GW150914 : the birth of gravitational astronomy · GW150914 : the birth of gravitational astronomy...
Transcript of GW150914 : the birth of gravitational astronomy · GW150914 : the birth of gravitational astronomy...
GW150914 : the birth of gravitational astronomy
Patrice Hello Laboratoire de l’Accélérateur Linéaire, Orsay.
CNRS/IN2P3 et Université de Paris-Sud
Colloquium ENS 10 mars 2016
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Public announcement on Feb.11th.
“We have detected gravitational waves. We did it!”
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Gravitational Waves
Gravitational Wave (GW) : prediction of the General Relativity (1916!)
Motion of masses => perturbation of spacetime that can propagate (Motion of electric charges => electromagnetic wave) Linearisation of Einstein equations Metric tensor approximation Leads to a propagation equation (far from sources)
µνµνµν η hg +≈
02 =∇ µνh
1 ⟨⟨µνh
µνµνµνµνπ Tc
GRgRG 4
821
=−=
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Gravitational Waves
• Propagation at the speed of light.
• Transverse and traceless (tensor) waves.
• 2 polarisation states (« plus » and « cross »).
• Quadrupolar radiation.
• Dimensionless amplitude h.
• Luminosity
G/5c5 ~10-53 W-1 5
5 QQcGP µν
µν=
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Gravitational Waves
GW = propagating perturbation of the spacetime metric ⇒Effect on measurements of distances between 2 mass-tests.
A B
OG
L xdt
hddt
xd 2
2
2
2
21 ν
µµν−=
Geodesic equation (weak field) :
hLδL 21
max =⇒ Distance measured between A and B varies as
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Gravitational Waves
One period
Effect of h+
Effect of hx
Effect of an incident GW on a circle of test-masses
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Astrophysical sources
« High frequency » sources (> 1Hz in the bandwidth of ground-based detectors) Transient sources • Supernovae (gravitational collapses) • Compact binary coalescences (CBC) black holes (BH) / neutron stars (NS) • … Permanent sources • Isolated NS (pulsars) • Stochastic backgrounds (cosmological or astrophycal origin) • …
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Astrophysical sources
In particular the CBC signals are well predicted: • Analytical PN expansion for the « spiral » phase (chirp signal) • Numerical relativity waveforms for the BH-BH merger • Quasi-normal modes of the possible final BH after merger
Binary NS GW signal (spiral phase)
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One century of developments
• 1916: prediction (Einstein) • 1957: Chapel Hill conference (Pirani, Bondi …) • 1963: rotating BH solution (Kerr) • 90’s: CBC PN expansion (Blanchet, Damour, Deruelle, Iyer, Will, Wiseman …) • 2000: BBH Effective One Body (Buonanno, Damour) • 2006: BBH mergers simulations (Pretorius, Baker, Lousto …)
• 1960: Weber’s bars • 1970: 1st itf prototype (Forward) • 1972: design studies (Weiss) • 1973: Discovery of PSR1913+16 (Hulse&Taylor) • 80’s: itf prototypes (10 meters class) (Glasgow, Garching, Caltech) • End of 80’s: Virgo proposal, LIGO proposal • 90s: LIGO and Virgo funded • 2005-11: « science »runs • 2007: LIGO-Virgo MoU • 2012: Advanced LIGO, Advanced Virgo funded • 2015: LIGO first science run (O1)
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Interferometric detection
[ ] )cos( 1 2 0det φ∆+= CPP
Suspended mirrors test-masses
Incident GW optical paths are modified variation of the light detected power
λπ
λπδφφφ )( 4 2 (t) GWOP
thLL+
∆=+∆=∆ [ ] (t))sin( )cos( 1
2 GWOPOP
0det δφφφ ×∆−∆+≈ PP
The detected signal is proportional to h(t) !
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Interferometric detection
Environmental perturbations : monitoring by many probes (seismometers, magnetometers, microphones etc…)
Seismic noise : mirrors must be isolated from ground motions.
Thermal noise : large mirrors, large incident light beams, high Q materials.
Optical read-out noise : high power laser, optical configuration (arm cavities, power recycling)
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A world-wide network of detectors
G1: 600 m GEO
L1: 4 km
V1: 3 km H1: 4 km
K1: 3 km
LIGO-India
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A world-wide network of detectors
What for?
