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Scientific Report 2007-2009
Particle physics
P30. Experimental Search of Gravitational Waves
Gravitational waves (GW) are space-time ripples such
that the distance between free masses will alternately decrease
and increase during their transit out of phase in
two perpendicular directions. For astrophysical events
such as a supernova explosion at the galactic center,
the GW amplitude, the dimensionless strain parameter
h,could range between 10 −19 - 10 −21 . The observation
of gravitational waves will complement the observation
of electromagnetic waves and astro-particles (as cosmic
rays and neutrinos). It will reveal aspects of the Universe
not reachable by these means and will extend the
observable domain even in the cosmic regions darkened
by dust and masked by other phenomena.
With one large interferometer VIRGO (located near
Pisa) and two cryogenic resonant antennas, EXPLORER
installed at CERN and NAUTILUS at the INFN laboratory
of Frascati, the G23 group is at the forefront of
research on gravitational waves.
EXPLORER has been the first large-mass cryogenic
GW antenna to perform long-term continuous operation.
NAUTILUS is an ultracryogenic resonant-mass
GW detector, cooled for the first time at 100 mK in
1991. The data produced continously by our detectors
[1] are made available for the network analysis in the
context of the International Gravitational Event Collaboration
(IGEC). The typical detector sensitivities are of
the order of h ∼ 10 −19 in a bandwidth of few tens Hz
around 900 Hz.
VIRGO is the result of an international effort of eleven
research groups supported by INFN-Italy and CNRS-
France. The detector consists of a laser interferometer
with two orthogonal arms each 3 kilometers long[2]. In
each arm, a two mirror Fabry-Perot (F-P) resonant cavity
extends the optical length from 3 to about 100 km
and therefore amplifies the tiny effect due to an impinging
gravitational wave. VIRGO is sensitive to gravita-
which plays a crucial role in defining the thermal noise
limit of the interferometer and permits the mirror position
control through the very small forces applied to the
optical element [3]. Further improvement of this limit
will require cooling the interferometer to very low temperatures,
in order to minimize the thermal energy. This
approach is under study in the context of the conceptual
design of a third generation of gravitational wave detector
( FP7-ET research program of European Union).
The data taken by VIRGO are analysed in common
with those of two similar instruments installed in USA,
the LIGO interferometers sensitive in the frequency
range 50 - 6,000 Hz.
Figure 2: The spectral sensitivities of VIRGO and LIGO
versus frequency, compared with the VIRGO design sensitivity
curve.
Taking advantage of the larger bandwidth, these antennae
should allow the detection of a large variety of GW
signals, as those generated by the coalescence of binary
systems (stars or black holes), supernovae and the
stochastic GW [4]. In particular the Sapienza group is
analysing the data for detecting continuous GW signals
as those generated by pulsars, a data analysis challenge
pursued using the GRID technology.
References
1. P. Astone et al., Class.Quant.Grav.25,114048 (2008).
2. F. Acernese et al., Phys. Rev. A 79, 053824 (2009).
3. F. Acernese et al. Astropart. Phys. 30, 29 (2008)
4. B. Abbott et al., Nature 460, 990 (2009).
Figure 1: A F-P mirror suspended to its last stage (left).
The VIRGO optical scheme (right).
tional waves in a wide frequency range, from 10 to 10,000
Hz with a typical sensitivity of h ∼ 10 −21 . The remarkable
low frequency sensitivity of VIRGO, only results
from its peculiar suspension system. Since the beginning
of its construction phase the G23 group keeps the
responsability of the last suspension stage of the mirror,
Authors
P Astone 1 , A. Colla, A. Corsi, S. Frasca, E. Majorana 1 ,
C. Palomba 1 , P. Puppo 1 , G.V. Pallottino, P. Rapagnani,
F.Ricci
http://www.virgo.infn.it/
http://www.roma1.infn.it/rog/index.html
Sapienza Università di Roma 137 Dipartimento di Fisica