Ripples from the Dark Side of the Universe the Search for Gravitational Waves

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Ripples from the Dark Side of the Universe the
Search for Gravitational Waves

• Jim Hough
• Institute for Gravitational Research
• University of Glasgow

Frontiers 2006
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The importance of our work
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Theories of Gravitation
Newtons Theory instantaneous action at a
distance
Einsteins Theory information carried at the
speed of light there must be gravitational
radiation
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What are Gravitational Waves?

• A Prediction of Einsteins Theory of General
  Relativity
• Maxwells Equations gt production of radio waves
  by accelerated charges
• Einsteins Equations gt production of
  gravitational waves by accelerated masses

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Gravitation - curvature in space time

• Weak Field Equations gt Wave Equation
• in nearly flat space time, where hab is the
  amplitude or tidal strain associated with the
  wave and is the double integral w.r.t. time of
  the curvature Ra0b0

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Two approaches
Easier approach
The formal approach
Since mass distorts space time surface,
accelerated mass gt ripples in the surface
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Production

• Two important factors
• gravitational force is VERY WEAK
• mass comes in only one variety (c.f. charge - two
  types) - so radiation from 2 halves of
  accelerating system tends to cancel out - no
  dipole radiation
• So to produce significant intensity requires
  huge masses and accelerations
• gt ASTROPHYSICAL SOURCES

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Gravitational Waves- possible sources

• Pulsed
• Compact Binary Coalescences
• NS/NS NS/BH BH/BH
• Stellar Collapse (asymmetric) to NS or BH 
• CW
• Pulsars
• Low mass X-ray binaries (e.g. SCO X1)
• Modes and Instabilities of Neutron Stars 
• Stochastic
• Inflation
• Cosmic Strings

Binary stars coalescing
Supernovae
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Indirectdetection of gravitational waves
The evidence for gravitational waves
PSR 191316
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Detection of Gravitational waves

• WHY?
• ASTRONOMY
• obtain information about violent astrophysical
  events involving black holes and neutron stars,
  obtainable in no other way
• gt A new window on the Universe

Crab Nebula
Different wavelengths
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Gravitational Waves A Strain in Space
Gravitational Wave Amplitude h 2 x
L
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Sources

• Amplitudes from expected sources are tiny

ADVANCED GROUND - BASED DETECTORS
Detectors are split into space-based and ground
based systems
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How can we detect them?
L

• Using a large bar is one way to measure this

L DL
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Detection Techniques

• Field originated in the 1960s with J. Weber
  using aluminium bars at room temperature
• Joined by other groups in Germany, Italy,UK and
  USA

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Further developments of bar detectors

• Low temperature bar detectors in Italy (Rome,
  Legnaro), CERN, USA (Stanford (no longer),
  Louisiana), Australia (Perth) and Netherlands
• Move to separated mass detectors with laser
  interferometric sensors

Auriga detector at Legnaro

• Interesting coincidences between detectors in
  Rome and at CERN

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Move to low temperature bars and crystals..
Dr Who, BBC
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Detection of Gravitational Waves

• Detection by laser interferometry between free
  test masses
• Requires strain sensitivities of 10-21 to 10-23
  over timescales of 10-3 to 104 seconds
• Detectors needed on earth and in space to cover
  the range of sources

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Detectors using interferometry
Consider the effect of a wave on a ring of
particles
One cycle
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Michelson Interferometer
Mirror
Beam splitter
laser
Observer
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Addition of Light Waves (Interference)
CONSTRUCTIVE (BRIGHT)
DESTRUCTIVE (DARK)
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Detection again
Interferometer
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Detection of Gravitational Waves
Consider the effect of a wave on a ring of
particles
One cycle
Michelson Interferometer
Gravitational waves have very weak effect
expect movements of less than 10-18 m over 4km
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The Michelson Interferometer

• Consider light running up and down the two arms
• If 2L1 - 2L2 0 would expect beams to lie on top
  of each other and so get interference maximum
• If 2L1 - 2L2 n ? again would expect maximum
• So 2?L n ? for maximum
• So one fringe shift is equivalent to ?L changing
  by ? /2
• NB In reality there is an extra half wavelength
  shift at the beamsplitter but that does not
  materially alter the argument above.

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Interaction of Gravitational Wave with Michelson

• Suppose the Michelson is optimally oriented with
• respect to the polarisation of the incoming wave
• i.e. arms along the semi-major and semi-minor
  axes of the quadrupole ellipse
• Then ?L1 h.L1/2 and ? L2 h.L2/2 of opposite
  sign
• Or ?L h.(L1 L2)/2 h.L where L L1 L2
• So if h 10-22 and L 3 .103 m what is ?L?

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Null in Sensitivity

• To get a null in the response need the arms to
  change length in exactly the same way
• This only happens if wave is incident along the
  bisector of the two arms? You check!

