Gravitational Waves: A new window to the universe

Gravitational Waves:A new window to the

universePresented by:

John S. Jacob

Sr. Research Fellow, School of Physics, University of Western Australia

Project Engineer,Australian International Gravitational Research Center

Gravitational Waves:A new window to the

universeAcknowledgementsACIGA: Australian Consortium for Interferometric Gravitational Astronomy:

David McClelland (ANU), Jesper Munch (U. Adelaide), Tony Lun (Monash).

Gabriella Gonzales for some slides

Prof. David Blair for some slides and inspired leadership of the center

Dr. Ju Li for help with some slides

All members of the UWA Gravity Waves Group

LIGO Scientific Collaboration

International Advisory Committee, High Optical Power Project

The VIRGO Project, The TAMA Project and the GEO Collaboration

Gamma RaysBursts (BATSE)

Looking Out at the Universe

MicrowaveBackground(COBE)

Each new window yields dramatic new

insights!

Infrared

Gamma RaysBursts (BATSE)

These are all parts of the same Electromagnetic Spectrum.

MicrowaveBackground(COBE)

Each new window yields dramatic new

insights!

Infrared

The Electromagnetic Spectrum?

What’s That?

In 1820, Hans Christian Oersted discovered that electric currents (moving electric charges) create

magnetic fields.

In 1831, Michael Farraday discovered that varying magnetic fields create

electric currents.

In 1864, James Clerk Maxwell put two and two together.

• Oscillating electric fields create oscillating magnetic fields.

• Oscillating magnetic fields create oscillating electric fields.

• Together, electromagnetic waves propagate through empty space at a speed Maxwell calculated to be 3 x 108 meters per second.

In 1886, Heinrich Hertz experimentally demonstrated creation and

propagation of electromagnetic waves at the speed of light.

It was eventually realized that:

• Radio• Microwave• Infrared Light• Visible Light• Ultraviolet Light• X-Ray Radiation• Gamma Ray Radiation

are all forms of Electromagnetic Waves.

It was eventually realized that:

• Radio – 100 km to 1 meter• Microwave – 10 cm to 1 mm• Infrared Light - 1/1000 mm• Visible Light – 500 micron• Ultraviolet Light – 100 nm to 10 nm• X-Ray Radiation - 10 nm to 0.01 nm• Gamma Ray Radiation – 0.01 nm to ??

are all forms of Electromagnetic Waves.

In 1915, Albert Einstein presented his theory of General Relativity.

• Matter changes the shape of space.

• Space changes the path of matter.

• Waves in the continuum of space can propagate forward at the speed of light. “Gravity waves.”

Gravitational Waves: a different kind of waves

They were predicted by Einstein: all moving masses change space time around them!

Matter tells space how to curve. Space tells matter how to move.

Not light waves!! But they also have different wavelengths…

Gravitational waves: a new window

Gravitational wave sources:

• periodic sources: binary systems,• rotating stars…

• burst sources: supernovae, collisions, black hole formations, gamma ray bursts?…

• stochastic sources: clutter of signals from the entire universe, early moments of the big bang, cosmic strings?...

GWs are produced by accelerated masses.

They represent an entirely new spectrum.

Gravity wave detectors are the ears that will allow us to listen to the sounds of the universe.

But they are very weak….We’ve been looking for them with detectors sensitive to changes in distance A BILLION TIMES smaller than an atomic diameter… and nothing yet!?

Gravitational waves produce larger effects if the detectors are VERY long. We also want to try different wavelengths!

Astronomers are not surprised: most strong sources are VERY far away!

Resonant bar detectors:

University of Western Australia :NIOBE

Lousiana State University (USA)

ALLEGRO

What do we know about gravitational waves?

That they exist!…That they exist!…

Nobel PrizePhysics 1993Hulse & Taylor

There is There is indirectindirect evidence for evidence for the existence of Gravity Waves. the existence of Gravity Waves.

Nobel PrizePhysics 1993Hulse & Taylor

However, no one has yet been able to observe them directly.

The ability to do so is important as a test of General Relativity and as a totally new

kind of astronomy!

However, no one has yet been able to observe them directly.

The ability to do so is important as a test of General Relativity and as a totally new

kind of astronomy!

Gravitational waves:an international dream

Resonant

Mass

Detectors

Gravitational waves:an international dream

GEO600 (British-German)Hannover, Germany LIGO (USA)

Hanford, WA and Livingston, LA

TAMA (Japan)Mitaka

VIRGO (French-Italian)Cascina, Italy

AIGO (Australia), Wallingup Plain, 85km north of Perth

Gravitational Waves: what else do we know?

