virgo cascina 6 may 2008 - ego-gw.it · •2 semi-independent 5 10 6km michelson interferometers...

LISA

William Joseph Weber

Dipartimento di Fisica, Università di Trento

LISA / LISA Pathfinder Project

3rd VESF School on Gravitational Waves

VIRGO Cascina

6 May 2008

Laser Interferometer Space Antenna

NASA / ESA Mission, to be launched in 2018

• 2 semi-independent 5 106 km Michelson

interferometers with laser transponders

(measurement noise 40 pm/Hz1/2)

• 3 pairs of “free falling” test masses in 3

“Drag-Free” spacecraft shields

( acceleration noise < 3 10-15 m/s2/Hz1/2)

LISA: Laser Interferometer Space Antenna

( acceleration noise < 3 10-15 m/s2/Hz1/2)

LISA goals:

GW Band: 0.1 mHz – 1 Hz

Sensitivity: Sh1/2 ~ 10-20 Hz-1/2 at 1 mHz

∆∆∆∆(h) ~ 2 10-24 for 1 year integration

5 106 km

LISA Constellation

• 5 million km equilateral triangle

• 60° tilt with respect to ecliptic

• 1 AU from sun, 20° behind earth

LISA Orbits

• 3 phased orbits with eccentricity ε ≈ .01 and inclination θINC ≈1°

• Maintains equilateral configuration within 1°

• Sweeps antenna sensitivity through the sky, frequency and amplitude

modulation for source location

SEND

1 W

RECEIVE

~200 pW (< 100 pW final)

Telescope

D ~ 30 cmArriving Beam

~20 km

LISA Interferometry

Laser divergence:

YAG 1.06 µm

L1 L23 5 million km arms: 33 sec 2-way light time

(1st interferometry null at 30 mHz)

1 W ~200 pW (< 100 pW final)

1/2

4

222/1

pm/Hz 1042

≈==D

L

P

c

P

cS

sentreceived

L

λ

η

λ

π

λ

πδ

hh

Goal: keep all optical path errors within 40 pm/Hz1/2

Shot Noise:

Laser transponding: outgoing light phase locked to incoming beam

LISA astronomy: source location

Axis of max

φ

θhkhf ˆ,ˆ,,

Axis of max

sensitivity Constellation

orbital velocity

(v/c ~ 10-4)

• sensitivity lobes of antenna pattern

sweep through sky

• signals doppler shifted by orbital velocity

of observatory

c

vff GW±∆ ~

2~f

T∆ ±

Use frequency and amplitude

modulation to locate sources in

the sky

Synthetic aperture telescope

with diameter D=2 AU!!(T = 1 year)

D

GWλθ ≈∆ For S/N = 1

L1

L2 L3

( )

( )321

21

23

1LLL

LL

−+

−2 independent

interferometry

signals

Measure both

gravitational

wave strain

polarizations

h+ hx

LISA astronomy: wave polarization

• Other combination relatively insensitive to gravity wave signal (L1 + L2 + L3)

Discriminate instrumental noise from a noisy GW background!

(ie, we can turn off the gravitational wave signal)

Ground and Space GW Observatories Complementary

LISA Signals: mass, separations,distance

Keplerian orbit

frequency ( x 2)

[equal mass

binaries with

circular orbits]

( ) rf

ch~

GW 22τπ

Product of measured strain and measured

decay time gives distance to source!

Black hole

merger

Energy

decay

time τ

( )3

22

a

GMf TOT=π

LISA Gravitational Wave Astronomy:

Compact Object Mergers

Astronomers tell us ...

Most stars are in binary systems

Many stars “collapse” to compact

relativistic stars:

Neutron stars, White dwarfs, black holes

... but they are hard to... but they are hard to

“see” electromagnetically

Only 5 merging NS-NS systems have been found

(need to be lucky to see the pulsar)

Only roughly 50 ultra-compact binaries observed (mostly WD-

WD)

LISA and Galaxial Binaries

• Known “calibration” signals

Signal “guaranteed” for a functioning LISA!

