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VIRGO and the search for
Gravitational Waves
Giovanni Losurdo
INFN Firenze-Urbino
e-mail: [email protected]
PSR1913+16: GW Exist
• Pulsar bound to a “dark companion”,
7 kpc away.
• Extremely accurate clock: f=17 Hz,
Df/Dt=10-15 s-2
• Relativistic clock: vmax/c ~ 10-3
• GR predicts such a system to loose
energy via GW emission  orbital
period decrease
Pobs  corr.
= 1.0032 0.0035

Pth
Cascina – May. 6th, 2003
G.Losurdo – INFN Firenze-Urbino
Ripples in the Cosmic Sea
• Linearized Einstein eqs. (far from big masses) admit wave solutions
(perturbations to the background geometry)
g =   h with h
Cascina – May. 6th, 2003
 2 1 2 
 1     2 2 h = 0
c t 

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Energy-Time Scales
Coupling constants
strong
0.1
e.m.
1/137
weak
10-5
gravity
10-39
Gmp
c
• In SN collapse  withstand 103 interactions before leaving the star,
GW leave the core undisturbed
• Decoupling after Big Bang
– GW ~ 10-43 s (T ~ 1019 GeV)
–  ~ 1 s (T ~ 1 MeV)
– e.m. ~ 1012 s (T ~ 0.2 eV)
GW brings unique information out of the Universe
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G.Losurdo – INFN Firenze-Urbino
2
Sources of GW
• Luminosity:
• Amplitude:
G ij 
erg  RS 
P = 5 Q
Qij   4  1059
 
5c
s R
h =
2
v
 
c
6
DE ~ 1052 erg
for NS/NS
2G 1 
G Ens

Q


4
4

c r
c
r
• M ~ 1.4 M, R ~ 20 km, r ~ 15 Mpc, forb ~ 400 Hz
M
h ~ 10-21
Cascina – May. 6th, 2003
R
r
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Coalescing Binaries
• Compact stars (NS/NS, NS/BH, BH/BH)
• Inspiral signal accurately predictable
– Newtonian dynamics
– Post-Newtonian corrections (3PN, (v/c)11/2)
1996]
[L.Blanchet et al., CQG 13,
chirp
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Supernovae
•
•
•
•
Final evolution of big mass stars
Core collapses to NS or BH
Neutrinos and GW emission
Rate: several per year in the VIRGO cluster
GW emitted
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Kerr Black Holes – Torus System
• A BH-torus system may originate in a NS-BH coalescence
• GRBs can be emitted during BH accretion by torus matter
• A non-axisymmetric spinning torus, powered by BH angular
momentum, is expected to emit a GW burst of T~15 sec, f~1-2/(1+z)
kHz
• If the observed GRBs are originated in such events a rate of ~105 /yr
within 100 Mpc is expected
g
(Van Putten, PRL, 87, 2001)
GW
GW
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g
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Interferometric Detection of GW
• GW acting on a ring of freely falling masses
L-DL
L+DL
DL =
Measurable DL ~ 1018 m
1
hL
2
L ~ 103 m
Target h ~ 1021
(NS/NS @10 Mpc)
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Noise in Interferometric Detectors
• Suspended mirrors (free falling test masses)
• GW induce ITF phase shift:
• Shot noise:
~
=
 =
4

