1 wei-tou ni center for gravitation and cosmology purple mountain observatory chinese academy of...

1

Wei-Tou NICenter for Gravitation and

CosmologyPurple Mountain Observatory Chinese Academy of Sciences

Nanjing, China

On Technologies Common to DECIGO,

LISA and ASTROD

2008.11.12. First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 2

OUTLINE General Concept of --- ASTROD I, ASTROD Super-

ASTROD A comparison of technologies needed for DECIGO, LISA

and ASTROD with focus on common technologies. Drag-free technology (including sensors and micro-

thrusters), charge management system, thermal diagonostic and thermal control, and laser optics are the technology common to all.

Time delay interferometry is the technology common to LISA and ASTROD.

Laser metrology is the technology common to DECIGO and ASTROD. All three requires stringent suppression of spurious noises. This commonness serves as a natural basis for feedbacks and collaborations.

Outlook


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The Gravitational Wave Background from Cosmological Compact BinariesAlison J. Farmer and E. S. Phinney (Mon. Not. RAS [2003])

Optimistic (upper dotted), fiducial (Model A, lower solid line) and pessimistic (lower dotted) extragalactic backgrounds plotted against the LISA (dashed) single-arm Michelson combination sensitivity curve. The‘unresolved’ Galactic close WD–WD spectrum from Nelemans et al. (2001c) is plotted (with signals from binaries resolved by LISA removed), as well as an extrapolated total, in which resolved binaries are restored, as well as an approximation to the GalacticMS–MS signal at low frequencies.

Super-ASTRODRegion DECIGO

BBO Region


The General Concept of ASTROD

The general concept of ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) is

to have a constellation of drag-free spacecraft navigate through the solar system and range with one another using optical devices

to map the solar-system gravitational field, to measure related solar-system parameters, to test relativistic gravity, to observe solar g-mode oscillations, and to detect gravitational waves.


Gravitational Field in the Solar System

The solar-system gravitational field is determined by three factors:

the dynamic distribution of matter in the solar system; the dynamic distribution of matter outside the solar system

(galactic, cosmological, etc.) and gravitational waves propagating through the solar system. ------------------------- Different relativistic theories of gravity make different

predictions of the solar-system gravitational field. Hence, precise measurements of the solar-system gravitational

field test these relativistic theories, in addition to enabling gravitational wave observations, determination of the matter distribution in the solar-system and determination of the observable (testable) influence of our galaxy and cosmos.


Summary of the scientific objectives in testing relativistic gravity of the ASTROD I and ASTROD missions


A Recent Development of LISA

Last talk in LISA7 Symposium: General Relativistic treatment of LISA optical links and TDI S. Dhurandhar Ground interferometers also need to de-convolute the orbit motion and include GR when the precision

increases


ASTROD I (Cosmic Vision 2015-25) submitted to ESA by H. Dittus (Bremen)

arXiv:0802.0582 v1 [astro-ph]

Scaled-down version of ASTROD 1 S/C in an heliocentric orbit Drag-free AOC and pulse ranging Launch via low earth transfer orbit to

solar orbit with orbit period 300 days First encounter with Venus at 118

days after launch; orbit period changed to 225 days (Venus orbit period)

Second encounter with Venus at 336 days after launch; orbit period changed to 165 days

Opposition to the Sun: shortly after 370 days, 718 days, and 1066 days


Laser ranging / Timing: 3 ps (0.9 mm)

Pulse ranging (similar to SLR / LLR) Timing: on-board event timer (± 3 ps)

reference: on-board cesium clock For a ranging uncertainty of 1 mm in a distance of 3

× 1011 m (2 AU), the laser/clock frequency needs to be known to one part in 1014 @ 1000 s

Laser pulse timing system: T2L2 (Time Transfer by Laser Link) on Jason 2

Single photon detector

Jason 2 S/C


Drag-free AOC requirements

Thruster requirements

Proof massProof mass-S/C couplingThrust noise Control loop

gain

Atmospheric (terrestrial) air column exclude a resolution of better than 1 mm This reduces demands on drag-free AOC by orders of magnitude Nevertheless, drag-free AOC is needed for geodesic orbit integration.


Two GOCE sensor heads (flight models) of the ultra-sensitive accelerometers (ONERA’s

courtesy) 2 × 10^-12 m s^-2 Hz^-1/2 resolution in presence of more than 10^-6 m

s^-2 acceleration


ASTROD configuration (baseline ASTROD after 700 days from launch)

Sun

Inner OrbitEarth Orbit

Outer Orbit

Launch Position

.

