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Probing the Dark Side of the Universe with Gravitational Waves
Martin Hendry
Astronomy and Astrophysics Group and Institute for Gravitational ResearchSUPA, Dept of Physics and Astronomy, University of Glasgow, UK
2
Outline of talk
• Introduction: what are gravitational waves?
• Astrophysical motivation: possible sources and overview of science case
• GW detection: general principles; noise limitations; and current status (ground-based)
• The advent of GW astronomy: some examples
• Coming attractions (next 5 years) and future developments (next 20 years)
• Case study: cosmology with GW sources
Cambridge, Jan 2010
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Who am I?…
Jim Hough and Ron Drever, 1978
Institute for Gravitational ResearchInstitute for Gravitational Research~50 research staff and students, with activity spanning advanced materials, optics and interferometry, data analysis, for ground-and space-based GW detectors.
Cambridge, Jan 2010
5
Who am I?…
“There is nothing new to be discovered in physics now.
All that remains is more and more precise measurement”(1900)
William Thompson(Lord Kelvin)1824 - 1907
Cambridge, Jan 2010
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µνµν κTG =
Spacetimecurvature
Matter (and energy)
Gravity in EinsteinGravity in Einstein’’s Universes Universe“The greatest feat of human thinking about nature, the most amazing combination of philosophical penetration, physical intuition and mathematical skill.” Max Born
Cambridge, Jan 2010
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Spacetime tells matter how to move, and matter tells spacetimehow to curve
Gravity in EinsteinGravity in Einstein’’s Universes Universe
Cambridge, Jan 2010
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Gravitational WavesProduced by violent acceleration of mass in:
neutron star binary coalescencesblack hole formation and interactionscosmic string vibrations in the early universe (?)
and in less violent events:pulsarsbinary stars
Gravitational waves‘ripples in the curvature of spacetime’that carry information about changing gravitational fields – or fluctuating strains in space of amplitude h where:
LLh ∆
=2
Cambridge, Jan 2010
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PulsedCompact Binary Coalescences: NS/NS; NS/BH; BH/BHStellar Collapse (asymmetric) to NS or BH
Continuous WavePulsarsLow mass X-ray binaries (e.g. SCO X1)Modes and Instabilities of Neutron Stars
StochasticInflationCosmic Strings
Gravitational Waves: possible sources
Cambridge, Jan 2010
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Science goals of the gravitational wave field
Fundamental physics and GR• What are the properties of gravitational waves?• Is general relativity the correct theory of gravity?• Is GR still valid under strong-gravity conditions?• Are Nature’s black holes the black holes of GR?• How does matter behave under extremes of
density and pressure?
Cosmology• What is the history of the accelerating
expansion of the Universe?• Were there phase transitions in the early
Universe?
Cambridge, Jan 2010
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Astronomy and astrophysics• How abundant are stellar-mass black holes?• What is the central engine that powers GRBs?• Do intermediate mass black holes exist?• Where and when do massive black holes form
and how are they connected to galaxy formation?• What happens when a massive star collapses?• Do spinning neutron stars emit gravitational waves?• What is the distribution of white dwarf and
neutron star binaries in the galaxy?• How massive can a neutron star be?• What makes a pulsar glitch?• What causes intense flashes of X- and gamma-
ray radiation in magnetars?• What is the star formation history of the Universe?
