Stefan HildNIKHEF, May 2009
How to listen to the Universe?
Optimising future GW observatories forastrophysical sources
Stefan Hild Amsterdam, May 2009 Slide 2
Overview
Microphones to detect gravitational waves⇒ Why haven’t we heard GW so far?⇒ How does a microphone for GW work?⇒ Michelson interferometer: a brief history
Optimisation of the Advanced Virgo sensitivity forastrophysical sources⇒ How can we optimise our microphones?⇒ What is quantum noise?
Signal Recycling and optical rigidity⇒ Which is the most promising source for the first detection?⇒ When will we hear the first tones from the Universe?
Einstein Telescope: The future microphone
Stefan Hild Amsterdam, May 2009 Slide 3
Looking at a dark spot in the sky
For ages mankind has been looking towards the starswondering about the origin of the Earth and the wholeUniverse.
Today we know the Universe is a zoo of exciting phenomena.
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Stefan Hild Amsterdam, May 2009 Slide 4
Gravitational waves: A new way ofexploring the Universe
Nearly all of our current knowledge of thecosmos is based on observation ofelectromagnetic radiation (visible light,radio astronomy, infrared, ...).
Gravitational astronomy can providecompletely new insight to the universe:
⇒ Multimessenger observations: We canlearn more about things we already seein the electromagnetic spectrum by alsoseeing their GW emission (for instancesupernovae).
⇒ Exclusive GW observations: There areobjects that can only be seen by theirGW emission
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Stefan Hild Amsterdam, May 2009 Slide 5
Gravitational Waves:Ripples in space time
GW are consequence ofGeneral Relativity.
GW are caused by asymmetricaccelerated masses.
GW change the metric ofspace time.
Quadrupole waves. We know that GW exist:
Indirect detection by Taylorand Hulse (1993 NobelPrice).
No direct detection so far.On going search withkilometer-long Michelsoninterfero-meters looking fortiny length changes.
Stefan Hild Amsterdam, May 2009 Slide 6
Why haven’t we heard GW so far?
Space time is extremely stiff ! Length changes are really tiny (<10-21) !
Stress Energy Tensor Metric Tensor
Stiffness of space time
Analogon: Hooke’s law
Stefan Hild Amsterdam, May 2009 Slide 7
How can we detect gravitationalwaves?
A Michelson interferometeris the ideal instrument tomeasure relative lengthchanges.
L1 = L2 L1 = L2 L1 = L2L1 > L2 L1 < L2
destructive destructive destructiveLight @ outputLight @ output
Stefan Hild Amsterdam, May 2009 Slide 8
Interaction of GW and laser light
TT-gauge: Test masses do not move => but GWchanges the distance between test masses:
Only considering x-arm:
Travel timein x-arm
Stefan Hild Amsterdam, May 2009 Slide 9
Interaction of GW and laser light (2)
Phase Laser freq
Assumptionof GW signal
Phase shiftProduced by
GW
GW freqGW
amplitude
Geometry term
Stefan Hild Amsterdam, May 2009 Slide 10
Optimal arm length
Maximum Signal:
=1
Optimal Arm length: GWwavelength
Example: GW signal at 100 Hz=> optimal arm length of 750 km (!!)
For short arms: develop sine term⇒ Signal proportional to h0, w0, L⇒ Signal independent from GW frequency
Stefan Hild Amsterdam, May 2009 Slide 11
Going back to the starting point
The first Michelson interferometer: Experiment performed byAlbert Michelson in Potsdam 1881.
Measurement accuracy 0.02 fringe (expected Ether effect~0.04 fringes)
Outcome: Not conclusive
Stefan Hild Amsterdam, May 2009 Slide 12
Michelson in Cleveland, Ohio
2nd attempt in 1887, together with Morley. Increased optical pathlength (multiply-folded arms) Improved seismic isolation: Mercury bath (also stopping
traffic around the laboratory building).
Stefan Hild Amsterdam, May 2009 Slide 13
The first science derived from anMichelson interferometer
Measurement accuracy 0.01 fringes, expected Ether effect~0.4 fringes
Stefan Hild Amsterdam, May 2009 Slide 14
Michelson Interferometer for GWdetection
1970s: Weiss/Forward: first
idea and realisationof a Michelson-basedgravitational-wavedetector
Sensitivity: 10-8 of a fringe
Stefan Hild Amsterdam, May 2009 Slide 15
State-of-the-art Michelson
Amsterdam, May 2009 Slide 16
Today’s network of GW detectors
Today: LIGO, GEO600 and Virgo Sensitivity: 10-13 of a fringe
GEO600: measures the 600m long arms to an accuracy of 0.0001 proton diameter @ 500 HzS. Hild for the LSC: “The Status of GEO600” , Class. Quantum Gravity 23 (2006)
Stefan Hild Amsterdam, May 2009 Slide 17
Status and future of GW observatories
1st generation successfully completed:⇒ Long duration observations (~1yr)
in coincidence mode of 5oberservatories.
