1 Detecting Gravitational Waves: How does LIGO work and how well does LIGO work? Barry C. Barish Caltech University of Kentucky 4-March-05 Colliding Black Holes" redit: ational Center for Supercomputing Applications (NCSA)
Jan 15, 2016
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Detecting Gravitational Waves: How does LIGO work and how well does LIGO work?
Barry C. BarishCaltech
University of Kentucky4-March-05
"Colliding Black Holes"
Credit:National Center for Supercomputing Applications (NCSA)
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Einstein’s Theory of Gravitation
a necessary consequence of Special Relativity with its finite speed for information transfer
gravitational waves come from the acceleration of masses and propagate away from their sources as a space-time warpage at the speed of light
gravitational radiationbinary inspiral
of compact objects
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General RelativityEinstein’s equations have form similar to the
equations of elasticity.
P = Eh (P = stress, h = strain, E = Young’s mod.)
T = (c4/8πG)h T = stress tensor, G = Curvature tensor and c4/8πG ~ 1042N is a space-time “stiffness” (energy density/unit curvature)
• Space-time can carry waves.
• They have very small amplitude
• There is a large mismatch with ordinary matter, so very little energy is absorbed (very small cross-section)
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Einstein’s Theory of Gravitationgravitational waves
0)1
(2
2
22
htc
• Using Minkowski metric, the information about space-time curvature is contained in the metric as an added term, h. In the weak field limit, the equation can be described with linear equations. If the choice of gauge is the transverse traceless gauge the formulation becomes a familiar wave equation
)/()/( czthczthh x
• The strain h takes the form of a plane wave propagating at the speed of light (c).
• Since gravity is spin 2, the waves have two components, but rotated by 450 instead of 900 from each other.
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Detectionof
Gravitational Waves
Detectors in space
LISA
Gravitational Wave
Astrophysical Source
Terrestrial detectorsVirgo, LIGO, TAMA, GEO
AIGO
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International Network on Earth
LIGO
simultaneously detect signal
detection confidence
GEO VirgoTAMA
AIGOlocate the sourcesdecompose the polarization of
gravitational waves
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Detecting a passing wave ….
Free masses
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Detecting a passing wave ….
Interferometer
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Interferometer Concept Laser used to measure
relative lengths of two orthogonal arms
As a wave passes, the arm lengths change in different ways….
…causing the interference
pattern to change at the photodiode
Arms in LIGO are 4km Measure difference in
length to one part in 1021 or 10-18 meters
SuspendedMasses
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Simultaneous DetectionLIGO
3002 km
(L/c = 10 ms)
Hanford Observatory
Caltech
LivingstonObservatory
MIT
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LIGO Livingston Observatory
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LIGO Hanford Observatory
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LIGO Facilitiesbeam tube enclosure
• minimal enclosure
• reinforced concrete
• no services
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LIGObeam tube
LIGO beam tube under construction in January 1998
65 ft spiral welded sections
girth welded in portable clean room in the field
1.2 m diameter - 3mm stainless50 km of weld
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Vacuum Chambersvibration isolation systems
» Reduce in-band seismic motion by 4 - 6 orders of magnitude» Compensate for microseism at 0.15 Hz by a factor of ten» Compensate (partially) for Earth tides
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Seismic Isolation springs and masses
ConstrainedLayer
damped spring
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LIGOvacuum equipment
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Seismic Isolationsuspension system
• support structure is welded tubular stainless steel • suspension wire is 0.31 mm diameter steel music wire
• fundamental violin mode frequency of 340 Hz
suspension assembly for a core optic
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LIGO Opticsfused silica
Caltech data CSIRO data
Surface uniformity < 1 nm rms Scatter < 50 ppm Absorption < 2 ppm ROC matched < 3% Internal mode Q’s > 2 x 106
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Core Optics installation and
alignment
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LIGO Commissioning and Science Timeline
Now
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Lock Acquisition
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Tidal Compensation DataTidal evaluation 21-hour locked section of S1 data
Residual signal on voice coils
Predicted tides
Residual signal on laser
Feedforward
Feedback
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Controlling angular degrees of freedom
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Interferometer Noise Limits
Thermal (Brownian)
Noise
LASER
test mass (mirror)
Beamsplitter