Verification of GR predictions for GW strain

Recent binary neutron star discovery

PSR J0737-3039

• 2 neutron stars (MTOT ~ 2.8 M◉ )

• 2.4 hour orbital period

• 3 times faster than HT, doubles

strain signal, easier detection at

higher frequency .25 mHz

• 2006 � orbital decay detected,

confirms GR at 1 % level

• possibly detectable by LISA (strain of order 10-21 at .25 mHz)?

• changes estimates of population of galaxial NS-NS binary mergers

1 / 5000 years in our galaxy � 200 with τ < 1 million years (f > 2 mHz)

• LISA should provide a real measurement of populations of galactic binaries

• “only” 2000 light years away

• 10 times closer than HT

Stochastic GW noise: galaxial binaries and primordial backgrounds

1 year measurement: µHz 03.

year 1

1≈≈∆f

• 105 frequency bins up to 3 mHz

• many galactic white dwarf binaries (perhaps 108 ), lots per

frequency bin below 3 mHz, produces “noisy” background

Discrimination of noisy confusion limited galactic binary

“foreground”

• Sagnac variable to characterize instrument noise from noisy gravitational

foreground

• Annual modulation of noisy from galactic center

Sample data with instrument noise

Gravitational Wave Astronomy:

Massive Black Hole Mergers

Astronomers tell us ...

Many galaxies have massive black

hole at core

Most galaxies merge

... but we can’t “see” them... but we can’t “see” them

Our Milky Way appears to have

a 3 106 M◉ black hole at its core

Valtonen et al, Nature, 2008

• Observation of quasi-periodic (12 year) quasar light bursts since 1913, occuring in pairs

Quasar OJ287: gravitational radiation in a massive black hole system

Valtonen et al, Nature, 2008

September 2007 burst � without gravitational

radiation, burst would arrive 20 days later!

10 %-level validation of general relativity

description of gravitational radiation

• Optical bursts from an orbiting object

penetrating accretion disk of a massive black hole

• Mass – 18 109 M◉ — determined by geodesic

precession of eccentricity, 39° / orbit

The next major periodic outburst is expected in early January 2016, by which

time there may be methods to measure the gravitational waves directly.

-- Valtonen, et al, Nature

• Massive black hole binaries from cores of

merging galaxies (104 -108 M◉ )

• SNR up to 2000 in one year at z ≈1 – 3 �

observable anywhere in the universe

• visibility up to one year before merger

• chirp rate and amplitude combine to give

the luminosity distance (0.2 % -1%

uncertainties)

Coalescence of Massive Black Hole Pairs

uncertainties)

• frequency and amplitude modulation

combine to give angular resolution (to within a

square degree)

�well calibrated source distances

� formation of MBH as function of

redshift

� with optical counterpart, measure

distance – redshift relationship

Simulated strain time series for a MBH merger at redshift z = 5

S/N ratio >> 1 even for single cycles near the end of a MBH merger

(2 105 M◉ at z = 5)

� High S/N observation of MBH mergers anywhere in the relevant universe!

• Gravitational capture of compact

object (1-10 M◉ BH, NS)

• High rate (order 10 per year)

• SNR 10-20

• Trajectory of “point particle” near

event horizon of a BH � test of

Sources: Black Hole Gravitational Captures

event horizon of a BH � test of

relativity in strongly relativistic limit

Gravitational waves physics

• Gravitational wave observation (phase, polarization, amplitudes)

can probe general relativity in limit of strong gravitational fields,

near black hole event horizons

• Gravitational waves drive dynamics in such systems

• need compact “test particle” – NS or BH – not tidally disrupted

near MBH

Example: small “test

particle” black hole falling

into a massive black hole

New result in numerical relativity:

complete GW waveform of BH-BH merger

Successfully “bridged the gap”

between inspiral and ringdown phases

High precision tests of GR demand high precision GR radiation solutions!!

• Early universe opaque to EM radiation until “recombination” of

neutral atoms liberated the cosmic microwave background

photons (400,000 years after Big Bang)

• Universe transparent to GW since much earlier

LISA Gravitational Wave Astronomy:

Cosmic Gravitational Wave Background?

