Physics of LIGO, lecture 1a

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News from the Laser Interferometer
Gravitational-Wave Observatory (LIGO)
Dennis Ugolini, Trinity University for the LIGO
Science Collaboration SMU Physics Seminar March
29, 2010 Document no. LIGO-G100214
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Gravitational Waves

• Gravitational waves are transverse distortions of
  spacetime due to the motion of massive
  astronomical bodies.
• Expected sources
• Inspiraling neutron stars/black holes
• (Asymmetric) supernovae
• Rotating pulsars
• Cosmic gravitational-wave background
• Expected properties
• Quadrupole polarization
• Propagating at speed of light
• Strains of ?L/L 10-21 or less

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Hulse-Taylor Binary Pulsar

• PSR 1913 16, measured in 1975
• System should lose energy through gravitational
  radiation
• Stars get closer together
• Orbital period gets shorter

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Why Are We Looking?
Chirp Signal
We can use weak-field gravitational waves to
study strong-field general relativity.
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The Fabry-Perot Michelson Interferometer

• Uses light interference to measure path length
  difference between the two arms
• Each arm is a Fabry-Perot cavity, effectively
  increasing arm length
• Geometry ideally suited for quadrupole radiation

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The LIGO Project

• LIGO Laser Interferometer Gravitational-Wave
  Observatory
• Detection, followed by astronomy
• LIGO Science Collaboration (LSC) includes many
  institutions ?
• Funded by US National Science Foundation

Max Planck Institute Andrews University Australian
National Univ. Caltech Cardiff University
Carleton College Charles Sturt Univ. Columbia
University Embry-Riddle Aero. Univ. E?tv?s
University Hobart William Smith Institute of
Applied Physics, Nizhny Novgorod Inter-University
Centre for Astronomy and Astrophysics,
Pune Leibniz Universität Hannover LIGO Hanford
Observatory LIGO Livingston Observatory Massachuse
tts Inst. of Technology Louisiana State Louisiana
Tech McNeese State Univ. Montana State
Univ. Moscow State Univ. NASA/Goddard Flight
Ctr. Nat. Astronomical Observatory of
Japan Northwestern University Rochester Inst. of
Technology Rutherford Appleton Lab. San Jose
State Univ. Sonoma State Univ. Southeastern
Louisiana Southeastern Univ. Southern
University Stanford University Syracuse
University Penn State Univ. University of
Melbourne Univ. of Mississippi Univ. of
Sheffield Univ. of Texas at Austin Univ. of Texas
at Brownsville Universitat de les Illes
Balears Trinity University Univ. of
Adelaide University of Birmingham Univ. of
Florida Univ. of Glasgow University of
Maryland Univ. of Mass. Amherst University of
New Hampshire Univ. of Michigan Univ. of
Minnesota University of Oregon Univ. of
Rochester Univ. of Salerno Univ. of
Southhampton Univ. of Sannio at
Benevento University of Strathclyde University
of Western Australia University of
Wisconsin-Milwaukee Washington State University
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The LIGO Observatories
LIGO Hanford Observatory (LHO) (4km and 2km in
same vacuum)
LIGO Livingston Observatory (LLO)
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LIGO Vacuum System

• Vacuum at 10-9 torr to reduce light scattering
  and momentum kicks to optics.
• One meter diameter arms, with chambers
  separated by 4x4 gate valves
• Serrated baffles included to disperse light
  scattered at optics
• Lengthy bake to remove adsorbed water vapor

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Seismic Isolation
Passive (to reduce noise in sensitive freq. band)
Active (to improve lock acquisition/maintenance)
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Suspended Test Masses
Optics are 25 cm diameter, 10 cm thick, 10.7 kg,
of high purity fused silica. They must have lt50
ppm scattering losses, lt1 ppm absorption losses.
The optics are suspended to attenuate seismic
motion above the pendulum frequency.
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Science Run Timeline
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Shot noise and pole frequency
Seismic
Internal thermal
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So Have We Detected Gravitational Waves?
Nope.

• But the lack of detections puts interesting
  constraints on our universe
• The properties of certain astronomical objects
• The populations of gravitational-wave sources
• The total energy density of gravitational waves

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Search Classifications
Short Duration Long Duration
Waveform Known Binary Inspirals Search via matched filtering with pre-generated waveforms Periodic (Pulsars, rotating neutron stars) Integrate sinusoidal signal
Waveform Unknown Burst (supernovae, gamma ray bursts) Search for excess power Stochastic Cross-correlation between multiple detectors
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Low Mass Binary Inspiral Search Results

• Covers first 18 months of S5 data no
  detections for total mass lt 35 M?
• Limits assume NS 1.35 solar masses, BH 5.0
  solar masses
• L10 1010 L? (1 Milky Way 1.7 L10)

Expected rates (yr-1 L10-1) Expected rates (yr-1 L10-1) Measured range Measured range Upper limits (yr-1 L10-1) Upper limits (yr-1 L10-1)
Source optimistic realistic Mpc L10 no spin spin
NS-NS 5 10-4 5 10-5 30 490 1.4 10-2 ---
BH-BH 6 10-5 4 10-7 100 11000 7.3 10-4 9 10-4
BH-NS 6 10-5 2 10-6 60 2100 3.6 10-3 4.4 10-3
Kalogera et al., ApJ 601, L179 (2004) Kalogera et
al., ApJ 614, L137 (2004) OShaughnessy et al.,
ApJ 633, 1076 (2005) OShaughnessy et al., ApJ
672, 479 (2008) B. Abbott et al., PRD 80, 047101
(2009)
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Bursts GRB 070201
GRB 070201 was short (0.15s), intense, and from
direction of M31 (770 kpc). Both Hanford
detectors operating, exclude inspiral within 3.5
Mpc at 90 CL. Thus the gamma-ray burst was
extremely unlikely to be an inspiral in M31.
B. Abbott et al., ApJ 681, 1419 (2008)
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Other Burst Searches
Other GRBs One GRB every few days, 212 total
during S5
All Sky Survey Search for any signal between
64-2000 Hz in first year of S5 data. 90 CL rate
limits shown at left. Also limits on strength
10 kpc lt 1.9 10-8 M? Virgo cluster (16 Mpc)
lt 0.05 M?
B. Abbott et al., PRD 80, 102001 (2009)
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Crab Pulsar Search