Science goals of the gravitational wave field
Cambridge, Jan 2010
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“Indirect” detection from orbital decay of binary pulsar: Hulse & Taylor
PSR 1913+16
Evidence for gravitational waves
Cambridge, Jan 2010
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How can we detect them?Gravitational wave amplitude h ~
LL∆
L + ∆L
LSensing the induced excitations of a large bar is one way to measure this
Field originated with J. Weber looking for the effect of strainsin space on aluminium bars at room temperature
Claim of coincident events between detectors at Argonne Lab and Maryland – subsequently shown to be false
Cambridge, Jan 2010
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laserBeam splitter
Mirror
Observer
It’s all done with mirrors…
Michelson Interferometer
Cambridge, Jan 2010
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laser
CONSTRUCTIVE(BRIGHT)
+
DESTRUCTIVE(DARK)
+
path 2
path
1
Michelson Interferometer
It’s all done with mirrors…
Cambridge, Jan 2010
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Detecting gravitational waves
GW produces quadrupolar distortion of a ring of test particles
h =2∆L
LDimensionless strain Expect movements of
less than 10-18 m over 4km
Cambridge, Jan 2010
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Principal limitations to sensitivity – ground based detectors
Photon shot noise (improves with increasing laser power) and
radiation pressure (becomes worse with increasing laser power)There is an optimum light power which gives the same limitation expected by application of the Heisenberg Uncertainty Principle –the ‘Standard Quantum limit’
Seismic noise (relatively easy to isolate against – use suspended test masses)
Gravitational gradient noise, - particularly important at frequencies below ~10 Hz
Thermal noise – (Brownian motion of test masses and suspensions)
• All point to long arm lengths being desirableLIGO 4km; Virgo 3km; GEO 600m, TAMA 300m
Cambridge, Jan 2010
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Ground based Detector Network – audio frequency range
GEO600TAMA, CLIO
LIGO Livingston
LIGO Hanford
4 km2 km
600 m300 m100 m
P. Shawhan, LIGO-G0900080-v1
4 km
VIRGO 3 kmLIGO Livingston
Cambridge, Jan 2010
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Sources – the gravitational wave spectrum
Gravity gradient wall
ADVANCED GROUND - BASED DETECTORS
Cambridge, Jan 2010
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• After first studies in 1980s, M3 proposal for 4 S/C ESA/NASA collaborative mission
in 1993
• LISA selected as ESA Cornerstone in 1995
• 3 S/C NASA/ESA LISA appears in 1997
• Baseline concept unchanged ever since!
LISA – a joint ESA/NASA Mission to study Black hole physics, and much more, in the frequency range 10-4 Hz -10-1 Hz
LISA: Laser Interferometric Space Antenna
Cambridge, Jan 2010
28Pulsar Timing: nano-Hz search for stochastic background and super-massive black hole coalescences
Australia
Multi-country
Full data sharingNorth America
Courtesy G. Hobbs
Cambridge, Jan 2010
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• Gravitational waves distort spacetime as they propagate.
• A periodic gravitational wave passing across the line of sight to a pulsar will produce a periodic variation in the time of arrival (TOA) of pulses.
If the strain along the line-of-sight is h, then the fractional change in the pulse arrival rate due to the gravitational wave just depends on the strain at emission and reception.
Pulsar timing arrays as a probe of GWs
Cambridge, Jan 2010
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Real progress in GW astronomy over past few years
Operation of six ground based interferometers (in addition to three cryogenic bar detectors)
Advances in waveform predictions from Numerical Relativity
Significant advances in Space Borne Detectors – LISA and DECIGO
Pulsar Timing coming to the fore
Importance of Multi-messengerAstronomy
Using wider interest in relativity, cosmology and fundamental physics to bring science to schools and the public.
Cambridge, Jan 2010
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Current status 2: the advent of GW astronomy
Initial Science Runs Complete (LIGO, Virgo, GEO 600, TAMA)
Upper Limits set on a range of sources (no detections as yet)
Credit: AEI, CCT, LSU
Coalescing Binary Systems• Neutron stars, low mass black holes, and NS/BS systems
Credit: Chandra X-ray Observatory
‘Bursts’• galactic asymmetric core collapse supernovae• cosmic strings• ???
NASA/WMAP Science Team
Cosmic GW background
• stochastic, incoherent background
• unlikely to detect, but can bound in the 10-10000 Hz range Casey Reed, Penn State
Continuous Sources• Spinning neutron stars
• probe crustal deformations, ‘quarki-ness’
Cambridge, Jan 2010
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• Remnant from supernova in year 1054
• Spin frequency νEM = 29.8 Hz
νgw = 2 νEM = 59.6 Hz
• observed luminosity of the Crab nebula
accounts for < 1/2 spin down power
• spin down due to:
• electromagnetic braking
• particle acceleration
• GW emission?
• LIGO S5 result: h < 3.9 x 10-25 GW amplitude ~ 4X below spin down limit• Upper limit on the ellipticity: ε < 2.1 x 10-4
• GW energy upper limit < 6% of radiated energy is in GWs
Example: The Crab Pulsar – Beating the Spin Down Limit
Abbott, et al., “Beating the spin-down limit on gravitational wave emission from the Crab pulsar,” Ap. J. Lett. 683, L45-L49, (2008).
