Experimental search for Gravitational Waves

Experimental search for Gravitational Waves

Geppo [email protected]

INFN - Firenze

University of Glasgow

Physik-Institut der Universität Zürich/ETH 28th June 2006

28th June 2006 Physik-Institut der Universität Zürich / ETH 2 of 53

The GR prediction

Newton’s Theory“instantaneous action

at a distance”

Einstein’s Theoryinformation carried

by gravitational radiation at the speed of light

G= 8


Sources of Gravitational Waves

• Compact object binaries

• Pulsars

• Neutron Star internal dynamics

• Non symmetrical supernovae

• Cosmological gravitational waves

Small amplitudeapproximation

Mass quadruplemoment


New potential sources

January 05: A swarm of 10,000 or more black holes may be orbiting the Milky Way's supermassive black hole, according to new results from NASA's Chandra X-ray Observatory. This would represent the highest concentration of black holes anywhere in the Galaxy.


Detection Principles -1

• In the reference frame of the lab (Fermi’s coordinates) the effect of GW is pure mechanical. The potential is:

• 3 types of detectors

– Resonators

– Interferometers

– RF cavities

)yx(mh41

)y,x( 22

Inertia Dimensions



Effect of a sinusoidal gravitational wave going through the slideon the space-time frame and on a circular distribution of free masses

Figure: M.Lorenzini

LL

L/L < 10 -21

Expected from astronomical sources



Two detectors fully developed:Resonant Masses Interferometers

Figure: S. Reid


Theory of GW Detectors - 1

Detectorh x

Read-out

V

f

Detectorinternal

noise

Readoutinternal

noise


First attempt of buildinga resonant detector

Joseph Weber(~1960)

Resonant barsuspended in the middle

Piezoelectrictransducers

Sensitivitypattern


The Band Width of a resonant detector

x/h

Detector noise

Read-out noise

DetectorBW


Bars

Resonant detectors today

Spheres

GW burstsexcite the

resonances of the test masses

mechanicalsignal

enhancement

Capacitive + SQUIDor optical readout


Bar

SQUIDAmplifier

MatchingTransformer

CapacitiveResonantTransducer

DecouplingCapacitor

Transducer Charging Line

L Ls Li

CryogenicSwitch

Mi

CT

CdM

A capacitive Read-out systemof a resonant detector



Interferometric detectors:the concept

• Monitoring the distances between free-flying masses with laser interferometer

• The background noise comes from the readout and from the internal motion of the masses


A bit of history…

• Gertsenshtein M E and Pustovoit V I 1962 Sov. Phys.—JETP 16 433

• Moss G E, Miller L R and Forward R L 1971 Appl. Opt. 10 2495b

• Weiss R 1972 Q. Prog. Rep. Res. Lab. Electron. 105 54


The Band Width of an interferometric detector

Detector noise

Read-out noise

DetectorBW

x = h · L / 2 for each end mirror

)yx(mh21

)y,x(f


Interferometers today - 1

LaserPhotodiode

Pendulumsuspensions

Beamsplitter

Fabry – Perotcavities

• End mirrors positioned in theDark Fringe condition: laser beam is frequency modulated, the sidebands are detected

• Multiple bouncingphase accumulation:laser power increasesfrom 20W to 1kW

• Power recycling: number ofphotons in the interferometerincreases

• Signal recycling:just the side bands are reflectedback in the interferometerGEO600 is the onlydetector that uses thistechnique to enhance the detector response in a narrow band



LaserPhotodiode

Pendulumsuspensions

Beamsplitter




LaserPhotodiode

Pendulumsuspensions

Beamsplitter


The optics and suspensions arein vacuum to minimize fluctuation of index of refraction



TAMA600 m

300 m4 & 2 km

4 km

AIGO

3 km


Real data from LIGO


Real data from GEO600

100 1000 [Hz]

10 -19

10 -18

10 -17

Dis

pla

cem

en

t [m

]


Real data from Virgo


Detectors of 1st Generation

1 10 100 1k 10kFrequency [Hz]

10 -22

10 -23

10 -24

10 -25

10 -19

10 -20

10 -21

h

[ H

z –1

/2 ]