• The pulsar in the Crab has a rotational frequency
  of 29.78 Hz, and is slowing
• df/dt -3.7 10-10 Hz s-1
• dE/dt -4.4 1031 W
• How much of this energy loss is due to
  gravitational wave radiation?
• Apply matched filtering with templates at or near
  twice rotational frequency.
• Lack of detection implies
• Less than 6 of energy loss due to
  gravitational waves
• Internal mag. field lt 1016 G

B. Abbott et al., ApJ Lett. 683, 45 (2008)
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Other Periodic Searches

• All-sky survey search for periodic sources
• First eight months of S5
• fgw 500-1100 Hz
• df/dt -5 10-9 Hz s-1 to zero
• 95 CL strain limits shown at right (best and
  worst spin orientations).
• Search is sensitive to neutron stars within 500
  pc with eccentricity 10-6.

B. Abbott et al., PRL 102, 111102 (2009)
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Stochastic GW Background
95 CL on gravitational-wave energy density from
S5 data
Limit supercedes Big Bang Nucleosynthesis bound,
constrains certain cosmic string and pre-Big Bang
models.
B. Abbott et al., Nature 460, 990 (2009)
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Developments Since S5

• Data sharing agreement with VIRGO collaboration
  beginning in 2007
• Trigger passing real-time alerts to
• Swift satellite (X-ray)
• TAROT, QUEST wide-field telescopes (optical)
• Program began in December 2009
• Enhanced LIGO improved sensitivity
• x4 increase in laser power
• DC demodulation
• Thermal lensing compensation

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RF Heterodyne Demodulation
In Initial LIGO, an electro-optic modulator
applied radio-frequency sidebands to the carrier
light. The interferometer is operated at the
dark fringe to minimize shot noise. The carrier
light is resonant in the arms, while the
sidebands are not. The output is electronically
mixed with the applied RF frequency, giving a
linear correction signal.
From S. Hild et al., Class. Quantum Grav. 26,
055012 (2009).
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DC Homodyne Demodulation

• In DC demodulation, the interferometer is
  operated slightly off the dark fringe, and this
  light mixes optically with the sidebands.
• Advantages
• Simplified electronics
• Reduced phase noise
• Larger non-RF photodiodes
• Requires good laser intensity stabilization
  output mode cleaner (OMC). The OMC in turn
  requires better seismic isolation.

From S. Hild et al., Class. Quantum Grav. 26,
055012 (2009).
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Active Seismic Isolation
New seismic isolation stacks installed in output
mode cleaner chamber at each site. Six sets of
position and velocity sensors (GS-13
seismometers) feed back to coil actuators. Order
of magnitude improvement over wide frequency
range.
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Isolation Stack Installed
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Thermal Compensation System

• Fused silica is a poor conductor of heat, and the
  higher power laser delivers a lot of heat!
• Uneven heating causes reflective properties to
  become a function of position a translation of
  the beam creates a phase shift that mimics a
  signal.
• In Enhanced LIGO, 25W carbon dioxide lasers scan
  the optical surface in an annulus pattern,
  flattening the surface temperature profile.

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Sensitivity Improvement
PRELIMINARY
S6 began on July 7, 2009, coincident with VIRGOs
second science run. S6 will continue through Oct.
2010.
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The Need for Advanced LIGO

• Goal factor of ten improvement in sensitivity
  at all frequencies
• x10 increase in sensitivity x1000 volume of
  sky searched
• Inspiral event rate from one every few years to
  one every few days!
• Resolution improved for astronomy
• Assembly underway, transition begins this fall

Initial LIGO
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Projected Sensitivity
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180 Watt Laser
Laser Zentrum Hannover e.V.
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Signal Recycling
Add optic at output to make cavity resonant for
beats between carrier and desired signal
frequency. Can tune to particular source, or to
follow thermal noise for maximum sensitivity.
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New Optic Suspensions

• 40 kg fused silica optics
• Quadruple suspension with reaction mass
• Last stage suspended by fused silica ribbons
  for higher Q

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Electrostatic Drive
GEO prototype MIT LASTI prototype

• Gold coating on reaction mass
• Forms pair of electrodes in each quadrant
• Fringing fields attract optic proportional to
  V2

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My Contribution Charging

• Charge buildup on optic surfaces
• Mechanical contact with other materials
• Friction with dust during pumpdown
• Exposure to electrostatic drive
• Particle showers from cosmic rays?
• Potential concerns
• Electric fields interfere with positioning
  control
• Dust held to surface, increasing absorption
• Motion generates low-frequency suspension noise

The goal is to measure the charging magnitude,
relaxation time constant, and spatial variation,
and find a noncontact discharging method.
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Kelvin Probe Measurements
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Summary

• No detections yet, but results of S5 science run
  have put interesting constraints on our nearest
  neighbors
• Enhanced LIGO science run ongoing
• Advanced LIGO construction already underway,
  aiming for sensitivity to detect GW sources with
  regularity by 2014-5