•Gravitational Waves are ripples in space-time.•They cause distortions of distances.•Their strength is measured by strain, the fractional change in distance, called h. Typical value of h is ~10-21.•The size of the waves is tiny because space is very “stiff”! •Because gravity only attracts - never repels, gravity waves have “quadrupole polarizations” pictured above.

How to do it?

But how can we detect Gravity Waves?

What measurable effect could Gravity Waves have?

What kind of instrument could directly observe them?

Gravitational Waves: what else do we know?

•Gravitational Waves are ripples in space-time.•They cause distortions of distances.•Their strength is measured by strain, the fractional change in distance, called h.•The size of the waves is tiny because space is very “stiff”! •A NS binary, oscillating at ~100 Hz, ~100 Mpc away, produces h~10-21

Simply measure the distance between two objects floating in

space.

If it’s that easy, why hasn’t it already been done?

Problems:• The marks on the ruler would have to be a

billion times smaller and closer together than the atoms the ruler is made of.

• The objects might be drifting away from each other. We need a way to control them without constraining them too much.

• The length of our ruler will also be affected by the gravity waves.

Make the distance between controlled objects

very large.The greater the distance, the greater the effect of

gravity waves.

Use a laser beam to measure the distance.

For achange in distance due to gravity waves of 1 mm, the objects only have to be about onemillion billion kilometers apart.

Problem: It would take more than one hundred years just to make one

measurement.

The distance must also be smaller than the wavelengthwe are trying to measure

Solution: It’s much easier to measure differences between

two large distances than it is to measure the large distances

themselves.

A device called an Interferometer does this with utterly ridiculous

accuracy.

An interferometer compares the distances traveled by two laser beams. It is sensitive to

changes in length smaller than the wavelength of its

light.

Example of a simple Fabry—Perot Cavity Interferometer

Laser

Fabry-Perot cavities

photo detector

beamsplitter

Are not both arms of the interferometer affected equally by

gravity waves?

Laser

Fabry-Perot cavities

photo detector

beamsplitter

Answer: Yes. However, due to the “quadrupole polarization” of gravity waves, the effects do not

happen to both sides at the same time!

Laser

Fabry-Perot cavities

photo detector

beamsplitter

By having its arms at right angles, an interferometer’s sensitivity to gravity waves is

effectively doubled!

Problem: Noise

Many sources of “noise” reduce the sensitivity of an interferometer:

• Laser fluctuations• Photon noise effects• Thermal vibrations of mirrors• Seismic noise

Noise Solutions

Laser Stabilization

Frequency stabilization Laser geometry fluctuation stabilization

ReferenceCavity

Laser

Isolator PBS

PBS

To laserinterferometer

PC

RFOscillator

Mixer

Mode cleaner—long optical cavity

Photon Noise Effects:

• Statistical sampling of photons: precision of the phase measurement increases as N1/2.

• Radiation pressure of photons exerts random forces on mirrors, also increasing as N1/2.

Solution:• Use an optimum number of photons.

• Present detectors use 100 times too few photons.

• Use a very powerful laser (100watts and build it up by resonance to 1 Megawatt).

Problems with High Light Power

• Powerful lasers cause Thermal Lensing.

• The radiation pressure forces push the mirrors apart and create stability problems (before they cause photon noise.)

Solutions:

• Develop thermal lensing compensation techniques.

• Develop better control systems.

AIGO High Optical Power Test

Facility

ACIGA

ACIGA arm

80m high power test cavity

10m Mode cleaner

Injection locked100W laser

-Pre-stabilisation

cavity

Beam expander

SapphireInput Mirror

Power recycling

Detection bench

Injection bench

Sapphire end mirror

Problems:• Seismic noise is a 1012 times stronger

than gravity waves.

• Ocean waves, people, cars and kangaroos!

• AIGO site is 1000 times better than UWA.

Full vibration isolation system

vertical Euler

springs

thin fibre pendulum link

Eddy current viscous

coupling

copper

simple wire pivot

2-d gimbal pivot

concentric with wire

pivot Eddy current damped rocker

magnets

to next stage

Self damped pendulum

LIGO: Now testing, planning upgrade

Estimated noise sources

Australian International Gravitational Observatory

AIGO (opened in 2000) and Wallingup Plain

Gravity Discovery Centre

SCCC: opened in 2001

Conclusion

• AIGO is developing vital technology for the upgrade of detectors to reach a sensitivity where known sources are detectable.

• It will be an essential element in the world array of detectors.

• It offers opportunities to promote science for the benefit of all West Australians