Detection of Gravitational Waves
PX436 – General Relativity – Detection of Gravitational Waves
Strong, indirect evidence
• Strong indirect
evidence comes from
observed spin-down of
binary pulsars,
compared to the
predictions for energy
loss due to gravitational
waves
e.g. PSR1916+013 (Nobel Prize
1993, Hulse and Taylor)
Figure from Weisberg et al, 2010, ApJ, 722, 1030
PX436 – General Relativity – Detection of Gravitational Waves
Signatures of Gravitational Waves
Steady state systems are going to be hard to distinguish from
background - need distinctive patterns that can be matched
against models:
PX436 – General Relativity – Detection of Gravitational Waves
Signatures of Gravitational Waves
• Inspiralling binary systems
produce a ‘chirp’ – rising in
frequency and amplitude.
• Newly merged systems settling
to a new equilibrium tend to
produce a ‘ringdown’ effect.
PX436 – General Relativity – Detection of Gravitational Waves
Example
Sounds from
LIGO
Strain Amplitudes
• The strength of the gravitational perturbation is
usually given in the form of a strain amplitude, h
PX436 – General Relativity – Detection of Gravitational Waves
Resonant Bars
•  First used in the early 1960s by Weber and collaborators.
Now run at cryogenic temperatures
•  Examples running through to 2005ish include AURIGA,
ALLEGRO, miniGRAIL, NAUTILUS, reach h~10-21
Joseph Weber (1919-2000) made
several claims now believed to be
experimental error
PX436 – General Relativity – Detection of Gravitational Waves
Resonant Bars
•  First used in the early 1960s by Weber and collaborators.
Now run at cryogenic temperatures
•  Examples running through to 2005ish include AURIGA,
ALLEGRO, miniGRAIL, NAUTILUS, reach h~10-21
NAUTILUS (Italy) – 2300kg of
aluminium, cooled to 0.1K, in a
vacuum, resonant at 908 and 924 Hz
PX436 – General Relativity – Detection of Gravitational Waves
Michelson Interferometers
• Laser interference used to measure change in arm length
• Arms can be longer than in resonant bars, but still subject
to thermal and seismic noise
PX436 – General Relativity – Detection of Gravitational Waves
First Generation Detectors: LIGO
•  Laser Interferometer Gravitational-Wave Observatory (USA)
•  2 detectors, 4km arms, 1m diameter vacuum tubes
•  Initial experiment completed 2010
Last few runs were
in cooperation with
3km long FrenchItalian detector
VIRGO
No clear detections
PX436 – General Relativity – Detection of Gravitational Waves
First Generation Detectors: LIGO
PX436 – General Relativity – Detection of Gravitational Waves
First Generation Detectors: Einstein@home
PX436 – General Relativity – Detection of Gravitational Waves
Second Generation Detectors
Advanced LIGO
and Advanced
VIRGO aim for
h~10-22
Improved
software and
hardware
(stronger lasers,
heavier, more
stable mirrors)
10-fold increase
in sensitivity =>
1000x volume
PX436 – General Relativity – Detection of Gravitational Waves
Second Generation Detectors
Advanced LIGO and VIRGO will work together closely, and with
optical/radio telescopes for electromagnetic follow-up.
Ultimately, they will be
joined by LIGO-India
Multiple interferometers:
-  confirmation of signal
-  triangulation on source
-  improved sensitivity
Triggering EM follow-up
=> better characterisation
of source
PX436 – General Relativity – Detection of Gravitational Waves
Third Generation Detectors: LISA
• LISA is a proposed
space interferometer
• 2 million km arms
• Spacecraft will have to
orbit in very tight
formation
• Free-floating test
masses
• Can work at low
frequencies
• Currently unfunded
PX436 – General Relativity – Detection of Gravitational Waves
Third Generation Detectors: Einstein
•  Proposed thirdgeneration groundbased detector
(Europe)
•  Currently unfunded
($1bn) but has a
funded design
study
•  Would be half a
mile underground
•  Still several options
under consideration
PX436 – General Relativity – Detection of Gravitational Waves
Other Wave Detectors: SKA and Planck
•  The cosmic microwave
background can be ‘twisted’ by
primordial (early Universe)
gravitational waves
•  An early ‘detection’ by BICEP
has been disproved by the
more sensitive Planck
telescope
•  An alternative method is using
measurements of distant
pulsars as a sensitive map of
distance in the local Universe
•  The Square Kilometre Array
(SKA) will attempt this
experiment.
PX436 – General Relativity – Detection of Gravitational Waves
Gravitational Waves
•  Q10.2 – Can we use a local source to calibrate the firstgeneration interferometer detectors?
PX436 – General Relativity – Experimental Evidence for GR
Gravitational Waves
•  Q10.2 – Can we use a local source to calibrate the firstgeneration detectors?
If we have two equal masses, separated by 2a, we can assign them
coordinates: x=±a cos ωt
Assuming the masses are point-like, then:
Ixx = m(a cos ωt)2 + m(a cos ωt)2 = ma2(1+cos 2ωt)
so
Ïxx = -4ma2ω2 cos 2ωt
and
hxx has an amplitude 2G/c4r x 4ma2ω2 = 8Gma2ω2/c4r
hyy, hxy and hyx will have the same amplitude, but differ in phase
PX436 – General Relativity – Experimental Evidence for GR
Gravitational Waves
•  Q10.2 – Can we use a local source to calibrate the firstgeneration detectors?
We want this to be h~10-21
We can rearrange this to get mass and separation, but it’s easier to
think in terms of energy requirements.
Assuming we put our calibration source r~100km away from the
detector (we want a plane wave):
kinetic energy needed = ½ma2w2 = c4hr / 16G ~1026J
… i.e. 300 MW per person on Earth for a year
Gravitational potential is one of the most powerful energy
sources in the Universe
PX436 – General Relativity – Experimental Evidence for GR