Reduce the background (coincidences) Estimate the background for a coincidence experiment (time slides) Source sky localisation (triangulation) Source parameters inference GW polarisation determination Tests of General Relativity Astrophysics of the sources
A single detector is not directional: at least 3 are needed for a complete reconstruction (localisation, polarisations…)
Detector beam pattern
Spatial response to each polarisation
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VIRGO HANFORD LIVINGSTON
The LIGO-VIRGO Network :
Angular resolution~ 1o
(can be much worse)
Beam patterns Virgo LIGO-Hanford LIGO-Livingston
Distances: HL ~ 10 msec., VL ~ 26 msec. and VH ~ 27 msec.
A world-wide network of detectors
tLivingston
tHanford
tVirgo
SOURCE
GHOST
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A world-wide collaboration
Formal agreement (MOU) between LIGO and Virgo since 2007
• Total sharing of data. • Joint data analysis (4 « Physics groups »). • Concerted planning for upgrades and data takings. • Run S5/VSR1 : > 4 months of data taking LIGO/Virgo (2007). • Run S6/VSR2-3 : 2009-2010. • Run O1 (Sept. 2015- Jan. 2016) – LIGO instruments only.
Initially CNRS + INFN APC Paris – 2008 LPM-ESPCI Paris LAL Orsay LAPP Annecy LKB Paris - 2010 LMA Lyon ARTEMIS-OCA Nice + NIKHEF (Amsterdam) - 2006 + POLGRAW (Varsovie) - 2008 + RMKI (Budapest) – 2008 ~ 200 members
INFN Pise INFN Rome 1 INFN Rome 2 - 2006 INFN Perugia INFN Florence/Urbino INFN Naples INFN Genes - 2008 INFN Padoue/Trento - 2007
Builders labs (the « LIGO lab ») CALTECH et MIT + the 2 sites LIGO-Hanford et LIGO-Livingstone The LIGO Science Collaboration (LSC) : 85 labs, USA, UK, Germany, Australia … ~ 1000 members
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A world-wide collaboration
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10h54 (Paris) 12h55: first email +20mns: no injected signal +30mns: BBH ! +55mns: data quality OK +70mns: Mchirp ~27 Msun FAR ~10-10Hz
What happened on Monday Sept. 14th?
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o Later that day, Dave Reitze (LIGO executive director) sent an email at 17h59 “The BI team has indicated that they have not carried out a blind injection nor an untagged hardware injection” ...
o Detectors / data quality check list procedure for GW alert sending to EM follow-up partners (MOU privacy)
o GCN (Gamma-ray Coordinate Network) alert sent on Sep 16th at 14h39 (Paris)
What happened on Monday Sept. 14th?
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• 2015 September 14th, 9h50 UTC: the 2 aLIGO instruments have detected in coincidence (time lag ~7 ms) a signal of astrophysical origin (online robust “burst” algorithm).
• Analysis shows the signal corresponds to the merger of a Binary Black Hole of masses around 30 M⊙
• The detection is statistically non ambiguous (high SNR, “clean” data …). References: detection paper Phys. Rev. Lett. (LSC+Virgo, PRL 116, 061102, 2016) + 12 “companion” papers posted on ArXiv.
GW150914: executive summary
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GW150914: time series and time-frequency maps
Freq
uenc
y (H
z)
h (x
10-2
1 )
h (x
10-2
1 )
GW150914
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GW150914: sensitivity of the instruments
2010
2018 2015
2020+
Gain of factor 3-4 in sensitivity (30-60 in rate of events)
September 2015 aLIGO sensitivity
GW150914
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GW150914: data analysis
• Event first detected ONLINE (processing and alert within 3 minutes !) by a « burst » robust pipeline.
• Then offline complete analysis with CBC searches (2 independent ones) and burst searches. Different pipelines CAN look at the same kind of events.
• Instruments frozen for ~ 1 month => 16 days of coincident data in the same stable conditions.
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GW150914: CBC search
GW150914
Waveform is well predicted => matched (Wiener) filtering (noise-weighted cross-correlation of data with a template of the expected signal)
FT of the data Signal template
Noise power spectral density
Template bank (span the parameter space) Analytical for NS-NS, BH-NS. Analytical+numerical for BH-BH. ~ 250,000 templates.
