LIGO and the Search for Gravitational Waves Barry Barish Stanford Colloquium 15-Jan-01.

LIGOand the

Search for Gravitational Waves

Barry Barish

Stanford Colloquium15-Jan-01

15-Jan-02 Stanford Colloquium 2

Sir Isaac NewtonUniversal Gravitation

Three laws of motion and law of gravitation (centripetal force) disparate phenomena» eccentric orbits of comets» cause of tides and their variations» the precession of the earth’s axis» the perturbation of the motion of the

moon by gravity of the sun

Solved most known problems of astronomy and terrestrial physics» Work of Galileo, Copernicus and Kepler

unified.


Einstein’s Theory of Gravitation

Newton’s Theory“instantaneous action at a distance”

Einstein’s Theoryinformation carried by gravitational radiation at the speed of light


General Relativity the essential idea

Overthrew the 19th-century concepts of absolute space and time

Einstein: gravity is not a force, but a property of space & time» Spacetime = 3 spatial dimensions + time» Perception of space or time is relative

Concentrations of mass or energy distort (warp) spacetime

Objects follow the shortest path through this warped spacetime; path is the same for all objects


General Relativity

Einstein theorized that smaller masses travel toward larger masses, not because they are "attracted" by a mysterious force, but because the smaller objects travel through space that is warped by the larger object

Imagine space as a stretched rubber sheet.

A mass on the surface will cause a deformation.

Another mass dropped onto the sheet will roll toward that mass.


Einstein’s Theory of Gravitationgravitational waves

• a necessary consequence of Special Relativity with its finite speed for information transfer

• time dependent gravitational fields 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


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

• 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. )/()/( czthczthh x


Gravitational Waves the evidence

Neutron Binary SystemPSR 1913 + 16 -- Timing of pulsars

17 / sec

~ 8 hr

Neutron Binary System• separated by 106 miles• m1 = 1.4m; m2 = 1.36m; = 0.617

Prediction from general relativity• spiral in by 3 mm/orbit• rate of change orbital period


Hulse and Taylorresults

due to loss of orbital energy period speeds up 25 sec from 1975-98 measured to ~50 msec accuracy deviation grows quadratically with time

emission of

gravitational waves


Direct DetectionLaboratory Experiment

ExperimentalGeneration and Detection

of Gravitational Waves

gedanken experiment

a la Hertz


Radiation of Gravitational Waves

Radiation of Gravitational Wavesfrom binary inspiral

system

LISA

Waves propagates at the speed of lightTwo polarizations at 45 deg (spin 2)


Interferometers space

The Laser Interferometer

Space Antenna (LISA)

• The center of the triangle formation will be in the ecliptic plane • 1 AU from the Sun and 20 degrees

behind the Earth.


Astrophysics Sourcesfrequency range

EM waves are studied over ~20 orders of magnitude» (ULF radio HE -rays)

Gravitational Waves over ~10 orders of magnitude

» (terrestrial + space)

Audio band


Suspended mass Michelson-type interferometerson earth’s surface detect distant astrophysical sources

International network (LIGO, Virgo, GEO, TAMA) enable locating sources and decomposing polarization of gravitational waves.

Interferometersterrestrial


Laser

Beam Splitter

End Mirror End Mirror

ScreenViewing

Michelson Interferometer


Recycling Mirror

Optical

Cavity

4 km or

2-1/2 m

iles

Beam Splitter

Laser

Photodetector

Fabry-Perot-Michelson with Power Recycling

Suspended

Test Masses


Sensing a Gravitational Wave

Laser

signal

Gravitational wave changes arm lengths and amount of light in signal

Change in arm length is ~10-18 meters

h = L/L ~ 10-21

4 km


How Small is 10-18 Meter?

Wavelength of light, about 1 micron100

One meter, about 40 inches

Human hair, about 100 microns000,10

LIGO sensitivity, 10-18 meter000,1

Nuclear diameter, 10-15 meter000,100

Atomic diameter, 10-10 meter000,10


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


Noise Floor40 m prototype

• displacement sensitivityin 40 m prototype. • comparison to predicted contributions from various noise sources

sensitivity demonstration


Phase Noisesplitting the fringe

• spectral sensitivity of MIT phase noise interferometer

• above 500 Hz shot noise limited near LIGO I goal

• additional features are from 60 Hz powerline harmonics, wire resonances (600 Hz), mount resonances, etc

expected signal 10-10 radians phase shift

demonstration experiment


Interferometer Data40 m prototype

Real interferometer data is UGLY!!!(Gliches - known and unknown)

LOCKING

RINGING

NORMAL

ROCKING


The Problem

How much does real data degrade complicate the data analysis and degrade the sensitivity ??

