Questions and answers Questions and answers

Questions and answers Questions and answers

I. I. Clarify the techniques used to position the Clarify the techniques used to position the free free - - falling LISA test masses, and how falling LISA test masses, and how

surface charging is dealt with.

surface charging is dealt with.

II. II. Explain how cosmic ray Explain how cosmic ray - - induced induced

backgrounds are managed in LISA, and backgrounds are managed in LISA, and their effect on the sensitivity (amplitude their effect on the sensitivity (amplitude - -

frequency plane).

frequency plane).

III. What order of magnitude for gravitation wave amplitudes do the Hulse & Taylor

observations infer? If such amplitudes are not observed experimentally, what

alternatives are being advocated ?

• proof masses have a double role:

1 - mirrors for the interferometer

2 - inertial references for the drag-free control system.

• The drag-free control system consists of:

1 - inertial sensor

2 - proportional micro-propulsion system 3 - control loop

• The displacement of the cubes with respect to their housing is measured by capacitive sensing in three dimensions.

• These position signals are used in a feedback loop to command proportional micro-propulsion

thrusters to enable the spacecraft to remain centered on the proof mass.

• Field Emission Electric Propulsion (FEEP) thrusters

will be used as actuators.

• cosmic rays and solar flare particles can

significantly charge the proof mass surfaces

(see “Cosmic-ray spectra near the LISA orbit”, Grimani et al. 2004 Class. Quantum Grav., 21 S629)

• A system of fiber-coupled UV lamps will discharge the proof masses at regular intervals

• In order not to damage the coating during

launch, a caging mechanism is used to maintain the proof masses in a safe position during launch.

Questions and answers Questions and answers

I. Clarify the techniques used to position the free-falling LISA test masses, and how

surface charging is dealt with.

II. Explain how cosmic ray-induced

backgrounds are managed in LISA, and their effect on the sensitivity (amplitude- frequency plane).

III. III. What order of magnitude for gravitation What order of magnitude for gravitation wave amplitudes do the Hulse & Taylor wave amplitudes do the Hulse & Taylor

observations infer? If such amplitudes are observations infer? If such amplitudes are

not observed experimentally, what

not observed experimentally, what

alternatives are being advocated ?

alternatives are being advocated ?

What order of magnitude for gravitation wave amplitudes do the Hulse + Taylor

observations infer?

PSR B1913+16

d = 6.44 pc --> about 0.1 – 1 Hz

h ~ 10

– 10

Type Range Run Time Sources Instrument HF 10 Hz →

1000 Hz

Compact stars

bars, LIGO, VIRGO MF 0.1 Hz →

10Hz 10 μHz →

10 mHz 1 nHz →

10 μHz 10 nHz →

0 Hz

NS binary

inspiral Advanced LIGO

LF binaries

SMBHs LISA

one per day one per a few days one per year

VLF ULF

once in a lifetime

cosmic

astrophysics PTA snapshots

only

cosmic structure

COBE, MAP Planck, etc.

The gravitational waves are described by:

Frequency, ν

If such amplitudes are not observed

experimentally, what alternatives are being advocated ?

From the Occam’s razor:

“entia non sunt multiplicanda praeter necessitatem”

(entities should not be multiplied beyond necessity)

Actually, any new alternative has been Actually, any new alternative has been

formulated to explain the orbital period decay formulated to explain the orbital period decay

of the Hulse & Taylor pulsar!

of the Hulse & Taylor pulsar!