What are GW’s ??

• Fluctuation in the curvature of space time, propagating outward form the source at the speed of light

• Predicted by Einstein’s GTR• Gravitational waves should penetrate regions of space

that electromagnetic waves cannot.• It is hypothesized that they will be able to provide

observers on Earth with information about black holes and other mysterious objects in the distant Universe.

• Such systems cannot be observed with more traditional means such as optical telescopes and radio telescopes

Sources of GW’s

• Sources of gravitational waves include binary star systems composed of white dwarfs, neutron stars, or black holes.

Frame from a 3D simulation of gravitational waves produced by merging black holes, representing the largest astrophysical calculation ever performed on a NASA supercomputer.

What happens when a GW passes?

• Exhibits quadruple polarization (cross, plus)

• The fraction of stretching or squeezing, i.e the amplitude is the order of h~ 10-19 !!

Detection requirements

AIM:• Minimum sensitivity of (h= 10-19) a for GW detection

- Range = 10Mpc- Expected annual rate = 1/20

• Sensitivity greater than (h= 10-21) for several detections per year- Range = 200Mpc- Expected annual rate = 100-200

Methods of Detection of GWs

• Earlier: (1990s)– solid bar of metal isolated from outside vibrations!– gravitational wave excite the bar's resonant frequency

(Δν=1Hz)– cryogenically cooled, with SQUIDs to detect vibration– They reached detection limits of 10-19 but no detection

ever occurred.– Eg: NIOBE (UWA, Australia), MiniGRAIL (Leiden University,

Netherlands), Explorer(CERN) etc.

NIOBE (Univ. Western Australia)

Modern detectors

Principle:• Michelson interferometers detect change in length

AIM:• Minimum sensitivity of h< 10-19 for GW detection• Sensitivity of h< 10-21 for several detections per

year

Basic principles: Michelson Int.

Basic principles: Michelson Int.

• Suspended mirrors as test-masses• Vibration isolation• Large arm lengths (few kms)• High power recycling cavities• Large bandwidth (Δν=1 kHz)• LIGO (2km, 4km USA), VIRGO(3km Italy),

TAMA(300m Japan), GEO(600m Germany) , AIGO(80m Australia)

Example: AIGO layout

Functional Detectors

LIGO Livingston, USA VIRGO Pisa, Italy

World Wide ArrayA single gravitational wave detector is unable to tell the

direction of a gravitational waveDirection of a wave is calculated from the varying arrival times

of signals at each locationAdding a southern detector improves detectable sources by

270% and the directional precision by 400%

Challenges !

Detection of very small motion: h~ 10-21

• Noise limits• High laser power (~1 MW )• High precision optics (test mirrors etc)• Thermal lensing (??)• Parametric Instability (??)

Vibration Isolation

• Noise limitations:– Limited at high frequencies by shot noise– Radiation pressure due to the laser– Thermal noise e.g. Brownian motion– Limited at low frequencies by seismic noise

(ground-based detectors )

Parametric Instability

• GW detectors need high optical power cavities• Parametric instabilities naturally arise at such

powers• The coupling of optical modes with the mirror

acoustic modes causing lock breaking of cavity• This will impose serious sensitivity limitations

on second generation detectors like Advanced- LIGO (2014)

Opto-acoustic interaction in the Laser cavity

PI studies at AIGO

• Pre-Mode-Cleaner (PMC) locked to TEM01 mode to generate TEM01 mode output

• Mach Zehnder recombines the TEM00 and TEM01 mode beams together and injects to the main cavity

• Compensation Plate (CP) is used to tune the cavity to enforce TEM01 and TEM00 are simultaneously resonant with frequency difference of ETM acoustic mode frequency

• The feedback makes the TEM01 mode transmission close to zero when ETM acoustically excited.

PI experimental setup using a MZ int.

Thermal Lensing

• Absorption of a even a small fraction optical power, causes strong thermaly induced wavefront distortion

• Causes heating of mirrors:– Introducing a refractive index profile– Distortion of curvature

• Reduction of contrast due to distortion of wavefront• Cavity locking is not feasible due to constantly

changing optical path

Thermal compensation• Use of compensation plates with cancel out the

optical path difference introduced• CO2 lasers for peripheral heating of these plates

Detection of GWs in future

• Next:– Advanced Ground-based detectors:

eg: Advanced LIGO– Space interferometers: LISA (NASA-ESA) • Arm-length ~ 5 million km• f ~ 0.1 – 100mHz• Galactic binaries, massive black hole• Orbiting the Sun, and trailing the Earth by about 20o

LISA

References

• A Takamori et al, Class. Quantum Grav. 19 (2002) 1615–1621• D G Blair, Building a GW detector, ISEG Kochi (2009)• J C Dumas, Honors Thesis, School of Physics,UWA (2002)• K.A Strain et al, Physics Letters A 194,124-132(1994)• Jérřome Degallaix et al, General Relativity and Gravitation, 1581-1589

(2005)• Picture Credit: http://cgwp.gravity.psu.edu/• Picture Credit: C. Henze, NASA• Animation credit: http://en.wikipedia.org/wiki/Gravitational_wave