|Subject: [Hangout - NYLXS] The extroordinary universe from a new perspective
Black Hole at Heart of Our Galaxy Is on Crash Course, Space-Time Ripples
Gravitational waves coming from supermassive black holes like the one at
the center of the Milky Way are offering clues to their fates
Supermassive black holes all over the universe are merging, a fate that
will eventually come for the black hole at the center of our galaxy.
These mysterious cosmic structures at the heart of nearly every galaxy
consume light and matter and are impossible to glimpse with traditional
But now, for the first time, astrophysicists have gathered knowledge
directly from these titans, in the form of gravitational waves that
ripple through space and time. What they learned suggests that the
population of massive black hole pairs that are merging numbers in the
hundreds of thousands—perhaps even millions. The gravitational waves
from these mergers are all contributing to an underlying background hum
of the universe that researchers can detect from Earth.
The findings, from a collaboration of more than 100 scientists, help
confirm what will one day happen to the supermassive black hole at our
galaxy’s center known as Sagittarius A*, as it crashes into the black
hole at the heart of the Andromeda galaxy.
“The Milky Way galaxy is on a collision course with the Andromeda
galaxy, and in about 4.5 billion years, the two galaxies are set to
merge,” said Joseph Simon, a University of Colorado, Boulder,
astrophysicist and a member of the North American Nanohertz Observatory
for Gravitational Waves, or Nanograv, which helped lead the new work
with support from the National Science Foundation.
That merger, he said, will eventually result in the black hole at the
center of Andromeda and Sagittarius A* sinking into the center of the
newly combined galaxy and forming what is known as a binary system. The
results were announced in a series of papers published Wednesday in the
Astrophysical Journal Letters.
“Before now, we didn’t even know if supermassive black holes merged, and
now we have evidence that hundreds of thousands of them are merging,”
said Chiara Mingarelli, a Yale University astrophysicist and another
member of Nanograv.
The new work could answer questions such as how these black holes grow,
and how often their host galaxies merge, the researchers said.
“These are some of the craziest objects in our universe,” said Masha
Baryakhtar, a physicist at the University of Washington in Seattle, who
wasn’t involved in the research. “There’s no kind of consensus yet for
how they get so big.”
If scientists understand more about the history of merging supermassive
black holes, it could help reveal how they form in the first place,
Essential to these findings is the detection of elusive gravitational
waves, and understanding how they are produced.
Any object with mass that is moving causes these waves—invisible
distortion in time and space that were first theorized by Albert
Einstein in 1916 but not detected until roughly 100 years later.
(Imagine the universe as a trampoline rippling as a bowling ball rolls
around on the surface.) In 2015, scientists used the ground-based Laser
Interferometer Gravitational-Wave Observatory, or LIGO, to detect how
short, high-frequency gravitational waves from one merger between less
massive black holes jiggled the Earth by less than the width of a single
subatomic particle. The effort won them a Nobel Prize.
LIGO can measure waves from colliding objects such as neutron stars that
change on short time scales, according to Sarah Vigeland, a University
of Wisconsin-Milwaukee physicist who oversees the gravitational-waves
searches for Nanograv.
“You get this burst of gravitational waves, and then it’s over,” she said.
The observatory can’t detect low-frequency gravitational waves that
change on longer time scales—on the order of months to decades coming
from more massive objects. So the Nanograv group, as part of an
international consortium including groups doing similar work in Europe,
Asia and Australia, decided to use a different method to measure these
space-time ripples: tracking how they mess with the emissions of star
remnants known as pulsars.
Pulsars are effectively like cosmic clocks, according to Columbia
University astrophysicist Slavko Bogdanov, who wasn’t involved in the
work. These remnants of dead stars rapidly rotate hundreds of times per
second, emitting radio waves at regular intervals that can be detected
from radio telescopes on Earth.
Because the regularity of these radio wave pulses can be calculated with
great precision, any deviation in their arrival to Earth, whether they
are just a little bit late or a little bit early, can be chalked up to
the effect of gravitational waves—the strength and source of which can
then be calculated.
For 15 years, Vigeland said, the Nanograv group observed the timing of
radio waves from pulsars in our galaxy using the Arecibo Observatory in
Puerto Rico, the Green Bank Telescope in West Virginia and the Very
Large Array in New Mexico.
“We monitor our pulsars on a regular basis, about once a month,” she
said, adding that the findings included data from 68 pulsars.
While 15 years might seem like a long time to collect data, such a time
span is necessary for measuring the type of slowly undulating
gravitational waves coming from supermassive black holes, according to
Simon, who said the arrival time of the pulses from these clocklike
spinning stars change by just hundreds of billionths of a second over
the span of a decade.
Bogdanov said that finding and adding more pulsars into the data set
would be essential to improving how sensitive these gravitational wave
detections are. There are still other things in the universe producing
gravitational waves that haven’t been detected yet, according to Julie
Comerford, an astrophysicist at the University of Colorado, Boulder, and
Nanograv member. One of those other sources, she said, could be ripples
in space time from the big bang itself.
Nearly 14 billion years ago, the early universe had a lot of curvature,
a bit like a crumpled-up blanket, Comerford said, before expanding at
the speed of light or faster, spreading and smoothing out.
“So you could see remnant gravitational waves from that process,” she said.
Write to Aylin Woodward at aylin.woodward-at-wsj.com
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