Laser Interferometer Gravitational-Wave
Observatory (LIGO)!
A Brief Overview!
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Sharon Brunett!
California Institute of Technology!
Pacific Research Platform Workshop!
October 15, 2015!
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LIGO G1101200-v2
Credit: AEI, CCT, LSU
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What is LIGO?
LIGO is the world’s leading facility for conducting
gravitational-wave science!
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LIGO Livingston Observatory
LIGO Hanford Observatory
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LIGO is discovery science. It will open the field of gravitationalwave astronomy through the direct detection of gravitational
waves from compact sources and conduct a long term
astrophysical observing program. !
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LIGO Organization
LIGO Laboratory
‘LIGO’ = LIGO Laboratory + LIGO Scientific Collaboration (LSC)!
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LIGO Laboratory, jointly managed by Caltech and MIT, is
responsible for operating LIGO Hanford and Livingston
Observatories under a cooperative agreement from the NSF!
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MIT
Caltech
~200 staff members!
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LIGO science conducted through the LIGO Scientific Collaboration !
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International collaboration of ~1000 members at 80+ institutions located
in 16 countries !
The LSC is the LIGO ‘User Community’ and includes LIGO
Laboratory staff (scientists, engineers, and technicians) !
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Advanced Ground-based GW Network
The LSC collaborates with other large gravitational-wave
collaborations as part of a global network – essential for multimessenger gravitational-wave astronomy !
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The LSC has a full data sharing agreement with the Virgo Collaboration!
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The LSC and Virgo signed an intent to share data with KAGRA !
LIGO
Hanford
2015
LIGO
Livingston
2015
KAGRA
2018
LIGO
India
2022
Virgo
2016
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Measuring Gravitational-waves
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Gravitational waves are propagating dynamic
fluctuations in the curvature of space-time!
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Physically manifested as strains!
Emitted from accelerating mass distributions, unimpeded by
matter; need astrophysical sources to generate detectable strains !
Travel at the speed of light (according to general relativity)!
GW interferometers use enhanced Michelson
interferometry to detect the strains!
Passing GWs dynamically modulate (‘stretch’ and
‘compress’) the distance between the end test
mass and the beam splitter!
The interferometer acts as a transducer, turning
GWs into photocurrent !
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h( f ) =
ΔL( f )
L
Vacuum
A coherent detector à signal is proportional to amplitude of
GW!
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LIGO Instruments
LIGO recently completed an
upgrade to Advanced LIGO
detectors that are designed to
be a factor of 10x more
sensitive than Initial LIGO!
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Transient events that would
have been seen once per
decade with Initial LIGO will,
therefore, be detected once
every few days.!
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Initial LIGO
Image courtesy of Beverly Berger
Cluster map by Richard Powell
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Fundamental Questions that
LIGO Observations can Answer
Is general relativity the correct theory of gravity?
What is the nature of one of the four fundamental forces?
What happens when two black holes collide?
Do black holes really have no hair?
What are the progenitors of short gamma ray bursts?
What is the engine that powers them?
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Fundamental Questions that
LIGO Observations can Answer
How does core collapse power a supernova?
Is there a mass gap between neutron stars and black holes?
What is the maximum mass of a neutron star?
What is the nuclear equation of state at very high densities?
Do neutron star mergers power kilonovae?
What is the origin of r-process elements (gold, platinum, ...)?
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Observational Targets for LIGO: Energetic and
Violent Compact Astrophysical Events
Gravitational Wave ‘Bursts’
Coalescing
Compact Binary
Systems: Neutron
Star-NS, Black
Hole-NS, BH-BH
- Strong emitters,
well-modeled,
template-based
searches
Credit: Chandra X-ray Observatory
- transient
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not well-modeled,
excess power
searches
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Galactic core collapse
supernovae, cosmic
strings, soft gamma
repeaters, pulsar
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transient
Credit: AEI, CCT, LSU
Spinning neutron stars
and pulsars
Stochastic Gravitationalwave Background
- (effectively) monotonic
waveform
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stochastic background
from incoherent
ensemble of point
emitters, (& primordial
universe)
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Long duration
- Long duration
NASA/WMAP Science Team
Casey Reed, Penn State
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LIGO Computing Model
Analysis methods and search algorithms are specifically tailored and
tuned to each source class. All can be efficiently decomposed into
“embarrassingly parallel’” tasks, using three basic classes of
computing matched to science goals:!
●  Dedicated LIGO Laboratory resources for detector characterization
and astrophysical searches that need low-latency results to meet
their science goals !
