Status of the Laser Interferometer Gravitational-Wave Observatory

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Status of the Laser Interferometer
Gravitational-Wave Observatory

• Reported on behalf of LIGO colleagues by
• Fred Raab,
• LIGO Hanford Observatory

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Laser Interferometer Gravitational-wave
Observatory

• LIGO Mission To sense the distortions of
  spacetime, known as gravitational waves, created
  by cosmic cataclysms and to exploit this new
  sense for astrophysical studies
• Over nearly 400 years, astronomy has developed
  many clever ways to see and study the universe
  LIGO hopes to add a sound track using
  interferometers that function like large
  terrestrial microphones, listening for vibrations
  of space.

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Gravitational Collapse and Its Outcomes Present
LIGO Opportunities
fGW gt few Hz accessible from earth fGW lt several
kHz interesting for compact objects
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Potential LIGO Sources

• Supernovae, with strength depending on asymmetry
  of collapse
• Inspirals and mergers of compact stars, like
  black holes, neutron stars
• Starquakes and wobbles of neutron stars and black
  holes
• Stochastic waves from the early universe, cosmic
  strings, etc.
• Unknown phenomena

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Gravitational Waves

• Gravitational waves are ripples in space when it
  is stirred up by rapid motions of large
  concentrations of matter or energy

• Rendering of space stirred by two orbiting black
  holes

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Catching WavesFrom Black Holes
Sketches courtesy of Kip Thorne
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Energy Loss Caused By Gravitational Radiation
Confirmed

• In 1974, J. Taylor and R. Hulse discovered a
  pulsar orbiting a companion neutron star. This
  binary pulsar provides some of the best tests
  of General Relativity. Theory predicts the
  orbital period of 8 hours should change as energy
  is carried away by gravitational waves.
• Taylor and Hulse were awarded the 1993 Nobel
  Prize for Physics for this work.

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Sounds of Compact Star Inspirals

• Neutron-star binary inspiral
• Black-hole binary inspiral

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How does LIGO detect spacetime vibrations?
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Important Signature of Gravitational Waves
Gravitational waves shrink space along one axis
perpendicular to the wave direction as they
stretch space along another axis perpendicular
both to the shrink axis and to the wave direction.
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Sketch of a Michelson Interferometer
Viewing
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The Laser Interferometer Gravitational-Wave
Observatory
LIGO (Washington)
LIGO (Louisiana)
Supported by the U.S. National Science
Foundation operated by Caltech and MIT the
research focus for more than 400 LIGO Scientific
Collaboration members worldwide.
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The Four Corners of the LIGO Laboratory
LHO

• Observatories at Hanford, WA (LHO) Livingston,
  LA (LLO)
• Support Facilities _at_ Caltech MIT campuses

LLO
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Part of Future International Detector Network
Simultaneously detect signal (within msec)
Virgo
GEO
LIGO
TAMA
detection confidence locate the
sources decompose the polarization of
gravitational waves
AIGO
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Spacetime is Stiff!
gt Wave can carry huge energy with miniscule
amplitude!
h (G/c4) (ENS/r)
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Some of the Technical Challenges

• Typical Strains less than 10-21 at Earth 1
  hairs 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
• Engineer structures to mitigate recoil from
  atomic vibrations in suspended mirrors
• Provide clear optical paths within 4-km UHV beam
  lines

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Fabry-Perot-Michelson with Power Recycling
Optical
4 km or
Cavity
2-1/2 miles
Beam Splitter
Recycling Mirror
Photodetector
Laser
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Vacuum Chambers Provide Quiet Homes for Mirrors
View inside Corner Station
P lt 10-6 Torr in chambers to reduce acoustics
molecular Brownian motion
Standing at vertex beam splitter
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Beam Tube Provides Distortion-Free 4-km Pathway
for Light

• Requires P 10-9 Torr H2 Equivalent
• Baked out by insulating tube and driving 2000
  amps from end to end

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What Limits Sensitivityof 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

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Design for Low Background Specd From Prototype
Operation
For Example Noise-Equivalent Displacement of
40-meter Interferometer (ca1994)
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Vibration Isolation Systems

• Reduce in-band seismic motion by 4 - 6 orders of
  magnitude
• Little or no attenuation below 10Hz
• Large range actuation for initial alignment and
  drift compensation
• Quiet actuation to correct for Earth tides and
  microseism at 0.15 Hz during observation

BSC Chamber
HAM Chamber
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Seismic Isolation Springs and Masses
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Seismic System Performance
HAM stack in air
BSC stackin vacuum
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Core Optics Suspension and Control
Optics suspended as simple pendulums
Local sensors/actuators provide damping and
control forces
Mirror is balanced on 1/100th inch diameter wire
to 1/100th degree of arc
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Suspended Mirror Approximates a Free Mass Above
Resonance
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Frequency Stabilization of the Light Employs
Three Stages

• Pre-stabilized laser delivers light to the long
  mode cleaner
• Start with high-quality, custom-built NdYAG
  laser
• Improve frequency, amplitude and spatial purity
  of beam

• Actuator inputs provide for further laser
  stabilization
• Wideband
• Tidal

4 km
15m
10-Watt Laser
Interferometer
PSL
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Interferometer Control System

• Multiple Input / Multiple Output
• Three tightly coupled cavities
• Ill-conditioned (off-diagonal) plant matrix
• Highly nonlinear response over most of phase
  space
• Transition to stable, linear regime takes plant
  through singularity
• Employs adaptive control system that evaluates
  plant evolution and reconfigures feedback paths
  and gains during lock acquisition
• But it works!

