News from the Laser Interferometer Gravitational-Wave Observatory (LIGO)

Dennis Ugolini, Trinity University for the LIGO Science Collaboration SMU Physics Seminar March 29, 2010

Document no. LIGO-G100214 1

Gravitational Waves

Gravitational waves are transverse distortions of spacetime due to the motion of massive astronomical bodies.

Expected sources: • Inspiraling neutron stars/black holes • (Asymmetric) supernovae • • Rotating pulsars Cosmic gravitational-wave background Expected properties: • Quadrupole polarization • Propagating at speed of light • Strains of ΔL/L = 10 -21 or less LIGO-G1000214

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

Hulse-Taylor Binary Pulsar

17 / sec



~ 8 hr

  PSR 1913 + 16, measured in 1975 System should lose energy through gravitational radiation » Stars get closer together » Orbital period gets shorter LIGO-G1000214

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Why Are We Looking?

“Chirp Signal”

LIGO-G1000214

We can use weak-field gravitational waves to study strong-field general relativity.

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The Fabry-Perot Michelson Interferometer

 Uses light interference to measure path length difference between the two arms  Each arm is a Fabry-Perot cavity, effectively increasing arm length  Geometry ideally suited for quadrupole radiation 5 LIGO-G1000214

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The LIGO Project

LIGO: L aser I nterferometer

  

G ravitational-Wave

Detection , followed by astronomy LIGO Science Collaboration (LSC) includes many institutions → Funded by US National Science Foundation LIGO-G1000214

O bservatory

Max Planck Institute Andrews University Caltech Cardiff University Charles Sturt Univ.

Eӧtvӧs University Australian National Univ. Carleton College Columbia University Embry-Riddle Aero. Univ.

Hobart & William Smith Institute of Applied Physics, Nizhny Novgorod Inter-University Centre for Astronomy and Astrophysics, Pune Leibniz Universität Hannover LIGO Hanford Observatory LIGO Livingston Observatory Louisiana State Massachusetts Inst. of Technology Louisiana Tech McNeese State Univ. Montana State Univ. Moscow State Univ. NASA/Goddard Flight Ctr.

Nat. Astronomical Observatory of Japan Rochester Inst. of Technology Northwestern University Rutherford Appleton Lab.

San Jose State Univ. Sonoma State Univ.

Southeastern Univ.

Southern University Syracuse University Penn State Univ.

Southeastern Louisiana Stanford University University of Melbourne Univ. of Mississippi Univ. of Sheffield Univ. of Texas at Brownsville Univ. of Texas at Austin Universitat de les Illes Balears Trinity University Univ. of Adelaide University of Birmingham Univ. of Florida Univ. of Glasgow Univ. of Mass. – Amherst University of Maryland University of New Hampshire Univ. of Michigan Univ. of Rochester Univ. of Minnesota Univ. of Salerno Univ. of Sannio at Benevento University of Oregon Univ. of Southhampton University of Strathclyde University of Western Australia University of Wisconsin-Milwaukee Washington State University

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The LIGO Observatories

LIGO Hanford Observatory (LHO) (4km and 2km in same vacuum) LIGO-G1000214 LIGO Livingston Observatory (LLO)

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LIGO Vacuum System

Vacuum at 10 -9 • torr to reduce light scattering and momentum kicks to optics. One meter diameter arms, with chambers separated by 4’x4’ gate valves • • Serrated baffles included to disperse light scattered at optics Lengthy bake to remove adsorbed water vapor LIGO-G1000214

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Seismic Isolation

Passive (to reduce noise in sensitive freq. band) LIGO-G1000214

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Active (to improve lock acquisition/maintenance) 9

Suspended Test Masses

Optics are 25 cm diameter, 10 cm thick, 10.7 kg, of high purity fused silica. They must have <50 ppm scattering losses, <1 ppm absorption losses.

The optics are suspended to attenuate seismic motion above the pendulum frequency. LIGO-G1000214

f



f

0

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Science Run Timeline

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LIGO-G1000214 Seismic Internal thermal

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Shot noise and pole frequency 12

So Have We Detected Gravitational Waves?

