G040437 00

Slide1: 

The Quest to Detect Gravitational Waves Peter Shawhan California Institute of Technology / LIGO Laboratory Donald E. Bianchi Planetarium California State University, Northridge September 10, 2004 LIGO-G040437-00-G

Outline: 

Outline Different Views of the Universe Gravitational Waves The New Era of Large Gravitational Wave Detectors Laser Interferometry

Different Views of the Universe: 

Different Views of the Universe Image of the spiral galaxy M100 from “An Atlas of the Universe” http://www.anzwers.org/free/universe

Slide4: 

Optical Astronomy Stars in the Tarantula Nebula Photo: NASA

Radio Astronomy: 

Images courtesy of National Radio Astronomy Observatory / Associated Universities, Inc. / National Science Foundation Very Large Array The Crab Nebula Radio Astronomy

X-Ray Astronomy: 

X-Ray Astronomy The Crab Nebula Chandra X-Ray Observatory Illustration: CXC/NGST Image: NASA/CXC/ASU / J. Hester et al.

Cosmic Ray Astronomy: 

Cosmic Ray Astronomy A high-energy particle from outside our Galaxy interacts in atmosphere, producing a “shower” of lower-energy particles Light emitted by the shower, and/or charged particles reaching the ground, allows trajectory and energy of original particle to be determined www.auger.org Pierre Auger Project

Neutrino Astronomy: 

Neutrino Astronomy n m Antarctic Muon and Neutrino Detector Array in South Pole ice http://www.amanda.uci.edu/results.html A neutral particle, interacting only through the “weak” nuclear force, travels a long distance before finally interacting inside the Earth Muon or electron, detectable by Čerenkov light emission, follows trajectory of original neutrino 19 neutrinos were detected from supernova 1987A

Gravitational Wave Astronomy ???: 

Gravitational Wave Astronomy ??? “Ripples in the geometry of space-time” produced by massive, rapidly-moving objects Penetrate all matter May carry unique information about black holes, neutron stars, supernovae, the early evolution of the universe, and gravity itself But… The waves are extremely weak when they reach Earth Gravitational waves have not been directly detected – yet Courtesy University of Oklahoma History of Science Collections

Gravitational Waves: 

Gravitational Waves Albert Einstein, January 2, 1931 Courtesy of The Archives, California Institute of Technology

Gravitational Waves: 

Gravitational Waves A consequence of Einstein’s general theory of relativity Emitted by a massive object, or group of objects, whose shape or orientation changes rapidly with time Waves travel away from the source at the speed of light Waves deform space itself, stretching it first in one direction, then in the perpendicular direction

Gravitational Waves in Action: 

Gravitational Waves in Action

Aside: Pulsars: 

Aside: Pulsars Sources of repeating radio and/or x-ray pulses with a regular period First discovered in 1968 – a few thousand known now Pulse period can be extremely stable, with a gradual slow-down in many cases  must be a small, spinning object a neutron star with a radio “hot spot” on its surface ! (a supernova remnant, more massive than the sun but with r < 10 km)

The Binary Pulsar PSR1913+16: 

The Binary Pulsar PSR1913+16 A radio pulsar in a close orbit around an unseen companion Discovered in 1974 by Russell Hulse and Joseph Taylor Long-term radio observations have yielded object masses (1.44 and 1.39 M) and orbital parameters System shows very gradual orbital decay – just as general relativity predicts!  Very strong indirect evidence for gravitational radiation

Potential Sources of Directly-Detectable Gravitational Waves: 

Potential Sources of Directly-Detectable Gravitational Waves “Inspiral” (orbital decay) of a compact binary system Two neutron stars, two black holes, or one of each One of the most promising sources, since: ● Binary neutron-star systems are known to exist ● The waveform and source strength are fairly well known (until just before merging) Merger of two compact objects Gravity in the extreme strong-field limit Waveforms unknown – a subject for numerical relativity calculations

Potential Sources of Directly-Detectable Gravitational Waves: 

