To Catch a Wave – The Hunt for
Gravitational Radiation with LIGO
Keith Riles
University of Michigan
REU seminar
June 24, 2013
Outline

Nature & Generation of Gravitational Waves

Detecting Gravitational Waves with the LIGO Detector

Data Runs and Results to Date

Looking Ahead – Advanced LIGO
2
Nature of Gravitational Waves

Gravitational Waves = “Ripples in space-time”

Perturbation propagation similar to light (obeys same wave equation!)
 Propagation speed = c
 Two transverse polarizations - quadrupolar:
+ and x
Example:
Ring of test masses
responding to wave
propagating along z

Amplitude parameterized by (tiny)
dimensionless strain h: ΔL ~ h(t) x L
3
Why look for Gravitational Radiation?

Because it’s there! (presumably)

Test General Relativity:
 Quadrupolar radiation? Travels at speed of light?
 Unique probe of strong-field gravity

Gain different view of Universe:
 Sources cannot be obscured by dust
 Detectable sources some of the most interesting,
least understood in the Universe
 Opens up entirely new non-electromagnetic spectrum
4
What will the sky look like?
Has BICEP-2 seen fossil GWs?5
Generation of Gravitational Waves

Radiation generated by quadrupolar mass movements:
(with Imn = quadrupole tensor, r = source distance)

Example: Pair of 1.4 Msolar neutron stars in circular orbit of radius 20 km
(imminent coalescence) at orbital frequency 400 Hz gives 800 Hz
radiation of amplitude:
6
Generation of Gravitational Waves

Strong indirect evidence for GW generation:
Taylor-Hulse Pulsar System (PSR1913+16)
Two neutron stars (one=pulsar)
in elliptical 8-hour orbit
Measured periastron advance
quadratic in time in agreement with
absolute GR prediction
 Orbit decay due to GW energy loss
17 / sec


~ 8 hr
7
Generation of Gravitational Waves
Can we detect this radiation directly?
NO - freq too low
Must wait ~300 My for
characteristic “chirp”:
8
What makes Gravitational Waves?
•
“chirps”
Compact binary inspiral:
– NS-NS waveforms are well described
– Recent progress on BH-BH waveforms
•
“bursts”
Supernovae / GRBs:
– burst signals in coincidence with signals in electromagnetic
radiation / neutrinos
– all-sky untriggered searches too
•
Pulsars in our galaxy:
“periodic”
– search for observed neutron stars
– all-sky search (computing challenge)
•
Cosmological Signals
9
“stochastic background”
Generation of Gravitational Waves
Most promising periodic source: Rotating Neutron Stars (e.g., pulsar)
But axisymmetric object rotating about symmetry axis
Generates NO radiation
Need an asymmetry or perturbation:

Equatorial ellipticity (e.g., – mm-high “mountain”):
h α εequat

Poloidal ellipticity (natural) + wobble angle (precessing star):
h α εpol x Θwobble
(precession due to different L and Ω axes)
10
Periodic Sources
Serious technical difficulty: Doppler frequency shifts
 Frequency modulation from earth’s rotation (v/c ~ 10-6)
 Frequency modulation from earth’s orbital motion (v/c ~ 10-4)
Additional, related complications:
 Daily amplitude modulation of antenna pattern
 Spin-down of source
 Orbital motion of sources in binary systems
Modulations / drifts complicate analysis enormously:
 Simple Fourier transform inadequate
 Every sky direction requires different demodulation
 All-sky survey at full sensitivity = Formidable challenge
11
Periodic Sources of GW
But two substantial benefits from modulations:
 Reality of signal confirmed by need for corrections
 Corrections give precise direction of source

Difficult to detect spinning neutron stars!

