1 detecting gravitational waves: how does ligo work and how well does ligo work? barry c. barish...

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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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detectors




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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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detectors




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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

PDF

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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detectors




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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

1 detecting gravitational waves: how does ligo work and how well does ligo work? barry c. barish...

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