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SNR ~ 23.6 FAR < 1/ 203000 years FAP < 2x10-7
(> 5.1 σ)
Search by correlation of the data with templates. Templates BBH : analytical+numerical («merger ») => inspiral+merger+ringdown
GW150914
GW150914: CBC search
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FAR: < 1 event /67400 years FAP: < 2x10-6 (> 4.6 sigma)
GW150914
GW150914: burst search Time-frequency robust methods
(without assumption on the details of the expected waveform)
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GW150914: data analysis Ruling out noise artefacts
Noise investigation: 200,000 auxiliary channels scrutinized • Un-correlated noises: human activities, seismic activity, radio-
frequency modulation, unknown origin / known family glitches. • Possible correlated noises: potential EM noise sources (lightning
exciting Schumann resonances, solar wind, …).
Detector's control systems have been checked for hacking hazard (thorough investigation to rule out that no-one has injected a signal).
Data quality around GW150914: good and stable over weeks.
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GW150914: data analysis Ruling out noise artefacts
CBC analysis: individual itf triggers and data quality flags
GW150914 is the loudest event in each itf. Effect of data quality flags (pb with H1 corrected since)
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GW150914: data analysis Ruling out noise artefacts
GW150914 in H1 A “glitch” in L1
Which is the level of coherence between these 2 «signals»?
Typical noise artefact in the GW150914 frequency/duration range
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GW150914: data analysis Ruling out noise artefacts
The noise transient (« blip ») in L1 doesn’t match any astrophysical signal. Note: single itf SNR ~ 9.
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GW150914: data analysis Estimation of parameters
15 parameters ! Initial masses, initial spins, final mass, final spin, distance, inclination angle+precession angle (if exists). MCMC methods => probability density functions for each parameter => mean value+statistical errors. +2 models involved => systematic errors. Statistical errors are the dominating ones.
θJN
m
1
m2 d
L
S1
S2
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GW150914: data analysis Estimation of parameters
Individual masses Final BH mass and spin m1 = 36+5-4 M⊙
m2 = 29+4-4 M⊙
Mf = 62+4-4 M⊙
af = 0.67+0.05-0.07
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GW150914: data analysis Estimation of parameters
Degeneracy luminosity distance and inclination angle. Face-on binary favored.
GW150914
Luminosity distance ~ 400 Mpc
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Spins aligned with orbital angular momentum constrained to be small (compatible with 0).
Precession angle un-constrained.
GW150914: data analysis Estimation of parameters
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GW150914: data analysis Estimation of parameters
Sky position: 90% contour ~ 590 deg.2
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GW150914: data analysis Estimation of parameters: waveform reconstruction
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GW150914: data analysis Parameters summary table
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GW150914: tests of General Relativity
Previous tests : solar system, binary pulsars, cosmology. Weak fields, linear regime … With GW150914 : strong field, non-linear regime, relativistic velocities => new tests !
Simplest test : data substracted with closest predicted IMR waveform. Residuals are compatible with Gaussian noise within measurement accuracy. Deviations from GR constrained to be less than 4%.
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GW150914: tests of General Relativity Consistency tests
The reconstructed waveform has 3 distinct regimes : inspiral+merger+ringdown.
Consistency of parameters from different regimes Best ringdown paramaters (f~250Hz, τ~4ms)
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GW150914: tests of General Relativity
Search for deviations from GR prediction for PN expansion of the inspiral signal phase ( xPN (v/c)2x )
Weak constraints but the best up to now except lowest order (few number of cycles).
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GW150914: tests of General Relativity Massive graviton
• Massive graviton => dispersion relation => propagation velocity depends on energy :
−= 2
42 22
21
Ecm
cv gg
• Additional terms in the phase of the inspiral signal : fz
Dcfg
1)1(
)( 2λπδϕ+
=
where D is the distance, z the redshift and cmh
gg =λ is the graviton Compton wavelength.
GW150914 => km1013>gλ or equivalently eV10 22−<gm BEST LIMIT !
• Best previous limit in solar system tests (Mars) : km103 12×>gλ
(Yukawa correction to the Newtonian potentiel ).
−=
g
rr
GMrVλ
exp)(
• Binary pulsars tests: not competitive km10-10 109>gλ
(threshold effect, emission if ) T
cm orbgπωωω 222 with 2 ×==>
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Highest luminosity ever observed ! ~3 M⊙ emitted during the merger
GW150914: summary
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GW150914: electromagnetic follow-ups
Sky maps sent to partners
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GW150914: electromagnetic follow-ups
⇒ A possible gamma-ray counterpart by Fermi GBM ? V. Connaughton et al., arXiv:1602.03920
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CONCLUSIONS
• First direction detection of gravitational waves.
• First direct observation of black holes.
• Binary black holes exist and do merge (in a time < Hubble time).
