VIRGO is an international collaboration for the detection of

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The Mirror suspension the super attenuator
VIRGO is an international collaboration for the
detection of
Gravitational Waves

• ITALY - INFN
• Firenze
• Frascati
• Napoli
• Perugia
• Pisa
• Roma
• Urbino

• FRANCE - CNRS
• Paris
• Lyon
• Orsay
• Annecy
• Nice

General relativity describes gravity as a
manifestation of the curvature of space-time.
This description has been tested in the solar
system, where gravity is weak and changes slowly
due to the orbital motions of planets and their
satellites. Einstein's theory predicts the
existence of gravitational waves, that is,
perturbations of the gravitational field, which,
as it is for electromagnetic field, spread out
through space at the speed of light. Right from
their source, these waves radiate, like ripples
on the surface of a pond. Spreading out, the
waves decrease very slightly when interacting
with matter, thus, unlike electromagnetic
radiations, they are not stopped either by the
stars, nor by interstellar matter.
VIRGO layout
The Virgo project consists mainly in a Michelson
laser interferometer made of two orthogonal arms
being each 3 kilometers long. Multiple
reflections between mirrors located at the
extremities of each arm extend the effective
optical length of each arm up to 120 kilometers.
Virgo will be located at Cascina, near Pisa on
the Arno plain. The frequency range of Virgo
extends from 10 to 6,000 Hz. In order to reach
the extreme sensitivity required, the whole
interferometer should attain optical perfection
and be completely isolated from the rest of the
world in order to be only sensitive to the
gravitational waves. To achieve it, Italian and
French scientists involved in the project, are
developing most advanced techniques in the field
of high power ultrastable lasers, high
reflectivity mirrors, seismic isolation and
position and alignment control.
Aerial view of VIRGO at Cascina (Tuscany in Italy)

• VIRGO will permit scientists to test this theory
  for rapidly changing, dynamical gravity and also
  for the extremely strong, dynamical gravity of
  two black holes as they collide.
• VIRGO has the possibility to
• Verify directly general relativity's prediction
  that gravitational waves exist and propagate
• - at the same speed as light , thus the graviton
  has zero rest mass.
• the forces the waves exert on matter are
  perpendicular to the waves' direction of travel,
  and stretch matter along one perpendicular
  direction while squeezing it along the other
• thereby, test general relativity's prediction
  that the graviton has twice the rate of spin as
  the photon.
• Moreover it can firmly verify that black holes
  exist, and test general relativity's predictions
  for the violently pulsating space-time curvature
  accompanying the collision of two black holes.
  This will be the most stringent test ever of
  Einstein's general relativity theory.

• Interferometer Control and
• Data production
• Fully digital control robust on long periods
• 20 kHz main signal acquisition
• Data flow 4 Mbyte/s

The beam splitter in place view from the bottom
Central interferometer results

• Best sensitivity10-16 m/Hz1/2 _at_ 1 kHz
• 10-13 m/Hz1/2 _at_ 10 Hz
• 5 orders of magnitude gained in 1 year
• Some of the main achievements
• Interferometer Control and reliability of VIRGO
  superattenuator
• Locking force partition over two points
  (hyerarchical control)
• Locking of the recycled CITF (lock acq. time
  sec. to min.)
• Automatic alignment
• Local control noise reduction
• Stable ISYSCITF operation
• Limiting noise identified over a large part of
  the spectrum
• Sources of lock losses are identified
• Integration of all subsystems achieved

Duty cycles 98 85 98 98 73
The 3 km vacuum of the North arm
The Virgo Central area

• VIRGO status
• - The vacuum system completed and within
  specifications
• - The mirrors are in place
• - VIRGO 3 km commissioning is starting in mid
  2003 and it will benefit of the experimental
  results already obtained in the Central
  interferometer commissioning

Sensitivity improvements of the central
Interferometer of VIRGO
Sensitivity goal of VIRGO
The super attenuator
Beam splitter paylaod assembly
Beam light arriving at the North tube end
Front view of the beam splitter