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Laser Interferometric detectors

• For best performance want arm length
• i.e. for 1kHz signals, length 75 km
• Such lengths not really possible on earth, but
  optical path can be folded
• Much longer arm lengths are possible in space

Fabry Perot Cavities
Delay lines
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Principal limitations to sensitivity ground
based detectors

• Photon shot noise (improves with increasing
  laser power) and radiation pressure (becomes
  worse with increasing laser power)
• There is an optimum light power which gives the
  same limitation expected by application of the
  Heisenberg Uncertainty Principle the Standard
  Quantum limit
• Seismic noise (relatively easy to isolate
  against use suspended test masses)
• Gravitational gradient noise, - particularly
  important at frequencies below 10 Hz
• Thermal noise (Brownian motion of test masses
  and suspensions)
• Several long baseline interferometers are now
  operating

• All point to long arm lengths being desirable

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Principal Limitations to Sensitivity

• Space borne
• Acceleration noise of test masses
• Photon shot noise in detected light
• Now, all detectors
• Require long baselines to achieve high
  sensitivity

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Gravitational Wave Detectors

• 5 detector systems approved/now being developed
• LIGO (USA) - 2 detectors of 4km arm length
• 1 detector of 2km arm length - WA and LA
• VIRGO (Italy/France) - 1 detector of 3km arm
  length - Pisa
• GEO 600 (UK/Germany) - 1 detector of 600m arm
  length - Hannover
• TAMA 300 (Japan) - 1 detector of 300m arm length
  - Tokyo
• LISA (NASA/ESA) - Spaceborne detector of 5 x
  106km arm length

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Interferometers - international network
Simultaneously detect signal (within msec)
Virgo
GEO
LIGO
TAMA
detection
confidence
locate the
sources
decompose the
polarization of
gravitational
waves
AIGO
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LIGO USA
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Initial LIGO detectors

• LIGO project (USA)
• 2 detectors of 4km arm length 1 detector of 2km
  arm length
• Washington State and Louisiana

Each detector is based on a Fabry-Perot
Michelson
NdYAG laser 1.064mm
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LIGO Hanford
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VIRGO The French-Italian Project 3 km armlength
at Cascina near Pisa
The Super Attenuator filters the seismic noise
above 4 Hz
3km beam tube
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Temptation
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Other Detectors and Developments TAMA 300 and
AIGO
AIGO Gingin, WA 80 m arm test facility
TAMA 300 Tokyo 300 m arms
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GEO 600

• UK-German collaboration
• Univ. of Glasgow
• Hough, Strain, Robertson, Rowan, Ward, Woan,
  Cagnoli, Heng and colleagues
• Cardiff Univ.
• Sathyaprakash, Romano, Schutz, Grishchuk and
  colleagues
• Univ. of Birmingham
• Cruise, Vecchio, Freise and colleagues
• AEI Hannover and Golm
• Danzmann, Schutz and colleagues
• Colleagues in Univ. de les Illes Balears

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GEO 600
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GEO600 optical layout
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Unique GEO Technology 1 - Advanced Interferometry

• One of the fundamental limits to interferometer
  sensitivity is photon shot noise
• Power recycling effectively increases the laser
  power
• Signal recycling a GEO invention trades
  bandwidth for improved sensitivity

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Unique GEO Technology 1 - Advanced Interferometry

• The interferometer is operated with the output
  port held at an interference minimum

• The only light at the output is (ideally) that
  containing information about
• differential length changes of the arms (the
  gravitational wave signal)
• The SR mirror reflects most of this light back
  into the interferometer
• The interferometer behaves like optical cavity
  in which the gw signal
• amplitude builds up

• Resonant enhancement of the signal occurs at a
  Fourier frequency and over a band width
  determined by the position and transmittance of
  the SR mirror

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Unique GEO Technology 2 - Monolithic Silica
Suspension
reduces thermal noise
Ultra-low mechanical loss suspension at the
heart of the interferometer
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Recent GEO600 commissioning noise curve
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Typical Feb. 2006 GEO600 commissioning noise
curve
10-17
h/?Hz
10-21
Frequency (Hz)
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Evolution of GEO Sensitivity
h/?Hz
10-17
10-18
10-19
10-20
10-21
102
103
Frequency (Hz)
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LIGO Performance 2005
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Range for Neutron star/neutron star inspirals
during 2005
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LIGO and GEO science runs

• Since Autumn 2001 GEO and LIGO have completed 4
  science runs
• Analysis completed for S1/2 and (most) papers
  published
• For S3/4 analysis in progress
• Some runs done in coincidence with TAMA and bars
  (Allegro)
• LIGO now at design sensitivity
• Upper Limits have been set for a range of
  signals
• Coalescing binaries
• Pulsars
• Bursts
• Stochastic background
• 14 major papers published or in press since 2004
  (work from a
  collaboration (LSC) of more than 400 scientists)

• S5 started on 4th Nov. 2005 at Hanford
  (LLO a few weeks later) - GEO joined in Jan 06
  initially for overnight data taking, then 24/7
• 18 months data taking in coincidence

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LIGO/GEO results highlights
Known pulsars
Stochastic backgrounds
Crab spin-down upper-limit within reach during S5
Nucleosynthesis bound on ?gw 5 x 10-6 within
reach for S5
Abbott et al, PRL 94, 181103 (2005)
Abbott et al. PRL, 95, 221101 (2005)
Slide from A. Vecchio
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http//einsteinathome.org
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Expectations

• With initial detector array optimistic event rate
  predictions suggest the possibility of a few
  detections in a 3 year period
• Kalogera et al. Dec 2003
• For initial LIGO the most probable detection
  rates for DNS inspirals are 1 event per
  (5-250)yr at 95 confidence we obtain rates up
  to 1 per 1.5 yr
• but really want to have frequent observations,
  even based on pessimistic predictions
• to achieve this requires 10x improvement in
  sensitivity (1000 times increase in rate)
• How is this to be achieved?