� Gravitational waves could allow study of big bang,

inflation, early universe phase transitions

BOOMERANG map of

CMB

LISA Sensitivity Curve

Sensitivity curve for 1 year integration and S/N=5

Photon shot

noise

Test mass

acceleration

noise

Decreased

interferometer

response

LISA Interferometry: TM separation as 2 part measurement

� long interferometer and (2) short interferometer

*** pm precision requires subtracting nm spacecraft motion (thruster noise)

Gerhard Heinzel, AEI

Interferometry challenges: frequency noise with unequal arm IF

Combat laser noise with Time Delay Interferometry

�Don’t need to hold interferometer arms equal to 10’s of meters

… rather measure arm lengths to within 10’s of meters and compare

measured phase shifts at offset times!

� Cancel frequency noise without cancelling GW signal

LISA Optical Bench

� Astrium Germany design, ESA study

Light from 2 lasers

L1 � to remote SC (1 W), local TM

L2 (beam for 2nd arm) � local oscillator

for incoming beam and TM readout

3 interferometers

TM readout (L1, L2 as LO)

Remote beam readout (far laser, L2 as LO)

L1 – L2 measurement of relative phase noise

Interferometry challenges: Keplerian breathing of orbital formation

Classical orbital dynamics do not produce “rigid rotation” of equilateral triangle

∆φ ~ 1 ° � telescope angle must breathe

∆L ~ 50000 km � unequal arm interferometer

∆v ~ 20 m/s � relative velocity causes Doppler shift up to 20 MHz (fringe rates)

interferometer signals are RF beat notes, with science signal as a mHz phase modulation

Interferometry challenges: frequency noise with unequal arm IF

fx L

f

∆∆ ≈ ∆

• With ∆L ~ 50000 km, need ∆f ~ µHz/Hz1/2

• More than 7 orders of magnitude

improvement from cavity stabilized laser!

Unequal arms � Time Delay Interferometry50000 km length difference 7 orders of magnitude too big

• Instead of reducing ∆L to order 10 m, measure ∆L to 10 m

precision

• Recombine phase data with opportune delays to cancel laser

phase noise

� “synthesize” an effectively equal arm

interferometer

[Tinto & Armstrong, PRD, 1999]( ) ( )2 1

1 1 2 2

2 2L Lz t z t z t z t

c c

− − − − −

• Can synthesize different combos (including Sagnac)

• Can also handle relative SC motion

Interferometry challenges: further frequency stabilization to relax TDI

• Cavity pre-stabilization is limited by the optical cavity

length stability.

� δL/L ~ 10-13 /√Hz

• Take advantage of 5 million km LISA arm stability:

� δL/L ~ 10-20 /√Hz

Daniel Shaddock, JPL

+ arm locking

Daniel Shaddock, JPL

Purity of free-fall critical to LISA science

Example: massive black hole (MBH) mergers

Integrated SNR at 1 week intervals for year before merger

Assuming LISA goal:

Sa1/2 < 3 fm/s2/Hz1/2

at 0.1 mHz

Acceleration noise at and below 0.1 mHz determines how well, how far,

and how early we will see the most massive black hole mergers.

� do we see the merger for long enough to use orbital

modulation to pinpoint it? To search with optical telescopes?

LISA Low Frequency Sensitivity:

Importance of drag-free control

2

2/1

minmin

1

ωm

S

TLL

Lh

f≈

∆≈

Stray acceleration

noise (1/f2 ) for flat

spectrum

hmin ~ 10-23 at 1 mHz (S/N=5) requires Sf1/2/m ~ 3 10 -15 m/s2 / Hz1/2

Spacecraft shield

(mass M)

Stray forces and drag-free control

µNewton Thrusters

“Drag Free” loop gain

MωDF2

• Solar radiation pressure would give

10 nm / s2 acceleration to 1 kg test

mass

Springlike coupling to spacecraft

motion (“stiffness”) mωp2

“internal” stray forces fstr

Relative position

measurement xm

m

mωp

external forces on

satellite Fstr

Common problem for several precision space experiments: LISA,

GPB, STEP ...

