Clouds of ultralight particles can form around rotating
black holes. A team of physicists from the University of Amsterdam and Harvard
University now show that these clouds would leave a characteristic imprint on
the gravitational waves emitted by binary black holes.
Black holes are generally thought to swallow all forms of
matter and energy surrounding them. It has long been known, however, that they
can also shed some of their mass through a process called superradiance. While
this phenomenon is known to occur, it is only effective if new, so far
unobserved particles with very low mass exist in nature, as predicted by
several theories beyond the Standard Model of particle physics.
When mass is extracted from a black hole via superradiance,
it forms a large cloud around the black hole, creating a so-called
gravitational atom. Despite the immensely larger size of a gravitational atom,
the comparison with sub-microscopic atoms is accurate because of the similarity
of the black hole plus its cloud with the familiar structure of ordinary atoms,
where clouds of electrons surround a core of protons and neutrons.
In a publication that appeared in Physical Review Letters
this week, a team consisting of UvA physicists Daniel Baumann, Gianfranco Bertone,
and Giovanni Maria Tomaselli, and Harvard University physicist John Stout,
suggest that the analogy between ordinary and gravitational atoms runs deeper
than just the similarity in structure. They claim that the resemblance can in
fact be exploited to discover new particles with upcoming gravitational wave
interferometers.
In the new work, the researchers studied the gravitational
equivalent of the so-called 'photoelectric effect'. In this well-known process,
which for example is exploited in solar cells to produce an electric current,
ordinary electrons absorb the energy of incident particles of light and are
thereby ejected from a material -- the atoms 'ionize'. In the gravitational
analogue, when the gravitational atom is part of a binary system of two heavy
objects, it gets perturbed by the presence of the massive companion, which
could be a second black hole or a neutron star. Just as the electrons in the
photoelectric effect absorb the energy of the incident light, the cloud of
ultralight particles can absorb the orbital energy of the companion, so that
some of the cloud gets ejected from the gravitational atom.
The team demonstrated that this process may dramatically
alter the evolution of such binary systems, significantly reducing the time
required for the components to merge with each other. Moreover, the ionization
of the gravitational atom is enhanced at very specific distances between the
binary black holes, which leads to sharp features in the gravitational waves that
we detect from such mergers. Future gravitational wave interferometers --
machines similar to the LIGO and Virgo detectors that over the past few years
have shown us the first gravitational waves from black holes -- could observe
these effects. Finding the predicted features from gravitational atoms would
provide distinctive evidence for the existence of new ultralight particles.
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