Gravitational Wave Astronomy: Gravitational Wave Astronomy: Past, Present, and FuturePast, Present, and Future

~or~~or~

How To Get Lots of Science Out How To Get Lots of Science Out of Null Resultsof Null Results

Jeffrey SilvermanJeffrey Silverman

Ay 250: Transient UniverseAy 250: Transient Universe

April 24, 2007April 24, 2007

22

Introduction to Gravity Waves (GW)Introduction to Gravity Waves (GW) E-M radiation observations biased towards E-M radiation observations biased towards

“hot” objects.“hot” objects. GW observations biased toward compact, GW observations biased toward compact,

massive, rapidly moving objects.massive, rapidly moving objects. GW astronomy will open “a whole new GW astronomy will open “a whole new

window” on the sky (much like the advent window” on the sky (much like the advent of radio and X-ray astronomy).of radio and X-ray astronomy).

GWs probe places with extreme gravity:GWs probe places with extreme gravity:• Usually opaque to E-M radiation.Usually opaque to E-M radiation.• Great places to test GR and other gravitational Great places to test GR and other gravitational

theories.theories.

33

The Physics of GWsThe Physics of GWs Like Maxwell’s Equations (E-M waves), Like Maxwell’s Equations (E-M waves),

Einstein’s field equations have radiative Einstein’s field equations have radiative solutions.solutions.

In Einstein’s formulation, GWs propagate at c In Einstein’s formulation, GWs propagate at c ((i.e.i.e. the graviton has zero mass). the graviton has zero mass).

GWs are produced by acceleratedGWs are produced by accelerated mass-energy (like E-M mass-energy (like E-M

waves produced by waves produced by accelerated charges). accelerated charges).

Also like E-M waves, flux Also like E-M waves, flux falls as rfalls as r-2-2..

GW exist in all theories of GW exist in all theories of gravity.gravity.

c. 1947, Princeton University

44

More Physics of GWsMore Physics of GWs The first non-vanishing radiative multipole is the The first non-vanishing radiative multipole is the quadrupolequadrupole.. There are two independent polarizations separated by 45˚: hThere are two independent polarizations separated by 45˚: h (“h-cross”) and h (“h-cross”) and h++ (“h-plus”). (“h-plus”). In general, GWs consist of a linear combination of the two states.In general, GWs consist of a linear combination of the two states.

Einstein never took GWs seriously, he thought their effect was just too small to ever be Einstein never took GWs seriously, he thought their effect was just too small to ever be detected.detected.

Arthur Eddington commented, “Gravitational waves propagate at the speed of thought.”Arthur Eddington commented, “Gravitational waves propagate at the speed of thought.”

Wikipedia Wikipedia

hh hh++

LIGO website

55

Strength of GWs best expressed as a Strength of GWs best expressed as a dimensionless quantity, the strain:dimensionless quantity, the strain:

h ≡ h ≡ L / LL / L

((i.e.i.e. the fractional length change). the fractional length change). Assuming GWs couple only to the quadrupole Assuming GWs couple only to the quadrupole

moment:moment:

If the energy ~ MIf the energy ~ Mcc22 then: then:

h ~ 5h ~ 51010-14-14 / (r / pc) / (r / pc)

rc

c

EG

rc

QGh

symmnonKin

2

2

..

4~~

What Will We See?What Will We See?

66

What Will We See?What Will We See? 1M1M of GWs of GWs h ~ 5 h ~ 51010-14-14 / (r / pc) / (r / pc)

Galactic center (8kpc) Galactic center (8kpc) h ~ 10 h ~ 10-17-17

Virgo cluster (17Mpc) Virgo cluster (17Mpc) h ~ 10 h ~ 10-21-21

Hubble distance (c/HHubble distance (c/H00~4Gpc) ~4Gpc) h ~ 10 h ~ 10-23-23

This OoM estimate is optimistic; most This OoM estimate is optimistic; most sources will radiate much less than 1Msources will radiate much less than 1M in in GWs.GWs.

Notice strain goes as rNotice strain goes as r-1-1 (flux goes as r (flux goes as r-2-2).). Note that the strain is tiny NOT because the Note that the strain is tiny NOT because the

radiated energy is small (it’s huge), but radiated energy is small (it’s huge), but because space-time is a “stiff medium”.because space-time is a “stiff medium”.

77

What Else Will We See?What Else Will We See? GW sources cannot be much smaller GW sources cannot be much smaller

than a Schwarzschild radius: 2GM / cthan a Schwarzschild radius: 2GM / c22

They also cannot emit strongly at They also cannot emit strongly at periods shorter than the light travel periods shorter than the light travel time around the circumference:time around the circumference:

P P 2 2RRSchSch / c / c 4 4GM / cGM / c33

This implies frequencies (f = PThis implies frequencies (f = P-1-1):):

f f c c33 / 4 / 4GM ~ 1.6GM ~ 1.6101044 Hz (M Hz (M / M) / M)

88

(Indirect) Evidence of GWs(Indirect) Evidence of GWs PSR B1913+16 was the first binary pulsar PSR B1913+16 was the first binary pulsar

to be discovered (Hulse & Taylor 1975).to be discovered (Hulse & Taylor 1975). Observed for over 30 years.Observed for over 30 years. Weak radio source (1 mJy at 1400 MHz).Weak radio source (1 mJy at 1400 MHz). Joseph H. Taylor Jr. and Russell A. Hulse Joseph H. Taylor Jr. and Russell A. Hulse

shared the Nobel Prize in Physics in 1993:shared the Nobel Prize in Physics in 1993:

"for the discovery of a new "for the discovery of a new type of pulsar, a discovery type of pulsar, a discovery that has opened up new that has opened up new

possibilities for the study of possibilities for the study of gravitation."gravitation."

c. 1993, Nobel Foundation c. 1993, Nobel Foundation

99

GWs Are Out There!GWs Are Out There! Orbital parameters for PSR B1913+16 are known Orbital parameters for PSR B1913+16 are known

to extremely high accuracy (both relativistic and to extremely high accuracy (both relativistic and non-relativistic measurables).non-relativistic measurables).

