GW150914 : the birth of gravitational astronomy · GW150914 : the birth of gravitational astronomy Patrice Hello . Laboratoire de l’Accélérateur Linéaire, Orsay. CNRS/IN2P3 et

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=

5

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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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.


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.

GW150914

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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.

GW150914

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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)

GW150914

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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

GW150914

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The noise transient (« blip ») in L1 doesn’t match any astrophysical signal. Note: single itf SNR ~ 9.

GW150914

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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

GW150914

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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

GW150914

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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

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Sky position: 90% contour ~ 590 deg.2

GW150914

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GW150914: data analysis Estimation of parameters: waveform reconstruction

GW150914

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GW150914: data analysis Parameters summary table

GW150914

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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%.

GW150914

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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)

GW150914

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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).

GW150914

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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 ×==>

GW150914

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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

GW150914

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GW150914: electromagnetic follow-ups

⇒ A possible gamma-ray counterpart by Fermi GBM ? V. Connaughton et al., arXiv:1602.03920

GW150914

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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.

Documents

GW150914 : the birth of gravitational astronomy · GW150914 : the birth of gravitational astronomy Patrice Hello . Laboratoire de l’Accélérateur Linéaire, Orsay. CNRS/IN2P3 et