++=

2

2

DF

strnp

strres

M

Fx

m

fa

ωω

Residual acceleration noise:

Relative spacecraft – TM

LISA Drag-free Control

Role of LISA drag-free control is to reduce test mass acceleration noise, with

respect to distant test mass

NOT to minimize relative spacecraft motion

NOT to produce most precise spacecraft orbit

Relative spacecraft – TM

motion

LISA control: spacecraft follows 2 masses at once


1: Move the spacecraft and centre the masses along laser beams


2: Re-center the masses along “orthogonal” axes using electrostatic forces


Need to sense all 6 degrees of freedom of the test mass

Need to apply (electrostatic) actuation forces on non-interferometry degrees of

freedom

Key LISA test mass acceleration noise sources

dx

Residual acceleration noise:

Springlike coupling to spacecraft:sensor readout stiffness (ωp

2xn ~ d)gravity gradients

10-6 N/m

External forces on

SC, finite control

loop bandwidth

Gap

++=

2

2

DF

strnp

strres

M

Fx

m

fa

ωω

gas damping

magnetic noise

readout back action (~ d-2)

Stray electric fields + charge/dielectric noise (~ d-1 ,d-2 )

∆T� radiation pressure, radiometric, outgassing effects

Local gravitational noise

6 fN/Hz1/2

Sensor noise

Low frequency stability!

2.5 nm/Hz1/2

Drag-free Control: Microthrusters

• Forces in range 0-100 µN

• Force noise below 0.1 µN/Hz1/2

� Will likely limit SC control level to

several nm/Hz1/2

Gravitational Reference Sensor Design

• 46 mm cubic Au / Pt test mass (1-2 kg)

• 6 DOF “gap sensing” capacitive sensor

• Contact free sensing bias injection

• Resonant inductive bridge readout (100 kHz)

• Defines TM environment

• Provide nm/Hz1/2 measurement on all axes

• Provides electrostatic voltages (force, measurement)

VACT1

VACT2

VM

Cs1

Cs2

VAC

100 kHz L

L

Cp

Cp

• Resonant inductive bridge readout (100 kHz)

• ~ 1 nm/Hz1/2 thermal noise floor

• Audio frequency electrostatic force actuation

�avoid DC voltages

• Large gaps (2 – 4 mm)

� limit electrostatic disturbances

• High thermal conductivity metal (Mo) / sapphire

construction

� limit thermal gradients

In orbit!

2010 ?To launch soon!

What levels of free-fall have been (or are being) achieved?

2010

� Need to verify acceleration noise levels for LISA low-frequency performance

� on ground torsion pendulum testing

� LISA Pathfinder flight test

Torsion pendulum ground testing of LISA Free-fall

Measure stray surfaces forces as

deflections of pendulum angular

Light-weight test mass suspended as

inertial member of a low frequency

torsion pendulum, surrounded by

sensor housing

deflections of pendulum angular

rotation

to within < 100 LISAgoal

<10 LTP goal

Precision coherent measurement of

known disturbances

Single mass torsion pendulum

for LISA ground testing

• 110 gm TM + mirror (hollow Al, Au

coated)

• 25 µm, 1 m long W fiber � 2 mHz

resonant frequency, Q 3000

• Passive magnetic damping of swing

mode (τ 100 s)mode (τ 100 s)

• Autocollimator and capacitive readouts

• On demand electrostatic damping

/actuation of swing mode

• Turbo vacuum pump 10-7 mB

• Thermally controlled foam room (50 mK

long term stability)

Upper limits on GRS force noise: • 1 mass pendulum, deflection monitored with capacitive and optical

autocollimator for 3 days

• Period 591 s, Q = 3400

8B

k T

Γ

Typical thermal peak-

peak pendulum

oscillation

Near 1 mHz, close to thermal noise

High frequencies � readout noise

Low frequency excess…

( ) 00

[ ]EXT

II N

Q

ωφ φ φ γφ γ+ Γ − + = ≈&& &

Upper limits on GRS force noise: Time domain conversion into external torque acting on

pendulum with transfer function:

Upper limits on GRS force noise: Distinguish true torque on pendulum from background

sensor noise with cross-correlation analysis:{ },AC SN N NS S= ℜ

•Less than factor 2 in power from Brownian noise for decade of frequency around 1 mHz

•Unexplained excess at lower frequencies (coupling to environment? Sensor itself?)