Binary should emit energy as GWsBinary should emit energy as GWs system loses energysystem loses energy orbit should shrinkorbit should shrink the period should decreasethe period should decrease

GR says the rate of period decrease is:GR says the rate of period decrease is:

(Weisberg & (Weisberg & Taylor 2004)Taylor 2004)

Using the measured values and correcting for the Using the measured values and correcting for the relative acceleration between the solar system relative acceleration between the solar system and the binary:and the binary:

0.00211.0013corrected

measured

P

P

1010

Building Up Our ConfidenceBuilding Up Our Confidence The dominant The dominant

dissipation in the dissipation in the binary is energy loss binary is energy loss by GWs (not mass by GWs (not mass loss or tidal drag).loss or tidal drag).

No GWs directly No GWs directly detected yet.detected yet.

However, the Hulse-However, the Hulse-Taylor PSR has Taylor PSR has convinced us they convinced us they exist (and that we exist (and that we understand them understand them relatively well).relatively well).

Weisberg & Taylor 2004

1111

Orbital ShrinkageOrbital Shrinkage The two NSs will merge in about 300 Myr.The two NSs will merge in about 300 Myr.

Weisberg et. al, 1981

1212

Sources of GWs ISources of GWs I Coalescing Compact BinariesCoalescing Compact Binaries

• Binary companions will eventually Binary companions will eventually merge (since orbit is shrinking).merge (since orbit is shrinking).

• Can consist of NS/NS (Can consist of NS/NS (e.g.e.g. Hulse-Taylor), Hulse-Taylor), BH/BH, or BH/NS.BH/BH, or BH/NS.

• Small sizes, large masses, and huge Small sizes, large masses, and huge orbital velocities orbital velocities efficient GW efficient GW emission.emission.

• Signal will look like a chirp (a technical Signal will look like a chirp (a technical term).term).

1313

Coalescing Compact BinariesCoalescing Compact Binaries To To first orderfirst order, a chirp signal is , a chirp signal is

described by its amplitude and change described by its amplitude and change in frequency over time:in frequency over time:

where Mwhere Mcc is the “chirp mass”: is the “chirp mass”:

Measure f, f-dot, and A, solve for MMeasure f, f-dot, and A, solve for Mcc and get r (distance to source).and get r (distance to source).

13235 rfMA c

31135 fMf c

51

21

5321

MM

MMM c

1414

Coalescing Compact BinariesCoalescing Compact Binaries Can learn much from the exact waveform:Can learn much from the exact waveform:

• Harmonic content Harmonic content eccentricity of orbit eccentricity of orbit• Overall modulation Overall modulation mass ratio of the two objects and mass ratio of the two objects and

spin-orbit coupling (spin-orbit coupling (i.e.i.e. frame-dragging) frame-dragging)• Higher-order corrections Higher-order corrections mass and spin of constituents mass and spin of constituents• hh versus h versus h++ orbital inclination orbital inclination• Timing of end point (merger) Timing of end point (merger) equation of state of nuclear equation of state of nuclear

mattermatter

Sigg 1998

1515

Coalescing BHsCoalescing BHs The BH/BH chirp waveform has three The BH/BH chirp waveform has three

phases:phases:• Inspiral (BHs approach each other)Inspiral (BHs approach each other)• Merger (actual coalescence of BHs)Merger (actual coalescence of BHs)• Ringdown (new BH relaxes from excited Ringdown (new BH relaxes from excited

state)state)

MPI for Gravitational Physics/W.Benger-ZIB

1616

Sources of GWs IISources of GWs II Normal Binary StarsNormal Binary Stars

• Orbital periods Orbital periods 1 hour 1 hour f fGWGW 10 10-3-3 Hz Hz

• Can only be detected in space (discussed Can only be detected in space (discussed later)later)

LISA website

Sources of GWs ISources of GWs I

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Sources of GWs IIISources of GWs III Core Collapse Supernovae (CC SNe), if they explode Core Collapse Supernovae (CC SNe), if they explode

asymmetrically:asymmetrically:• Some explosions are asymmetric.Some explosions are asymmetric.• How asymmetric, we don’t know very well.How asymmetric, we don’t know very well.• Initial neutrino emission drains much of the energy that Initial neutrino emission drains much of the energy that

could go into GWs.could go into GWs.

LISA website

• Possibly ~10Possibly ~10-3 -3 MMcc22 could go into GWs.could go into GWs.

• If we observe a CC If we observe a CC SN in E-M radiation SN in E-M radiation and GWs, we can and GWs, we can compare the compare the propagation speed propagation speed of GWs to c.of GWs to c.

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Sources of GWs IVSources of GWs IV (Super) Massive BHs(Super) Massive BHs

• M M 10 105 5 MM BH swallowing a nearby object BH swallowing a nearby object (especially another Massive BH).(especially another Massive BH).