But search is nonetheless intriguing:
 Unknown number of electromagnetically quiet, undiscovered
neutron stars in our galactic neighborhood
 Realistic values for ε unknown
 A nearby source could be buried in the data, waiting for just the
right algorithm to tease it into view
12
Outline

Nature & Generation of Gravitational Waves

Detecting Gravitational Waves with the LIGO Detector

Data Runs and Results to Date

Preparing for Advanced LIGO
13
Gravitational Wave Detection

Suspended Interferometers (IFO’s)
Top
view
 Suspended mirrors in “free-fall”
 Michelson IFO is
“natural” GW detector
 Broad-band response
(~50 Hz to few kHz)
  Waveform information
(e.g., chirp reconstruction)
14
The Global Interferometer Network
The three (two) LIGO, Virgo and GEO interferometers are part of a Global Network.
Multiple signal detections will increase detection confidence and provide better
precision on source locations and wave polarizations
V1
L1
H1, H2
LIGO
G1
GEO
K1
Virgo
KAGRA
15
LIGO – India (approved)
LIGO Observatories
Hanford
Observation of nearly
simultaneous signals 3000 km
apart rules out terrestrial artifacts
Livingston
16
LIGO Detector Facilities
•Stainless-steel tubes
(1.24 m diameter, ~10-8 torr)
•Gate valves for optics isolation
•Protected by concrete enclosure
Vacuum System
17
LIGO Detector Facilities
LASER


Infrared (1064 nm, 10-W) Nd-YAG laser from Lightwave (now commercial product!)
Elaborate intensity & frequency stabilization system, including feedback from
main interferometer
Optics



Fused silica (high-Q, low-absorption, 1 nm surface rms, 25-cm diameter)
Suspended by single steel wire
Actuation of alignment / position via magnets & coils
18
LIGO Detector Facilities
Seismic Isolation


Multi-stage (mass & springs) optical table support gives 106 suppression
Pendulum suspension gives additional 1 / f 2 suppression above ~1 Hz
102
100
10-2
10-6
10-4
Horizontal
10-6
10-8
Vertical
10-10
19
What Limits the Sensitivity
of the Interferometers?
•
Seismic noise & vibration
limit at low frequencies
•
Atomic vibrations (Thermal
Noise) inside components
limit at mid frequencies
•
Quantum nature of light (Shot
Noise) limits at high
frequencies
•
Myriad details of the lasers,
electronics, etc., can make
problems above these levels
achieved
Best design sensitivity:
~ 3 x 10-23 Hz-1/2 @ 150 Hz
< 2 x 10-23
20
The road to design sensitivity at Hanford…
21
Harder road at Livingston…
Livingston Observatory
located in pine forest popular
with pulp wood cutters
Spiky noise (e.g. falling trees) in
1-3 Hz band creates dynamic
range problem for arm cavity
control

Solution:
40% livetime
Retrofit with active feed-forward isolation system
(using technology developed for Advanced LIGO)
 Fixed
22
LIGO Scientific Collaboration
University of Michigan
University of Minnesota
The University of Mississippi
Massachusetts Inst. of Technology
Monash University
Montana State University
Moscow State University
National Astronomical
Observatory of Japan
Northwestern University
University of Oregon
Pennsylvania State University
Rochester Inst. of Technology
Rutherford Appleton Lab
University of Rochester
San Jose State University
Univ. of Sannio at Benevento,
and Univ. of Salerno
University of Sheffield
University of Southampton
Southeastern Louisiana Univ.
Southern Univ. and A&M College
Stanford University
University of Strathclyde
Syracuse University
Univ. of Texas at Austin
Univ. of Texas at Brownsville
Trinity University
Universitat de les Illes Balears
Univ. of Massachusetts Amherst
University of Western Australia
Univ. of Wisconsin-Milwaukee
Washington State University
University of Washington

Australian Consortium
for Interferometric
Gravitational Astronomy
The Univ. of Adelaide
Andrews University
The Australian National Univ.
The University of Birmingham
California Inst. of Technology
Cardiff University
Carleton College
Charles Sturt Univ.
Columbia University
Embry Riddle Aeronautical Univ.
Eötvös Loránd University
University of Florida
German/British Collaboration for
the Detection of Gravitational Waves
University of Glasgow
Goddard Space Flight Center
Leibniz Universität Hannover
Hobart & William Smith Colleges
Inst. of Applied Physics of the
Russian Academy of Sciences
Polish Academy of Sciences
India Inter-University Centre
for Astronomy and Astrophysics
Louisiana State University
Louisiana Tech University
Loyola University New Orleans
University of Maryland
Max Planck Institute for
Gravitational Physics