•Excess at higher frequencies – rotational motion of apparatus (order 10 nrad/Hz1/2)

� desire to improve sensitivity with lower thermal noise and better (interferometric) readout

Upper limits on GRS surface force noise: • Subtraction of “background” pendulum thermal noise

• Conversion into force – use of “suitable” armlength (10.75 mm) – and finally

TM acceleration (M=2 kg)

LISA PF

LISA

• Factor 2 from LISA PF goal at 1 mHz!!

• Sufficient to allow observation of GW from several known galactic

binaries (see PRD 75, 042001)

Force noise upper limits with 4-TM

pendulum

100 fN /Hz1/2 level near 1 mHz

An improved torsion pendulum for

higher sensitivity force measurement

High Q fused silica fiber

(35-40 micron diameter)

Collaboration U. GlasgowCollaboration U. Glasgow

Wavefront sensing

interferometric readout

Possible improved sensitivity � factor 10

Noise source: spacecraft coupling

+−=∆−=

2

22 DF

strnpresp

M

Fxxa

ωωω

springlike coupling

(“stiffness”)

Relative spacecraft / TM position jitter

(sensor noise and external forces)

LISA goal: 4 10-7 /s2 x 2.5 nm/Hz1/2 ~ 1 fm/s2/Hz1/2LISA goal: 4 10-7 /s2 x 2.5 nm/Hz1/2 ~ 1 fm/s2/Hz1/2

Known source: negative electrostatic spring from 100 kHz VAC (< 10-7 /s2 or 100 nN/m)

Other less-known sources ... Electrostatic or magnetic contamination, DC patch fields

x

VAC

100 kHz

SENSOR ON

SENSOR OFF

• Coherent torque excited

by square wave oscillation of

sensor rotation angle

• Search for all sources of

stiffness, not just from

sensing bias

Spacecraft coupling: rotational stiffness measurement

SENSOR ON

SENSOR OFF

Sensor ON stiffness

ΓON = -119.4 +/- .5 pN m / rad

�� Roughly as modelled!

Sensor OFF stiffness

ΓON = 1.3 +/- 1 pN m / rad

�� Extra stiffness negligible!

Noise source: Cosmic ray charging

Q

Cosmic rays deposit charge in test mass in

Poissonian fashion (biased random walk)

∆V

Poissonian fashion (biased random walk)

Deterministic: λNET ~ 100 +e/s

Shot noise: λEFF ~ 1000 +e/s[Araujo, 2004]

1. (Deterministic) Negative, unstable electrostatic spring

xtxQFQ

22 ∝∝

2. (Random) Mixing of charge shot noise with DC electric field

VQFQ ∆−∝

Noise source: Cosmic ray charging

• To compensate the deterministic charging of test

mass, need a discharging system

�UV light photoelectric discharging

• With appropriate biasing of electrodes, can both

measure and remove TM charge

• Charge becomes important for LISA at level of 106 e

254 nm Hg lamp

hν = 4.9 eV

φAu = 4.4 – 5.2 eV

• Charge becomes important for LISA at level of 10 e

UV light fibers illuminating TM and electrode surfaces

Electrostatic stiffness from TM charging

< 1% total LISA

stiffness

• As expected from electrostatic model (roughly 30% below infinite plate model)

• Note: minimum magnitude obtained for VTM ~ 60 mV (NOT 0 V)

� DC biases effect charge measurement and stiffness

60 mV (107 charges)

~ one day charging

Bipolar UV photoelectric

discharge test

•Measure charge with applied voltages and

coherent torque detection (as in flight)