• Can only be detected in space (discussed Can only be detected in space (discussed later).later).

LISA website

1919

Sources of GWs VSources of GWs V Isolated PSRs:Isolated PSRs:

• Asymmetric about their rotation axis Asymmetric about their rotation axis nonzero quadrupole moment.nonzero quadrupole moment.

Stochastic BackgroundStochastic Background• Could be caused by density fluctuations in Could be caused by density fluctuations in

the early universe (like a GW CMB).the early universe (like a GW CMB).• If measured, it links us to the Planck Era and If measured, it links us to the Planck Era and

can discriminate between different can discriminate between different cosmological models!cosmological models!

• Most models predict that the strain would be Most models predict that the strain would be quite small.quite small.

2020

Joseph Weber (1919 – 2000)Joseph Weber (1919 – 2000) Father of GW astronomyFather of GW astronomy Started on GW detection in 1958 (Weber Started on GW detection in 1958 (Weber

1960)1960) Built first detector in 1966 at Univ. of MDBuilt first detector in 1966 at Univ. of MD ““Resonant Mass” detector (or “Weber Bar”)Resonant Mass” detector (or “Weber Bar”)

• Two 1.5-ton Al cylinders (Two 1.5-ton Al cylinders (i.e.i.e. bars) with bars) with piezoelectric crystals glued on.piezoelectric crystals glued on.

• When squeezed (by a passing GW) the crystals When squeezed (by a passing GW) the crystals develop electric voltages.develop electric voltages.

• Strung many crystals together to amplify the Strung many crystals together to amplify the signal.signal.

• Bars were placed at separate locations sift out Bars were placed at separate locations sift out random noise.random noise.

• A GW could excite the fundamental longitudinal A GW could excite the fundamental longitudinal mode of the bar, ~1657 Hz, which would induce a mode of the bar, ~1657 Hz, which would induce a voltage across the bar.voltage across the bar.

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Direct “Detection” of GWsDirect “Detection” of GWs Weber claimed extremely strong GW detections, ~1 per Weber claimed extremely strong GW detections, ~1 per

month, from the late 60’s through the 70’s.month, from the late 60’s through the 70’s. No one could explain the large amplitudes theoretically.No one could explain the large amplitudes theoretically. Other groups built bigger and more sensitive Weber Bars, Other groups built bigger and more sensitive Weber Bars,

but could not reproduce his results.but could not reproduce his results.

AIP Emilio Segrè Visual Archives

By 1975, nearly everyone in the By 1975, nearly everyone in the field agreed that Weber’s detections field agreed that Weber’s detections were simply noise.were simply noise.

Through the early 80’s, better Through the early 80’s, better Weber bars were built using better Weber bars were built using better digital electronics and cryogenic digital electronics and cryogenic cooling techniques (to decrease cooling techniques (to decrease noise), but none successfully noise), but none successfully detected GWs.detected GWs.

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Using Lasers to See GWsUsing Lasers to See GWs As early as 1956, laser interferometry was As early as 1956, laser interferometry was

proposed to search for GWs.proposed to search for GWs. Even Weber suggested this in the late 60’s.Even Weber suggested this in the late 60’s. If a laser is bounced between two mirrors, If a laser is bounced between two mirrors,

the distance between them can be the distance between them can be accurately measured.accurately measured.

If the separation is large compared to the If the separation is large compared to the GW, then it will appear as a plane wave.GW, then it will appear as a plane wave.

The GW will stretch and compress the space-The GW will stretch and compress the space-time distance between the mirrors.time distance between the mirrors.

This tiny change in distance can be detected This tiny change in distance can be detected using interferometry.using interferometry.

2323

Michelson InterferometersMichelson Interferometers Most designs are based on the Michelson Most designs are based on the Michelson

interferometer with response functioninterferometer with response functionA(A() ) sinc( sinc(L/c)L/c)

To effectively increase L, can bounce the light many To effectively increase L, can bounce the light many times (off-axis) before detection, but this degrades times (off-axis) before detection, but this degrades the signal.the signal.

Instead, use a Fabry-Perot cavity:Instead, use a Fabry-Perot cavity:• Partially transmitting input mirrorPartially transmitting input mirror• Highly reflective rear mirrorHighly reflective rear mirror• Adjust length to multiple of laser wavelength and Adjust length to multiple of laser wavelength and

cavity becomes resonantcavity becomes resonant• This increases power without degrading signal!This increases power without degrading signal!

Response becomes A(Response becomes A() ) sinc( sinc(L/c)FPI(L/c)FPI(L/L/c) c) where FPI(x) = |twhere FPI(x) = |t11 / 1-r / 1-r11rr22eeixix||22

LL

Sigg 1998

2424

More Michelson InterferometersMore Michelson Interferometers Recycle lost input power with a Power-Recycling mirror:Recycle lost input power with a Power-Recycling mirror:

• Place another partially transmitting mirror at the input.Place another partially transmitting mirror at the input.• Form another resonant cavity.Form another resonant cavity.• Recycle light that would be lost back out the input.Recycle light that would be lost back out the input.