⇒ Spin-down upper limit of the Crab-Pulsar beaten!
2nd generation on the way:⇒ End of design phase, construction
about to start (or even started)⇒ 10 times better sensitivity than 1st
generation. => Scanning 1000times larger volume of theUniverse
3rd generation at the horizon:⇒ FP7 funded design study⇒ 100 times better sensitivity than
1st generation. => Scanning1000000 times larger volume ofthe Universe
Stefan Hild Amsterdam, May 2009 Slide 18
Overview
Microphones to detect gravitational waves⇒ Why haven’t we heard GW so far?⇒ How does a microphone for GW work?⇒ Michelson interferometer: a brief history
Optimisation of the Advanced Virgo sensitivity for astrophysicalsources⇒ How can we optimise our microphones?⇒ What is quantum noise?
Signal Recycling and optical rigidity⇒ Which is the most promising source for the first detection?⇒ When will we hear the first tones from the Universe?
Einstein Telescope: The future microphone
Stefan Hild Amsterdam, May 2009 Slide 19
Overview of Advanced Virgo
The Virgo is currently the second largest gravitational wavedetector in the world (3km).
Advanced Virgo will be the 2nd generation upgrade. Main new techniques: Signal recycling, high optical power,
non-degerate recycling cavities, monolithic suspension. Thermal compensation and DC-readout.
Start of Construction in 2009,Design sensitivity in 2015(?)
Stefan Hild Amsterdam, May 2009 Slide 20
Optical system designfor Advanced Virgo
Focus of my current work:Optical design of the AdvancedVirgo core interferometer.
Some examples of the topics weare working on:⇒ Definition of the optical configuration⇒ Optimisation of the sensitivity curve⇒ System integrity and interfaces to all
other subsystems of Advanced Virgo
Advanced Virgocore interferometer
Topic for the next minutes: How to optimise the Advanced Virgo sensitivity ?
Stefan Hild Amsterdam, May 2009 Slide 21
How to listen to the Universe?
Advanced Virgo is a hyper-sensitivity microphone to listen to theUniverse.
Each astrophysical source has its own sound or tone. This microphone can be tuned ‘similar’ to a radio receiver.
Pulsar
Supernova
Binary NeutronStar inspiral
Stefan Hild Amsterdam, May 2009 Slide 22
Fundamental noise limits forAdvanced Virgo
Advanced Virgo will belimited by quantumnoise at nearly allfrequencies of interest.
GOAL: Optimisequantum noise formaximal scienceoutput.
Stefan Hild Amsterdam, May 2009 Slide 23
Limits of the optimization
Our optimisation is limited by Coating thermal noise and Gravity Gradientnoise.
Quantum noise to be optimised!
Stefan Hild Amsterdam, May 2009 Slide 24
What is quantum noise?
Quantum noise is comprised of photon shot noise at highfrequencies and photon radiation pressure noise at lowfrequencies.
The photons in a laser beam are not equally distributed, but followa Poisson statistic.
photon shot noisephoton radiation pressure noise
wavelength
optical power
Arm lengthMirror mass
Stefan Hild Amsterdam, May 2009 Slide 25
The Standard Quantum Limit (SQL)
While shot noise contribu-tion decreases withoptical power, radiationpressure level increases:
The SQL is the minimal sum of shot noise and radiation pressure noise. Using a classical quantum measurement the SQL represents the lowest
achievable noise.
wavelength
optical power
Arm lengthMirror mass
V.B. Braginsky and F.Y. Khalili: Rev. Mod. Phys. 68 (1996)
Stefan Hild Amsterdam, May 2009 Slide 26
Advanced Virgo optical layout
knob 1
microscopic po-sition of SRM1
(nm scale)
knob 2
opticaltransmittance
of SRM1
knob 3
InputLight power
Signal Recyclingresonancefrequency
SignalRecyclingbandwidth
We have threeknobs availablefor optimisation:
Stefan Hild Amsterdam, May 2009 Slide 27
Optimization Parameter 1:Signal-Recycling (de)tuning
Frequency of pure optical resonance goes down with SR-tuning. Frequency of opto-mechanical resonance goes up with SR-tuning
Advanced Virgo, Power = 125W, SR-transmittance = 4%
Pure opticalresonance
Opto-mechanicalResonance (Optical spring)
Photon ra-diation pres-sure noise Photon shot
noise
knob 1
Stefan Hild Amsterdam, May 2009 Slide 28
Optical Springs & Optical Rigidity
Detuned cavities can beused to create opticalsprings.
Optical springs couple themirrors of a cavity with aspring constant equivalentto the stiffness of diamond.
In a full Michelsoninterferometer detunedSignal Recycling causes anoptical spring resonance.