●  Dedicated LIGO Scientific Collaboration and national/international
shared resources, e.g., XSEDE, for production offline (high
latency) searches and search development!
●  Einstein@Home community computing for offline searches with
low data-to-processing ratios that might otherwise be prohibitively
expensive and for which very high latency of scientific results is
acceptable !
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LIGO Computing Model
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Maintain sufficient flexibility – gravitational-wave physics is still in a
discovery phase and the first gravitational-wave signals detected may be
different than expected!!
Different astrophysical and detector characterization analyses rely on
different methods and algorithms – versatility comes from running on
heterogeneous compute platforms.!
Increase the portability of existing data analysis pipelines so that they can
take advantage of shared resources in addition to dedicated LIGO Data
Grid resources.!
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LIGO Modes of Operation
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Production: analysis during science observing runs!
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Simulation: simulations needed to measure the sensitivity to detections!
GW follow-up: resources needed to measure significance of strong
gravitational wave signals!
Development: development of improved and optimized codes!
Some analyses require large amounts of data, others only small slices from
reduced data sets. !
Low latency compute demands are mainly from two source classes:
compact binary coalesences (CBC) and continuous waves (CW) ~30
Million Service Units (MSU) for 2015-2016 compute needs. !
Totals for all compute classes in years 2015/2016/2017 – 61/194/390
MSUs!
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»  1 SU = 1 core hr of execution time on reference Intel Xeon E5-2670 !
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LIGO Computing Latencies
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LIGO Computing Model – Follow the Flow
of aLIGO data
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Calibrate aLIGO data (LIGO Lab)!
Aggregate data from multiple geographic locations (LIGO Lab)!
Run and interpret data-quality pipelines to generate summary information
(LIGO Scientific Collaboration – LSC)!
Run Detection and parameter estimation pipelines (LSC)!
Run large-scale simulations required by the scientific interpretation of the
data (LSC)!
Deliver validated alerts of transient GW candidates within minutes of data
acquisition (LSC)!
Archive data and results (LIGO Lab)!
Deliver validated catalogs of GW sources, data quality, and artifacts in the
GW data stream (LSC)!
Distribute data to the broader research community (LIGO Lab)!
Improve efficiency and performance of scientific analysis (LIGO Lab and
LSC)!
Workflow and job management via HT Condor
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LIGO Computing Model
Other Facilities
LIGO Laboratory Facilities
Tier-1-Caltech
Months
Data Buffer
Data Buffer
Alerts!
In
Alerts!
Out
Low-latency!
Pipelines
GW
Catalog
Candidate!
Database
Triggers
PE/Validation
Pipelines
RDS
Triggers
Legend
Control & Diagnostic System
Data Files (Stream, Trigger, etc)
Data Quality!
Databases
Data Quality!
Pipelines
SFT!
Pipeline
SFT
Calibration!
Pipeline
Strain V2+
Processing Pipelines
Stream
Data
Public Facing Data & Services
Other
Catalogs
GW
Catalog
Data Pathway
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Deep Search
Pipelines
PE/Validation
Pipelines
Triggers
CW Search!
Pipelines
PE/Validation
Pipelines
Triggers
LIGO Open Science Center
Databases
Data Transfer Service
Storage: Strain, RDS, SFT, Triggers
Weeks
RDS!
Pipeline
Strain V1
Main Data Archive: All Data
Days
Trend
Data Quality!
Pipelines
Site
Archive
Hours
Raw
Data Buffers
Minutes
Operator/Scientists: Operations and Engineering
Seconds
CDS: DAQ
Calibration!
Pipeline
XSEDE, Tier-2 LDG &!
Einstein@Home
From Virgo and/or
other detectors
Scientists: Key Science
Projects,!
Publications & Data Releases
Candidate!
Database
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Deep Search that relies primarily on GW data
From Virgo and/or
other detectors
Time-critical Joint GW-EM Observing
Tier-1-Observatories
Responses To Misc Questions
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Data generated by interferometers (quantity 2): 1.6M files/yr, 800 TB/yr (after
compression) !
Data generated by search groups (in TB/year):!
»  Burst – 254!
»  Compact Binary Coalescences – 1167!
»  Continuous Wave - 493 !
»  Stochastic GW background – 25!
»  Detector Characterization – 223 !
Archive storage via SAM-QFS (tape plus in-demand files on disk)!
A subset of the tools used to manipulate data: !
»  Custom LIGO Data Replicator – used by offline codes - detects files, publishes
metadata, transfers files, tracks progress, etc.!
»  Gridftp!
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