Black holes are among the most exotic phenomena in the universe because
they are enigmatic, fascinating, and unavoidable. The chirp sound that two
black holes make when they merge may be heard using gravitational-wave
detectors; about 70 such chirps have been discovered so far.
The "ocean of voices" in which chirps are thought to predominately occur in
two universal frequency ranges, according to a team of researchers from the
Heidelberg Institute for Theoretical Studies (HITS). The Astrophysical
Journal Letters is where the work has been published.
It was Einstein who first proposed gravitational waves in 2015, which led
to the 2017 Nobel Prize in Physics and the beginning of gravitational-wave
astronomy. The so-called chirp signal, or gravitational waves of increasing
frequency, are produced by the merger of two stellar-mass black holes and
may be "heard" on Earth. Scientists may determine the so-called "chirp
mass," a mathematical amalgamation of the two distinct black hole masses, by
analyzing this frequency variation (the chirp).
Black holes that are merging are currently thought to be capable of having
any mass. However, according to the team's simulations, some black holes
arrive in normal masses and produce universal chirps.
"The existence of universal chirp masses not only tells us how black holes
form," says Fabian Schneider, who oversaw the research at HITS, "it can also
be used to infer which stars explode in supernovae." In addition, it offers
new ways for researchers to evaluate the rapid cosmic expansion of the
universe as well as insights into the supernova mechanism, hazy nuclear and
stellar physics, and the supernova mechanism itself.
"Severe repercussions for the celestial destinies"
Massive stars that do not explode in supernovae but instead collapse into
black holes are the ends of stellar-mass black holes, which have masses of
around 3-100 times that of the sun. Binary star systems give birth to the
precursors of black holes that eventually combine, and these systems go
through multiple events of mass exchange between the parts. In specifically,
the two progenitors of a black hole are produced from stars that have lost
their protective envelopes.
"The stripping of the envelope has serious repercussions for the ultimate
destiny of stars. For instance, as now anticipated by our simulations, it
also results in universal black hole masses, according to Philipp
Podsiadlowski from Oxford University, the study's second author and the
Klaus Tschira Guest Professor at HITS.
The "stellar graveyard"—a collection of all known masses of the
neutron-star and black-hole remnants of large stars—is rapidly expanding as
a result of continuing gravitational-wave searches and ever-increasing
sensitivity of gravitational-wave detectors. Particularly, a gap in the
distribution of the chirp masses of merging binary black holes appears, and
evidence for peaks at between eight and fourteen solar masses appears. These
characteristics line up with the universal chirps the HITS team
expected.
According to Eva Laplace, the third author of the paper, "any features in
the distributions of black-hole and chirp masses can tell us a great deal
about how these objects have formed."
In our galaxy, no: far more massive black holes
Since the initial finding of merging black holes, it has been clear that
black holes with masses far greater than those observed in the Milky Way
exist. This is a direct result of these black holes developing from stars
that were created with a different chemical makeup than the ones in our
Milky Way Galaxy. The HITS team could now demonstrate that stars that become
envelope-stripped in near binaries create black holes of fewer than nine and
higher than 16 solar masses but essentially none in between, independent of
the chemical makeup of the stars.
The universal black-hole masses, which range between nine and 16 solar
masses, logically imply universal chirp masses, or universal noises, in
merging black holes.
Fabian Schneider explains, "When revising my presentation on
gravitational-wave astronomy, I noticed that the gravitational-wave
observatories had uncovered the first clues of an absence of chirp masses
and an abundance at exactly the universal masses anticipated by our
theories. "It is not clear yet whether this signal in the data is just a
statistical fluke or not," the authors write, "because the number of
observed black-hole mergers is still rather low."
Whatever the outcomes of upcoming gravitational-wave observations, they
will be fascinating and aid in the understanding of the origin of the
singing black holes in this sea of voices.