Test with real data by setting an upper limit on galactic neutron star inspiral rate using 40 m data


“Clean up” data stream

Effect of removing sinusoidal artifacts using multi-taper methods

Non stationary noise Non gaussian tails


Inspiral ‘Chirp’ Signal

Template Waveforms

“matched filtering”687 filters

44.8 hrs of data39.9 hrs arms locked25.0 hrs good data

sensitivity to our galaxyh ~ 3.5 10-19 mHz-1/2

expected rate ~10-6/yr


Optimal Signal DetectionWant to “lock-on” to one of a set of known signals

Requires:• source modeling• efficient algorithm• many computers


Detection Efficiency

• Simulated inspiral events provide end to end test of analysis and simulation code for reconstruction efficiency

• Errors in distance measurements from presence of noise are consistent with SNR fluctuations


Results from 40m Prototype

Loudest event usedto set upper-limit onrate in our Galaxy:

R90% < 0.5 / hour


Setting a limit

Upper limit on event rate can be determined from SNR of ‘loudest’ event

Limit on rate:R < 0.5/hour with 90% CL = 0.33 = detection efficiency

An ideal detector would set a limit:R < 0.16/hour


LIGOastrophysical sources

LIGO I (2002-2005)

LIGO II (2007- )

Advanced LIGO


Interferometersinternational network

LIGO

Simultaneously detect signal (within msec)

detection confidence locate the sources

decompose the polarization of gravitational waves

GEO VirgoTAMA

AIGO


Astrophysical Signaturesdata analysis

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 Signals “stochastic background”


“Chirp Signal”binary inspiral

•distance from the earth r•masses of the two bodies•orbital eccentricity e and orbital inclination i

determine


Binary Inspiralssignatures and sensitivity

Compact binary mergers

LIGO sensitivity to coalescing binaries


Signals in Coincidence

Hanford Observatory

LivingstonObservatory


Detection Strategycoincidences

Two Sites - Three Interferometers» Single Interferometer non-gaussian level ~50/hr

» Hanford (Doubles) correlated rate (x1000) ~1/day

» Hanford + Livingston uncorrelated (x5000) <0.1/yr

Data Recording (time series)» gravitational wave signal (0.2 MB/sec)

» total data (16 MB/s)

» on-line filters, diagnostics, data compression

» off line data analysis, archive etc

Signal Extraction» signal from noise (vetoes, noise analysis)

» templates, wavelets, etc


gravitational waves

’s

light

“Burst Signal”supernova


Supernovaegravitational waves

Non axisymmetric collapse ‘burst’ signal

Rate1/50 yr - our galaxy3/yr - Virgo cluster


pulsar proper motions

Velocities - young SNR(pulsars?) > 500 km/sec

Burrows et al

recoil velocity of matter and neutrinos

Supernovaeasymmetric collapse?


Supernovaesignatures and sensitivity


“Periodic Signals”pulsars sensitivity

Pulsars in our galaxy»non axisymmetric: 10-4 < < 10-6»science: neutron star precession; interiors»narrow band searches best


“Stochastic Background”cosmological signals

‘Murmurs’ from the Big Bangsignals from the early universe

Cosmic microwave background


LIGO Sites

Hanford Observatory

LivingstonObservatory


LIGO Livingston Observatory


LIGO Hanford Observatory


LIGO Plansschedule

1996 Construction Underway (mostly civil)

1997 Facility Construction (vacuum system)

1998 Interferometer Construction (complete facilities)

1999 Construction Complete (interferometers in vacuum)

2000 Detector Installation (commissioning subsystems)

2001 Commission Interferometers (first coincidences)

2002 Sensitivity studies (initiate LIGOI Science Run)

2003+ LIGO I data run (one year integrated data at h ~ 10-21)

2006 Begin LIGO II installation


LIGO Facilitiesbeam tube enclosure

• minimal enclosure

• reinforced concrete

• no services


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

NO LEAKS !!


LIGO I the noise floor

Interferometry is limited by three fundamental noise sources

seismic noise at the lowest frequencies thermal noise at intermediate frequencies shot noise at high frequencies

Many other noise sources lurk underneath and must be controlled as the instrument is improved


Beam Tube bakeout

• I = 2000 amps for ~ 1 week

• no leaks !!

• final vacuum at level where not limiting noise, even for future detectors


LIGOvacuum equipment


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


Seismic Isolation springs and masses

damped springcross section


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


Thermal Noise ~ kBT/mode

Strategy: Compress energy into narrow resonance outside band of interest require high mechanical Q, low friction


LIGO Noise Curvesmodeled

wire resonances


Core 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


Core Optics installation and alignment


ITMx Internal Mode Ringdowns

14.3737 kHz; Q = 1.2e+79.675 kHz; Q ~ 6e+5


LIGO laser

Nd:YAG

1.064 m

Output power > 8W in TEM00 mode


Commissioning configurations

Mode cleaner and Pre-Stabilized Laser 2km one-arm cavity short Michelson interferometer studies

Lock entire Michelson Fabry-Perot interferometer

“First Lock”


Why is Locking Difficult?

