Princeton University and New York University researchers have simulated the effect of a primordial black hole passing through a star. Primordial black holes are among the objects hypothesized to make up dark matter—the invisible substance thought to constitute much of the universe—and astronomers could use the researchers’ model to finally observe the elusive black holes. This image illustrates the resulting vibration waves as a primordial black hole (white dots) passes through the center of a star. The different colors correspond to the density of the primordial black hole and strength of the vibration. Image: Tim Sandstrom
Scientists looking to capture evidence of dark matter—the invisible
substance thought to constitute much of the universe—may find a helpful tool in
the recent work of researchers from Princeton
University and New York
The team unveiled in a report in Physical Review Letters on a ready-made
method for detecting the collision of stars with an elusive type of black hole
that is on the short list of objects believed to make up dark matter. Such a
discovery could serve as observable proof of dark matter and provide a much
deeper understanding of the universe’s inner workings.
Postdoctoral researchers Shravan Hanasoge of Princeton’s
Department of Geosciences and Michael Kesden of NYU’s Center for Cosmology and
Particle Physics simulated the visible result of a primordial black hole
passing through a star. Theoretical remnants of the Big Bang, primordial black
holes possess the properties of dark matter and are one of various cosmic
objects thought to be the source of the mysterious substance, but they have yet
to be observed.
If primordial black holes are the source of dark matter, the sheer number
of stars in the Milky Way galaxy—roughly 100 billion—makes an encounter
inevitable, the authors report. Unlike larger black holes, a primordial black
hole would not “swallow” the star, but cause noticeable vibrations on
the star’s surface as it passes through.
Thus, as the number of telescopes and satellites probing distant stars in
the Milky Way increases, so do the chances to observe a primordial black hole
as it slides harmlessly through one of the galaxy’s billions of stars, Hanasoge
says. The computer model developed by Hanasoge and Kesden can be used with
these current solar-observation techniques to offer a more precise method for
detecting primordial black holes than existing tools.
“If astronomers were just looking at the sun, the chances of observing
a primordial black hole are not likely, but people are now looking at thousands
of stars,” Hanasoge says.
“There’s a larger question of what constitutes dark matter, and if a
primordial black hole were found it would fit all the parameters—they have mass
and force so they directly influence other objects in the universe, and they
don’t interact with light. Identifying one would have profound implications for
our understanding of the early universe and dark matter.”
Although dark matter has not been observed directly, galaxies are thought
to reside in extended dark-matter halos based on documented gravitational
effects of these halos on galaxies’ visible stars and gas. Like other proposed
dark-matter candidates, primordial black holes are difficult to detect because
they neither emit nor absorb light, stealthily traversing the universe with
only subtle gravitational effects on nearby objects.
Because primordial black holes are heavier than other dark-matter
candidates, however, their interaction with stars would be detectable by
existing and future stellar observatories, Kesden says. When crossing paths
with a star, a primordial black hole’s gravity would squeeze the star, and
then, once the black hole passed through, cause the star’s surface to ripple as
it snaps back into place.
“If you imagine poking a water balloon and watching the water ripple
inside, that’s similar to how a star’s surface appears,” Kesden says.
“By looking at how a star’s surface moves, you can figure out what’s going
on inside. If a black hole goes through, you can see the surface vibrate.”
Eyeing the sun’s surface for hints of dark matter
Kesden and Hanasoge used the sun as a model to calculate the effect of a
primordial black hole on a star’s surface. Kesden, whose research includes
black holes and dark matter, calculated the masses of a primordial black hole,
as well as the likely trajectory of the object through the sun. Hanasoge, who
studies seismology in the sun, Earth, and stars, worked out the black hole’s
vibrational effect on the sun’s surface.
Video simulations of the researchers’ calculations were created by NASA’s
Tim Sandstrom using the Pleiades supercomputer at the agency’s Ames Research
Center in California. One clip shows the vibrations of
the sun’s surface as a primordial black hole—represented by a white trail—passes
through its interior. A second movie portrays the result of a black hole grazing the sun’s surface.
Marc Kamionkowski, a professor of physics and astronomy at Johns Hopkins
University, says that the
work serves as a toolkit for detecting primordial black holes, as Hanasoge and
Kesden have provided a thorough and accurate method that takes advantage of
existing solar observations. A theoretical physicist well known for his work
with large-scale structures and the universe’s early history, Kamionkowski had
no role in the project, but is familiar with it.
“It’s been known that as a primordial black hole went by a star, it
would have an effect, but this is the first time we have calculations that are
numerically precise,” Kamionkowski says.
“This is a clever idea that takes advantage of observations and
measurements already made by solar physics. It’s like someone calling you to
say there might be a million dollars under your front doormat. If it turns out
to not be true, it cost you nothing to look. In this case, there might be dark
matter in the data sets astronomers already have, so why not look?”
One significant aspect of Kesden and Hanasoge’s technique, Kamionkowski
says, is that it narrows a significant gap in the mass that can be detected by
existing methods of trolling for primordial black holes.
The search for primordial black holes has thus far been limited to masses
too small to include a black hole, or so large that “those black holes
would have disrupted galaxies in heinous ways we would have noticed,”
Kamionkowski says. “Primordial black holes have been somewhat neglected
and I think that’s because there has not been a single, well-motivated idea of
how to find them within the range in which they could likely exist.”
The current mass range in which primordial black holes could be observed
was set based on previous direct observations of Hawking radiation—the
emissions from black holes as they evaporate into gamma rays—as well as of the
bending of light around large stellar objects, Kesden says. The difference in
mass between those phenomena, however, is enormous, even in astronomical terms.
Hawking radiation can only be observed if the evaporating black hole’s mass is
less than 100 quadrillion grams. On the other end, an object must be larger
than 100 septillion (24 zeroes) grams for light to visibly bend around it. The
search for primordial black holes covered a swath of mass that spans a factor of
1 billion, Kesden explains—similar to searching for an unknown object with a
weight somewhere between that of a penny and a mining dump truck.
He and Hanasoge suggest a technique to give that range a much-needed trim
and established more specific parameters for spotting a primordial black hole.
The pair found through their simulations that a primordial black hole larger
than 1 sextillion (21 zeroes) grams—roughly the mass of an asteroid—would produce
a noticeable effect on a star’s surface.
“Now that we know primordial black holes can produce detectable
vibrations in stars, we could try to look at a larger sample of stars than just
our own sun,” Kesden says.
“The Milky Way has 100 billion stars, so about 10,000 detectable
events should be happening every year in our galaxy if we just knew where to