Detection of gravitational waves Miquel Nofrarias Institut de Ciències de l’Espai (IEEC-CSIC)
Gravity gradient • Coupling of the suspended test mass with density fluctuations • Dominant source comes from seismic surface waves near the location of the test mass • Solutions to these come from active isolation (using seismograph) to underground facilities ICE 02/07/18 GW detection - M. Nofrarias 2
Parametric instabilities • Energy can couple from the optical modes resonating in the cavity to the acoustic modes in the test mass • Solution is design to avoid this instabilities. • Suppression can be achieved, for instance, by means of thermal compensation ICE 02/07/18 GW detection - M. Nofrarias 3
Parametric instabilities - thermal compensation ICE 02/07/18 GW detection - M. Nofrarias 4
Active isolation • Test mass required to be located ~10 -6 of a wavelength • Residual motion is ~ microns - makes difficult to lock on mirror because large forces need to be applied • Makes it impossible to act on the test mass - noise limit on the actuator typically 10 -9 of the max signal to be applied - 10 -9 x 10 -6 = 10 -15 (large!) • Two options: either reduce residual motion or attenuate test mass motion by means of control stages ICE 02/07/18 GW detection - M. Nofrarias 5
Active isolation • The equation of motion for a suspended pendulum • We use the error signal • Our transfer function is then • Suppression factor ICE 02/07/18 GW detection - M. Nofrarias 6
Ultra low frequency pre-isolator Equation of motion for inverted pendulum Equation of motion for folded pendulum ICE 02/07/18 GW detection - M. Nofrarias 7
Inverted pendulum dynamics ICE 02/07/18 GW detection - M. Nofrarias 8
VIRGO Superattentuator ICE 02/07/18 GW detection - M. Nofrarias 9
External Injection Bench - Seismic Attenuation System ICE 02/07/18 GW detection - M. Nofrarias 10
Suspended detection benches (Virgo) ICE 02/07/18 GW detection - M. Nofrarias 11
Suspended detection benches (Virgo) ICE 02/07/18 GW detection - M. Nofrarias 12
LIGO ICE 02/07/18 GW detection - M. Nofrarias 13
Resonant mass/Acoustic detectors ICE 02/07/18 GW detection - M. Nofrarias 14
Acoustic detectors ICE 02/07/18 GW detection - M. Nofrarias 15
Weber pioneering work • Consider an harmonic oscillator driven by gravitational waves • Derive equations of motion • Exploit piezoelectric effect: stress in a piezoelectric will induce a signal that can be recorded, measure this and you can compute terms of the Riemann • As suggested by Dirac, what if anomalies in Earth rotation would be due to gravitational wave radiation? 5 x 10 8 erg/cm2 : ’The Earth rotation is not a useful detector unless the anomaly can be reduced’ ICE 02/07/18 GW detection - M. Nofrarias 16
Resonant mass detectors network ICE 02/07/18 GW detection - M. Nofrarias 17
Resonant bar ICE 02/07/18 GW detection - M. Nofrarias 18
Resonant bar ICE 02/07/18 GW detection - M. Nofrarias 19
Sensitivity to GW sources • Energy deposited in a resonant antenna • Total intrinsic noise of the antenna • Optimal result reaching the amplifier limit K B T N • Quantum limit • M = 1200kg, D = 3m, ω s =900Hz => T N = 0.04 uK , h min = 10 -21 ICE 02/07/18 GW detection - M. Nofrarias 20
Sensitivity to GW sources ICE 02/07/18 GW detection - M. Nofrarias 21
Spherical GW detectors ICE 02/07/18 GW detection - M. Nofrarias 22
Omnidirectional GW detectors • Various geometries proposed for omnidirectional GW detectors - Truncated Icosahedron (TIGA) - Pentagonal Hexacontahedron (PHC) • Some interesting properties derived from the geometry. In particular, multimode analysis. PHC, for instance, shown to differentiate GR from BD theories from the monopole ICE 02/07/18 GW detection - M. Nofrarias 23
Omnidirectional GW detectors What can we learn about GW Physics with an elastic spherical antenna? J.A. Lobo PRD (1995) ICE 02/07/18 GW detection - M. Nofrarias 24
DUAL wideband proposal ICE 02/07/18 GW detection - M. Nofrarias 25
The gravitational wave spectra ICE 02/07/18 GW detection - M. Nofrarias 26
Pulsar Timing Array A pulsar timing network of ~100 • pulsars each with better than 100 ns timing precision Observing with weekly regular • cadence would enable nHz – μ Hz band ICE 02/07/18 GW detection - M. Nofrarias 27
