Download Physics of LIGO, lecture 1a

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 ΔL/L = 10-21 or less
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Hulse-Taylor Binary Pulsar

~ 8 hr
17 / sec

 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
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
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
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Max Planck Institute Andrews University Australian National Univ.
Caltech
Cardiff University
Carleton College
Charles Sturt Univ. Columbia University Embry-Riddle Aero. Univ.
Eӧtvӧs 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 Massachusetts 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)
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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 4’x4’ 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)
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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 <50 ppm scattering losses, <1 ppm absorption losses.
The optics are suspended to
attenuate seismic motion above
the pendulum frequency.
f  f 0
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Science Run Timeline
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Seismic
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Internal thermal
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Shot noise and
pole frequency
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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
Waveform
Known
Waveform
Unknown
Short Duration
Long Duration
Binary Inspirals
Periodic
(Pulsars, rotating neutron stars)
Search via matched filtering with
pre-generated waveforms
Integrate sinusoidal signal
Burst
Stochastic
(supernovae, gamma ray bursts)
Search for excess power
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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 < 35 Mʘ
• Limits assume NS = 1.35 solar masses, BH = 5.0 solar masses
• L10 = 1010 Lʘ (1 Milky Way = 1.7 L10)
Expected rates (yr-1 L10-1)
Measured range
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)
O’Shaughnessy et al., ApJ 633, 1076 (2005)
B. Abbott et al., PRD 80, 047101 (2009)
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Kalogera et al., ApJ 614, L137 (2004)
O’Shaughnessy et al., ApJ 672, 479 (2008)
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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: < 1.9 × 10-8 Mʘ
Virgo cluster (16 Mpc): < 0.05 Mʘ
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.
B. Abbott et al., ApJ Lett. 683, 45 (2008)
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Lack of detection implies:
• Less than 6% of energy loss due to
gravitational waves
• Internal mag. field < 1016 G
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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:
41.5 Hz  f  161.25 Hz
GW  f   6.9 10 6
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.
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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.
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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 VIRGO’s second science run.
S6 will continue through Oct. 2010.
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The Need for Advanced LIGO
Initial LIGO
 Goal: factor of ten
improvement in sensitivity
at all frequencies
 x10 increase in sensitivity
= x1000 volume of sky
searched
E-LIGO
 Inspiral event rate from one
every few years to one
every few days!
 Resolution improved for
astronomy
 Assembly underway,
transition begins this fall
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Projected Sensitivity
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180 Watt Laser
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SMU Hannover
Physics Seminar,
March 29, 2010
Laser Zentrum
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
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