Today’s topic: Gravitational Wave Detectors Text:
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Wednesday, March 18, 2020 General Relativity: Einstein Field Equations
Einstein curvature tensor: stress-energy tensor: describes curvature of space- describes density of mass- time energy
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Wednesday, March 18, 2020 Some Predictions of General Relativity
1. Precession of Mercury’s orbit should be 1.555°/century
Prediction from Newtonian gravity: 1.544°/century Measured precession: 1.555°/century
Greatly exaggerated effect
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Wednesday, March 18, 2020 Some Predictions of General Relativity
2. The path of a light ray is deflected by a massive object;
“gravitational lensing”
Confirmed by Sir Arthur Eddington in 1919
Alignment Lensed Image
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Wednesday, March 18, 2020 Some Predictions of General Relativity
3. Gravitational radiation is emitted by accelerating masses (like electromagnetic radiation emitted by accelerating charges)
Gravitational radiation: time-dependent gravitational field that propagates away from the source at velocity = c as a warp in space-time
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Wednesday, March 18, 2020 Indirect Evidence for Gravitational Waves pulsar in binary system discovered by Hulse & Taylor in 1974
earned them 1993 Nobel Prize in Physics
orbit is decaying with time
matches prediction of GR from emission of gravitational radiation 7
Wednesday, March 18, 2020 Upon merger, will emit a huge blast of gravitational radiation
How can we measure such a signal?
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Wednesday, March 18, 2020 Electromagnetic Waves Gravitational Waves
- Oscillations through spacetime - Oscillations of spacetime itself
Requires a new kind of “observatory” to detect gravitational waves
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Wednesday, March 18, 2020 Electromagnetic Waves Gravitational Waves
- Oscillations through spacetime - Oscillations of spacetime itself
- Interact strongly with matter - Interact weakly with matter
Con: All our current telescopes and detectors are made of matter
Pro: If we can detect it, gravitational radiation can give us information
on locations we could never “see” into 10
Wednesday, March 18, 2020 Electromagnetic Waves Gravitational Waves
- Oscillations through spacetime - Oscillations of spacetime itself
- Interact strongly with matter - Interact weakly with matter
- Incoherent superposition of - Coherent emission from bulk emission from multiple charges movement of mass-energy
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Wednesday, March 18, 2020 Gravitational Electromagnetic Waves Waves
- Oscillations through spacetime - Oscillations of spacetime itself
- Interact strongly with matter - Interact weakly with matter
- Incoherent superposition of - Coherent emission from bulk emission from multiple charges movement of mass-energy
- Frequencies of 107 ~ 1027 Hz - Frequencies of 10-18 ~ 104 Hz (astronomical sources)
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Wednesday, March 18, 2020 Many observatories needed to Large objects : low frequencies probe full frequency range Small objects : high frequencies 13
Wednesday, March 18, 2020 Gravitational Wave “Polarization”
deformation of a circle of “test masses” by the forces induced by a gravitational wave
“plus” “cross” 14
Wednesday, March 18, 2020 Weber Bar (resonant gravitational wave detector)
designed to search for specific frequency that matches the resonant frequency of the bar (1660Hz)
gravitational wave would cause change in Prof. Weber working on his -16 antenna at U Maryland (c. cylinder length by tiny amount (10 m) 1965)
Weber claimed to detect many signals, including SN1987A Results never verified
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Wednesday, March 18, 2020 Modern bar detectors
- isolated in vacuum chambers - superconducting materials - low-noise amplifiers
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Wednesday, March 18, 2020 Gravitational Wave Interferometer
Initial setup: destructive interference at detector
no signal = no gravitational waves 17
Wednesday, March 18, 2020 Gravitational Wave Interferometer
Gravitational wave changes distances between mirrors → pulses of constructive and destructive interference at detector
signal = gravitational wave detection! (???) 18
Wednesday, March 18, 2020 Noise ground-borne seismic noise mechanical noise
thermal noise in mirrors & suspensions radiation pressure P = 2 I recoil on mirrors c
shot noise
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Wednesday, March 18, 2020 LIGO Laser Interferometric Gravitational-Wave Observatory Began science operations in 2002
Advanced LIGO upgrades finished late 2015, data runs now ongoing
LIGO Hanford, WA LIGO Livingston, LA 4km Fabry-Perot vacuum lines effective path length (multiple reflections) ~300km Nd:YAG laser, λ=1064nm (neodymium: yttrium aluminum garnet)