Residual gas scattering
Wavelength & amplitude fluctuations photodiode
Seismic Noise
Quantum Noise
"Shot" noise
Radiation pressure
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What Limits LIGO Sensitivity? Seismic noise limits low
frequencies
Thermal Noise limits middle frequencies
Quantum nature of light (Shot Noise) limits high frequencies
Technical issues - alignment, electronics, acoustics, etc limit us before we reach these design goals
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Evolution of LIGO Sensitivity
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Science Runs
S2 ~ 0.9Mpc
S1 ~ 100 kpc
E8 ~ 5 kpc
NN Binary Inspiral Range
S3 ~ 3 Mpc
Design~ 18 Mpc
A Measure of Progress
Milky WayAndromedaVirgo Cluster
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Astrophysical Sourcessignatures
Compact binary inspiral: “chirps”» NS-NS waveforms are well described» BH-BH need better waveforms » search technique: matched templates
Supernovae / GRBs: “bursts” » burst signals in coincidence with signals in
electromagnetic radiation » prompt alarm (~ one hour) with neutrino
detectors
Pulsars in our galaxy: “periodic”» search for observed neutron stars
(frequency, doppler shift)» all sky search (computing challenge)» r-modes
Cosmological Signal “stochastic background”
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Compact binary collisions
» Neutron Star – Neutron Star
– waveforms are well described
» Black Hole – Black Hole – need better waveforms
» Search: matched templates
“chirps”
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Template Bank
Covers desiredregion of massparam space
Calculatedbased on L1noise curve
Templatesplaced formax mismatchof = 0.03
2110 templatesSecond-orderpost-Newtonian
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Optimal Filtering
Transform data to frequency domain : Generate template in frequency domain : Correlate, weighting by power spectral density of
noise:
)(~fh
)(~ fs
|)(|)(
~)(~ *
fSfhfs
h
|)(| tzFind maxima of over arrival time and phaseCharacterize these by signal-to-noise ratio (SNR) and effective distance
dfefSfhfs
tz tfi
h
2
0
*
|)(|)(
~)(~
4)(
Then inverse Fourier transform gives you the filter output
at all times:
frequency domain
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Matched Filtering
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Loudest Surviving Candidate Not NS/NS inspiral event 1 Sep 2002, 00:38:33 UTC S/N = 15.9, 2/dof = 2.2 (m1,m2) = (1.3, 1.1) Msun
What caused this? Appears to be due to
saturation of a photodiode
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Results of Inspiral Search
Upper limit binary neutron starcoalescence rate
LIGO S2 DataR < 50 / yr / MWEG
Previous observational limits» Japanese TAMA R < 30,000 / yr / MWEG » Caltech 40m R < 4,000 / yr / MWEG
Theoretical prediction R < 2 x 10-5 / yr / MWEG
Detectable Range of S2 data reaches Andromeda!
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Astrophysical Sourcessignatures
Compact binary inspiral: “chirps”» NS-NS waveforms are well described» BH-BH need better waveforms » search technique: matched templates
Supernovae / GRBs: “bursts” » burst signals in coincidence with signals in
electromagnetic radiation » prompt alarm (~ one hour) with neutrino
detectors
Pulsars in our galaxy: “periodic”» search for observed neutron stars
(frequency, doppler shift)» all sky search (computing challenge)» r-modes
Cosmological Signal “stochastic background”
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Detection of Burst Sources Known sources -- Supernovae & GRBs
» Coincidence with observed electromagnetic observations.
» No close supernovae occurred during the first science run» Second science run – We analyzed the very bright and close GRB030329
Unknown phenomena » Emission of short transients of gravitational radiation of unknown waveform (e.g. black hole mergers).
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‘Unmodeled’ Burstssearch for waveforms from sources for which we cannot currently make an accurate prediction of the waveform shape.
GOAL
METHODS
Time-Frequency Plane Search‘TFCLUSTERS’
Pure Time-Domain Search‘SLOPE’
freq
uen
cy
time
‘Raw Data’ Time-domain high pass filter
0.125s
8Hz
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Coincidences and Efficiency
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Directed Burst Sources
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GRB030359
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Astrophysical Sourcessignatures
Compact binary inspiral: “chirps”» NS-NS waveforms are well described» BH-BH need better waveforms » search technique: matched templates
Supernovae / GRBs: “bursts” » burst signals in coincidence with signals in
electromagnetic radiation » prompt alarm (~ one hour) with neutrino
detectors
Pulsars in our galaxy: “periodic”» search for observed neutron stars
(frequency, doppler shift)» all sky search (computing challenge)» r-modes
Cosmological Signal “stochastic background”
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Detection of Periodic Sources
Pulsars in our galaxy: “periodic”» search for observed neutron stars » all sky search (computing challenge)» r-modes
Frequency modulation of signal due to Earth’s motion relative to the Solar System Barycenter, intrinsic frequency changes.
