The Search for Gravitational Waves with LIGO Duncan Brown California Institute of Technology On Behalf of the LIGO Scientific Collaboration VI Rencontres Du Vietnam, 2006

LIGO Science Goals • Direct verification of two dramatic predictions of Einstein’s theory of general relativity » Gravitational waves and black holes

• Physics and astronomy » Detailed tests of properties of gravitational waves (speed, polarization, graviton mass, …) » Probe strong field gravity around black holes and in the early universe » Probe the neutron star equation of state » Perform routine astronomical observations to understand compact binary populations, supernovae rates, test gamma-ray burst models, …

• LIGO provides a new window on the universe 8/8/2006

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Astrophysical Sources of Gravitational Waves • Compact binaries » Black holes and neutron stars » Inspiral, merger (and ringdown) phases » Probe internal structure, populations, and spacetime geometry

• Isolated pulsars » Neutron stars with mountains or wobbles » Low mass X-ray binaries » Probe structure and populations 8/8/2006

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Astrophysical Sources of Gravitational Waves • Bursts » » » »

Supernovae Cosmic strings Black hole mergers Correlations with electromagnetic observations » Surprises! cosmic gravitational-wave background (10-22s)

cosmic microwave background (10 +12s )

• Stochastic Background » Big Bang and early universe » Background of gravitational wave bursts

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Gravitational Waves • Gravitational waves are distortions of spacetime metric caused by time changing mass quadrupole moment • Cause distances between freely falling masses to vary Time t=0

(period)/4

(period)/2

3(period)/4

(period)

• Strength of gravitational wave given by strain h(t) = ΔL(t)/L 8/8/2006

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Detection of Gravitational waves • Michelson-type interferometers can detect spacetime distortions produced by gravitational waves

• Amplitude of gravitational waves produced by inpiralling binary neutron stars in the VIRGO cluster have h ~ 10-21 8/8/2006

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The LIGO Observatories •

LIGO: The Laser Interferometer Gravitational Wave Observatory

•

Two observatories 3002 km apart » »

Hanford, WA Livingston, LA

• • •

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One 4 km interferometer at each observatory Additional 2 km interferometer at Hanford Need to be measure distances ΔL = hL ~ 10-21 x 4 km = 10-18 m

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Noise Sources in LIGO Ground motion couples into motion of mirrors Thermal excitations of mirror suspensions

Counting statistics of photons at photodiode

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Evolution of Sensitivity

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The Search for Gravitational Waves is in Progress

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S5 Duty Cycle • Fifth science run started November 2005 and is ongoing • Over 3400 hours of coincident data taken!

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Compact Binary Searches • LIGO is sensitive to binaries containing neutron stars and black holes • Waveforms depend on spins and masses • Binary neutron stars » Rate estimates give upper bound 1/3 yrs for S5

• Binary black holes » Rate estimates give upper bound 1/yr for S5

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Binary Neutron Star Search

cumulative number of events

S2 Observational Result Phys. Rev. D. 72, 082001 (2005)

• S3 search complete Rate < 47 per year per Milky-Way-like galaxy; » Under internal review » 0.09 yr of data 0.04 yr data, 1.27 Milky-Ways » ~3 Milky-Way like galaxies • S4 search complete signal-to-noise ratio squared 8/8/2006

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» Under internal review » 0.05 yr of data » ~24 Milky-Way like galaxies 13

Binary Black Hole Search •

Stellar mass black holes » S2 result < 38 per year per Milky Way-like galaxy – Phys. Rev. D. 73, 062001 (2006)

» S3 search complete and under internal review – 0.09 yrs of data, 5 Milky Way-like galaxy for (5,5) Msun binaries

» S4 search complete and under internal review – 0.05 yrs of data, 150 Milky Way-like galaxy for (5,5) Msun binaries

•

Sub-solar mass black holes (primordial black holes) » S2 result < 63 per year per Milky Way Halo – Phys. Rev. D. 72, 082002 (2005)

» S3 search complete and under internal review – 0.09 yrs of data, 1 Milky Way Halo

» S4 search complete and under review – 0.05 yrs of data, 3 Milky Way Halos visible

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Preliminary S5 BNS Efficiency • First epoch of S5 (Nov 2005 - Feb 2006) • 32.6 days of triple coincident data • 24.0 days of Hanford double coincident data 8/8/2006

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Continuous Wave Sources Bumpy Neutron Star