Potential Sources of Directly-Detectable Gravitational Waves Supernova explosion Wave emission depends on asymmetry of explosion Example numerical simulation “Ringing” oscillations of a newly formed black hole Rapidly-spinning neutron star Will radiate continuously if slightly asymmetric Stochastic radiation from the early universe Shows up as correlated noise in different detectors “Unexpected” sources ? This is a new observational science ! Tony Mezzacappa Oak Ridge National Laboratory

The Experimental Challenge: 

The Experimental Challenge Sources are expected to be rare Have to be able to search a large volume of space Have to be able to detect very weak signals Typical strain at Earth: h ~ 10-21 ! 0.000000000000000000001 Stretches the diameter of the Earth by ~ 10-14 m (about the size of an atomic nucleus) How can we possibly measure such small length changes ???

First Type of GravitationalWave Detectors: 

First Type of Gravitational Wave Detectors Resonant aluminum “bar” detectors Suspended in the middle Ring if excited by a gravitational wave First built by Joseph Weber in the 1960s A few “bar” detectors currently operate at ultra-cold temperatures and are very sensitive at their resonant frequencies AURIGA detector

Laser Interferometry: 

Laser Interferometry

Interference of Light: 

Interference of Light Light consists of oscillating electric and magnetic fields When two light beams meet, the electric & magnetic field amplitudes add Depending on the relative phase, can get constructive or destructive interference + + = = nothing

Interference at a Beam Splitter: 

Interference at a Beam Splitter A beam splitter reflects half of the incoming beam power (1/ of the EM field amplitude) and transmits other half

Basic Michelson Interferometer: 

Basic Michelson Interferometer Basic design first used by Albert A. Michelson in 1881 Light intensity on photodetector depends on difference in light travel times in the two perpendicular “arms” Can measure length differences which are a small fraction of the wavelength of the light

Demonstration Interferometer: 

Demonstration Interferometer Beam splitter Steerable mirror Steerable mirror Laser pointer Mirror Diverging lens Screen

The New Era of LargeGravitational Wave Detectors: 

The New Era of Large Gravitational Wave Detectors

The LIGO Project: 

The LIGO Project LIGO = Laser Interferometer Gravitational-Wave Observatory Has constructed three large interferometers at two sites Funded by the National Science Foundation Construction cost ~ $300 million Operating cost ~ $30 million per year Led by the “LIGO Laboratory”, based at Caltech and MIT Scientific activities (data analysis, advanced detector R&D) are the responsibility of the “LIGO Scientific Collaboration” (LSC) Over 400 scientists at over 30 institutions around the world

LIGO Hanford Observatory: 

LIGO Hanford Observatory Located on DOE Hanford Nuclear Reservation north of Richland, Washington Two separate interferometers (4 km and 2 km arms) coexist in the beam tubes

LIGO Livingston Observatory: 

LIGO Livingston Observatory Located in a rural area of Livingston Parish east of Baton Rouge, Louisiana Has one interferometer with 4 km arms

Design Requirements: 

Design Requirements Even with 4-km arms, the length change due to a gravitational wave is very small, typically ~ 10-18 - 10-17 m Wavelength of laser light = 10-6 m Need a more sophisticated interferometer design to reach this sensitivity Add partially-transmitting mirrors to form resonant optical cavities Use feedback to lock mirror positions on resonance Need to control noise sources Stabilize laser frequency and intensity Use large mirrors to reduce quantum position uncertainty Isolate interferometer optics from environment Focus on a “sweet spot” in frequency range

Optical Layout(not to scale): 

Optical Layout (not to scale) Pre-Stabilized Laser “Antisymmetric” photodiode Mode cleaner Fabry-Perot arm cavity “Reflected” photodiode “Pick-off” photodiode Recycling mirror Input mirror Beam splitter Main interferometer is basically a Michelson design, with the addition of three semi-transparent mirrors to form optical cavities Input optics stabilize laser frequency & intensity, and select fundamental mode End mirror

Servo Controls: 