• Black holes can have masses about 20-30 Msun. • Astrophysical implications (formation of “heavy” stellar black holes, of binary black holes …). • No measured deviation from General Relativity predictions.
• Rate of events poorly estimated (need more events !).
LIGO/Virgo have opened a new window on the Universe and started to probe some of its most violent phenomena.
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Virgo is contributing to the analysis of the LIGO/Virgo data since 2007. Advanced Virgo installation should finish in the coming months:
• ~ 1 year of commissioning is foreseen, • will join LIGO for science runs in 2017.
3 detectors mandatory to • better localize sources (~O(100) deg2 → ~O(10) deg2), • constrain polarization prediction of GR. • cover the whole sky
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Toward next scientific run : O2
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Et après la seconde generation ? La troisième génération se prépare déjà ! Détecteurs triangulaires, base ~ 10 km Détecteurs cryogéniques (bruit thermique) Détecteurs souterrains (bruit sismique) Optiques uniquement réflectives Contrôle capacitif des miroirs ….
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Le projet japonais KAGRA
Interféromètre de 3 km, souterrain (mine de Kamioka) et cryogénique !
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Noise spectral density
Autocorrelation of process x(t) : ∫−
+=∞→
2/
2/)()( 1 ) ( lim
T
Tx txtxdtTT
A ττ
Power Spectral Density (PSD) : Sx( f ) = Fourier Transform of Ax(t)
Dimension of Sx( f ) = (dimension of x)2 / frequency
Amplitude Spectral Density : ) ( ) (~ ffx Sx=
If x(t) corresponds to a stochastic process (noise), its DSA gives the contribution of each frequency to the total noise
∫∞
=0
2 ) ( dffSxσ Link between PSD and RMS :
22/
2/
2)( 1 ) ( lim ∫−
−
∞→=
T
T
ftix etxdt
TTfS πIn practice, we use the estimator:
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Optical readout noise
2 aspects: photon counting noise (or shot noise) and radiation pressure noise
x
y
Photons detected by photodiode (PD) at the output ⇒ Shot noise limited sensitivity :
0shot
4 ~
PLh ω
πλ
≈
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Radiation pressure noise
Decreasing the shot noise => increasing the power ! Power fluctuations => radiation pressure (RP) force fluctuations => Mirror position fluctuations !
cPF =rpRP force on a mirror where P is the incident on a mirror (=P0/2)
cP
Fσσ =The force fluctuation is then
τωτωσ
2/ 0
P
PN ==Where (as derived in previous slide)
The RP force spectral density is then (white). cPP
cfF
λπω2
22
1)(~ 00RP
==
The mirror response to this force is then: c
Pmf
fFfm
fxλππ 3
02RP2 4
1)(~)2(
1)(~ ==
In term of GW amplitude sensitivity: c
PmLf
fxL
fhλπ 3
02RP 2
1)(~2)(~ ==
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Optical readout noise as quadratic sum of Shot and Radiation Pressure noises
2RP
2shotreadout )(~)(~)(~ fhfhfh +=
L=3000; % arm length (meters) lambda=1.06e-6; % wavelength (meters) P0=20; % laser power (Watts) mass=10; % mirror mass (kg)
Shot noise
RP noise is limiting at low frequencies ⇒ Non relevant for first generation detectors ⇒ But relevant for Advanced detectors
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Standard quantum limit
00
2RP
2shot
2readout )(~)(~)(~
PBAPfhfhfh +=+= is minimum for 2
min ,0 cfMBAP λπ==
To each frequency corresponds one optimum and the envelope of all the optima for the readout noise defines the standard quantum limit:
min ,0P
222sql 4)(~
fmLfh
π
=
The SQL is a 1/f noise The SQL depends only on the mirror masses (and arm length) The SQL is not a real limit. It can be beaten in some frequency band (other optical configurations, squeezed states of light ….)
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Background estimation
t
t
IFO 2
IFO 1 A “zero-lag trigger (true coincidence)
t
t
IFO 2
IFO 1 A “time-lag trigger (accidental coincidence)
∆T
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GW Sources : gravitational collapse
Zwerger & Müller, 1997.
Dimmelmeier et al., 2007.
The signal corresponding to the collapse:
Prompt signal ~ a few ms.
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GW Sources : gravitational collapse
And after the collapse:
Ott and Burrows, 2006.
collapse
Marek et al., 2008.
GW signal induced by instabilities around the proto-neutron star (turbulence, role of neutrinos …) Time scale > 1 s.