Nope.

But the lack of detections puts interesting constraints on our universe: • The properties of certain astronomical objects • • The populations of gravitational-wave sources The total energy density of gravitational waves 13 LIGO-G1000214

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Search Classifications

Waveform Known Short Duration Long Duration

Binary Inspirals

Search via matched filtering with pre-generated waveforms

Periodic

(Pulsars, rotating neutron stars) Integrate sinusoidal signal Waveform Unknown

Burst

(supernovae, gamma ray bursts) Search for excess power

Stochastic

Cross-correlation between multiple detectors 14 LIGO-G1000214

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Low Mass Binary Inspiral Search Results

• • • Covers first 18 months of S5 data – no detections for total mass < 35 M ʘ Limits assume NS = 1.35 solar masses, BH = 5.0 solar masses L 10 = 10 10 L ʘ (1 Milky Way = 1.7 L 10 ) Source NS-NS BH-BH BH-NS Expected rates (yr -1 L 10 -1 ) optimistic realistic 5 × 10 -4 5 × 10 -5 6 × 10 -5 6 × 10 -5 4 × 10 -7 2 × 10 -6 Measured range Mpc ~30 L 10 490 ~100 ~60 11000 2100 Upper limits (yr -1 L 10 -1 ) no spin spin 1.4 × 10 -2 -- 7.3 × 10 -4 3.6 × 10 -3 9 × 10 -4 4.4 × 10 -3 Kalogera

et al

., ApJ

601

, L179 (2004) O’Shaughnessy

et al.

, ApJ

633

, 1076 (2005) B. Abbott

et al

., PRD

80

, 047101 (2009) Kalogera

et al

., ApJ

614

, L137 (2004) O’Shaughnessy

et al.

, ApJ

672

, 479 (2008) 15 LIGO-G1000214

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Bursts: GRB 070201

GRB 070201 was short (0.15s), intense, and from direction of M31 (770 kpc).

Both Hanford detectors operating, exclude inspiral within 3.5 Mpc at 90% CL.

Thus the gamma-ray burst was extremely unlikely to be an inspiral in M31.

LIGO-G1000214 B. Abbott

et al

., ApJ

681

, 1419 (2008)

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Other Burst Searches

Other GRBs:

One GRB every few days, 212 total during S5

All Sky Survey:

Search for any signal between 64-2000 Hz in first year of S5 data.

90% CL rate limits shown at left. Also limits on strength: 10 kpc: < 1.9 × 10 -8 M ʘ Virgo cluster (16 Mpc): < 0.05 M ʘ B. Abbott

et al

., PRD

80

, 102001 (2009) LIGO-G1000214

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Crab Pulsar Search

The pulsar in the Crab has a rotational frequency of 29.78 Hz, and is slowing: df/dt = -3.7 × 10 -10 Hz s -1 dE/dt = -4.4 × 10 31 W How much of this energy loss is due to gravitational wave radiation?

Apply matched filtering with templates at

or near

twice rotational frequency.

Lack of detection implies: • Less than 6% of energy loss due to • gravitational waves Internal mag. field < 10 16 G B. Abbott

et al

., ApJ Lett.

683

, 45 (2008) LIGO-G1000214

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Other Periodic Searches

All-sky survey search for periodic sources: • First eight months of S5 • • f gw = 500-1100 Hz df/dt = -5 × 10 -9 Hz s -1 to zero 95% CL strain limits shown at right (best and worst spin orientations).

Search is sensitive to neutron stars within 500 pc with eccentricity ~ 10 -6 .

B. Abbott

et al

., PRL

102

, 111102 (2009) LIGO-G1000214

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LIGO-G1000214

Stochastic GW Background

95% CL on gravitational-wave energy density from S5 data: 41 .

5 Hz 

f

 161 .

25 Hz 

GW

 6 .

