Galileo: Exotic astronomies

EXOTIC ASTRONOMIES

Gravitational waves, the weakest signals in the Universe

The irradiation of gravitational waves by an accelerated mass is a phenomenon in accordance with General Relativity, but it is very difficult to identify it experimentally, due to its low intensity. The major cosmic sources of gravitational waves are catastrophical events that emit a huge amount of energy: star explosions that lead to supernovae, active galactive nuclei with large mass black holes in the centre, collision and fusion of stars with a large mass, interactions of black holes.
The study of gravitational waves, although they have not yet been definetly detected, has been added to that of other cosmic signals, such as electromagnetic waves and particles, so that we can understand better what we already know, and maybe discover new unexpected aspects.

A gravitational wave produces on every object a weak force that lasts about a thousandth of a second; as a result of such force, the object is temporarily deformed and is induced to vibrate with the same frequency as the wave. The resonant antennae are detectors of gravitational waves; they measure the mechanical vibrations of a body with a high elastic factor and a large mass (various tons).

A transducer, which is similar to a very sensitive microphone, converts the weak shape variations of the antenna into electric signals, that are later amplified and registered. The maximum amplitude of the oscillation is smaller than the thousandth part of the dimension of an atomic nucleus, and in can be detected only with an extreme mechanical and thermic isolation. The isolation of vibrations is obtained by suspending the antenna by the median section using cables, thus forming a multi stage pendulum.
In order to reduce spontaneous vibrations the antenna is cooled as much as possible: the ultracryogenic antennae reach the temperature of one tenth of a degree above absolute zero (-273 °C).

AURIGA

AURIGA, a detector of gravitational waves, operates at the National Laboratories of Legnaro (Padua), of the National Institute of Nuclear Physics (INFN). The detector works with an ultracryogenic antenna (which means it works at a very low temperature).

The various containers that form the cryostat (cooling device) and that constitute the AURIGA ultracryogenic antenna. (JPEG, 230 K)
(INFN)

LISA

The LISA probe (Large Interferometry Space Antenna) of ESA, which will be launched after the year 2005, is part of a group of six identical satellites which have the task to detect the gravitational waves of Einstein's theories.

They will occupy in pairs the vertices of an imaginary equilateral triangle, with sides that measure 5 million kilometres, orbiting around the Sun. Laser rays will travel from one vertex of the triangle to the other to measure with infinitesimal precision the variations of the distances of the satellites?
This system is a kind of gigantic antenna in space that will be able to detect the gravitational waves which, in accordance with Einstein's theory of General Relativity, propagate across the Universe at the speed of light, with the effect of determining a deformation of space. Was Einstein right?
Cosmic sources of neutrini

because of their nature, neutrinos interact very weakly with any kind of matter. This causes so many delicate problems as to their detection that the "telescopes" are placed in very deep tunnels. Seeing as they are electrically neutral, the neutrini cannot be directly observed but it is possible to recognise the consequences of the very rare collisions with other particles: if a neutrino hits an electron, it can transfer to the electron part of its kinetic energy. The electron acquires motion and can easily be observed.
In particular, if the collision takes place in a fluid medium, the electron, moving at high speed, emits a luminous radiation that allows its detection and the measurement of its energy, and so the energy of the primary neutrino can be calculated. Seeing as neutrini do not decay to other particles, one would expect them to be extremely abundant in the Universe, with a flux dependent on their energy.
Neutrini are produces particularly in the centre of the Sun and the stars, associated to thermonuclear reactions that take place at the very high temperatures and densities of the stars. Very high spurts of neutrini accompany the explosions of supernovae. Besides, the Universe should be full of "fossil neutrini" coming from the Big Bang, the primogenial explosion.

LVD

The LVD detector of neutrini (Large Volume Detector) is installed at National Laboratories of Gran Sasso of the National Institute of Nuclear Physics. LVD uses the technique of liquid scintillation counters. The scintillation mass reaches 2000 tons, and this makes LVD the greatest detector ever realized in the world. (INFN)

Artistic view of the underground rooms of the Gran Sasso National Laboratories of INFN.(JPEG, 413 K)
(INFN)

Mechanical structure of the LVD experiment in the assembly phase in the underground hall A of the Gran Sasso National Laboratories.(JPEG, 442 K)
(INFN)

The cosmic Rain

A continuous flux of cosmic rays, which are not electromagnetic radiation but elementary particles, that is atomic nuclei and subnuclear particles, reaches our atmosphere from the depths of space: then the particles multiply in a fantastic fall of consecutive interactions in order to create an invisible and harmless "cosmic rain" that continuously hits each one of us at the frequency of 4000 charged particles per minute, approximately.
Cosmic rays consist of principally protons and Helium nuclei, in addition there is a neutrini component, difficult to detect, and a high energy photon component. Cosmic Rays come from the Sun, the Supernovae and other sources within the Milky Way, but they can also come from farther sources.
The study of cosmic rays enables us to inquire into the Universe by using high energy signals, to study fundamental physics at energies that could not be reached by the accelerators on the Earth, and to obtain precious indications on the early phases of the evolution of the Universe.

CLUE

CLUE (Cerenkov Light Ultraviolet Experiment) is an ultraviolet light telescope, projected fo the observation of the radiation emitted by the atmospherical swarms produced by the high energy cosmic rays.
4 of such telescopes are operating at present at the Roque de los Muchachos on the La Palma island of the Canary islands (Spain).
CLUE was realized by groups of the National Institute of Nuclear Physics of Padua, Pisa, Trieste, Naples.