hL
2
P
• Thermal noise:
~
F 2 ( f ) = 4kBT ( f )
DL = 
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hL
2
DL = 
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hL
2
•
•
•
•
3 km vacuum performance within requirements
Final mirrors suspended
Start of commissioning: June 2003
Data taking: 2004+
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G.Losurdo – INFN Firenze-Urbino
Sensitivity Goal
seismic
bars
thermal
shot
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A Real Interferometer
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Vacuum
•
Requirements:
•
Status:
– 10-9 mbar for H2, 10-14 mbar for hydrocarbons
– 10-6 mbar in the suspension chambers
– Outgassing rate: 5 10-15 mbar l s-1 cm-2
– Whole tube welded, leak-tested and baked
– 160 steel baffles for reflected light shielding installed
– Vacuum performance within requirements
1.00E-07
Residual Gases in tube
900m - 1500m N
October 2001
partial pressure, mbar
1.00E-08
After bake
Ptot=2E-10 mbar
before bake
Ptot=3E-8 mbar
1.00E-09
2: hydrogen
18: water
12,28: carbon monoxide
44: carbon dioxide
55, 57: hydrocarbon contamination
1.00E-10
1.00E-11
1.00E-12
1.00E-13
1.00E-14
0
10
20
30
Cascina – May. 6th, 2003
40
50
mass/charge
60
70
80
90
100
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Superattenuators
•
•
•
•
Goal: move the seismic wall down to a few Hz
Expected attenuation: 10-14 @ 10 Hz
3 actuation points for hierarchical control of the
mirror: inverted pendulum,marionette, recoil mass
Inertial damping on IP, angular local controls on
marionette
10-14 @ 10 Hz
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SA Control
LVDTs,
accelerometers
Tide
control
Inertial
damping
• Residual low frequency motion of the
mirrors ~ 10-4 m
• Mirror motion after local control < 10-6 m
• Mirror motion after locking ~ 10-12 m
CCD
ITF
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Angular
controls
Pendulum
damping
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Inverted pendulum
Marionette
Mirror – ref. mass
filters
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The superattenuator and the VIRGO vacuum
system
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Injection System
• 20 W laser
• 144 m MC cavity to reduce jitter noise and
select TEM00 mode
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Detection
Output mode cleaner
• Suspended bench with optics for beam
adjustement and output mode cleaner
• Detection, amplification and demodulation
on the external bench
• Output mode cleaner locked in CITF
External
bench
Suspended
bench
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G.Losurdo – INFN Firenze-Urbino
Mirrors
•
•
Coater VIRGO in a class 1 clean room, unique in the world (2.2*2.2*2.4 m)
Coating features:
– very low losses: scattering < 5 ppm, absorption < 1 ppm
– Uniformity on large dimension: < 10-3  400 mm
•
•
Optical metrology adapted to large components
Type of controls: scattering, absorption (bulk, surface), birefringence,
reflection, transmission, wavefront (stitching), roughness, defects detection
Coater VIRGO
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Absorption, Birefringence,
wavefront measurement
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Global Control
•
•
•
•
•
•
Receives the photodiodes (10 kHz) and quadrants (500 Hz) signals
Simple implementation of complex algorithm for lock acquisition
Computes lengths and angles, applies filters
Sends corrections to mirrors
Used in the CITF commissioning
Working robustly over long periods
Cascina – May. 6th, 2003
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Environment Monitoring
Temperature probes, 88, 1 Hz
Humidity probes, 4, 1 Hz
Pressure probes, 4, 1 Hz
Episensors, 6, 1 kHz
N
I
WI
B
S
P
R
Accelerometers, 9, 10 kHz
S
R
D
B
Magnetometers, 3, 20 kHz
Microphones, 3, 10 kHz
Night-day seismic noise on site
IB
B
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Data Acquisition/Storage
• Data flow: 4.1 Mbytes/sec
– 39 % from detection channels (detection bench + global control)
– 46 % from suspension channels
– 15 % from monitoring channels
• Storage:
– Last 6 months (~64 Tbytes) to be stored on disk at site
– Last 2 years stored on disk at Bologna and Lyon
– All data copied on tapes on site
– 20 Tbytes on disk will be available on site by the end of 2002
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G.Losurdo – INFN Firenze-Urbino
The Central Interferometer
• 6 m recycled Michelson. Commissioning: april 2001-june 2002
• Integration of most of the VIRGO subsystems
• 5 engineering runs (E0-E4)
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CITF Progress
Duty cycles:
98% 85% 98% 98% 73%
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Locking Accuracy
• How good we are in keeping the mirror “still”?
• The dark fringe signal at low frequency measures the relative motion
of the mirrors:
DL = 2 · 10-12 m
• VIRGO requirement fulfilled!
RMS
2·10-12 m
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A long way to go…
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Network
600 m
3 km
TAMA
4 & 2 km
300 m
AIGO
4 km
• False alarm rejection will require coincidences
• ITFs have little directionality: 3 well separated detectors are necessary
to reconstruct the source direction
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The Other Detectors
LIGO
TAMA
• TAMA and LIGO are close to
the target sensitivity
• 2003: LIGO-TAMA S2 science
run
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How Many Events
• Expected rate of coalescences: 3/yr out of 40  200 Mpc
[Grishchuk et al. Astro-ph/0008481]
• VIRGO can detect a NS/NS event at ~ 20 Mpc
• Detection rate, best estimates:
–
–
<1/yr for NS/NS
2-3/yr for BH/BH
• Estimated rate of supernovae: several /yr in the VIRGO cluster, but the
efficiency of GW emission is strongly model dependent
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Expanding the Accessible Universe
Where and how can we reduce the detector noise?
•
•
Thermal noise reduction:
– Fused silica fibers
– Sapphire test masses
– Cryogenic interferometers?
Shot noise:
– High power laser
– Better optics
seismic
thermal
shot
Cascina – May. 6th, 2003
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Advanced Interferometers
Initial Interferometers
Open up wider band
~ 15 in h
~3000 in rate
Advanced Interferometers
From LIGO
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LISA
• Space interferometer
• LISA will explore a different frequency range
• Launch of test mission SMART 2: 2006
Cascina – May. 6th, 2003
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