9  10  6 Limit supercedes Big Bang Nucleosynthesis bound, constrains certain cosmic string and pre-Big Bang models.

B. Abbott

et al

., Nature

460

, 990 (2009)

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Developments Since S5

 Data sharing agreement with VIRGO collaboration beginning in 2007  “Trigger passing” – real-time alerts to: » Swift satellite (X-ray) » TAROT, QUEST wide-field telescopes (optical) » Program began in December 2009  Enhanced LIGO – improved sensitivity » x4 increase in laser power » DC demodulation » Thermal lensing compensation LIGO-G1000214

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RF Heterodyne Demodulation

In Initial LIGO, an electro-optic modulator applied radio-frequency sidebands to the carrier light.

The interferometer is operated at the dark fringe to minimize shot noise.

The carrier light is resonant in the arms, while the sidebands are not.

The output is electronically mixed with the applied RF frequency, giving a linear correction signal.

LIGO-G1000214 From S. Hild

et al

., Class. Quantum Grav.

26

, 055012 (2009).

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DC Homodyne Demodulation

In DC demodulation, the interferometer is operated slightly off the dark fringe, and this light mixes optically with the sidebands.

Advantages: • Simplified electronics • Reduced phase noise • Larger non-RF photodiodes Requires good laser intensity stabilization & output mode cleaner (OMC). The OMC in turn requires better seismic isolation.

LIGO-G1000214 From S. Hild Grav.

26

SMU Physics Seminar, March 29, 2010

et al

., Class. Quantum , 055012 (2009).

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Active Seismic Isolation

New seismic isolation stacks installed in output mode cleaner chamber at each site.

Six sets of position and velocity sensors (GS-13 seismometers) feed back to coil actuators.

Order of magnitude improvement over wide frequency range.

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Isolation Stack Installed

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Thermal Compensation System

 Fused silica is a poor conductor of heat, and the higher power laser delivers a lot of heat!

 Uneven heating causes reflective properties to become a function of position; a translation of the beam creates a phase shift that mimics a signal.

 In Enhanced LIGO, 25W carbon dioxide lasers scan the optical surface in an annulus pattern, flattening the surface temperature profile.

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Sensitivity Improvement

PRELIMINARY

S6 began on July 7, 2009 , coincident with VIRGO’s second science run.

S6 will continue through Oct. 2010.

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LIGO-G1000214

The Need for Advanced LIGO

E-LIGO

Initial LIGO  Goal: factor of ten improvement in sensitivity at all frequencies  x10 increase in sensitivity = x1000 volume of sky searched  Inspiral event rate from one every few years to one every few days!

 Resolution improved for astronomy  Assembly underway, transition begins this fall

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Projected Sensitivity

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LIGO-G1000214

180 Watt Laser

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Signal Recycling

Add optic at output to make cavity resonant for beats between carrier and desired signal frequency.

Can tune to particular source, or to follow thermal noise for maximum sensitivity.

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New Optic Suspensions

LIGO-G1000214 • • • 40 kg fused silica optics Quadruple suspension with reaction mass Last stage suspended by fused silica ribbons for higher Q

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LIGO-G1000214

Electrostatic Drive

GEO prototype MIT LASTI prototype • • • Gold coating on reaction mass Forms pair of electrodes in each quadrant Fringing fields attract optic proportional to V 2

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My Contribution: Charging

 Charge buildup on optic surfaces » Mechanical contact with other materials » Friction with dust during pumpdown » Exposure to electrostatic drive » Particle showers from cosmic rays?

 Potential concerns » Electric fields interfere with positioning control » Dust held to surface, increasing absorption » Motion generates low-frequency suspension noise The goal is to measure the charging magnitude, relaxation time constant, and spatial variation, and find a noncontact discharging method.

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Kelvin Probe Measurements

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Summary

 No detections yet, but results of S5 science run have put interesting constraints on our nearest neighbors  Enhanced LIGO science run ongoing  Advanced LIGO construction already underway, aiming for sensitivity to detect GW sources with regularity by 2014-5 LIGO-G1000214

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