Make signal resonant with a Signal-Recycling mirror:Make signal resonant with a Signal-Recycling mirror:• Place another partially transmitting mirror to the anti-Place another partially transmitting mirror to the anti-

symmetric port.symmetric port.• Can shape interferometer response around a narrow Can shape interferometer response around a narrow

frequency band.frequency band. If both are used, it’s called Dual-Recycled.If both are used, it’s called Dual-Recycled. Response becomes A(Response becomes A() ) sinc( sinc(L/c)FPI(L/c)FPI(L/L/c)Gc)GRR where where

GGRR is the total gain from the recycling cavity(ies). is the total gain from the recycling cavity(ies). All combinations of recycling and Fabry-Perot arms All combinations of recycling and Fabry-Perot arms

can be used.can be used.

Sigg 1998

2525

The LIGO ProjectThe LIGO Project Laser Interferometer Gravitational wave Laser Interferometer Gravitational wave

ObservatoryObservatory Collaboration between Cal Tech and MIT.Collaboration between Cal Tech and MIT. Two widely separated sites under common Two widely separated sites under common

management (to make coincidence management (to make coincidence measurements)measurements)

Hanford, WA (LIGO website) Livingston, LA (LIGO website)

2626

The LIGO ProjectThe LIGO Project Interferometers with 4km arms.Interferometers with 4km arms. Can operate several interferometers simultaneously.Can operate several interferometers simultaneously. Hanford site has 4km and 2km arm detectors.Hanford site has 4km and 2km arm detectors. Well collimated lasers.Well collimated lasers. Vacuum of 10Vacuum of 10-9-9 torr H and 10 torr H and 10-10-10 torr other gases. torr other gases. Lifetime of at least 20 years.Lifetime of at least 20 years.

One 4km arm (LIGO website) Beam splitter (LIGO website)

2727

The LIGO InterferometerThe LIGO Interferometer Power-recycled Fabry-Perot Power-recycled Fabry-Perot

interferometer.interferometer. Sensing and control systemSensing and control system

Interferometer (LIGO website)

Seismic Seismic isolation isolation system (not system (not shown)shown)

SuspensionsSuspensions

Interferometer (Abbott et al. 2007)

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The LIGO MirrorsThe LIGO Mirrors ~10 kg cylinders 25cm in ~10 kg cylinders 25cm in

diameter and 10cm thick.diameter and 10cm thick. Made of high-purity fused Made of high-purity fused

silica.silica. Permanent magnets glued Permanent magnets glued

to the back to control to the back to control longitudinal and angular longitudinal and angular orientation.orientation.

Permanent magnets glued Permanent magnets glued to the side to control to the side to control sideways motion.sideways motion.

Coil drivers mounted on Coil drivers mounted on suspension cage to adjust suspension cage to adjust force on magnets/mirrors force on magnets/mirrors (by changing current in (by changing current in the coils).the coils).Mirror and Cage (LIGO website)

2929

The LIGO LaserThe LIGO Laser

Diffraction Diffraction limited limited beam of beam of width ~30 width ~30 to 40mm.to 40mm.

Part of the laser setup

(LIGO website)

Solid state diode-pumped Nd:YAG Solid state diode-pumped Nd:YAG laser.laser.

Operates at 10W and 1064nm.Operates at 10W and 1064nm.

3030

The LIGO Scientific Collaboration!!!The LIGO Scientific Collaboration!!!

3131

Sources of Noise From the LaserSources of Noise From the Laser Shot noise: Shot noise:

fluctuations in the fluctuations in the number of number of photons in the photons in the beambeam

Light amplitude Light amplitude and laser and laser frequency noise: frequency noise: the laser’s beam the laser’s beam isn’t perfectisn’t perfect

Sigg 1998

3232

Sources of Noise From the LaserSources of Noise From the Laser Scattered light: Scattered light:

light can scatter light can scatter into and out of the into and out of the beam path (back beam path (back scatter is why scatter is why fiber optics aren’t fiber optics aren’t used)used)

Beam jitter: the Beam jitter: the laser’s output laser’s output angle and position angle and position isn’t perfectisn’t perfect

Sigg 1998

3333

Sources of Noise From the LaserSources of Noise From the Laser Residual gas Residual gas

column density column density fluctuations: if the fluctuations: if the vacuum isn’t vacuum isn’t perfect, changes perfect, changes in gas density will in gas density will change the index change the index of refractionof refraction

Sigg 1998

3434

Seismic NoiseSeismic Noise Affects mirror motion directlyAffects mirror motion directly The Earth is in constant motion fromThe Earth is in constant motion from

Sigg 1998

volcanoes, ocean volcanoes, ocean waves, wind, and waves, wind, and lunar tidal forceslunar tidal forces

Strongest from Strongest from 0.1Hz to 10Hz0.1Hz to 10Hz

Huge source of Huge source of noise on Earth at noise on Earth at low frequencieslow frequencies

Absent in spaceAbsent in space

3535

Thermal Noise in SuspensionsThermal Noise in Suspensions Also affects mirror motion directlyAlso affects mirror motion directly Heat excites motion in suspensionHeat excites motion in suspension

Sigg 1998

elementselements Damped by Damped by

suspending mirrors suspending mirrors as pendulums as pendulums hung with thin hung with thin wire.wire.

