Using neutrinos to look at the hotest objects in the Universe from the coldest place on Earth
In 1610 the four largest moons around Jupiter were found, unexpectedly, by
Galileo with the recently invented telescope. This proved to be
crucial evidence supporting the Copernican hypothesis that the Earth
revolves around the Sun. Penzias and Wilson were studying
electromagnetic noise in antennas used in the telephone system when
they stumbled upon the Cosmic Microwave Background, the echo from the
Big Bang explosion that created our Universe. American spy satellites
monitoring the surface of our planet for nuclear weapons tests
serendipitously discovered Gamma-ray bursts (GRBs), enormous flashes
of high energy photons (very short wavelength light, invisible to the
human eye) that occur in the Universe about once per day. The history
of astronomy is littered with similar stories of new instruments
enabling unexpected discoveries. Now, a new kind of instrument is
being built at the South Pole: IceCube, a neutrino telescope. It is
not guaranteed that IceCube will discover neutrinos from astronomical
objects, but history seems to be on our side.
Neutrinos are subatomic particles associated with radioactive
decays. In addition to nuclear reactors and man-made accelerators,
neutrinos have also been detected from Sun and, during a brief 10
seconds, from a supernova (the explosion of a dying, massive star) near
our own galaxy in 1987. Unlike protons and electrons, neutrinos
have no electric charge, and neutrinos hardly ever interact with
matter. Indeed, a neutrino produced by the Sun could
travel more than one light year in water before stopping. Neutrinos
travel through walls, planets or across the Universe without
difficulty. This ghost-like behavior of neutrinos is both their
advantage and disadvantage. Because they travel unimpeded from their
source, neutrinos can allow us to peer inside astronomical objects
where no light could escape. But a very large detector is needed in
order to detect just a handful of them. It is generally believed
that a kilometer-scale detector is needed to search the skies for
neutrino sources beyond our Sun.
A neutrino sky map will reveal the still unknown sources of the
highest energy cosmic rays. For over twenty years we have known that
somewhere in the Universe particles are being accelerated to energies
in excess of 1 Joule, or 10 million times more energy than particles
accelerated in machines built by human-kind.
We detect neutrinos by looking for a tiny amount of light that is
produced when a neutrino collides with matter. IceCube is being built
at the South Pole because we need a large volume of highly transparent
material, and the ice down there is so pure that we can see light from
several hundred meters away. IceCube's detection principle is
simple: drill holes into the glacial ice and sink in optical sensors
(Photo-multiplier tubes) in a three dimensional array that can see the
light due to a neutrino interaction.
Theoretical physicists have a long list of objects that
can produce neutrinos. Active Galactic Nuclei, which are driven by
supermassive black holes at the center of galaxies; micro-quasars in
which a solar mass black holes devour matter from a companion star;
the remanents of supernova explosions; and, the subject of my
research, Gamma-ray bursts.
A detection of neutrinos in the direction of and in coincidence with a
Gamma-ray burst would be a great advance for science. It would provide
with a smoking gun for the source of the highest energy cosmic rays,
it would give us a fantastic tool to study these mysterious objects
and would increase our knowledge into the properties and behavior of
neutrinos themselves. With IceCube already operating at half its final
size, and with the end of construction only three years away, the
promise of extragalactic neutrino astronomy is within our grasp.
