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 is discovery science. It will open the field of gravitational-wave astronomy through the direct detection of gravitational
waves from compact sources and conduct a long term astrophysical observing program. !
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LIGO Livingston Observatory
LIGO Hanford Observatory
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LIGO Organization
‘LIGO’ = LIGO Laboratory + LIGO Scientific Collaboration (LSC)!!● LIGO Laboratory, jointly managed by Caltech and MIT, is
responsible for operating LIGO Hanford and Livingston Observatories under a cooperative agreement from the NSF!
» ~200 staff members!!
● LIGO science conducted through the LIGO Scientific Collaboration !» 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) !
!● The LSC collaborates with other large gravitational-wave
collaborations as part of a global network – essential for multi-messenger gravitational-wave astronomy !
» The LSC has a full data sharing agreement with the Virgo Collaboration!» The LSC and Virgo signed an intent to share data with KAGRA !
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Caltech
MIT
LIGO Laboratory
LIGO Hanford 2015
LIGO Livingston 2015
LIGO India 2022
Virgo 2016
KAGRA 2018
Advanced Ground-based GW Network
Measuring Gravitational-waves
● Gravitational waves are propagating dynamic fluctuations in the curvature of space-time!
» 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 !
» A coherent detector à signal is proportional to amplitude of GW!
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h( f ) = ΔL( f )L
Vacuum
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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!!Transient events that would have been seen once per decade with Initial LIGO will, therefore, be detected once every few days.!!!!! Image courtesy of Beverly Berger
Cluster map by Richard Powell
Initial LIGO
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
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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
Casey Reed, Penn State
Credit: AEI, CCT, LSU
Coalescing Compact Binary Systems: Neutron Star-NS, Black Hole-NS, BH-BH
- Strong emitters, well-modeled, template-based searches
- transient Credit: Chandra X-ray Observatory
Gravitational Wave ‘Bursts’
- not well-modeled, excess power searches
- Galactic core collapse supernovae, cosmic strings, soft gamma repeaters, pulsar
- transient
NASA/WMAP Science Team
Stochastic Gravitational-wave Background - stochastic background
from incoherent ensemble of point emitters, (& primordial universe)
- Long duration
Spinning neutron stars and pulsars - (effectively) monotonic waveform - Long duration
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
!● 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
● Production: analysis during science observing runs!
● 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!
» 1 SU = 1 core hr of execution time on reference Intel Xeon E5-2670 !
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LIGO Computing Model – Follow the Flow of aLIGO data
!● 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
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Triggers
Triggers
Triggers
Triggers
CDS:
DAQ
Calibration!Pipeline
Data Quality!Pipelines
Deep Search Pipelines
Strain V1
PE/Validation Pipelines
Alerts!Out
Data
Buf
fers
RDS!Pipeline RDS
Raw
SFT!Pipeline SFT
Data Quality!Databases
Trend
Data Quality!Pipelines
Alerts!In
Low-latency!Pipelines
Candidate!Database
PE/Validation Pipelines
Calibration!Pipeline Strain V2+
Mai
n Da
ta A
rchi
ve: A
ll Da
ta
LIGO Open Science Center
Data
Buf
fer
CW Search!Pipelines
Tier-1-Observatories Tier-1-Caltech XSEDE, Tier-2 LDG &!Einstein@Home
Other Catalogs
Stream Data
GW Catalog
Minutes
Hours
Days
Weeks
GW Catalog
Candidate!Database
Scientists: Key Science Projects,!
Publications & Data Releases
Months
Ope
rato
r/Sci
entis
ts: O
pera
tions
and
Eng
inee
ring
Seconds
Data
Buf
fer
Site
Ar
chiv
e
Stor
age:
Stra
in, R
DS, S
FT, T
rigge
rs
PE/Validation Pipelines
Control & Diagnostic System
Legend
Processing Pipelines
Data Files (Stream, Trigger, etc)
Databases
Public Facing Data & Services
Data PathwayData Transfer Service From Virgo and/or
other detectors
Tim
e-cr
itica
l Joi
nt G
W-E
M O
bser
ving
Deep
Sea
rch
that
relie
s pr
imar
ily o
n G
W d
ata
From Virgo and/or other detectors
LIGO Laboratory Facilities Other Facilities
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Responses To Misc Questions
!● 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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