3636

Thermal Noise in MirrorsThermal Noise in Mirrors Again affects mirror motion directlyAgain affects mirror motion directly Heat excites normal modes in theHeat excites normal modes in the

Sigg 1998

mirrors themselvesmirrors themselves Frequencies are Frequencies are

more toward kHz more toward kHz range (sorta)range (sorta)

Can be modeled Can be modeled well if the mirrors well if the mirrors are fabricated are fabricated accuratelyaccurately

3737

Radiation Pressure ImbalanceRadiation Pressure Imbalance Number of photons hitting a mirror will Number of photons hitting a mirror will

fluctuate based on photon countingfluctuate based on photon counting

Sigg 1998

statisticsstatistics The recoil of these The recoil of these

photons will photons will introduce a small introduce a small force on the force on the mirrorsmirrors

3838

Gravity Gradient NoiseGravity Gradient Noise Any mass place near a mirror will exert Any mass place near a mirror will exert

a gravitational force on the mirrora gravitational force on the mirror

Sigg 1998

The Earth’s internal The Earth’s internal seismic waves and seismic waves and density fluctuations density fluctuations in the atmosphere in the atmosphere are the main are the main concernconcern

Sets the ultimate Sets the ultimate limit in sensitivity limit in sensitivity for Earth-based for Earth-based detectorsdetectors

3939

Other (Extreme) Sources of NoiseOther (Extreme) Sources of Noise Radiometer noiseRadiometer noise Electric field fluctuationsElectric field fluctuations Magnetic field fluctuationsMagnetic field fluctuations Cosmic ray muonsCosmic ray muons

4040

Noise SummaryNoise Summary Red curve is LIGO goal (current LIGO).Red curve is LIGO goal (current LIGO). Blue curve is theoretical limit on Earth.Blue curve is theoretical limit on Earth.

Sigg 1998

4141

Noise SummaryNoise Summary

LIGO website

4242

Noise Summary (on Earth)Noise Summary (on Earth) Red curve is Initial (current) LIGO.Red curve is Initial (current) LIGO. Blue curve is theoretical limit on Earth.Blue curve is theoretical limit on Earth.

Sigg 1998

Can possibly (one Can possibly (one day) model or day) model or eliminate noise eliminate noise from all sources from all sources and get down to and get down to blue curve on blue curve on Earth. (A LIGO)Earth. (A LIGO)

Or, you can go into Or, you can go into space!!! (LISA)space!!! (LISA)

4343

LIGO Results: Fourth Science Run (S4)LIGO Results: Fourth Science Run (S4) Most recently released data set.Most recently released data set. 22 Feb 2005 to 23 Mar 2005.22 Feb 2005 to 23 Mar 2005. Used all three LIGO interferometers Used all three LIGO interferometers

(2km and 4km at Hanford, 4km at (2km and 4km at Hanford, 4km at Livingston) and GEO 600 in Germany Livingston) and GEO 600 in Germany (more on GEO later).(more on GEO later).

~570 hours of observation time on ~570 hours of observation time on each LIGO interferometer.each LIGO interferometer.

~402 hours when all three were ~402 hours when all three were collecting data simultaneously.collecting data simultaneously.

4444

LIGO Results: Stochastic BackgroundLIGO Results: Stochastic Background A stochastic background of GWs has been A stochastic background of GWs has been

predicted from many possible sources:predicted from many possible sources:• Amplification of quantum vacuum fluctuations Amplification of quantum vacuum fluctuations

(Grishchuk 1975, Grishchuk 1997, Starobinsky 1979)(Grishchuk 1975, Grishchuk 1997, Starobinsky 1979)

• Pre-Big-Bang modelsPre-Big-Bang models (Gasperini & Veneziano 1993, Buonanno et (Gasperini & Veneziano 1993, Buonanno et al. 1997)al. 1997)

• Phase transitionsPhase transitions (Kosowky et al. 1992, Apreda et al. 2002) (Kosowky et al. 1992, Apreda et al. 2002)

• Cosmic stringsCosmic strings (Caldwell & Allen 1992, Damour & Vilenkin 2005) (Caldwell & Allen 1992, Damour & Vilenkin 2005)

• Rotating NSsRotating NSs (Regimbau 2001) (Regimbau 2001)

• SupernovaeSupernovae (Coward et al. 2002) (Coward et al. 2002)

• Low-mass X-ray binariesLow-mass X-ray binaries (Cooray 2004) (Cooray 2004)

Despite much searching, none have been Despite much searching, none have been discovered thus far.discovered thus far.

4545

LIGO Results: Stochastic BackgroundLIGO Results: Stochastic Background 192 second long intervals of data with 1/32 192 second long intervals of data with 1/32

Hz frequency resolution.Hz frequency resolution. S4 results are beginning to differentiate S4 results are beginning to differentiate

between different cosmological models.between different cosmological models. S5 (and A LIGO) willS5 (and A LIGO) will

probe evenprobe evenmore parametermore parameterspace.space.

Results are aResults are a90% upper limit90% upper limiton the GWon the GWbackground.background.

Abbott et al. 2006

4646

LIGO Results: Stochastic BackgroundLIGO Results: Stochastic Background 10 trials were performed with artificially 10 trials were performed with artificially

injected amplitudes.injected amplitudes. Left (gray) error bars denote the Left (gray) error bars denote the

theoretical errors.theoretical errors.

Abbott et al. 2006

Right (black) Right (black) error bars denote error bars denote the standard the standard errors over the errors over the 10 trials.10 trials.

Seem to recover Seem to recover injected signal injected signal quite accurately!quite accurately!

4747

LIGO Results: GWs From Single PSRsLIGO Results: GWs From Single PSRs Appeared on astro-ph on 4 Apr 2007.Appeared on astro-ph on 4 Apr 2007. 95% upper limits on GW amplitudes for 78 PSRs.95% upper limits on GW amplitudes for 78 PSRs. Tightest strain upper limit is 3.2Tightest strain upper limit is 3.21010-25-25

Strain is directly related to Strain is directly related to , a PSR’s equatorial , a PSR’s equatorial ellipticity.ellipticity.

