remarks…
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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.
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…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
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Some of the Technical Challenges
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Typical Strains < 10-21 at Earth ~ 1 hair’s width at 4
light years
Understand displacement fluctuations of 4-km arms
at the millifermi level (1/1000th of a proton diameter)
Control arm lengths to 10-13 meters RMS
Detect optical phase changes of ~ 10-10 radians
Hold mirror alignments to 10-8 radians
Engineer structures to mitigate recoil from atomic
vibrations in suspended mirrors
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The International Interferometer
Network
Simultaneously detect signal (within msec)
LIGO
GEO
Virgo
TAMA
detection
confidence
locate the
sources
AIGO
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decompose the
polarization of
gravitational
waves
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Laser-Interferometer or “FreeMass” 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.
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LIGO Science Runs
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S1: 17 days in Aug-Sep 2002
» 3 LIGO interferometers in coincidence with GEO600 and ~2 days with
TAMA300
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S2: Feb 14 – Apr 14, 2003
» 3 LIGO interferometers in coincidence with TAMA300
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S3: Oct 31, 2003 – Jan 9, 2004
» 3 LIGO interferometers in coincidence with periods of operation of
TAMA300, GEO600 and Allegro
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S4: Feb 22 – Mar 23, 2005
» 3 LIGO interferometers in coincidence with GEO600, Allegro, Auriga
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S5: Nov 4, 2005 – until 1 year of coincidence data collected
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