One meter, about 40 inches

Human hair, about 100 microns000,10

Wavelength of light, about 1 micron100

LIGO sensitivity, 10-18 meter000,1

Nuclear diameter, 10-15 meter000,100

Atomic diameter, 10-10 meter000,10

Earthtides, about 100 microns

Microseismic motion, about 1 micron

Precision required to lock, about 10-10 meter


Laserstabilization

IO

10-WattLaser

PSL Interferometer

15m4 km

Tidal Wideband

Deliver pre-stabilized laser light to the 15-m mode cleaner• Frequency fluctuations• In-band power fluctuations• Power fluctuations at 25 MHz

Provide actuator inputs for further stabilization• Wideband

• Tidal

10-1 Hz/Hz1/2 10-4 Hz/ Hz1/2 10-7 Hz/ Hz1/2


Prestabalized Laser performance

> 18,000 hours continuous operation

Frequency and lock very robust

TEM00 power > 8 watts

Non-TEM00 power < 10%


LIGO“first lock”

signal

LaserX Arm

Y Arm

Composite Video


Watching the Interferometer Lock

signal

X Arm

Y Arm

Laser

X arm

Anti-symmetricport

Y arm

Reflected light

2 min


Lock Acquisition


2km Fabry-Perot cavity 15 minute locked stretch


An earthquake occurred, starting at UTC 17:38.

The plot shows the band limited rms output in counts over the 0.1- 0.3Hz band for four seismometer channels. We turned off lock acquisition and are waiting for the ground motion to calm down.

From electronic logbook 2-Jan-02

Engineering Rundetecting earthquakes


17:03:03

01/02/2002

=========================================================================

Seismo-Watch

Earthquake Alert Bulletin No. 02-64441

=========================================================================

Preliminary data indicates a significant earthquake has occurred:

Regional Location: VANUATU ISLANDS

Magnitude: 7.3M

Greenwich Mean Date: 2002/01/02

Greenwich Mean Time: 17:22:50

Latitude: 17.78S

Longitude: 167.83E

Focal depth: 33.0km

Analysis Quality: A

Source: National Earthquake Information Center (USGS-NEIC)

Seismo-Watch, Your Source for Earthquake News and Information.

Visit http://www.seismo-watch.com

=========================================================================

All data are preliminary and subject to change.

Analysis Quality: A (good), B (fair), C (poor), D (bad)

Magnitude: Ml (local or Richter magnitude), Lg (mblg), Md (duration),

=========================================================================


Detecting the Earth Tides Sun and Moon


LIGOconclusions


Noise Spectrum: 2K Recycled

Factor of 200 improvement(over E2 spectrum)

Recycling Reduction of electronics

noise Partial implementation of

alignment control


Initial LIGO Sensitivity

Frequency noise Improve PSL Table layout

(done) Tailor MC loop (done) Implement common-mode

feedback from arms

Electronics noise Non-linearities? Filters? Alignment? Others?


TAMAperformance


LIGOconclusions

LIGO construction complete

LIGO commissioning and testing ‘on track’

“First Lock” officially established 20 Oct 00

Engineering test runs begin now, during period when emphasis is

on commissioning, detector sensitivity and reliability

First Science Run will begin during 2003

Significant improvements in sensitivity anticipated to begin about

2006



TAMA1000 hour run 86% duty cycle


TAMAconclusions


TAMAinterferometer stability

• Signal to Noise Ratio

• Binary Inspirals at 10 Kpc


How LIGO Works LIGO is an interferometric detector

» A laser is used to measure the relative lengths of two orthogonal cavities (or 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» Current technology then allows one to

measure h = L/L ~ 10-21 which turns out to be an interesting target


Compact binary inspiral

Neutron star binaries» Equation of state» Size of stars?» Thro tidal disruption

Black hole binaries» Spins» Only way to see them


Spinning neutron stars

Isolated neutron stars with deformed crust

Newborn neutron stars with r-modes

X-ray binaries may be limited by gravitational waves


Short duration bursts

Supernova hangup Core collapse Other routes

BBH merger phase» Short duration, high SNR


Physical Effects of the Waves As gravitational waves pass, they change the distance between

neighboring bodies

• Fractional change in distance is the strain given by

h = L / L


Configuration of LIGO Observatories

2-km & 4-km laser interferometers @ Hanford

Single 4-km laser interferometer @ Livingston


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.


Spacetime is Stiff!

=> Wave can carry huge energy with miniscule amplitude!

h ~ (G/c4) (ENS/r)


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

•Requires adaptive control system that evaluates plant evolution and reconfigures feedback paths and gains during lock acquisition

•But it works!


Digital Interferometer Sensing & Control System


When Will It Work?Status of LIGO in Spring 2001

Initial detectors are being commissioned, with first Science Runs commencing in 2002.

Advanced detector R&D underway, planning for upgrade near end of 2006» Active seismic isolation systems» Single-crystal sapphire mirrors» 1 megawatt of laser power circulating in arms» Tunable frequency response at the quantum limit

Quantum Non Demolition / Cryogenic detectors in future? Laser Interferometer Space Antenna (LISA) in planning and

design stage (2015 launch?)

LIGO and the Search for Gravitational Waves Barry Barish Stanford Colloquium 15-Jan-01.

Documents

finite speed

larger masses

relativeconcentrations

smaller masses travel

general relativity spiral

acceleration of masses

mysterious force

hrneutron binary system