How to listen to the Universe? - Glashild/presentations/nikhef_hild.pdfHow to listen to the Universe? Advanced Virgo is a hyper-sensitivity microphone to listen to the Universe. Each

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


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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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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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.


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


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


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


Interaction of GW and laser light (2)

Phase Laser freq

Assumptionof GW signal

Phase shiftProduced by

GW

GW freqGW

amplitude

Geometry term


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


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


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).


The first science derived from anMichelson interferometer

Measurement accuracy 0.01 fringes, expected Ether effect~0.4 fringes


Michelson Interferometer for GWdetection

1970s: Weiss/Forward: first

idea and realisationof a Michelson-basedgravitational-wavedetector

Sensitivity: 10-8 of a fringe


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)


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


Overview


Optimisation of the Advanced Virgo sensitivity for astrophysicalsources⇒ How can we optimise our microphones?⇒ What is quantum noise?




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(?)


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 ?


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


Fundamental noise limits forAdvanced Virgo

Advanced Virgo will belimited by quantumnoise at nearly allfrequencies of interest.

GOAL: Optimisequantum noise formaximal scienceoutput.


Limits of the optimization

Our optimisation is limited by Coating thermal noise and Gravity Gradientnoise.

Quantum noise to be optimised!


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


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)


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:


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


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.


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


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


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


Example: Optimizing 2 Parameters

Inspiral ranges forfree SR-tuningand free SRM-transmittance, butfixed Input power

NSNS-range

BHBH-range


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 !


Example: Optimizing 3 Parameterfor Inspiral range

Scanning 3parameter atthe same time:⇒ SR-tuning⇒ SR-trans⇒ Input Power

Using a videoto display 4thdimension.


Optimal configurations

Curves show the optimal sensitivity for a single source type.


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.


When will we detect gravitationalwaves ??

When Advanced Virgoand Advanced Ligo comeonline WE WILL SEEGRAVITATIONALWAVES!

… if not, then somethingis completely wrongwith our understandingof General Relativity.


Overview


Optimisation of the Advanced Virgo sensitivity for astrophysicalsources⇒ How can we optimise our microphones?⇒ What is quantum noise?




Start around 2020(?)

Underground location

~30km integratedtunnel length (?)

Myriads of newpossibilities andchallenges !!

Plenty of newScience…

NIKHEF, ‘08


Tackling Gravity Gradient noise:going underground

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Surface (Pisa) Underground (Kamioka)

about about


Start Result

How to achieve the ambitious sensitivity?


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


… then will soon listen tothe symphony of the Universe !!

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If we do a good job over thenext few years …

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Documents

How to listen to the Universe? - Glashild/presentations/nikhef_hild.pdfHow to listen to the Universe? Advanced Virgo is a hyper-sensitivity microphone to listen to the Universe. Each