Servo Controls Optical cavities must be kept in resonance Need to control lengths to within a small fraction of a wavelength – “lock” Nearly all of the disturbance is from low-frequency ground vibrations Use a clever scheme to sense and control all four length degrees of freedom Modulate (wiggle) phase of laser light at very high frequency Demodulate electrical signals generated by photodiodes Disentangle contributions from different lengths, apply digital filters Feed back to coil-and-magnet actuators on various mirrors Arrange for destructive interference at “antisymmetric port”

Pre-Stabilized Laser: 

Pre-Stabilized Laser Based on a 10-Watt Nd:YAG laser (infrared) Uses additional sensors and optical components to locally stabilize the frequency and intensity Final stabilization uses feedback from average arm length

Mirrors: 

Mirrors Made of high-purity fused silica Largest mirrors are 25 cm diameter, 10 cm thick, 10.7 kg Surfaces polished to ~1 nm rms, some with slight curvature Coated to reflect with extremely low scattering loss (<50 ppm)

Vacuum System: 

Vacuum System 4 km laser 4 km antisymm photodiode 2 km laser Hanford shown; Livingston only has one detector 2 km antisymm photodiode

Vacuum System: 

Vacuum System

A Mirror in situ: 

A Mirror in situ

Mirror Close-Up: 

Mirror Close-Up Suspension wire Electromagnet mirror actuators

Vibration Isolation: 

Vibration Isolation Optical tables are supported on “stacks” of weights & damped springs Wire suspension used for mirrors provides additional isolation Active isolation now being added at Livingston

Fundamental Noise Sources(conceptual): 

Fundamental Noise Sources (conceptual) If detector is not perfectly tuned, other noise sources can easily dominate Ground motion Thermal vibrations of mirrors & wires Quantum nature of light (“shot noise”) Sensitive frequency range: ~ 40 – 2000 Hz 40 Hz 2000 Hz

LIGO Status: 

LIGO Status Commissioning and engineering runs started in 2000 Science runs S1 : August 23 – September 9, 2002 (17 days) S2 : February 14 – April 14, 2003 (59 days) S3 : October 31, 2003 – January 9, 2004 (70 days) S4 : Planned to begin in January 2005 Commissioning in between Working to reduce noise and improve robustness First analysis results published, many more in progress

Performance Improvements: 

Performance Improvements

Data Analysis: 

Data Analysis Goal is to detect weak signals buried in noisy data Antisymmetric photodiode is continuously sampled at 16384 Hz Use matched filtering if waveform is known Need a lot of CPU time, e.g. using “Einstein@home” for periodic sources Use more general techniques (e.g. “excess power”) to look for unknown waveforms “Veto” events which can be identified as environmental or instrumental glitches Powerful check: require coincidence (consistent signals at consistent times) between the different interferometers Analysis effort in LSC organized into four working groups according to source type: inspiral, periodic, burst, stochastic

The Worldwide Network ofGravitational Wave Interferometers: 

The Worldwide Network of Gravitational Wave Interferometers Simultaneous detection from multiple sites would give sky location and polarization information, and can check properties of the waves themselves There is a strong spirit of cooperation among the projects 3 km 300 m 600 m 4 km 2 km 4 km

Future Detectors: 

Future Detectors Advanced LIGO Complete upgrade of LIGO interferometers toward end of this decade Large interferometers being considered in Japan, China, Australia? LISA – Laser Interferometer Space Antenna Three spacecraft in solar orbit, to be launched in 2013 (?) by ESA / NASA Free of earthly environmental disturbances Arms 5 million km long  sensitive to signals at much lower frequencies

Slide45: 

Courtesy Jet Propulsion Laboratory

Summary: 

Summary There is a bold effort underway to get a new view of the universe Detecting weak signals is extremely challenging, but solvable! LIGO is now operating, getting close to design sensitivity TAMA operating too; GEO and VIRGO being commissioned

When will Gravitational Wavesbe Detected ?: 

When will Gravitational Waves be Detected ? We don’t know ! Event rates generally expected to be low There are no guaranteed sources for the current generation of detectors This is an exploratory science ! Advanced LIGO, LISA are certain to see sources – may have to wait until then to begin doing real gravitational wave astronomy