Earth L1 point S/C (700 days after launch)

S/C 2

S/C 1

.

.

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.


LISA LISA consists of a fleet of 3 spacecraft 20º behind earth in solar orbit keeping a triangular configuration of nearly equal sides (5 × 106 km). Mapping the space-time outside super-massive black holes by measuring the capture of compact objects set the LISA requirement sensitivity between 10-2-10-3 Hz. To measure the properties of massive black hole binaries also requires good sensitivity down at least to 10-4 Hz. (2017)


Example 3 of Li et al. (IJMPD 2008):the variations of the arm-lengths, trailing

angle, velocities in the measurement direction and the angles between arms

16

Doppler ShiftsDoppler shift is due to the line-of-sight velocity between the spacecraft

which we inspect in more detail in the next slides


Line-of-sight velocity between near-Earth and inner-orbit spacecraft

Peak velocities are of order 20 km/s, both ways


Line-of-sight velocity between near-Earth and outer-orbit spacecraft

Peak velocities are of order 20 km/s, both ways


Doppler shift due to line-of-sight velocity

With l.o.s. velocities of up to 20 km/s at certain epochs, for wavelength 1.064 µm one would get a Doppler shift of as much as 20 GHz, which needs to be synthesised.

Attempt to pull laser frequency according to Doppler shift (up to 0.01 %), might also be a possibility.


Time delay interferometry: Technology

common to LISA and ASTROD

Although the velocity in the Doppler shift direction differ by 200-300 times, LISA and ASTROD both need to use time delay interferometry

The issue of large differences in frequency is ideally solved by using optical comb generator and optical frequency synthesizer together with optical clock


Technology common to all

Drag-free technology (including sensors and micro-

thrusters), Charge management system, Thermal diagonostic and thermal

control, Lasers and optics.


Incoming Laser beam

Proof mass

Outgoing Laser beam

Optical readoutbeam

Large gap

Telescope

Dummy telescope

Telescope

Anchoring

LASER Metrology

Housing

Proof mass

Dummy telescope

Capacitive readout


Laser Metrology

Laser metrology is the technology common to the needs of DECIGO and ASTROD

LISA community is developing various laser metrology methods and various optical sensing methods


Super-ASTROD (1st TAMA Meeting1996)

W.-T. Ni, “ASTROD and gravitational waves” in Gravitational Wave Detection, edited by K. Tsubono, M.-K. Fujimoto and K. Kuroda

(Universal Academy Press, Tokyo, Japan, 1997), pp. 117-129.

With the advance of laser technology and the development of space interferometry, one can envisage a 15 W (or more) compact laser power and 2-3 fold increase in pointing ability.

With these developments, one can increase the distance from 2 AU for ASTROD to 10 AU (2×5 AU) and the spacecraft would be in orbits similar to Jupiter's. Four spacecraft would be ideal for a dedicated gravitational-wave mission (Super-ASTROD).


Primordial GW and Super-ASTROD

For detection of primordial GWs in space. One may go to frequencies lower or higher than LISA/ASTROD bandwidth where there are potentially less foreground astrophysical sources to mask detection.

DECIGO and Big Bang Observer look for gravitational waves in the higher range

Super-ASTROD look for gravitational waves in the lower range.

Super-ASTROD (ASTROD III) : 3-5 spacecraft with 5 AU orbits together with an Earth-Sun L1/L2 spacecraft and ground optical stations to probe primordial gravitational-waves with frequencies 0.1 μHz - 1 mHz and to map the outer solar system.


Sensitivity to Primordial GW The minimum detectable intensity of a

stochastic GW background is proportional to detector noise spectral power density Sn(f) times frequency to the third power

with the same strain sensitivity, lower frequency detectors have an f ^(-3)-advantage over the higher frequency detectors.

compared to LISA, ASTROD has 27,000 times (30^3) better sensitivity due to this reason, while Super-ASTROD has an additional 125 (53) times better sensitivity.


Primordial Gravitational Waves[strain sensitivity (ω^2) energy sensitivity]

28

Thank you !

Especially toProfessor Tsubono and his group

for inviting me to this meeting and organizers of this meeting for

giving me a chance to give this talk

1 wei-tou ni center for gravitation and cosmology purple mountain observatory chinese academy of...

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