23
Michigan LIGO Group Members
Old fogeys:
Dick Gustafson, Keith Riles
Graduate students:
Santiago Caride, Grant Meadors, Jaclyn Sanders
Undergraduates / high school students:
Weigang Liu, Daniel Mantica, Pranav Rao, Curtis Rau / David Groden
Graduated Ph.D. students:
*Continued GW research
after graduating
Dave Chin (now medical physicist)
Vladimir Dergachev* (now postdoc at Caltech)
Evan Goetz* (now postdoc at Albert Einstein Institute – Hanover, Germany)
Former undergraduates:
Jamie Rollins* (Caltech postdoc)
Alistair Hayden (Boston U.)
Joseph Marsano (Chicago postdoc)
Michael La Marca (Arizona State G.S.)
Jake Slutsky* (A.E.I. postdoc)
Phil Szepietowski (U. Virginia postdoc)
Tim Bodiya* (MIT G.S.)
Courtney Jarman (Wisconsin G.S.)
Ramon Armen (industry)
24
Alex Nitz* (Syracuse G.S.)
Michigan Group – Main Efforts
Search for Periodic Sources (rotating neutron stars)
Riles, Caride, Meadors, Sanders, Liu, Mantica, Rao
Detector Characterization (instrumentation, software)
Riles, Gustafson, Caride, Meadors, Liu, Groden
Commissioning & Noise Reduction
Gustafson, Meadors, Sanders (when in residence at Hanford)
Controls System Development
Gustafson
Public Outreach
Riles, Meadors, Rau
25
LIGO Scientific Collaboration
University of Michigan
University of Minnesota
The University of Mississippi
Massachusetts Inst. of Technology
Monash University
Montana State University
Moscow State University
National Astronomical
Observatory of Japan
Northwestern University
University of Oregon
Pennsylvania State University
Rochester Inst. of Technology
Rutherford Appleton Lab
University of Rochester
San Jose State University
Univ. of Sannio at Benevento,
and Univ. of Salerno
University of Sheffield
University of Southampton
Southeastern Louisiana Univ.
Southern Univ. and A&M College
Stanford University
University of Strathclyde
Syracuse University
Univ. of Texas at Austin
Univ. of Texas at Brownsville
Trinity University
Universitat de les Illes Balears
Univ. of Massachusetts Amherst
University of Western Australia
Univ. of Wisconsin-Milwaukee
Washington State University
University of Washington

Australian Consortium
for Interferometric
Gravitational Astronomy
The Univ. of Adelaide
Andrews University
The Australian National Univ.
The University of Birmingham
California Inst. of Technology
Cardiff University
Carleton College
Charles Sturt Univ.
Columbia University
Embry Riddle Aeronautical Univ.
Eötvös Loránd University
University of Florida
German/British Collaboration for
the Detection of Gravitational Waves
University of Glasgow
Goddard Space Flight Center
Leibniz Universität Hannover
Hobart & William Smith Colleges
Inst. of Applied Physics of the
Russian Academy of Sciences
Polish Academy of Sciences
India Inter-University Centre
for Astronomy and Astrophysics
Louisiana State University
Louisiana Tech University
Loyola University New Orleans
University of Maryland
Max Planck Institute for
Gravitational Physics

26
GEO600
Work closely with the GEO600 Experiment (Germany / UK / Spain)
• Arrange coincidence data runs when commissioning schedules permit
• GEO members are full members of the LIGO Scientific Collaboration
• Data exchange and strong collaboration in analysis now routine
• Major partners in proposed Advanced LIGO upgrade
600-meter Michelson Interferometer
just outside Hannover, Germany
27
Virgo
Have begun collaborating with Virgo colleagues (Italy/France)
Took data in coincidence for parts of last two science runs
Data exchange and joint analysis
Will coordinate closely on detector upgrades and future data taking
3-km Michelson
Interferometer just
outside Pisa, Italy
28
Outline