Abbott et al. 2007

Smallest ellipticity is Smallest ellipticity is 8.5 8.5 10 10-7-7

Strange quark stars Strange quark stars or hybrid stars have or hybrid stars have ~ 10 ~ 10-5-5

More conventional More conventional NS EOSs have NS EOSs have ~ 10~ 10-7-7

4848

LIGO Results: LIGO Results: TransientTransient GW Bursts GW Bursts Appeared on astro-ph on 6 Apr 2007.Appeared on astro-ph on 6 Apr 2007. 90% upper limit on mean rate of GW bursts: 90% upper limit on mean rate of GW bursts: 0.15 0.15

per dayper day Assuming isotropic emission of GWs (for OoM):Assuming isotropic emission of GWs (for OoM):

Abbott et al. 2007

2min

20

32

24

hfG

crEGW

hhminmin = best = best sensitivity and sensitivity and assume 50% assume 50% efficiencyefficiency

r ~ 10kpc r ~ 10kpc 881010-8-8 M M in GWs in GWs

r ~ 16Mpc r ~ 16Mpc 0.2 0.2 MM in GWs in GWs

4949

LIGO Results: LIGO Results: TransientTransient GW Bursts GW Bursts Monte Carlo was run to test efficiency.Monte Carlo was run to test efficiency. Statistical errors comparable to size of Statistical errors comparable to size of

symbols.symbols.

Abbott et al. 2007

5050

LIGO Results: LIGO Results: TransientTransient GW Bursts GW Bursts CC SN model with non-spinning 11MCC SN model with non-spinning 11M

progenitor (and 50% detection efficiency) progenitor (and 50% detection efficiency) visible to ~0.2kpc.visible to ~0.2kpc.

CC SN model with spinning 15MCC SN model with spinning 15M progenitor progenitor (and 50% detection efficiency) visible to (and 50% detection efficiency) visible to ~8kpc.~8kpc.

A pair of merging 10MA pair of merging 10M BHs visible to BHs visible to ~1.5Mpc.~1.5Mpc.

A pair of merging 50MA pair of merging 50M BHs visible to BHs visible to ~60Mpc.~60Mpc.

5151

The Other GuysThe Other Guys TAMA300 (http://tamago.mtk.nao.ac.jp/)TAMA300 (http://tamago.mtk.nao.ac.jp/)

• Tokyo, Japan and started in 1995Tokyo, Japan and started in 1995• 300m dual-recycled Fabry-Perot300m dual-recycled Fabry-Perot• Test-bed for new interferometer technologiesTest-bed for new interferometer technologies

GEO 600 (http://geo600.aei.mpg.de/)GEO 600 (http://geo600.aei.mpg.de/)• Hannover, Germany (British/German) and started Hannover, Germany (British/German) and started

in 2002in 2002• 600m dual-recycled Fabry-Perot600m dual-recycled Fabry-Perot• Comparable sensitivity to LIGOComparable sensitivity to LIGO• Between 2002 and 2006, participated in several Between 2002 and 2006, participated in several

data runs in coincidence with LIGOdata runs in coincidence with LIGO VIRGO (http://www.virgo.infn.it/)VIRGO (http://www.virgo.infn.it/)

• Pisa, Italy (Italian/French) and started in 2003Pisa, Italy (Italian/French) and started in 2003• 3km power-recycled Fabry-Perot3km power-recycled Fabry-Perot• Comparable sensitivity to LIGOComparable sensitivity to LIGO

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The Future of LIGO: S5The Future of LIGO: S5 Fifth Science Run began in Nov Fifth Science Run began in Nov

2005.2005. Expected to continue into late Expected to continue into late

2007.2007. Goal of collecting a full year of Goal of collecting a full year of

coincident LIGO data.coincident LIGO data. Uses all 3 LIGO interferometers and Uses all 3 LIGO interferometers and

GEO 600 as well.GEO 600 as well. First science run at design First science run at design

sensitivity.sensitivity.

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The Future of LIGO: Einstein@HomeThe Future of LIGO: Einstein@Home http://einstein.phys.uwm.edu/http://einstein.phys.uwm.edu/ Based on SETI@Home (Mo will talk Based on SETI@Home (Mo will talk

about this).about this). Uses idle computer time to search for Uses idle computer time to search for

GWs from PSRs in data from LIGO and GWs from PSRs in data from LIGO and GEO.GEO.

Supported by the American Physical Supported by the American Physical Society (APS) and by a number of Society (APS) and by a number of international organizations.international organizations.

Currently searching 840 hours of data Currently searching 840 hours of data from S5.from S5.

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The The REALREAL Future of LIGO Future of LIGO Advanced LIGO (A LIGO).Advanced LIGO (A LIGO). Sensitivity increase by ~10x.Sensitivity increase by ~10x. Lowest frequency goes from ~40 Hz to Lowest frequency goes from ~40 Hz to

~10 Hz.~10 Hz. During first several hours of operation During first several hours of operation

data will exceed S5.data will exceed S5. Observe binary NSs ~15x further Observe binary NSs ~15x further

event rate 3000x greater.event rate 3000x greater. NS-BH binaries visible to 650Mpc.NS-BH binaries visible to 650Mpc. BH-BH binaries visible to z=0.4.BH-BH binaries visible to z=0.4.

