Detecting gravitational waves with LISA Detecting gravitational waves with LISA
”Opening a new window on the universe”
Outline
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LISA mission - introduction
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Gravitational waves - properties and sources
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LISA mission - concept and techniques
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Other detectors (VIRGO, LIGO)
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Future
L aser I nterferometer S pace A ntenna
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Direct detection of (low-
frequency) gravitational waves
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Laser interferometry -
a giant space-based Michelson interferometer
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Frequency range: 10
-4– 1 Hz
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ESA & NASA joint mission
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Mission status: ”Phase A”
(formulation phase)
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Expected to launch in 2015 (?)
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Pathfinder launches in 2009 (?)
Scientific goals
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Explore powerful but invisible objects in our universe
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Discover thousands of exotic binary stars in our galaxy
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Understand the nature of space, time and gravity by precisely mapping the
warped space-time around giant black holes
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Probe the early universe –
the cosmic background of
Gravitational waves
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Oscillations in the fabric of space-time itself
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Predicted from Einstein's theory of general relativity
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No full theory of quantum gravity does yet exist
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Convincing indirect evidence for their existence
(study of binary pulsar, Hulse & Taylor, 1974)
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First direct detection using:
LISA, LIGO, VIRGO, GEO600
Gravitational waves II
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First-order expansion: g
μν= η
μν+ h
μν●
Wave-equation: h
μν= 0
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GWs propagate with the speed of light
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GWs are of quadrupole type with two independent polarization
modes, with their amplitudes labeled h
+and h
Χ, respectively
Gravitational waves III
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Sources of GWs should have a time-varying quadrupole moment (a spherically symmetric source does not radiate!)
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Order-of-magnitude estimate:
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Milky way sources: h ~ 10
-17Extragalactic sources: h ~ 10
-20Tiny amplitudes!
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The main problem in observing GWs is that the relative length change is exceedingly small
h ~G ¨Q/ d ~4 G E
kin/d
Gravitational waves IV
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Binary systems:
Lose energy as a result of the emission of gravitational radiation
→ spiral into one another (distance decreases, angular frequency
increases)
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Supernovae:
A spherically non-symmetric collapse
→intense (but brief) source
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Giant black holes (galactic centre)
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Fluctuations from the early universe
Detecting GWs with LISA - basic concept
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Three identical spacecraft - three independent Michelson interferometers
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Detect GWs by measuring the changes in distances between freely floating test masses
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If the ”arms” are not of the same length, the light waves will not be in phase
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Core technologies:
Laser interferometry
Gravitational reference sensors
Micronewton thrusters
Orbit
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Spacecraft arranged in an equilateral triangle of side length 5 million km
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Trails 20 degrees behind the Earth's orbit of the Sun
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Angled at 60 degrees to the ecliptic plane and rotates as it
orbits the Sun
Why these choices?
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Orbit: trade-off between communication distance and avoiding Earth's varying gravitational field. Also, good temperature at that distance. Orbital inclination assures stability of the formation
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Arm length: 5 million km is a compromise. Shorter arms lose the
low frequencies, whereas longer arms lose the high. Bottom of
the sensitivity curve only depends on the received power
Differences from Michelson interferometry
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Test masses are freely floating within the spacecraft (not suspended with wires as in a ground-based interferometer)
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LISA doesn't use beam splitters:
The two laser beams from the main spacecraft is transmitted to the other two spacecraft, which act as the end mirrors
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Because of the large distances between the spacecraft, rather
than reflecting the received beams back to the main spacecraft,
the secondaries transmit new laser beams (in phase)
Challenges
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The biggest challenge is disturbances that mask GWs: solar wind buffeting, solar
radiation pressure, spacecraft drift,
interference from interplanetary magnetic fields, test mass charging etc
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These have to be eliminated or damped down to an extremely low level
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Disturbance Reduction System (DRS):
Gravitational Reference Sensor (GRS) + Field Effect Electric Propulsion (FEEP) thrusters
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Each spacecraft is kept centered about its
test mass (so-called ”drag-free” operation)
Space- vs ground-based
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In space: avoid seismic noise at low frequencies and use longer interferometer arms
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Different frequency ranges
Instrument
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Short structural cylinder with a diameter of 1.8 m and a mass of 203 kg
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Each spacecraft carries two free-flying proof
masses with associated sensors, two identical
telescopes and two optical benches, all housed in an inner, Y-shaped tube
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Each telescope is a 30 cm
f/1 Cassegrain, used both
to transmit and receive
Design considerations
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The spacecraft and payload require careful layout and thermal design
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Sensitive payload components must be separated from noisier spacecraft systems
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Very few movable elements
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Power dissipation is kept constant
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Materials and construction details are selected to thermally isolate the inertial sensor and optical components
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Thermal shields cover the top and bottom of the cylinder to
prevent sunlight from striking the payload
Optical bench
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The optical bench contains the main optics: the laser beam injection, detection, beam shaping optics and the
gravitational reference sensor
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Thermally isolated from the
outside by a two-stage thermal filter made of ultra-low
expansion glass (ULE)
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Most components are passive
Gravitational reference sensor
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The inertial sensor consists of a free-falling proof mass inside a
reference housing, which is fixed to the spacecraft.