Stefan Hild Amsterdam, May 2009 Slide 29
Optimization Parameter 2:Signal-Recycling mirror transmittance
Advanced Virgo, Power = 125W, SR-tuning = 0.07
Resonances are less developed for larger SR transmittance.
knob 2
Stefan Hild Amsterdam, May 2009 Slide 30
Optimization Parameter 3:Laser-Input-Power
Advanced Virgo, SR-tuning=0.07, SR-transmittance = 4%
High frequency sensitivity improves with higher power (Shotnoise) Low frequency sensitivity decreases with higher power (Radiation pressure noise)
knob 3
Stefan Hild Amsterdam, May 2009 Slide 31
Figure of merit: Inspiral
Inspiral ranges for BHBH and NSNScoalesence:
Parameters usually used:⇒ NS mass = 1.4 solar masses⇒ BH mass = 10 solar masses⇒ SNR = 8⇒ Averaged sky location
[1] Damour, Iyer and Sathyaprakash, Phys. Rev. D 62, 084036 (2000).[2] B. S. Sathyaprakash, “Two PN Chirps for injection into GEO”, GEO Internal Document
Frequency of last stable orbit(BNS = 1570 Hz, BBH = 220 Hz)
Spectral weighting = f-7/3Total mass
Symmetricmass ratio
Detector sensitivity
Stefan Hild Amsterdam, May 2009 Slide 32
Example: Optimizing 2 Parameters
Inspiral ranges forfree SR-tuningand free SRM-transmittance, butfixed Input power
NSNS-range
BHBH-range
Stefan Hild Amsterdam, May 2009 Slide 33
Example: Optimizing 2 Parameters
MaximumNSNS-range
MaximumBHBH-range
Parametersfor maximum
Parametersfor maximum
Different source usuallyhave their maxima atdifferent operationpoints.
It is impossible to getthe maximum for BNSAND BBH both at thesame time !
Stefan Hild Amsterdam, May 2009 Slide 34
Example: Optimizing 3 Parameterfor Inspiral range
Scanning 3parameter atthe same time:⇒ SR-tuning⇒ SR-trans⇒ Input Power
Using a videoto display 4thdimension.
Stefan Hild Amsterdam, May 2009 Slide 35
Optimal configurations
Curves show the optimal sensitivity for a single source type.
Stefan Hild Amsterdam, May 2009 Slide 36
Binary neutron star inspirals:
Which is the most promising source?
Binary neutron star inspirals:
Expected event rates seen by Advanced Virgo: ~1 to 10 events per year.Binary neutron star inspirals are chosen to be the primary target forAdvanced Virgo.
Based on observations ofexisting binary stars
Based on models of binarystar formation and evolution
Binary black hole inspirals:
C.Kim, V.Kalogera and D.Lorimer: “Effect of PSRJ0737-3039 on the DNS Merger Rate and Implications for GW Detection”, astro-ph:0608280http://it.arxiv.org/ abs/astro-ph/0608280.K.Belczynski, R.E.Taam, V.Kalogera, F.A.Rasio, T.Buli:, “On the rarity of double black hole binaries: consequences for gravitational-wavedetection”, The Astrophysical Journal 662:1 (2007) 504-511.
Stefan Hild Amsterdam, May 2009 Slide 37
When will we detect gravitationalwaves ??
When Advanced Virgoand Advanced Ligo comeonline WE WILL SEEGRAVITATIONALWAVES!
… if not, then somethingis completely wrongwith our understandingof General Relativity.
Stefan Hild Amsterdam, May 2009 Slide 38
Overview
Microphones to detect gravitational waves⇒ Why haven’t we heard GW so far?⇒ How does a microphone for GW work?⇒ Michelson interferometer: a brief history
Optimisation of the Advanced Virgo sensitivity for astrophysicalsources⇒ How can we optimise our microphones?⇒ What is quantum noise?
Signal Recycling and optical rigidity⇒ Which is the most promising source for the first detection?⇒ When will we hear the first tones from the Universe?
Einstein Telescope: The future microphone
Stefan Hild Amsterdam, May 2009 Slide 39
Start around 2020(?)
Underground location
~30km integratedtunnel length (?)
Myriads of newpossibilities andchallenges !!
Plenty of newScience…
NIKHEF, ‘08
Stefan Hild Amsterdam, May 2009 Slide 40
Tackling Gravity Gradient noise:going underground
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Surface (Pisa) Underground (Kamioka)
about about
Stefan Hild Amsterdam, May 2009 Slide 41
Start Result
How to achieve the ambitious sensitivity?
Stefan Hild Amsterdam, May 2009 Slide 42
Xyolophone: More than one detector tocover the full bandwidth
Low Frequency IFO: low optical power, cryogenic test masses, sophisticatedlow frequency suspension, underground, heavy test masses.High Frquency IFO: high optical power, room temperature, surface location,squeezed light
Stefan Hild Amsterdam, May 2009 Slide 43
… then will soon listen tothe symphony of the Universe !!
http
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If we do a good job over thenext few years …
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