Nature & Generation of Gravitational Waves

Detecting Gravitational Waves with the LIGO Detector

Data Runs and (small sampling of ) Results to Date

Looking Ahead – Advanced LIGO
29
Data Runs
Have carried out a series of Engineering Runs (E1–E14) and Science
Runs (S1—S6) interspersed with commissioning & upgrades
S1 run:
17 days (Aug / Sept 2002) – Rough but good practice
S2 run:
59 days (Feb—April 2003) – Many good results
S3 run:
70 days (Oct 2003 – Jan 2004) -- Ragged
S4 run:
30 days (Feb—March 2005) – Another good run
S5 run:
23 months (Nov 2005 – Sept 2007) – Great!
S6 run:
“16” months (Jul 2009 – Oct 2010) – Better sensitivity but uneven
30
S1  S5 Sensitivities
hrms = 3 10-22
31
“Enhanced LIGO” (July 2009 – Oct 2010)
Displacement
spectral noise
density
Factor of 2 improvement above
300 Hz
S5
S6
32
Searching for Gravity Waves
Short-Lived
Known
waveform
Long-Lived
Binary Inspirals
Continuous waves
(NS-NS, NS-BH, BH-BH)
Spinning black-hole /
high-mass inspirals
(Spinning NS)
SGR ringdowns
Bursts
Unknown
waveform
Toda
y
(Supernovae, “mergers”)
Young pulsars
(glitchy)
Stochastic background
(Cosmological, astrophysical)
33
Searching for continuous waves
Model
Chandra image
Crab Pulsar
Bayesian
PDF
Use coherent, 9-month, time-domain matched filter
Strain amplitude h0
Upper limits on GW strain amplitude h0
Single-template, uniform prior: 3.4 × 10–25
Single-template, restricted prior: 2.7 × 10–25
Multi-template, uniform prior: 1.7 × 10–24
Multi-template,
restricted prior: 1.3 × 10–24
34
Implies that GW
emission accounts
for ≤ 4% of total
spin-down power
Ap. J. Lett 683 (2008) 45
Searching for continuous waves
Same algorithm applied to 195 known pulsars over
LIGO S5/S6 and Virgo VSR2/VSR4 data
Lowest upper limit on
strain:
h0 < 2.1 × 10−26
Lowest upper limit on
ellipticity:
ε < 6.7 × 10-8
Crab limit at 1% of total
energy loss
Vela limit at 10% of total
energy
loss
35
arXiv:1309.4027 (Sept 2013)
Searching for continuous waves
Linearly
polarized
Circularly polarized
All-sky search
for unknown
isolated neutron
stars
Semi-coherent,
stacks of 30-minute,
demodulated power
spectra
(“PowerFlux”)
Phys. Rev. Lett. 102 (2009) 111102
Carried out by Michigan
graduate student Vladimir
Dergachev (now at Caltech)
36
Recent results
Latest full-S5 all-sky results
Semi-coherent, stacks of
30-minute, demodulated power
spectra (“PowerFlux”)
Astrophysical reach
Phys. Rev. D85 (2012) 022001
37
Searching for continuous waves
First all-sky search for unknown binary CW sources
Uses TwoSpect* algorithm:
Sample spectrogram
(30-minute FFTs) for
simulated strong signal
(Earth’s motion already
demodulated)
Result of Fourier
transforming each
row of spectrogram
 Concentrates
power in orbital
harmonics
38
*E. Goetz & K. Riles, CQG 28 (2011) 215006
Initial search uses 30-minute FFTs
 Favors longer orbital periods:
Mod. Depth (Hz)

Searching for continuous waves
Period (hr) 
Search is severely
computationally bound
Upper limits based on summing
power in harmonics
Templates used only in follow-up
39
Not so limited in directed
searches, e.g., for Scorpius X-1
http://www.einsteinathome.org/