Low-mass x-ray binary

Wobbling pulsars

C redit: M. Kramer

C redit: Dana Berry/NASA 8/8/2006

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Pulsar Searches •

There are many known neutron stars that can produce GWs in the LIGO band » » » »

•

Observed spindown can set strong upper limits on gravitational wave emission Targeted search for 73 known (radio and x-ray) pulsars in S5 S2 search complete, Phys. Rev. Lett. 94 181103 (2005) S3, S4 paper in preparation, S5 under internal review

Also likely to be many neutron stars that are not visible via electromagnetic observations » All sky, unbiased searches

•

Search for a sine wave Doppler modulated by the Earth’s motion (and possible spin down) » Easy, but computationally expensive » S2 search complete, no detection, Phys. Rev. D. 72, 102004 (2005) 8/8/2006

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All Sky Pulsar Searches • Computationally intensive: http://www.einsteinathome.org/ • Use Einstein@Home • S3 data analyzed, no detections • S4 complete, under internal review • S5 search under way now 8/8/2006

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Preliminary S5 Known Pulsar Results

Crab Pulsar

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Stochastic Background Searches •

Cosmological background from Big Bang (analog of CMB)

•

Astrophysical backgrounds due to unresolved individual sources » E.g.: BH mergers, inspirals, supernovae

WMAP 3-year data GW spectrum due to cosmological BH ringdowns (Regimbau & Fotopoulos) 8/8/2006

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Cosmological Stochastic Background • A cosmological stochastic gravitational wave background is a prediction of most cosmological models • Given an energy density spectrum ΩGW(f) there is a GW strain spectrum

• This signal can be searched for by cross correlating different pairs of detectors 8/8/2006

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Stochastic Search Results LIGO S1: Ω0 < 44

0

PRD 69 122004 (2004)

LIGO S3: Ω0 < 8.4x10-4

Log

(Ω0)

-2 -4

Pulsar Cosmic stringsTiming

PRL 95 221101 (2005)

BB Nucleosynthesis

LIGO S4: Ω0 < 6.5x10-5 (new)

-6 -8

-12

Pre-big bang model Inflation

-14

Slow-roll

-10

CMB

-18 -16 -14 -12 -10 -8 8/8/2006

Initial LIGO, 1 yr data Expected Sensitivity ~ 4x10-6 Advanced LIGO, 1 yr data EW or SUSY Expected Sensitivity Phase transition ~ 1x10-9 Cyclic model -6 -4 -2 Log(f [Hz])

0

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4

6

8

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Untriggered Burst Searches • Search for triple coincident triggers with a wavelet algorithm • Measure confidence with waveform consistence test • Set threshold for low false alarm probability • Compare with efficiency for detecting sample waveforms • Analysis of S1 - S3 data published, no bursts found • S4 analysis complete, under final review » S2 < 0.26 bursts per day Phys. Rev. D. 72, 062001 (2005) » Preliminary S4 results < 0.148 bursts per day

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Untriggered Burst Upper Limits

Rate Limit (events/day)

Central Frequency

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Triggered Burst Searches • Follow up GRB triggers looking at cross-correlation from data in at least two detectors • HETE GRB030329 occurred during S2 » Search resulted in no detection » Phys. Rev. D. 72, 042002 (2005)

• SWIFT, IPN, HETE-2, Konus-Wind: 39 triggers during S2/S3/S4 » No loud event from any GRB

• S5 run: 53 GRBs (mostly SWIFT) in 5 months » Preliminary results indicate no GW events

• Triggered inspiral search analysis currently under development on S5 data 8/8/2006

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Joint Searches •

•

Several km-scale detectors, bars now in operation Network gives: »

» » »

»

Detection confidence Sky coverage Duty cycle Direction by triangulation Waveform extraction

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AURIGA INFN Legnaro, Italy 1 Bar detector

ALLEGRO Baton Rouge LA 1 Bar detector

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The Future: Advanced LIGO • Advanced LIGO has been approved by the NSF » Planned funding in FY 2008, installation scheduled to begin in 2011

• Factor of 10 improvement in sensitivity » Factor of 1000 improvement in rate!

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Conclusion •

These are exciting times for gravitational wave astrophysics!

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We are taking data of unprecedented sensitivity

•

We are getting ready for Advanced LIGO

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We are preparing for a detection (not if, but when!)

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LIGO will open a new window on the universe. What is out there…?

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You can join the search! Download Einstein@Home!

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