Cambridge, Jan 2010
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M31The Andromeda Galaxy
by Matthew T. RussellDate Taken:
10/22/2005 - 11/2/2005
Location:Black Forest, CO
Equipment:RCOS 16" Ritchey-Chretien
Bisque Paramoune MEAstroDon Series I Filters
SBIG STL-11000Mhttp://gallery.rcopticalsystems.com/gallery/m31.jpg
Refs:GCN: http://gcn.gsfc.nasa.gov/gcn3/6103.gcn3
X-ray emission curves (IPN)
Example: GRB070201, Not a Binary Merger in M31
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Example: GRB070201, Not a Binary Merger in M31
Inspiral (matched filter search:
Binary merger in M31 scenario excluded at >99% levelExclusion of merger at larger distances 90%
75%
50%
25%
Inspiral Exclusion Zone
99%
Abbott, et al. “Implications for the Origin of GRB 070201 from LIGO Observations”, Ap. J., 681:1419–1430 (2008).
Burst search:Cannot exclude an SGR in M31
SGR in M31 is the current best explanation for this emission
Upper limit: 8x1050 ergs (4x10-4 MŸc2) (emitted within 100 ms for isotropic emission of energy in GW at M31 distance)
(1<m1<3 Msun)
D. Reitze
Cambridge, Jan 2010
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Example: The Stochastic GW Background, Beating BBN
An isotropic stochastic GW background could come from:Primordial universe (inflation)Incoherent sum of point emitters isotropically distributed over the sky
Energy density:
Log-frequency spectrum:
Strain spectral density:
Published S5/VSR1 result, 95% C.L. limit:
UL consistent with no GW stochastic background (null result)
><= αβαβπ
ρ hhG
cGW
&&32
2
Ω0, LIGO < 6.9 x 10-6
)(ln1)(
fddf GW
critGW
ρρ
=Ω
32
20 )(
103)(
ffHfS GWΩ
=π
Nature, August 20th 2009
Cambridge, Jan 2010
D. Reitze
37
Current status 3: coming attractions!
Enhancements to LIGO and Virgo at end of commissioningaimed at a factor of two improvement in sensitivity
meanwhile GEO, LIGO and cryogenic bar detectors have maintained ‘astrowatch’
New science runs recently started (July 7th 2009)
2nd generation detectorsAdvanced LIGO fully funded (10 to 15 x improved sensitivity, operational ~2014)
Advanced Virgo close to approval
GEO-HF conversion starting
For Comparison: Neutron Star Binaries:Initial LIGO (S5): ~15 Mpc → rate ~1/50yrAdv LIGO: ~ 200 Mpc → rate ~ 40/year
Black Hole Binaries (Less Certain):Initial LIGO (S5): ~100 Mpc → rate ~1/100yrAdv LIGO: ~ 1 Gpc → rate ~ 20/year
D. Reitze
Cambridge, Jan 2010
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Need a network of detectors for good source location and improve overall sensitivity
Second Generation NetworkAdvanced LIGO/Advanced Virgo/Geo-HF/LCGT/AIGO
LCGT under review (proposed cryo, underground interferometer in Kamioka mine)
AIGO plans progressing (proposed interferometer in Western Australia)
Future developments – on the ground
Cambridge, Jan 2010
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Networking
Sky coverage at >50% maximum sensitivity
L/H+L/L L/H+L/L+V L/H+L/L+V+LCGT
LIGO – Hanford & Livingston LIGO – Hanford & Livingston+ Virgo
LIGO – Hanford & Livingston
+ Virgo + LCGT
Bernard Schutz, AEI
Cambridge, Jan 2010
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Third Generation Network — Incorporating Low Frequency Detectors
Third-generation underground facilities are aimed at having excellent sensitivity from ~1 Hz to ~104 Hz.
This will greatly expand the new frontier of gravitational wave astrophysics.
Recently begun:
Three year-long European design study, with EU funding, underway for a 3rd-generation gravitational wave facility, the Einstein Telescope (ET).