•Alternately expose TM and electrodes to

produce charge rates of +/- 12000 e/s

� enough to discharge 1 day’s

worth of charge in 10 minutes

Noise source: stray low frequency electrostatics

VM

∆x / 2

δV1 δV2

Electrostatic stiffness( )2

2

2

1

2

ii TM

i

CFk V V

x x

∂∂= − = − −

∂ ∂∑

2Q∝

2Vδ∝

[For zero net DC bias imbalance]

x

T

EFF

Fx

C

C

eS ∆

∂

∂=

ω

λ2

2/12

2/12/1

xS

x

C

C

QS

T

F ∆∂

∂=

Random charge noise mixing

with DC bias (∆x)

Noisy average “DC” bias (S∆x)

mixing with mean charge

iVii

F SVx

CS δδ∑

∂

∂= 2

2

2/1 Noisy “DC” biases interacting

with themselves

∑∂

∂= i

i

TOT

Vx

C

C

QF δ

Noise source: DC biases and charge shot noise

VVd

CF M ∆−=

VM

∆V ≈∆x/2

δV1 δV2

Fluctuating test mass charge (cosmic ray shot noise)

forced by stray DC electrostatic “patch” fields

VACTVACT

VMOD

( )

∆

−

fsfS xeff

a

Hz 10

mV 001/800 Hz/fm/s 6~

42/1

22/1λ

• λeff ~ 800 e/s (H. Araujo, LISA Symposium 2004) includes +/-, different charge number

∝

d

1

• Can be cancelled by application of correct compensation voltage

Charge feels integrated effect from all patch fields

• Can be measured by applying a coherent TM bias (simulated charge)

DC Bias: measurement and compensation

Torsion pendulum measurement of ∆φ as a

function of applied compensation voltage VCOMP

φφφ

∆∂

∂−≡

∂

∂−= ∑ x

Mii

M

CVV

CVN

• DC biases compensated with VCOMP = +15 mV (intrinsic ∆φ = -60 mV)

• Sub-mV measurement possible in 15 minutes integration

• Compensation possible to DAC resolution, in flight

• Random charging should not be problematic under normal conditions

δV1Α

δV2Α

δV1Β

δV2Β

V∆+VCOMP

+VCOMP

-VCOMP

-VCOMP

Verification of random charge model• Create charge noise by compensating photoelectric currents of +/- 12000 e/s

• Intentionally apply large fields (+/- 3 V on x electrodes)

• Clear and quantitatively consistent correlation between measured charge

fluctuations and measured pendulum torque noise

n vVd

CF δ−≈

vn

δV

Noise source: in-band voltage noise mixing with DC bias

DC voltage difference: δVVoltage noise: vn

LISA goal vn ≈ 20 µV/Hz1/2 at 0.1 mHz

DC voltage difference: δV

• Test mass charge

• Residual unbalanced patch effects

Voltage noise: vn

• Actuation amplifier noise

(electronics)

• Thermal voltage fluctuations (δ)

• Drifting (not Brownian) DC bias SδV1/2

––– Voltage fluctuations

––– Measurement noise (quad phase)

Measured noise in stray “DC” biases


––– Measurement noise floor (quad phase)

––– Measurement noise floor (theory)



LISA goal


––– Measurement noise floor (quad phase)

––– Measurement noise floor (theory)

3

1/2 160 µHz200 V/Hz 1

fµ

× +

• No excess voltage fluctuation noise observed above 0.1 mHz

• 1σ-limit of measurement: 200 µV/ Hz1/2 white noise near 0.2 mHz

• fit to 1/f 3/2 excess at lower frequencies

LISA goal

50 µV/Hz1/2

Noise source: Thermal gradients

T

TAPFradiom

4

∆=

∆T

Forces proportional to temperature gradient

come from imbalance in radiation pressure,

Tc

ATF pressrad ∆=

3

3

8 σ

outgasoutgas QT

TF Θ

∆∝

2 ???

