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PotentialGravitational Applications of Grid

• B.S. Sathyaprakash
• GridLab conference, 31 Mar-1 April, Eger, Hungary

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Modern Astronomy
Optical, radio, x- and gamma-ray telescopes have
revealed a lot of new objects and phenomena

• Cosmic micro-wave background and big bang

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Astronomy has taught us that more than 90 of the
Universe is dark
But ...
Even this dark matter interacts gravitationally
we should be able to see this matter via
gravitational radiation it might emit
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Plan of the talk

• Gravitational waves
• brief overview of gravitational waves
• astronomical sources
• interferometric detector projects around the
  world
• Gravitational wave data analysis and Grid
• large data sets
• big collaborations
• huge data base records

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Gravitational Waves - A simple and brief overview
of the theory
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Newtons law of Gravity

• The force of gravity between two masses m and M
  separated by a distance r is
• F G m M / r2
• Newtons law of gravity transmits force
  instantaneously - if body M changes its position
  it is felt by instantaneously by body m
• If Newtons gravity is right we will be able to
  build a gravitational telegraph which can
  transmit signals instantaneously - a violation of
  Einsteins special relativity

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Ripples in the Fabric of Spacetime

• Gravitational disturbances too travel at a finite
  speed - indeed the same speed as light. This is
  what we call gravitational waves
• According to Einstein gravity is nothing but
  warping of spacetime
• Therefore, gravitational waves are ripples in
  space-time warping that propagates at the speed
  of light.

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Do Gravitational Waves Exist? Inspiral in
Hulse-Taylor binary pulsar

• Two neutron stars in orbit
• Each has mass 1.4 times the mass of the Sun
  Orbital period 7.5 Hrs
• stars are whirling around each other at a
  thousandth the speed of light

• According to Einsteins theory the binary should
  emit GW
• GW carry rotational energy from the system which
  causes the two stars to spiral towards each other
  and a decrease in the period
• Observed period change is about 10 micro seconds
  per year
• This decrease in period is exactly as predicted
  by Einsteins theory

Eventually the binary will coalesce emitting a
burst of GW that will be observable
using instruments that are currently being built
But that will take another 100 million years
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Stellar mass GW sources - observable from ground
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GW Sources observable from space

• Merging super-massive black holes in galactic
  centers

• Signals from gravitational capture of small black
  holes by super-massive black holes

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Observing the origin of the Universe
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Gravitational Wave Detectors
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Interaction of Gravitational Waves
Plus polarization
Cross polarization
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Laser Interferometric Detectors Basic Principle
of Operation
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Searching for Gravitational WavesHow Grid
Technology Can Help
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A list of the problems

• Computationally limited searches - bigger
  computers means better science
• Hundreds of collaborators requiring to access
  data from a network of detectors distributed
  round the world
• Events are rare but data is poor with large false
  alarm rates - need to examine subsidiary channels
  of information
• A large number of database records - making sense
  out of garbage

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What are we up against?Large Data Rates

• environmental background
• seismic disturbances
• solar flares and magnetic storms, cosmic rays,
  ...
• instrumental noise
• electronic noise in feedback systems, laser
  frequency and intensity noise, thermal
  fluctuations in mirrors, vibration of suspension
  systems, ...

• Important to understand detectors before any
  analysis begins
• a large number of channels are collected to
  record detector state - any analysis should look
  at all this data
• Interferometers collect data at rates of order 10
  Mbytes per second, 24/7 300 Tbytes per year
• We want to be able analyse at least part of that
  data

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Distributed data

• Interferometer projects work collaboratively -
  all data is accessible everyone in the
  collaboration wherever in the world they may be
• How do we make all this data available to the
  community?
• data replication to multiple sites - GriPhyN,
  Triana
• guaranteeing data integrity
• data discovery tools and P2P data access

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Types of gravitational wave signals

• Transients - last for a short duration so that
  detector motion can be neglected
• Transients with known shape, e.g. black hole
  binaries
• Transients with unknown shape, e.g. supernovae
• Stochastic backgrounds
• population of astronomical sources
• primordial stochastic gravitational wave signals

• Continuous waves - last for a duration long
  enough so that detector motion cannot be
  neglected
• Typically very weak amplitude, signal power a
  billion times smaller than noise power
• long integration times needed
• slowly changing frequency depending on several
  parameters

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Near all-sky sensitivity

• All sky sensitivity
• Quadrupolar antenna pattern
• multiple detectors to determine direction to
  source
• Wide band sensitivity
• 1 kHz around 100 Hz

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Why GW data analysis challenging?

• Signals with known shapes but unknown parameters
• large parameter space
• for example, 10 parameters in black hole binary
  search
• great number of wave cycles to integrate
• for example, 1010 wave cycles in a year from a
  neutron star
• Signals of unknown shape
• uncertain and inaccurate, physical models
• for example waves from supernovae and black hole
  collisions
• Very weak signal strengths
• long integration times
• for example up to a year for neutron star signals
• a lot of pixels on the sky due to Doppler
  modulation
• Implies the need for large computational resources

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Compute-intensive searches - An example

• Searching for black hole binaries that last for
  about a few seconds in the detector band
• A pattern matching technique is employed since
  the signal shape is known, but ...
• signal parameters are not known before hand
• must filter the data through a large number of
  templates corresponding to different parameters
• a search in a 10-dimensional space

• Triana is currently implementing this search on a
  compute cluster to be extended using Grids
• issues - distributed data, on-line search, load
  balancing
• data serial search is preferred due to
  astrophysical reasons

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Knowledge discovery

• Not all problems are computational resource
  intensive - some can be handled computationally,
  for example short bursts of unknown shape as in
  supernovae, but produce huge data bases
• millions of records inserted into the database
  each day
• must go back to the original data set to veto out
  false alarms (that is, spurious non-GW events
  produced by instrumental and environmental
  background)
• need an automatic bridge between analysis
  pipeline and database
• Database query functionality built into Triana ...

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Two searches that urgently require grid technology

• Searching for black hole binaries
• large parameter space
• masses, spins, orientations,
• need to go back to numerical simulations that
  produced the templates and to refine the search
• need to analyse thousands of subsidiary channels
  to confirm or veto out events

• All sky search for neutron stars
• week signals warranting integration of large data
  sets
• Doppler modulation in the signal caused by the
  motion of the detector means billions of pixels
  in the sky
• currently the search is restricted to targeted
  known sources

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Scientific rewards from GW observations
Quantum theory
Fundamental physics
Extreme Gravity
Gravitational Wave Observations
(Very) Early Universe
Astrophysics
Solar, stellar interiors
Cosmology