GEO600

LIGOVIRGO

1ST GENERATION

FOR INTERFEROMETERS•STEEL SUSPENSIONS (APART GEO600)•ROOM TEMPERATURE

FOR RESONATORS• Al or AlCu•100mK < T < 4K

AURIGANAUTILUS MiniGRAIL

Pulsars

NS-NS 14 MpcBH-BH 67 Mpc

Supernovae

NSvibration

1ST GENERATION IS CLOSE TO REACH THEDETECTION RANGE FOR NS-NS COALESCENCE

AT THE DISTANCE OF THEVIRGO CLUSTER (17MPc)

BUT THEEVENT RATEIS TOO LOW !!

1 EVENT/3 YRSMOST OPTIMISTICCASE


Future Detectors of Gravitational Waves

• DUAL– Nested hollow cylinder resonant detector– AURIGA collaboration– Construction planned starting on 2009

• Ad. LIGO, Ad. Virgo and GEO HF– 2nd generation interferometers– Virgo + GEO600 collaboration– Commissioning starts on 2009

• 3rd Generation Interferometer– Cryogenic and underground interferometer– Construction envisaged by 2014


DUAL – the concept

read-out the differential deformations of two nested resonators

useful GW band

5.0 kHz

π Phase difference

The inner resonator is driven below resonance

The outer resonator is driven above resonance


Mo Dual 16.4 ton height 3.0m 0.94m

SiC Dual 62.2 ton height 3.0m 2.9mQ/T=2x108 K-1

M. Bonaldi et al. Phys. Rev. D 68 102004 (2003)

Antenna pattern: like 2 IFOs colocated and rotated by

45°

DUAL performance


Real data from Virgo

READOUTTHERMALNOISE

EARTHRELATEDNOISE

CONTROL RELATED NOISE


Readout noise – shot noise

• A fundamental limit to phase measurement is due to the quantum nature of light

• Phase measurements to a level of 10 -13 rad require about 1 MW of laser power in the optical cavities

• But more power = more fluctuating radiation pressure P=1 MW F=3 mN F=1.5

N · ≥ 1/2

fN√Hz


Readout noiseThe Standard Quantum Limit

For a simple Michelson interferometer (GEO HF parameters)

1 100Frequency [ Hz ]

10-21

10-23

Str

ain

[

1/√

Hz

]

Quantum limit onphase measurement

Rad

iation p

ressure n

oise

Quantum noisewith increased laser

power (x100)

SQL

RomanSchnabelMPG-AEIHannover


Beyond the SQL: Squeezed Light

• In one representation of the EM field the two orthogonal states are the Amplitude Quadrature X1 and the Phase Quadrature X2




• In one representation of the EM field the two orthogonal states are the Amplitude Quadrature X1 and the Phase Quadrature X2





1 100Frequency [ Hz ]

10-21

10-22

Str

ain

[

1/√

Hz

]

Quantum limit onphase measurement

Radia

tion p

ressu

re n

oise SQL

Noise reduction by squeezed light - 6 dB in variance


Squeezed light demonstrations

[Chelkowski et al., Phys. Rev. A 71, 013806 (2005)].

[Vahlbruch et al., Phys. Rev. Lett., submitted (2005)].


Intermediate frequencies

1 10 kFrequency [ Hz ]

10-19

10-25

Str

ain

[

1/√

Hz

]

THERMALNOISE

From the realm of Quantum to the realm of Statistical Physics


Thermal noise

• Non isolated system shows uncorrelated fluctuations of volume and temperature

• The equipartition principle states that each observable has a mean energy equal to kBT/2

• The observable– Optical readout: part of the mirror sensed by the laser– Capacitive readout: the average position of the

capacitor plates

TB

2

V

2B2

PV

TkVCTk

T


Thermal noise reduction strategy

• Linear systems & thermal equilibrium

• Each dynamic variable <E>= kT

• Fluctuation-Dissipation theorem

)(Tk4)(S Bff R.K.PatriaStatistical MechanicsPergamon Press

Log f

Noise

Log

[S

xx (

) ]

