Supermassive black holes can be used to measure the mass of
extremely light particles to unprecedented levels and rule out the existence of
new exotic particles, perhaps constraining the nature of dark matter. These are
the results of a recent study by Vitor Cardoso and Paolo Pani to be published in the prestigious Physical
Review Letters and covered by New Scientist, Expresso, Phys.Org. and selected for highlights of the American Physical Society.
Most of our knowledge of the Universe comes from electromagnetic
waves, which travel through vacuum at the speed of light (300 thousand
kilometers per second). Electromagnetic waves are carried by photons,
elementary particles whose mass must be exactly zero if they travel at
precisely the speed of light. If photons had mass their speed would be slower,
drastically affecting our understanding of particle physics and of the whole
Universe.
BLACK HOLES AS LABORATORIES FOR PARTICLE PHYSICS
Traditionally, particles are studied by colliding them in
accelerators. Ultra-light particles, however, are hard to study in this way.
This is one reason why researchers have started to investigate the possibility
of using precise astrophysical observations to probe the microscopic world. By
studying how particles interact with black holes it is possible to estimate not
only their masses, but even their very existence.
Supermassive black holes are huge gravitational objects
typically located at the center of galaxies, including our Milky Way. When
black holes rotate (and most do), they display an interesting effect known as
"superradiance": if one shines a lamp on a rotating black hole, the
beam reflected off the black hole is brighter! This happens at the expense of
the hole's kinetic energy: after the reflection, the black hole spin decreases.
BLACK HOLES AND MASSIVE PHOTONS
In recent work, an international team of researchers at IST
(Portugal), Rome (Italy), Mississippi (USA) and Osaka (Japan) showed that
ultralight photons with nonzero mass would produce a "black hole
bomb": a strong instability that would extract energy from the black hole
very quickly. Therefore the very existence of such particles is constrained by
the observation of spinning black holes. With this technique the authors have
succeeded in constraining the mass of the photon to unprecedented levels: the
mass must be smaller than 10^-20 electron-volt, or one hundred times better
than the current bound. To put this in context, this mass is one hundred
billion of billions times smaller than the present constraint on the neutrino
mass (about two electron-volt).
The results of this study can be used to investigate the
existence of new particles, as those possibly contributing to the dark matter
that are currently being searched for at the LHC at CERN. Can observations of
supermassive black holes provide new insights which are not accessible in
laboratory experiments? This would certainly be exciting. Perhaps these new
frontiers in astrophysics will give us a clearer understanding of the
microscopic Universe. |