Gravitational Waves: a new window to observe the Universe

Questions and answers
I. Clarify the techniques used to position the
free-falling 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. 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
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 (amplitudefrequency
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 ?

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 -18 –10 -20

The gravitational waves are described by:
Frequency, ν
Type Range Run Time Sources Instrument
HF
10 Hz →
1000 Hz
one per day
Compact
stars
bars, LIGO,
VIRGO
MF
0.1 Hz →
10Hz
one per
a few days
NS binary
inspiral
Advanced
LIGO
LF
10 μHz →
10 mHz
one per year
binaries
SMBHs
LISA
VLF
1 nHz →
10 μHz
once in a
lifetime
cosmic
astrophysics
PTA
ULF
10 nHz →
0 Hz
snapshots
only
cosmic
structure
COBE, MAP
Planck, etc.

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
formulated to explain the orbital period decay
of the Hulse & Taylor pulsar!