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The next step

• Need to improve sensitivity X10 by applying the
  GEO technology to longer detectors
• ? Advanced LIGO
• Many signals expected

• RD funded in US, UK and Germany
• Capital contributions funded in UK and Germany
• Advanced LIGO in Presidents budget for 2008 to
  allow re-construction on site starting 2010
• Full installation and initial operation of 3
  interferometers by 2013

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From initial LIGO detectors to planned upgrades

• Many research groups collaborating on RD for
  upgraded LIGO design (LIGO Scientific
  Collaboration - very strong contribution from GEO
  group

• Low thermal noise suspensions
• Fused silica fibres jointed monolithically to
  test masses
• Increased test mass sizes
• Improved dielectric mirror coatings

Elliffe and Jones
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UK Advanced LIGO - University of GlasgowSilica
ribbon CO2 laser fabrication and welding

• Silica ribbons now being produced by machine
  using CO2 laser for heating rather than gas flame

CO2 pulling welding machine development
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Advanced LIGO sensitivity goals
Advanced LIGO

• Advanced LIGO
• Seismic noise reduced by x40 at 10Hz
• Thermal noise reduced by x15
• Optical noise reduced by x10
• Design reaches limits set by quantum noise, (and
  noise from Newtonian gravity gradients)
• Sensible break point in what is achievable with
  current technologies on appropriate timescale

LIGO
Quantum
LIGO
Advanced LIGO
Test mass thermal
Estimated gravity gradients
Suspension thermal
Seismic
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The Future of Detectors on Earth

• 1st generation on ground are operating
• 2nd generation follows 2010-13, designs mature
• Advanced LIGO (USA/GEO Group/LSC)
• Advanced VIRGO (Italy/France GEO Group?)
• Large Cryogenic Gravitational Telescope (LCGT)
  (Japan)

Sapphire mirrors cooled to 40K
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Future (third generation) detectors

• Plans for advanced detectors have pushed
  technology design envelope to a portion of design
  space where interferometer sensitivity is limited
  by four main areas
• Thermal noise in the test mass substrates and
  their suspensions
• Quantum Effects becoming important
• Effects related to the use of high power lasers
• Seismic/gravity gradient noise at very low
  frequencies
• Upgraded detectors designs do not yet reach
  limits imposed by facilities -
  further sensitivity improvements possible
• Strategies to improve sensitivities in different
  frequency ranges produce conflicting requirements
  for the design of future detectors
• Research needed to inform design decisions

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For the Further Future

• More technology improvements
• Cooled materials silicon or sapphire
• Quantum non-demolition techniques
• All-reflective interferometry
• Application to
• GEO upgrade
• GEO-HF
• New European detector
• EGO

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Sources - reminder
ADVANCED GROUND - BASED DETECTORS

• To see sources at low frequencies need detector
  in space

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LISA - A Collaborative ESA/NASA Mission

• Cluster of 3 S/C in heliocentric orbit at 1 AU
• Equilateral triangle with 5 Mio km armlength
• Trailing the earth by 20
• S/C contain lasers and free-flying test masses
• Equivalent of Michelson interferometer
• Approved as an ESA Cornerstone Mission

LISA
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LISA -Cluster of 3 spacecraft in heliocentric
orbit at 1 AU
reference beams
Inertial proof mass shielded by
drag-free spacecraft
main transponded laser beams
LISA
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LISA ORBIT
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LISA Interferometry
LISA
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Arm length considerations

• LISA arm length is 5 x 109 metres.
• What wavelength of gravitational waves tends to
  give no signal if the plane of the detector is
  perpendicular to the source direction?
• set ?/2 5 x 109 or ? 1010 m
• This corresponds to a frequency f c/ ? 3 x
  10-2 Hz
• For a detector to operate up to 1 Hz what arm
  length would you choose?
• calculate ? and set L ?/4

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LISA Science Goals
Compact objects orbiting massive Black Holes
Massive Black Holes formation, binary orbit,
and coalescence
White dwarf, neutron star, and other compact
binary systems
LISA
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The LISA Mission
Credits Milde Marketing, Albert Einstein
Institute,Golm and Exozet Babelsberg GmBH
http//exozet-effects.com/
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Glasgows role..
Its time we face reality my friends. Were not
exactly rocket scientists
is directed at the payload
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LISA Pathfinder Concept Technology
demonstrator for launch in 2009
Demonstration of inertial sensing and drag free
control
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Glasgow Optical Bench Test Model
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Worldwide Interferometer Network
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Gravitational Wave Astronomy
A new way to observe the Universe