ATOM & COSMOS
How to make
black holes ‘sing’
Certain mergers would create
odd gravitational wave signals
BY EMILY CONOVER
When black holes collide, astronomers
expect to record a gravitational wave
“chirp.” But rapidly spinning black holes,
like the one featured in the 2014 film
Interstellar, might prefer to sing.
According to the calculations of
Caltech physicist Kip Thorne, who
served as a consultant for Interstellar, the
movie’s black hole, known as Gargantua,
must have had a mass 100 million times
that of the sun and whirled about its own
axis at breakneck speeds.
If such a rapidly spinning black hole
merges with a companion, it would produce a unique signal — one that gravitational wave detectors might be able to
observe, MI T physicist Niels Warburton
reported April 18. “There is a completely
different gravitational wave signature,”
said Warburton, who coauthored a
related paper posted online March 3 at
The standard signal of merging black
holes is a chirp, named for the increase
in frequency and amplitude of the gravitational waves produced as the black
holes spiral inward. When converted
into sound waves, this pattern sounds
like a bird’s chirp. Warburton and colleagues performed calculations to determine the gravitational wave signature
from a merger with a black hole spinning at nearly full tilt. Instead of a chirp,
the gravitational waves would maintain
a constant pitch but slowly fade away.
“It was certainly very unexpected to
see something that didn’t chirp,” said
MIT physicist Jolyon Bloomfield, who
was not involved with the research.
If such black hole mergers occur in
nature, next-generation gravitational
wave observatories like the Evolved Laser
Interferometer Space Antenna might
provide proof of their existence. Plans call
for eLISA to measure gravitational waves
from space beginning in 2034.
The Advanced Laser Interferometer
Gravitational-Wave Observatory, which
made the first detection of gravitational
waves (SN: 3/5/16, p. 6), might be able to
observe such mergers if the conditions
were just right. Although LIGO can’t
observe mergers of black holes as mas-
sive as Gargantua, smaller spinning black
holes would produce a similar effect.
Spinning black holes are “really inter-
esting from a fundamental physics point
of view,” said Samuel Gralla of the Univer-
sity of Arizona in Tucson, a coauthor on
the new paper.
Black holes can spin faster and faster
as they suck in matter, but there may be
Ancient dwarf galaxy was heavy-element factory
In the primeval universe, a violent event roiled a dwarf galaxy, leaving an
indelible mark on the stars that formed there. Scientists reached that conclusion after finding traces of heavy elements produced by the cataclysm in the
ancient dwarf galaxy Reticulum II.
Many of the universe’s heaviest elements form primarily through the
r-process, a chain of reactions through which atomic nuclei climb the
periodic table, swallowing up neutrons and decaying radioactively. But scientists don’t agree on where the seeds of these heavy elements are sown,
except that it must be an environment rich in neutrons.
Alexander Ji of MIT and collaborators used the Magellan telescopes in
Chile to catalog chemical elements in nine of Reticulum II’s stars. Seven contained heavy elements in the proportions produced by the r-process.
Since most similar dwarf galaxies show no conclusive evidence of r-process
elements, the scientists deduced that the event must be rare. The progenitor
was probably a collision of two neutron stars, or an unusual type of stellar
explosion that spews jets of material, Ji said April 19. — Emily Conover
Map charts previously unknown gamma-ray sources
A new map of the sky charts the origins of some of the highest-energy photons ever detected. Researchers from the High-Altitude Water Cherenkov
Observatory in Mexico released an analysis of their first year of observations
of gamma rays, ultrahigh-energy light particles blasted in our direction from
some of the most extreme environments in the universe.
The team found 40 gamma-ray sources, about a quarter of which hadn’t
previously been identified. The map is “revealing new information about
nature’s particle accelerators,” Brenda Dingus, a leader of the HAWC collaboration, said April 18. These accelerators include supernova remnants
and active galaxies known as blazars that shoot out blasts of particles.
The team found new sources in areas that had already been searched by
other high-energy gamma-ray telescopes. “That’s a little perplexing,” Dingus
said. The discrepancy could be due to the fact that HAWC observes higher-energy gamma rays, or that the sources are too spread out for the other
telescopes to find. — Emily Conover
MEE TING NOTES
a limit to how fast they can go. At a black
hole’s center is a singularity, or region
of infinite density, which is hidden by
an event horizon — the surface inside of
which nothing can escape the black hole’s
pull. But if the black hole twirls too fast,
the singularity becomes exposed. Such
a “naked singularity” is thought to be
impossible to reach, because the known
laws of physics would break down.
According to the scientists’ calculations, black hole mergers sing when the
larger black hole is rotating just below
the limit, at 99.99 percent of its maximum speed. This makes singing black
holes an enticing prospect for understanding physics at its extremes. s