Goal: 100 times better sensitivity than first generation instruments.
Future developments – on the ground
Cambridge, Jan 2010
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Third Generation Network — Incorporating Low Frequency Detectors
Third-generation underground facilities are aimed at having excellent sensitivity from ~1 Hz to ~104 Hz.
This will greatly expand the new frontier of gravitational wave astrophysics.
Recently begun:
Three year-long European design study, with EU funding, underway for a 3rd-generation gravitational wave facility, the Einstein Telescope (ET).
Goal: 100 times better sensitivity than first generation instruments.
Future developments – on the ground
Cambridge, Jan 2010
42
Third Generation Network — Incorporating Low Frequency Detectors
Future developments – on the ground
Cambridge, Jan 2010
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LISA (Laser Interferometer Space Antenna)
10-4 Hz – 10-1 Hz Our first priority for a space based mission
Mission Description– 3 spacecraft in Earth-trailing solar orbit,
separated by 5 x106 km.
– Gravitational waves are detected by measuring change in proper distance between fiducial masses in each spacecraft using laser interferometry
– Partnership between NASA and ESA
– Launch date: soon after 2020?...
Future developments – in space
Cambridge, Jan 2010
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LISA (Laser Interferometer Space Antenna)
10-4 Hz – 10-1 Hz Our first priority for a space based mission
Mission Description– 3 spacecraft in Earth-trailing solar orbit,
separated by 5 x106 km.
– Gravitational waves are detected by measuring change in proper distance between fiducial masses in each spacecraft using laser interferometry
– Partnership between NASA and ESA
– Launch date: soon after 2020?...
Future developments – in space
Cambridge, Jan 2010
45
LISA (Laser Interferometer Space Antenna)
10-4 Hz – 10-1 Hz Our first priority for a space based mission
LISA : A Universe Full of Strong Gravitational Wave Sources
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Cosmic backgrounds, superstring bursts?
Massive Black Hole Binary (BHB) inspiral and merger (10s‐100s)
Ultra‐compact binaries(thousands)
Extreme Mass Ratio Inspiral (EMRI) (hundreds)
K. Danzmann
Future developments – in space
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State of the Universe: September 2009
Some key questions for cosmology:
• What is driving the cosmic acceleration?
• Why is 96% of the Universe ‘strange’ matter and energy?
• Is dark energy = Λ ?
• How, and when, did galaxies evolve?
• Big bang + inflation + gravity = LSS?
What rôle could gravitational waves play in answering these questions?
Cambridge, Jan 2010
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Much recent interest in ‘Standard Sirens’:
e.g. SMBHs at cosmological distances, for which DL can in principle be determined to exquisite accuracy.
Inspiral waveform strongly dependent on SMBH masses.
Since amplitude falls off linearly with (luminosity) distance, measured strain determines the distance of the source to high precision.
Holz and Hughes 2005
Long tail due to parameter degeneracies
Gravitational Wave Sources as Cosmological Probes
Cambridge, Jan 2010
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What could we do with standard sirens?
• Completely independent, gravitational, calibration of the distance scale and the Hubble parameter
• Useful adjunct to existing constraints from CMBR, BAO,subject to completely different systematic errors.
• High precision probe of
• Extension of beyond the reach of SNIe and BAO.
Gravitational Wave Sources as Cosmological Probes
)(zH
)(zw
Are these goals realistic?...
Cambridge, Jan 2010
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Currently three major issues:
• Identification of E-M counterpart
• Impact of weak lensing
• Predicting merger event rates
Gravitational Wave Sources as Cosmological Probes
Cambridge, Jan 2010
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Identifying an E-M counterpart:
• GWs are redshifted, just like E-M radiation.Hence we determine (very precisely)
• If our goal is to probe e.g. how varies withwe can assume and break the
degeneracy. (See e.g. Hughes 02, Sesana et al. 07, 08)
)1( z+
zDL −z
• If we want to use sirens to measure , we must observe the E-M counterpart.
For this we need an accurate sky position!
zDL −
Gravitational Wave Sources as Cosmological Probes
Cambridge, Jan 2010
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So what exactly can we do with sirens?....
