Gravitational
Astronomy
The New FrontierB.S. Sathyaprakash
Gravitational Physics Group, School of Physics and Astronomy
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Gravitational Physics Users of ARCCA
Leonid Grishchuk
Stephen Fairhurst
Patrick Sutton
B Sathyaprakash
Bernard Schutz
Deepak Baskaran
James Clark
Alex Dietz
Gareth Jones
Craig Robinson
Van Den Broeck
Wen Zhao
David McKechan
Devanka Pathak
Ian Harry
Edmund Schluessel
Jack Yu
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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(t) = G m M / r(t)2
Newtons law of gravity
transmits force
instantaneously - if the
mass M changes its
position, it is felt by body
m instantaneously
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Newtons law of Gravity
When Earth reaches the
point A let us remove the
Sun from its position
At that very instant Earth
would no more feel the
force of Sun
Newtons law of inertia
states Earth would take
the path tangent to the
orbit at A
However, Sun will be
seen by Earthlings for
about 5 minutes as the
Earth zips across
A
..
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Einsteins Special Theory of Relativity
The speed of light is the
same for all
Woman on the ground
measures the same speed
for the light from the torch
held by the girl speeding on
a rocket
One famous consequence
of this is E = mc2
Another consequence is
the twin-paradox
The one we are interested
is nothing can travel
faster than light
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Newtons Gravity Vs Relativity
At the turn of the last century Einstein faced the
contradiction between Newtons theory of gravity and his
new special theory of relativity
According to Newton, Gravity acts instantaneously
Relativity, a very successful theory supported by
Maxwells theory of light, does not allow any signal to
propagate faster than light
Either abandon special relativity or find a new theory of
gravity
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Einsteins Gravity
According to Einstein,
gravity is not a force but a
warping of space & time
Space is said to be warped
if familiar laws of
geometry in flat space do
not hold
Time is warped if clocks at
different points in space
dont run at the same rate
Someone living close to a
black hole would age more
slowly relative to their
twin sister on Earth
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Einsteins Gravity
Earth Moves around sun
not because of there is
a force of gravity
Earth moves in
straightest possible
paths in the curved
geometry of space
caused by the Sun
Gravitational force,
just as all other forces,
is transmitted at a
finite speed
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Einsteins Gravity
Again, when the Earth reaches
point A let us remove the Sun
from its position
According to Einstein Earth
would continue to move in its
orbit around the Sun for about
5 minutes after the Sun has
been removed
It takes a path that is tangent
to orbit at B
Gravity travels at a finite
speed: implies wavelike
phenomenon must be
associated with gravity
A
..
B
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Einsteins Gravity & Gravitational Waves
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What are Gravitational Waves?
A natural consequence of
Einsteins gravity and special
theory of relativity
Ripples on space-time curvature
travelling at the speed of light
Anything that accelerates
produces gravitational waves
A falling cannon ball, an
astronomical binary system, an
exploding star
We know gravitational waves exist
Decaying orbit of the double
binary pulsar for which Hulse and
Taylor were awarded the Nobel
Prize in 1992
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Tidal gravitational forces
Gravitational effect of
a distant source can
only be felt through
its tidal forces
Gravitational waves
are traveling, time-
dependent tidal
forces.
Tidal forces scale with
size, typically produce
elliptical
deformations.
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Gravitational Waves & Tidal Forces
Cross polarizationPlus polarization
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Evidence for gravitational waves
In 1974 Hulse and Taylor observed
the first binary pulsar
Two neutron stars
Orbital period ~ 7.5 Hrs
Stars whirling around at a thousandth
the speed of light
Einsteins gravity says the binary
should emit gravitational
radiation
Causes the two stars to spiral in
towards each other
Observed decrease in period - about
10 s per year - is exactly as predicted by Einsteins theory
Eventually the two stars will coalesce, but that will take another 100 million years
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A world network of
Interferometric Gravitational
Wave Detectors
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Gravitational Wave Detectors
About 1 billion US $ investment worldwide
STFC has invested about US $ 50
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American LIGO at HanfordAmerican LIGO at LivingstoneBritish-German GEO
French Italian VIRGO near PISALaser Interferometer Space Antenna
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The LIGO Scientific Collaboration
Comprises ~ 30 institutions and > 400 people
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Recent Results from GW Searches
The largest mountain on the
Crab pulsar (or radius 10 km) is
no more than 10 cm
Primordial gravitational waves
did not limit primordial
synthesis of heavy elements
(helium, deuterium, etc.)
A recent (Feb 1, 2007) gamma-
ray burst that occurred in
Andromeda was a new
phenomena that needs further
explanation
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Gravitational waves: A New Window
Was Einstein right?
Is the nature of gravitational radiation as predicted by Einstein
or is it something different?
Are black holes hairless?
Are there naked singularities?
How did the black holes at galactic nuclei form?
The centre of our galaxy has a million solar-mass black hole?
How and when did it form? Galactic cannibalism via slow
accretion of smaller black holes, gas and other stars?
Fundamental questions about our existence
What were the physical conditions at the big bang?
What is the nature of quantum gravity, origin of space and
time?
How many spatial dimensions are there, only 3?
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Most of the Universe is Dark
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Astronomical Sources
Compact binary mergersBinary neutron stars
Binary black holes
Black hole-neutron star binaries
Gravitational wave burstsBlack hole collisions
Supernovae
gamma-ray bursts
Continuous waves Rapidly spinning neutron stars or other objects
Stochastic backgroundPrimordial background
Astrophysical background
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Slide by: P Shellard
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Why are GW searches challenging?
All sky sensitivity
Any one detector is sensitive to
a significant portion of the sky
Large data rates
Tens of terabytes per year from
a worldwide detector network
Wide band sensitivity
Different types of signals all
present in the same data set
Beam Pattern Functions of an American and a European Detector
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Computational Cost of our Searches
Current searches are limited by computational resources
Can only search for non-spinning black hole binaries
A month of search takes 60,000 CPU hours
Bigger computers afford better searches
Future SearchesSearch for black holes with spins
Would require at least 10-100 times more computational cost
ARCCA Cluster Can Help Detect Gravitational Waves
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Targeting the biggest discovery of
our times through ARCCAHere are some signals from colliding black holes as predicted by Einsteins theory
Black hole spins modulate the waveform
We use matched filtering to search for signals buried in noise
Pattern matching algorithm
But matched filters, i.e. templates used in the search, depend on many parameters
A search in 17-dimensional space involving the masses, spins of the stars, position on the sky, etc.
About 100 million shapes must be searched for in each piece of data
A
m
p
l
i
t
u
d
e
Time
Increasin
g Spin
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What do we need?
Compute cycles: a lot of themOur algorithms are embarrassingly parallel
Literally no communication between different compute nodes
Data Access:Each detector produces about 30 TBytes of data per year and will run for 2 to 3 years at a time
About 10% of this should be available for compute nodes
Multiple nodes (100s of them) might access data at any one time
Centralized data archives are a bottleneck and a nightmare in our game
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A typical pipeline: Searching for colliding
black holes
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All sky search for spinning neutron stars
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Gravitational Astronomy
(Very) Early Universe
GW observations with
ARCCA
Stellar interiorsCosmology
Quantum theory
Astrophysics
Fundamental physics Extreme Gravity
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