Amplitude modulation due to the detector’s antenna pattern.
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Two Search Methods
Frequency domain
• Best suited for large parameter space searches
• Maximum likelihood detection method + Frequentist approach
Time domain
• Best suited to target known objects, even if phase evolution is complicated
Bayesian approach
First science run --- use both pipelines for the same search for cross-checking and validation
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Directed Searches
OBSGWh0 /TfS4.11h
NO DETECTION EXPECTED
at present sensitivities
PSR J1939+2134
1283.86 Hz
Limits of detectability for rotating NS with equatorial ellipticity =I/Izz: 10-3 , 10-4 , 10-5 @ 8.5 kpc.
Crab Pulsar
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The Data
hS
days
hS
hS hS
days
days
days
time behavior
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The Data
hS
hShS
hS
Hz
Hz
Hz
Hz
frequency behavior
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Summary of S2 resultslimits on strain
S1
J1939+2134
S2J1910 – 5959D:
h0 = 1.7 x 10-24
Crab pulsar
Red dots: pulsars are in globular clusters - cluster dynamics hide intrinsic spin-down propertiesBlue dots: field pulsars for which spin-downs are known
h95
1
0
strain
Marginalized Bayesian PDF for
h
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Directed Pulsar Search
28 Radio Sources
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Detection of Periodic Sources
Known Pulsars in our galaxy
Frequency modulation of signal due to Earth’s motion relative to the Solar System Barycenter, intrinsic frequency changes.
Amplitude modulation due to the detector’s antenna pattern.
NEW RESULT28 known pulsars
NO gravitational waves
e < 10-5 – 10-6 (no mountains > 10 cm
ALL SKY SEARCH enormous computing challenge
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EM spin-down upper-limits
LIGO upper-limits from hmax
J1939+2134
S1
S2
Summary S2 results - ellipticity limits
Red dots: pulsars are in globular clusters - cluster dynamics hide intrinsic spin-down properties
Blue dots: field pulsars for which spin-downs are known
Best upper-limits:
• J1910 – 5959D: h0 < 1.7 x 10-24
• J2124 – 3358: < 4.5 x 10-6
How far are S2 results from spin-down limit? Crab: ~ 30X
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Einstein@Home
LIGO Pulsar Search using home pc’s
BRUCE ALLENProject Leader
Univ of Wisconsin Milwaukee
LIGO, UWM, AEI, APS
http://einstein.phys.uwm.edu
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Astrophysical Sourcessignatures
Compact binary inspiral: “chirps”» NS-NS waveforms are well described» BH-BH need better waveforms » search technique: matched templates
Supernovae / GRBs: “bursts” » burst signals in coincidence with signals in
electromagnetic radiation » prompt alarm (~ one hour) with neutrino
detectors
Pulsars in our galaxy: “periodic”» search for observed neutron stars
(frequency, doppler shift)» all sky search (computing challenge)» r-modes
Cosmological Signal “stochastic background”
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Signals from the Early Universe
Cosmic Microwave
background
WMAP 2003
stochastic background
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Signals from the Early Universe
Strength specified by ratio of energy density in GWs to total energy density needed to close the universe:
Detect by cross-correlating output of two GW detectors:
First LIGO Science Data
Hanford - Livingston
d(lnf)
dρ
ρ
1(f)Ω GW
criticalGW
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Results – Stochastic Backgrounds
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Gravitational Waves from the Early Universe
E7
S1
S2
LIGO
Adv LIGO
results
projected
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Advanced LIGOimproved subsystems
Active Seismic
Multiple Suspensions
Improved Optics
Higher Power Laser
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Advanced LIGOCubic Law for “Window” on the
Universe
Initial LIGO
Advanced LIGO
Improve amplitude sensitivity by a factor of 10x…
…number of sources goes up 1000x!
Virgo cluster
Today
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Advanced LIGO
Enhanced Systems• laser• suspension• seismic isolation• test mass Rate
Improvement
~ 104
+narrow band
optical configuration
2007 +
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LIGO Construction is complete & commissioning almost complete
New upper limits for neutron binary inspirals, a fast pulsar and stochastic backgrounds have been achieved from the first short science runs
Sensitivity improvements are rapid -- second data run was 10x more sensitive and 4x duration and results are beginning to be reported ----- (e.g. improved pulsar searches)
Enhanced detectors will be installed in ~ 5 years, further increasing sensitivity
Direct detection should be achieved and gravitational-wave astronomy begun within the next decade !
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Gravitational Wave Astronomy
LIGO will provide a new way to view the dynamics of the
Universe