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Steps to Locking an Interferometer
Y Arm
Laser
X Arm
signal
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Watching the Interferometer Lock
Y Arm
Laser
X Arm
signal
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Why is Locking Difficult?
One meter, about 40 inches
Human hair, about 100 microns
Earthtides, about 100 microns
Wavelength of light, about 1 micron
Microseismic motion, about 1 micron
Atomic diameter, 10-10 meter
Precision required to lock, about 10-10 meter
LIGO sensitivity, 10-18 meter
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Tidal Compensation Data
Tidal evaluation on 21-hour locked section of S1
data
Predicted tides
Feedforward
Feedback
Residual signal on voice coils
Residual signal on laser
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Microseism
Microseism at 0.12 Hz dominates ground
velocity
Trended data (courtesy of Gladstone High School)
shows large variability of microseism, on
several-day- and annual- cycles
Reduction by feed-forward derived from
seismometers
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Background Forces in GW Band Thermal Noise
kBT/mode
xrms ? 10-11 m f lt 1 Hz
xrms ? 2?10-17 m f 350 Hz
xrms ? 5?10-16 m f ? 10 kHz
Strategy Compress energy into narrow resonance
outside band of interest ? require high
mechanical Q, low friction
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Thermal Noise Checked in 1st Violins on H2, L1
During S1
Almost good enough for tracking calibration.
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Chronology of Detector Installation
Commissioning

• 7/98 Begin detector installation
• 6/99 Lock first mode cleaner
• 11/99 Laser spot on first end mirror
• 12/99 First lock of a 2-km Fabry-Perot arm
• 4/00 Engineering Run 1 (E1)
• 10/00 Recombined LHO-2km interferometer in E2 run
• 10/00 First lock of LHO-2km power-recycled
  interferometer
• 2/01 Nisqually earthquake damages LHO
  interferometers
• 4/01 Recombined 4-km interferometer at LLO
• 5/01 Earthquake repairs completed at LHO
• 6/01 Last LIGO-1 mirror installed
• 12/01 Power recycling achieved for LLO-4km
• 1/02 E7 First triple coincidence run first
  on-site data analysis
• 1/02 Power recycling achieved for LHO-4km
• 9/02 First Science Run (S1) completed
• 2/03 Second Science Run (S2) begun

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LIGO Sensitivity Livingston 4km Interferometer
May 01
Jan 03
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S1 Analysis Working Groups

• Data from S1 is being analyzed by LSC working
  groups for
• Detector Characterization monitors of data
  quality, instrument diagnostics, tuning displays
• Binary Inspirals modeled chirps from BHs,
  NSs, MACHOs
• Bursts unmodeled deviations from stationarity
  independent searches searches triggered on
  GRBs, SNs
• Periodic Sources known pulsar search work
  toward all-sky search for unknown pulsars
• Stochastic Background early universe

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Summary

• First triple coincidence run completed (17 days
  with 23 triple coincidence duty factor)
• On-line data analysis systems (Beowulf parallel
  supercomputer) functional at LHO and LLO
• S1 coincidence analyses with GEO TAMA are first
  science analyses with international laser-GW
  network
• First science data analysis ongoing
• Interferometer control system still being
  commissioned and tuned
• Working to increase immunity to high seismic
  noise periods (especially important at LLO)
• S2 running 14Feb03 14Apr03

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Future Plans

• Improve reach of initial LIGO to run 1 yr at
  design sensitivity
• Advanced LIGO technology under development with
  intent to install by 2008
• Planning underway for space-based detector, LISA,
  to open up a lower frequency band 2015

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Despite a few difficulties, science runs started
in 2002.
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Stochastic Background sensitivities and theory
E7
results
projected
S1
S2
LIGO
Adv LIGO
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We Have Continued to Progress
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Pre-stabilized Laser (PSL)
Custom-built 10 W NdYAG Laser, joint development
with Lightwave Electronics (now commercial
product)
Cavity for defining beam geometry, joint
development with Stanford
Frequency reference cavity (inside oven)
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Digital Interferometer Sensing Control System
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Core Optics

• Substrates SiO2
• 25 cm Diameter, 10 cm thick
• Homogeneity lt 5 x 10-7
• Internal mode Qs gt 2 x 106
• Polishing
• Surface uniformity lt 1 nm rms
• Radii of curvature matched lt 3
• Coating
• Scatter lt 50 ppm
• Absorption lt 2 ppm
• Uniformity lt10-3
• Production involved 6 companies, NIST, and LIGO