Lower dissipation Lower thermal noise

Thermal noise forDamped HarmonicOscillator

Lower T Lower thermal noise


The most severe limit for IFOs:thermal noise from the coatings

• Alternate layers of transparent materials with different index of refraction

• Impedance mismatch andinterference produce highcoefficient of reflectivity

• Its structure is not compact as the substrateDeposition with DIBS

• 10 m of coating produces morethermal noise than 10 cm of substrate 1 10 kFrequency [ Hz ]

10-19

10-25

Str

ain

[

1/√

Hz

]

QUANTUM

COATINGS

SUBSTRATESEGO


Suspensions at room temperature

• Best material:silica (SiO2)

• Silicate bonding

• Tested on GEO600


Silicon for mirrors and suspensions at low T

– Thermal expansion null at 124K and 18K main source of thermal noise is ruled out

– High thermal conductivity

– Monocrystal ingots up to 45cm diameter

– Possibility of monolithic suspensions

– Diffractive as well as transmissive interferometry allowed

2.5e-65000


• Test masses have to behave like free flying objects, yet they have to be suspended against gravity

• Seismic motion always present has to be filtered

Earth related noise - 1


Earth related noise - 2:Isolation short-circuit

Newtonian noise

00( ) . ( )

( )

Gh f const x f

H f

Figure: M.Lorenzini

SEISMIC NOISE

The Newtonian noisewill be dominant below 10 Hz for

cryogenic detectors

Surface waves dieexponentially with

depth:

GO UNDERGROUND!


Further considerations

• Building the most perfect inertial reference system

• A system subjected to the quantum problem of measurement

• All the fundamental parameters of the detector have to be CONTROLLED without introducing a significant noise


Detector Generations

1 10 100 1k 10kFrequency [Hz]

10 -22

10 -23

10 -24

10 -25

10 -19

10 -20

10 -21

h

[ H

z –1

/2 ]

GEO600

LIGOVIRGO

AURIGANAUTILUS MiniGRAIL Ad

VIRGO

Mo DUAL

SiC DUAL

3rd GENERATIONINTERFEROMETER

Distance

Rate

NS-NS 14 Mpc1/30ce

1/3yr

NS-BH 29 Mpc1/25ce

1/2yr

BH-BH 67 Mpc1/6ce

3/yr

NS-NS240 Mpc

3/yr

4/day

NS-BH500 Mpc

1/yr

6/day

BH-BH Z~0.31/month

30/day


NS-NS coalescence range

GRB050509B

3RD GENERATIONINTERFEROMETER

2ND

GENERATION

1ST GENERATION

BH-BH coalescence range


Audio band1 Hz – 10 kHz

Beyond Earth based detectors:LISA


A collaborative ESA NASA mission

• Cluster of 3 S/C in heliocentric orbit

• Trailing the earth by 20° (50 Mio km)

• Equilateral triangle with 5 Mio km arms

• Inclined against ecliptic by 60°


The spacecraft

• LISA needs a purely gravitational orbit

• Test masses have to be shielded from solar wind

• Capacitive sensing of the test masses

• Feedback loop to propulsion

• FEEP thrusters with micro-Newton thrust


The Payload


LISA technology demonstration

10-15

10-14

10-13

10-12

F

NS

Hz

Torsion pendulum

Flight test

LISA


LISA Path Finder Mission

Testing:

Inertial sensorCharge managementThrustersDrag-free controlLow frequency laser metrology 2

14 msa 3 10 f 5 mHz

Hz

Only one S/C with two test masses is needed


LISA sensitivity curve

10-4 10-3 10-2 10-1 100

Frequency (Hz)

10-23

10-22

10-21

10-20

10-19

10-18

Detection th

reshold

106Mo z=1104Mo z=1

RXJ1914.4+2456

4U1820-30

wav

e am

plit

ud

e h

10

5

0

-5

-10

-15

app

aren

t m

agn

itu

de

(GW

flu

x)

10 M o + 10

6 M o BH z=1

LISA will see all the compact white-dwarfand neutron-star binaries in the Galaxy. (Schutz)


Conclusions

A new way to observe the

Universe

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Experimental search for Gravitational Waves

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