on-sky source localization: accurate to 1° sensitive to 10-1,000Hz 20 Wednesday, March 18, 2020 Virgo GEO600 Cascina, Italy Sarstedt, Germany
Italian-French collaboration German-British collaboration physical arm length: 3 km physical arm length: 600 m effective path length: 100 km effective path length: 1.2 km sensitive to 10-10,000Hz sensitive to 100-10,000Hz
Agreement between LIGO - GEO600 - VIRGO allows detection confirmation and source triangulation 21
Wednesday, March 18, 2020 LISA Laser Interferometer Space Antenna Originally a joint project of ESA and NASA NASA pulled out in 2011 (lack of $)
3 spacecraft flying in formation with separations of 5,000,000 km, Earth-trailing orbit
Scaled down to become: LISA Pathfinder, launched 3 Dec 2015 to L1 began operations 1 March 2016 (16 month mission)
tested essential LISA technology --> 2 test masses and lasers ESA/NASA full mission reboot in --> free-fall rather than formation flying 2017, stay tuned... 22 Wednesday, March 18, 2020 Other Gravitational Wave Observatories
laser interferometer resonant bar
Observatory Location Status TAMA Japan Operational LCGT Japan Planned INDIGO India Planned AIGO Australia Planned DECIGO Space (Japanese) Planned ET Currently unknown Planned BBO Space Planned New designs AGIS and TOBA proposed Dimopoulos et al. 2008, Phys Rev D 78 Ando et al. 2010, Phys Rev Let 105 23
Wednesday, March 18, 2020 Gravitational “Spectrum” Extremely Low-Frequency Band
10-19 Hz 10-7 Hz 105 Hz
Very Low-Frequency Band Low-Frequency Band High-Frequency Band
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Wednesday, March 18, 2020 Expected Signals
Continuous Gravitational Waves
Examples: - binary systems - fast rotation of non-spherically symmetric objects (e.g., neutron star with a mountain)
Characteristics: - weak signal - slowly evolving 25
Wednesday, March 18, 2020 Expected Signals
Inspiral Gravitational Waves
Examples: - merging binary systems (pulsars, black holes, etc)
Characteristics: - finite duration (has a definite end) - frequency increases with time
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Wednesday, March 18, 2020 Expected Signals
Burst Gravitational Waves
Examples: - supernovae and GRBs (???)
Characteristics: - transient - non-periodic
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Wednesday, March 18, 2020 Expected Signals
Stochastic Gravitational Waves
Examples: - relic Big Bang radiation
Characteristics: - low amplitude “noise”
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Wednesday, March 18, 2020 Direct Detection of Gravitational Waves
11 February 2016 LIGO Collaboration announces discovery published in Physical Review Letters
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Wednesday, March 18, 2020 Total members: 1006 Total institutions: 83 Countries represented: 15 31
Wednesday, March 18, 2020 Marco Drago: an Italian working in Germany analyzing data from an experiment in USA
Total members: 1006 Total institutions: 83 Countries represented: 15 31
Wednesday, March 18, 2020 S/N = 24
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Wednesday, March 18, 2020 Advanced LIGO Progress to Date
O1: Sep 2015 - Jan 2016 Feb 2016 - announced 1st binary BH merger S/N~24: 29Msun + 36Msun, D~400Mpc
Jun 2016 - announced 2nd binary BH merger S/N~13: 8Msun + 14Msun, D~400Mpc
third event (unconfirmed) also observed
O2: Nov 2016 - Aug 2017
GW170104: 19Msun + 32Msun, D~1Gpc, S/N~13 GW170608: 7Msun + 14Msun, D~320Mpc, S/N~13 GW170814: 24Msun + 32Msun, D~580Mpc, S/N~18 ...
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Wednesday, March 18, 2020 All known stellar black holes and neutron stars 10 BH from GW and/or mergers EM (Dec 2018)
Not included are two more BH mergers that 1 NS were detected merger and announced already in O3 (40% increase in sensitivity over O1/O2) 38
Wednesday, March 18, 2020 Finding E-M Counterparts to GW Signals
Challenges poor localization from GW observations significant theoretical work needed not all GW signals have E-M counterparts (none for BHs)
Benefits significant increases in our understanding glimpses into otherwise invisible realms
LSST and other synoptic surveys may provide sky coverage, timing, and S/N necessary to identify counterparts
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Wednesday, March 18, 2020 First E-M Counterpart Discovered!
Binary neutron star merger detected in GWs, gamma rays, X- rays, UV, optical, IR, radio
GW170817: 1.1Msun + 1.9Msun, S/N~32
host galaxy: NGC 4993, D~41Mpc
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Wednesday, March 18, 2020 basically every telescope in the world looked at this source!
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Wednesday, March 18, 2020 What did we learn? test of GW speed (consistent with c)
constraint on H0 basically every ~70 km/s/Mpc telescope in the world estimated NS merger rate 3 looked at this ~1000/Gpc /yr source! binary NS are progenitors of short GRBs
production of heavy elements (Au, U, etc)
... analysis continues, even more to come!
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Wednesday, March 18, 2020