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Advanced LIGOAdvanced LIGO Laser power increased from 10W to Laser power increased from 10W to

~200W.~200W. Test masses larger in diameter (~34cm) Test masses larger in diameter (~34cm)

to reduce internal thermal noise.to reduce internal thermal noise. Test masses more massive (~40kg) to Test masses more massive (~40kg) to

reduce radiation pressure noise.reduce radiation pressure noise. Test masses suspended by fused silica Test masses suspended by fused silica

fibers to reduce suspension thermal fibers to reduce suspension thermal noise.noise.

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Advanced LIGOAdvanced LIGO Performance dominated at most frequencies Performance dominated at most frequencies

by quantum noise of sensing test mass by quantum noise of sensing test mass positions.positions.

Should take this architecture to its technical Should take this architecture to its technical endpoint; it is as sensitive as one can make endpoint; it is as sensitive as one can make an interferometer based on a Fabry-Perot an interferometer based on a Fabry-Perot Michelson configuration.Michelson configuration.

Further advances will come from R&D that is Further advances will come from R&D that is just beginning:just beginning:• cryogenic optics and suspensionscryogenic optics and suspensions• purely reflective opticspurely reflective optics• readouts that fully exploit the quantum nature of readouts that fully exploit the quantum nature of

the measurementthe measurement These developments will help instruments in These developments will help instruments in

the second decade of this century.the second decade of this century.

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Advanced LIGO…SomedayAdvanced LIGO…Someday 2008: Receipt of funding for the fabrication 2008: Receipt of funding for the fabrication

and construction project.and construction project. 2009: Delivery of first interferometer 2009: Delivery of first interferometer

hardware to the observatory staging facilities.hardware to the observatory staging facilities. 2011 (early): Decommissioning of initial LIGO 2011 (early): Decommissioning of initial LIGO

at Livingston and simultaneous start of at Livingston and simultaneous start of installation of Advanced LIGO there.installation of Advanced LIGO there.

2011 (late): Decommissioning of initial LIGO 2011 (late): Decommissioning of initial LIGO at Hanford and simultaneous start of at Hanford and simultaneous start of installation of Advanced LIGO there.installation of Advanced LIGO there.

2013: Both observatories in operation.2013: Both observatories in operation. GEO 600 is a full partner in Advanced LIGO, GEO 600 is a full partner in Advanced LIGO,

participating at all levels in the effort.participating at all levels in the effort.

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LISA: GWs LISA: GWs IN SPACE!!!!IN SPACE!!!! Laser Interferometer Space AntennaLaser Interferometer Space Antenna Space-based GW observations first Space-based GW observations first

proposed in 1977proposed in 1977 PLANNED LAUNCH DATE: 201XPLANNED LAUNCH DATE: 201X MISSION DURATION: Five years for MISSION DURATION: Five years for

nominal mission (10 years extended nominal mission (10 years extended mission)mission)

Jointly sponsored by the European Jointly sponsored by the European Space Agency (ESA)Space Agency (ESA)

Will measure the (change in) distance Will measure the (change in) distance between test masses separated by 5 between test masses separated by 5 million km with a precision of 10pm.million km with a precision of 10pm.

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The LISA Spacecraft(s)The LISA Spacecraft(s) 3 identical cylinders 1.8m in diameter and 0.5m high.3 identical cylinders 1.8m in diameter and 0.5m high. Each cylinder supports a Y-shaped tubular structure Each cylinder supports a Y-shaped tubular structure

that contains two instruments.that contains two instruments. A cover across the top (not shown) will protect the A cover across the top (not shown) will protect the

detector from photon pressure from the Sun and detector from photon pressure from the Sun and variable solar B field (two external sources of noise).variable solar B field (two external sources of noise).

The Y-shaped structure is gold-coated and suspendedThe Y-shaped structure is gold-coated and suspendedby stressed-fiberglass by stressed-fiberglass bands to thermally isolate bands to thermally isolate it from the spacecraft.it from the spacecraft.

Two radio antennas (not Two radio antennas (not shown) will be mounted to shown) will be mounted to each cylinder for each cylinder for communication with Earth.communication with Earth.

LISA website

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The LISA InterferometerThe LISA Interferometer Each spacecraft transmits light to the other two Each spacecraft transmits light to the other two

spacecraft.spacecraft. Rather than reflecting the received light, each Rather than reflecting the received light, each

spacecraft transmits a new beam back.spacecraft transmits a new beam back. Each spacecraft then compares the signals to Each spacecraft then compares the signals to

measure the variation in distance between them.measure the variation in distance between them. Thus, each cylinder acts as Thus, each cylinder acts as bothboth the light source the light source andand

mirrors at the end of a mirrors at the end of a Michelson interferometer Michelson interferometer arm arm it behaves like it behaves like three independent three independent interferometers.interferometers.

This allows GW This allows GW polarizations to be polarizations to be detected.detected.

LISA website

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The LISA PayloadThe LISA Payload The Y-payload includes two identical The Y-payload includes two identical

instruments, each including:instruments, each including:• 30cm telescope for transmission and reception of 30cm telescope for transmission and reception of

laser signalslaser signals• an optical bench that contains interferometer an optical bench that contains interferometer

opticsoptics• an inertial sensoran inertial sensor• a test mass shielded from non-gravitational a test mass shielded from non-gravitational

disturbancesdisturbances• a capacitor plate a capacitor plate

arrangement for arrangement for measuring the position measuring the position of the spacecraft with of the spacecraft with respect to the test respect to the test massmass LISA website

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The LISA LaserThe LISA Laser

LISA website

1W at 11W at 1mm

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The LISA AccelerometerThe LISA Accelerometer

LISA website

The test mass is in the center,it’s surrounded by capacitor plates.