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The relative motion of the proof mass is monitored by capacitive sensors.
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Feedback loop to propulsion
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The sensor surrounds the proof
mass with electrode plates that
facilitate both capacitive position
Proof masses
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2 kg Pt-Au alloy cubical 4 cm proof masses
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Highly polished to enable them to reflect laser light
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Shielded from all external and internal disturbances so that they detect only the force of gravity
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Electrostatic actuation perpendicular to the measurement axes
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Charge control by UV illumination
Ultrastable laser
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The light source is a 1 W diode- pumped 1064 nm Nd:YAG solid state laser, frequency-stabilized to an onboard ULE reference cavity and also to the inter-spacecraft arms
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A few mW is split off the 1 W main beam to serve as the local
reference for the measurement of the phase of the incoming beam
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Reliability > 5 years
Micronewton thrusters
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The spacecraft positions must be precisely controlled to follow the motion of the proof masses
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The noise-reduction system will detect their movement relative to the spacecraft and activate the Field Effect Electric
Propulsion thrusters
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Liquid cesium or indium from small reservoirs is ionized and accelerated electrically to efficiently provide thrust that can be very finely controlled by adjustment of the acceleration voltage
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Six thrusters,
3 - 40 μN each
Global network of ground observatories
VIRGO
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French-Italian ground-based
facility near Pisa, Italy, with 3 km long arms
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Frequency range: 10 – 6 000 Hz
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20 W Nd:YAG high power ultrastable laser
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Fabry-Perot cavity
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High reflectivity mirrors:
extremely low diffusion and absorption
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Recycling mirror
VIRGO
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Ultra high vacuum technology: The path of the light beam propagating between mirrors has to be
evacuated down to the extremely low pressure of 10 mbar.
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The seismic isolation: A chain of suspended
seismic filters made of triangular cantilever blade springs provide the vertical isolation while the
compound pendulum provides isolation against horizontal motions.
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The chain is attached to an actively stabilized
platform. Mirror and recoil mass are suspended by
extremely fine wires to a ”marioneta” at the end of
LIGO
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LIGO – Laser Interferometer Gravitational-wave Observatory
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Two observatories: Livingston
& Hanford, separated by 3200 km
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4 km Fabry-Perot arms
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Nov 2005: sensitivity has reached the primary design specification (10
-21)
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Similar to Virgo, but Virgo is built on soft sediment to better insulate the detector from seismic vibrations, and uses a
different, complex damping system to increase the detectable
frequency range of the observatory.
Future
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Present detectors may detect the first GW, but it is clear that realizing the full advantage of this new observing window will necessitate a substantial improvement of their sensitivity
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Already plans in USA and Japan for constructing second
generation detectors. Advanced LIGO (LIGO 2), if built, would improve the sensitivity by more than a factor of 10
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Discussions for a second generation detector in Europe
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Work at liquid helium temperature, have larger optics possibly made of new materials, use a very high power laser, and
possibly be located in deep underground to avoid cosmic and
environmental perturbations.
References
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LISA:
http://lisa.jpl.nasa.gov/
http://sci.esa.int/home/lisa
http://www.srl.caltech.edu/lisa/
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VIRGO:
http://www.ego-gw.it/virgodescription/
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LIGO:
http://www.ligo.caltech.edu/
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