Need to measure dF/d∆T to search for any excess coupling to a temperature gradient

come from imbalance in radiation pressure,

momentum from molecular impacts

Torsion pendulum testing: Thermal gradients

• Apply alternating thermal gradient across sensor, detect coherent torque

PRD, 76 102003 (2007)

Observe linear pressure dependence of radiometric effect

� in quantitative agreement with model (30% level due to

uncertainty in temperature distribution

Zero pressure torque small � radiation pressure and outgassing not threatening

to LISA goals

100 pN / K rough estimate � need 10-5 K/Hz1/2 temperature difference stability

LISA Pathfinder (2010): Einstein’s Geodesic Explorer

Mostly ESA test mission for free-

fall in LISA and other future

precision space missions

Shrink 1 LISA arm from 5 million km to 30 cm

Flight test of LISA free-fall at 30 fm/s2/Hz1/2 level at 1 mHz

Flight test of LISA local interferometry measurement at 10 pm/Hz1/2 level

Acceleration noise flight test: LISA Pathfinder (2010)

x

Xbase

~ 30 cm

TM1 TM2

drag-free

electrostatically

suspended

∆x12≡x2 - x1

• Compare relative noise in orbits of two “free-falling” test masses

• 1 spacecraft, 1 measurement axis (30 cm baseline)

• Relative displacement ∆x12 measured with interferometer to probe drag-free

performance

Optical interferometer Differential displacement ∆x12

LTP Goal: demonstrate ares < 30 10-15 m/s2/Hz1/2 for f > 1 mHz

(relaxed from LISA by factor 10 in both acceleration noise and frequency)

LISA Pathfinder: Performance limited by 2 TM in 1 SC � applied forces

• Local SC gravity modelled and

balanced to 100 pico-g level

• SC can only follow 1 TM along x (2 TM, 1 SC)

• Any differential DC acceleration must be balanced

by applied (electrostatic) forces

• Noise in applied voltage gives noisy force

2

1/ 2 1/ 2

/2F V V

F V

S FSδ

∝

=

• Actuation voltage carrier

amplitude stable to 2 ppm/Hz1/2

(electronics Contraves Space,

test U. Trento / ETH Zurich)

LTP as an orbiting laboratory for LISA disturbances:

Magnetic field effects• Coupling to permanent and induced magnetic moment

TM1 TM2

Thermal gradient effects• radiation pressure, radiometric, outgassing

+

∂

∂≈ B

BVM

xFx

rr

r.

0µ

χ

TM1 TM2

• radiation pressure, radiometric, outgassing

• Measurement of disturbance time series allows correlation analysis of noise

sources, measurement of actual coupling parameter allows possible correction

• LTP is a true experiment, “debuggable”

TM1 TM2

Coupling to spacecraft motion• “wiggle” spacecraft setpoint, detect force

This document is the property of Astrium. It shall not be communicated to third parties without prior written agreement. Its content shall not be disclosed. As

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In orbit!

2010 ?To launch soon!

LISA Pathfinder

2010LPF after

post-

processing

� LISA Pathfinder should be able to guarantee almost the entire LISA science return!

Going beyond LISA: The

Big Bang Observer

• Exploiting frequencies near 1 Hz --- few signals from galactic binaries

• Look for an extra-galactic cosmic gravitational wave background produced by the big

bang and inflation

The Big Bang Observer

• Shorter arms (5 104 km, not 5 106 km)

• More light (300 W, not 1 W)

• Bigger telescopes (3 m, not 30 cm)

• Better force isolation (.03 fm/s2/Hz1/2, not 3 fm/s2/Hz1/2)

• Multiple constellations for noise discrimination

� Need to subtract off signals from ALL NS-NS, BH-BH mergers in

universe in order to see background of gravitational radiation from big

bang .... Wow!

� year 2025 (????)

ESA LTP

Collaboration

Trento LISA Team

Stefano Vitale (LPF PI)

Matteo Benedetti, Daniele Bortoluzzi, Antonella

Cavalleri, Giacomo Ciani, Rita Dolesi, Mauro

Hueller, Daniele Nicolodi, David Tombolato,

Peter Wass, Bill Weber

virgo cascina 6 may 2008 - ego-gw.it · •2 semi-independent 5 10 6km michelson interferometers...

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