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The LISA Test Masses & ThrustersThe LISA Test Masses & Thrusters

4cm highly polished cubes Optical bench with test mass To keep test masses floating To keep test masses floating

freely, distance between freely, distance between them and surrounding them and surrounding spacecraft is constantly spacecraft is constantly monitored.monitored.

If there’s a shift, If there’s a shift, microthrusters fire to move microthrusters fire to move the spacecraft into position.the spacecraft into position.

Minimize drifting by adding Minimize drifting by adding biases in initial velocity and biases in initial velocity and position (Povoleri 2006).position (Povoleri 2006). Newton thruster, accurate to 10nm

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LISA’s OrbitLISA’s Orbit Each spacecraft will be in an Earth-like orbit Each spacecraft will be in an Earth-like orbit

around the Sun.around the Sun. They will maintain an equilateral triangular They will maintain an equilateral triangular

configuration with center 20° behind the Earth configuration with center 20° behind the Earth and side length 5 million km.and side length 5 million km.

The plane of the The plane of the formation is tilted 60° formation is tilted 60° below the ecliptic.below the ecliptic.

The changing The changing orientation makes it orientation makes it possible to determine possible to determine the direction of a GW the direction of a GW source.source.

LISA website

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LISA’s Sources of NoiseLISA’s Sources of Noise Internal sources:Internal sources:

• E field generated by the spacecraft computer E field generated by the spacecraft computer acting on the test massesacting on the test masses

• Effects from residual gas pressure near the test Effects from residual gas pressure near the test massesmasses

• Thermal radiation by the electrodes used to Thermal radiation by the electrodes used to measure the spacecraft positionmeasure the spacecraft position

External sources:External sources:• Solar wind buffetingSolar wind buffeting• Spacecraft driftSpacecraft drift• Test mass chargingTest mass charging• Interference from the interplanetary magnetic Interference from the interplanetary magnetic

fieldfield Any of these disturbances may cause Any of these disturbances may cause

movement of the test masses or changes in movement of the test masses or changes in the distance of the interferometer arms.the distance of the interferometer arms.

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What Will LISA See?What Will LISA See? Low-frequency GWs that will never be Low-frequency GWs that will never be

detectable by terrestrial detectors (due detectable by terrestrial detectors (due to gravity gradient noise).to gravity gradient noise).

Galactic compact binaries long before Galactic compact binaries long before coalescence.coalescence.

Extragalactic Extragalactic (S)MBH (S)MBH binaries in binaries in the final the final months of months of coalescence.coalescence.

LISA website

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What Might LISA See?What Might LISA See? If SNe Ia occur when WD binaries coalesce, LISA will If SNe Ia occur when WD binaries coalesce, LISA will

determine the directions to, and time of collision, for determine the directions to, and time of collision, for the next 500 SNe Ia in our galaxy. the next 500 SNe Ia in our galaxy.

Close encounters between SMBHs and compact or Close encounters between SMBHs and compact or MS stars will emit GWs:MS stars will emit GWs:• the encounters will the encounters will

produce produce transienttransient bursts of GWsbursts of GWs

• Models show an Models show an event rate of ~15/yr event rate of ~15/yr in our galaxy and in our galaxy and ~3/yr in the Virgo ~3/yr in the Virgo Cluster detectable Cluster detectable by LISA.by LISA.

Rubbo 2006

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What’s LISA’s Sweetspot?What’s LISA’s Sweetspot? GW observations GW observations calculated energy of GWs (M calculated energy of GWs (MGWGW)) Let Let MMGWGW be the error in M be the error in MGWGW

Consider a good detection if Consider a good detection if MMGWGW / M / MGWGW 5% 5% Monte Carlo simulations run as a function of MMonte Carlo simulations run as a function of MGWGW and and

distancedistance Best masses are Best masses are around Maround Mzz = 10 = 1066 ± (a ± (a few) Mfew) M where M where Mzz = = M(1+z)M(1+z)

Mass range shrinks Mass range shrinks with redshiftwith redshift

Event rates also Event rates also calculated: 1-5 calculated: 1-5 events/yr @ z~2-3events/yr @ z~2-3

Hughes 2005

z = 0.5

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GWs: Perpetually The GWs: Perpetually The WaveWave of the Future of the Future The theory of GWs is pretty stable and much is well-The theory of GWs is pretty stable and much is well-

understood.understood. There are places for improvement (post-Newtonian There are places for improvement (post-Newtonian

and other forms of gravity, numerical simulations of and other forms of gravity, numerical simulations of signal waveforms, further rate calculations, etc.).signal waveforms, further rate calculations, etc.).

Although there’s no direct detections yet, current Although there’s no direct detections yet, current detectors have placed quite strong limits on many detectors have placed quite strong limits on many astrophysical sources of GWs.astrophysical sources of GWs.

New technologies and upgrades of older detectors New technologies and upgrades of older detectors are coming (relatively) soon.are coming (relatively) soon.

Massive upgrades and space-based observations are Massive upgrades and space-based observations are in the further future.in the further future.

““In astronomy, it’s always the next telescope that’s In astronomy, it’s always the next telescope that’s gonna solve all your problems.”gonna solve all your problems.”

--James Graham--James Graham

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Gravitational Wave AstronomyGravitational Wave Astronomy