GEO-600 Hannover
LIGO Hanford
LIGO Livingston
Current search point
Current search
coordinates
Known pulsars
Known supernovae
remnants
Your
computer
can help
too!
40
Outline

Nature & Generation of Gravitational Waves

Detecting Gravitational Waves with the LIGO Detector

Data Runs and Results to Date

Looking Ahead – Advanced LIGO
41
Looking Ahead
Both LIGO and Virgo underwent significant upgrades since first joint
science run (S5/VSR1):
Initial LIGO  “Enhanced LIGO”
Initial Virgo  “Virgo +”
LIGO schedule: S6 data run July 2009 – October 2010
Began Advanced LIGO installation October 2010
 Aim for first data run fall (summer?) 2015
Virgo schedule: VSR2/3 data runs July 2009 – October 2010
Virgo+ upgrade ongoing
VSR4 data run – Summer 2011
Began Advanced Virgo installation fall 2011
 On schedule for 2016 data run
42
Advanced LIGO
Increased laser power:
10 W  200 W
Improved shot noise (high freq)
Higher-Q test mass:
Fused silica with better optical coatings
Lower internal thermal noise in band
Increased test mass:
10 kg  40 kg
Compensates increased radiation pressure noise
43
Advanced LIGO
New suspensions:
Single  Quadruple pendulum
Lower suspensions thermal noise in
bandwidth
Improved seismic isolation:
Passive  Active
Lowers seismic “wall” to ~10 Hz
44
Advanced LIGO
Neutron Star Binaries:
Average range ~ 200 Mpc
Most likely rate ~ 40/year
The science from the first
3 hours of Advanced LIGO
should be comparable to
1 year of initial LIGO
(Range x ~10  Volume x ~1000)
But that sensitivity will
not be achieved
instantly…
45
arXiv: 1304.0670
Summary
Bottom line:
No GW signal detected yet 
But
• Not all S5-6 VSR1-4 searches completed
• Advanced LIGO / Virgo will bring major sensitivity improvements
with orders of magnitude increase in expected event rates
46
Extra Slides
47
LIGO Interferometer Optical Scheme
Michelson interferometer
With Fabry-Perot arm cavities
end test mass
•Recycling mirror matches losses,
enhances effective power by ~ 50x
4 km Fabry-Perot cavity
recycling
mirror
150 W
LASER/MC
20000 W
6W
(~0.5W)
48
Generation of Gravitational Waves
Coalescence rate estimates based on two methods:
 Use known NS/NS binaries in our galaxy (three!)
 A priori calculation from stellar and binary system evolution
 Large uncertainties!
For initial LIGO design “seeing distance” (~15 Mpc):
Expect 1/(70 y) to 1/(4 y)
 Will need Advanced LIGO to ensure detection
49
Generation of Gravitational Waves
Super-novae
(requires asymmetry in explosions)
Examples of SN
waveforms
May not know exactly what
to look for – must be openminded with diverse
algorithms
Tony Mezzacappa -- Oak Ridge National Laboratory50
Major Interferometers world-wide
LIGO
Livingston, Louisiana &
Hanford, Washington
VIRGO
Near Pisa, Italy
GEO
Near Hannover, Germany
TAMA
Tokyo, Japan
51
2 x 4000-m
(1 x 2000-m)
Completed 2-year data
run at design sensitivity –
“enhanced” – running
again
1 x 3000-m
Took ~4 months
coincident data with
LIGO – near design
sensitivity - running
1 x 600-m
Took data during L-V
downtime, undergoing
upgrade
1 x 300-m
Used for R&D aimed at
future underground
detector
Search for binary systems
John Rowe, CSIRO
Use calculated templates for inspiral phase (“chirp”) with optimal
filtering.
Search for systems with different masses:
 Binary neutron stars (~1-3 solar masses):
~15 sec templates, 1400 Hz end freq
 Binary black holes (< ~30 solar masses):
shorter templates, lower end freq
 Primordial black holes (<1 solar mass):
longer templates, higher end freq
52
Searching for binaries
John Rowe, CSIRO