LISA recent chronology • 2011 Apr: “ESA has ended the partnership with NASA because NASA is financially unable to participate when ESA's funding is available” R. Stebbins (NASA). • 2013 Nov: ESA selects ‘The Gravitational Universe’ as scientific theme for L3 mission slot. • 2016 Feb: LIGO announcement (and LISA Pathfinder successfully releases TMs in free-fall) • 2016 Nov: ESA call for mission proposals for the L3 slot (1.000 ME, 2034) • 2017 Jan: LISA Consortium submits its proposal • 2017 Jun: ESA officially selects LISA as L3 mission of its scientific programme • 2017 Jul-Dec: LISA phase 0 study. Output: Payload Description Document (PDD) • 2018 June: LISA Phase A kick off (2 years). ICE 02/07/18 GW detection - M. Nofrarias 28
It’s a long story… NASA studies GWI (Gravity Wave Interferometer), an interferometer with a total launch mass of 16.4 t , which included four 1000kg test masses. The total cost of the project was estimated at that time to be $ 49.5 M R. Weiss, P.L. Bender, C.W. Misner and R.V. Pound , in Report of the Sub-Panel on Relativity and Gravitation, Management and Operations Working Group for Shuttle Astronomy, (NASA, Washington, DC, 1976 ). ICE 02/07/18 GW detection - M. Nofrarias 29
LISA schedule ICE 02/07/18 GW detection - M. Nofrarias 30
The LISA mission concept LISA is the concept selected for ESA L3 mission slot (2034, 1000M € ) LISA (Laser Interferometer Space Antenna) Constellation of 3 satellites in heliocentric orbit Space-craft are drag-free, ie. it follows a test mass inside which is in nominal free fall Micro-newton thrusters steers the space-frat to be entered around the test mass Differential arm-length measured by laser interferometry ICE 02/07/18 GW detection - M. Nofrarias 31
Black Hole Astronomy in the 2030s Redshift, z Future ground Future EM obs. Current SKA, ground Mass [log M/Mo] ICE 02/07/18 GW detection - M. Nofrarias 32
LISA science case L I S A P a t h fi n d e r Thermo-elastic modelling workshop 33
LISA measurement concept Thermo-elastic modelling workshop 34
LISA challenges — MOSA Going to 20uHz requires 14h measurements. Many things move, so it requires: very high pointing accuracy: 5 nrad/ √ Hz imaging systems and very good alignment between Optical Bench, Telescope and test mass (order of a few 10 μ m) The MOSA has to follow the constellation ‘breathing’, then necessary a backlink fiber harness with 100+ cables must be continuously moved One pm is 10 -6 of a wavelength stray light with an amplitude of 10 -6 compared to the wanted light produces pm errors ICE 02/07/18 GW detection - M. Nofrarias 35
LISA challenges — Gravitational Reference Sensor • Requirements on the test mass: - Allow TM to be a 3 fm/s 2 /Hz 1/2 level geodesic reference (relaxed 1/f2 from 20 mHz to 400 mHz) - Allow TM to be used as a mirror for < 10 pm/Hz 1/2 IFO readout • Forces applied to the test mass dominate noise budget at low frequency • Cold gas: initially 4 tanks 50kg each pulling outwards on two TMs. - Cannot deplete fuel without unbalancing g on two TMs - Costs ~ hundreds pm/s 2 in differential ICE 02/07/18 GW detection - M. Nofrarias 36
LISA challenges — interferometry Beam expands to 15 km → only 1 nW received in telescope Very accurate pointing (sub- μ rad) required → complicated link acquisition One-way light travel time is about 8 seconds, spacecraft are moving phasemeter needs to take into account Doppler and implement tracking schemes Difference between transmit and receive directions, changing over time Need point-ahead mechanism in the optical path Laser frequency noise couples into measurement with enormous amplification Time-Delay Interferometry (TDI) ICE 02/07/18 GW detection - M. Nofrarias 37
LISA challenges — Time Delay Interferometry (TDI) TDI is essential to remove frequency noise in post-processing Suppresses frequency noise by several orders of magnitude Fundamental step in post-processing LISA telemetry Based on pioneering work at JPL, Tinto et al. (1999) technique to ‘synthesize’ an equal arm interferometer, relying in accurate knowledge of satellite constellation TDI impacts in mission design since it requires: sub-m knowledge of absolute arm lengths ns-accurate time-stamping ranging function of phasemeter special requirements on spacecraft timing and clocks ICE 02/07/18 GW detection - M. Nofrarias 38
LISA Pathfinder — paving the way for LISA ICE 02/07/18 GW detection - M. Nofrarias 39
LPF - exploded view ICE 02/07/18 GW detection - M. Nofrarias 40
Gravitational Reference Sensor ICE 02/07/18 GW detection - M. Nofrarias 41
Gravitational Reference Sensor ICE 02/07/18 GW detection - M. Nofrarias 42
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