Adapted from Holz & Hughes (2005)
Gravitational Wave Sources as Cosmological Probes
Cambridge, Jan 2010
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So what exactly can we do with sirens?....
Gravitational Wave Sources as Cosmological Probes
Adapted from Holz & Hughes (2005)
Cambridge, Jan 2010
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GW sources will be (de-)magnified by weak lensing due to LSS.
Same treatment as for SN
[ See e.g. Misner, Thorne & Wheeler; Varvella et al (2004), Takahashi (2006) ].
However, WL has muchgreater impact for sirens,because of their muchsmaller intrinsic scatter.
Weak lensing may also limit identification of E-M counterpart
Gravitational Wave Sources as Cosmological Probes
Cambridge, Jan 2010
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But what will the counterpart signatures be, and when would we see them? Much detailed modelling required. See e.g. Kocsis et al (2008)
• Periodic variations in flux during inspiral phase, correlated with variations in potential (c.f. OJ287)
• Viscous dissipation of GW energy released duringcoalescence
• Shocks induced by sudden mass loss during final GW burst
• Shocks induced by a supersonic GW recoil ‘kick’
• Infall of gas onto SMBH merged remnant
Before GW peak
hours / days
days / weeks
months / years
years
Gravitational Wave Sources as Cosmological Probes
Cambridge, Jan 2010
55
But what will the counterpart signatures be, and when would we see them? Much detailed modelling required. See e.g. Kocsis et al (2008)
• Periodic variations in flux during inspiral phase, correlated with variations in potential (c.f. OJ287)
• Viscous dissipation of GW energy released duringcoalescence
• Shocks induced by sudden mass loss during final GW burst
• Shocks induced by a supersonic GW recoil ‘kick’
• Infall of gas onto SMBH merged remnant
Before GW peak
hours / days
days / weeks
months / years
years
Gravitational Wave Sources as Cosmological Probes
Strong argument for multimessenger approach
Cambridge, Jan 2010
56
Dalal et al. (2006):Short-duration GRBS, due to NS-NS mergers, will also be observed by ALIGO network.
What could be done from the ground?
First optical observation of a NS-NS merger?
GRB 080503 (Perley et al 2008)
Cambridge, Jan 2010
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Dalal et al. (2006):Short-duration GRBS, due to NS-NS mergers, will also be observed by ALIGO network.
Beaming of GRBs (blue curves), aligned with GW emission, could boost GW SNR.
All-sky monitoring of GRBs + 1 year operation of ALIGO network
⇒ H0 to ~2% ?
What could be done from the ground?
Cambridge, Jan 2010
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Nissanke et al. (2009):Very thorough treatment.
Considers impact of:
• siren true distance;
• no. of detectors in network;
Identifies strong degeneracy between distance and inclination.
Need E-M observations / beaming assumption to break this?
to 10 – 30% at 600 Mpc (NS-NS); 1400 Mpc (NS-BH).
Competitive with traditional ‘distance ladder’; probe of peculiar velocities?
What can be done from the ground?
LD
Cambridge, Jan 2010
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Fit , ,
Looking ahead to the Einstein Telescope…
Sathyaprakash et al. (2009):
~106 NS-NS mergers observed by ET. Assume that E-M counterparts observed for ~1000 GRBs, 0 < z < 2.
Weak lensing De-lensed
Competitive with ‘traditional’methods
Cambridge, Jan 2010
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Cutler and Holz (2009):
~3 x 105 sirens observed, with unique E-M counterparts, for 0 < z < 5.
…And even further ahead to BBO…
BBO schematic
Extremely good angular resolution, even at z = 5!
Robust E-M identification of host galaxy, for determining redshift
Cambridge, Jan 2010
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Cutler and Holz (2009):
~3 x 105 sirens observed, with unique E-M counterparts, for 0 < z < 5.
…And even further ahead to BBO…
Simulated Hubble diagram, including effects of lensing
Cambridge, Jan 2010
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…And even further ahead to BBO…
Hubble constant to ~0.1%
w0 to ~1%, wa to ~10%
Cambridge, Jan 2010
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…And even further ahead to BBO…
Hubble constant to ~0.1%
w0 to ~1%, wa to ~10%All of this lies far ahead, but the key is to work on
development of the science case now
Cambridge, Jan 2010