Use two or more detectors: search for double or triple coincident “triggers”
Can infer masses and “effective” distance.
Estimate inverse false alarm probability of resulting candidates: detection?
Triple
Double
Double
Blue – Coincident
Gray – Time lag
S5 Year 1 Search for “Low-Mass” Inspirals
54
Searching for binaries


John Rowe, CSIRO
No evidence of excess
Use detection efficiency and surveyed galaxies
 Set upper limit vs stellar mass
Phys. Rev. D 79 (2009) 122001
BH-BH
L10 = 1010 × blue solar luminosity
Milky Way = 1.7 L10
NS-BH
55
Searching for bursts

GRB 070201
Short, hard gamma-ray burst
 A leading model for short
GRBs: binary merger involving
a
neutron star
IPN
3-sigma error region from
56
Mazets et al., ApJ 680, 545

Position (from IPN) consistent
with being in M31 (Andromeda)

LIGO H1 and H2 were operating

Result from (several) LIGO
searches:
No plausible GW signal found;
therefore very unlikely to be
from a binary merger in M31
Ap. J. 681 (2008) 1419
Searching for bursts (untriggered)
Search for double or triple coincident triggers (three algorithms)
 Check waveform consistency among interferometers – apply vetoes
 Set a threshold for detection for low false alarm probability
 Evaluate efficiency for variety of simple waveforms

Parametrize strength in terms of “root sum square of h” : hRSS
Sampling of efficiency curves:
 hRSS 
2

  (| h (t ) |2  | h (t ) |2 )dt

hRSS
57
S5 Year 1 Search for Untriggered Bursts
Searching for bursts (untriggered)
Detected triggers and
expected background for
one algorithm (Coherent
WaveBurst – wavelet-based)
for triple-coincident triggers
with fcentral > 200 Hz
No candidates found above threshold
in any of the searches
 Set upper limits on rate vs hRSS
Threshold
58
Coherent
network amplitude
arXiv:0905.0020 (May 2009)
Searching for a stochastic
background
NASA, WMAP




A primordial isotropic GW stochastic background is predicted by
most cosmological theories.
Given an energy density spectrum Wgw(f), there is a strain power
spectrum:
The signal can be searched from cross-correlations in different pairs
of detectors: L1-H1 and H1-H2.
The59farther the detectors, the lower the frequencies that can be
searched.
Searching for a stochastic
background
Early-S5 H1-L1 Bayesian 90% UL:
Ω90% = 6.9 × 10-6 (42-169 Hz)
Nature 460
(2009) 990
60
NASA, WMAP
Other S5 Searches (released)
Search for Gravitational Wave Bursts from Soft Gamma
Repeaters
Phys Rev Lett 101 (2008) 211102
Search for High Frequency Gravitational Wave Bursts in the First
Calendar Year of LIGO's Fifth Science Run
arXiv:0904.4910
Stacked Search for Gravitational Waves from the 2006 SGR
1900+14 Storm
arXiv:0905.0005
Search for Gravitational Waves from Low Mass Compact Binary
Coalescence in 186 Days of LIGO's fifth Science Run
arXiv:0905.3710
61
Other S5 (S6) Searches Underway (planned)
Inspirals:
High-mass, spinning black holes
Year 2, joint LIGO-Virgo
Ringdowns
GRBs
Bursts:
Year 2, Joint LIGO-Virgo
GRBs
Continuous wave:
Full-S5 all-sky searches (semi-coherent, Einstein@Home)
Directed searches (Cassiopeia A, globular clusters, galactic center, SN1987A)
“Transient CW” sources
All-sky binary – Evan Goetz Ph.D. dissertation – this year
Stochastic:
Full-S5 isotropic – imminent
Directed (anisotropic)
H1-H2
High-frequency (37 kHz – LIGO arm free spectral range)
62