Dark matter is a mysterious substance composing most of the material
universe, now widely thought to be some form of massive exotic particle.
An intriguing alternative view is that dark matter is made of black
holes formed during the first second of our universe's existence, known
as primordial black holes. Now a scientist at NASA’s Goddard Space
Flight Center in Greenbelt, Maryland, suggests that this interpretation
aligns with our knowledge of cosmic infrared and X-ray background glows
and may explain the unexpectedly high masses of merging black holes
detected last year.
"This study is an effort to bring together a broad set of ideas and
observations to test how well they fit, and the fit is surprisingly
good," said Alexander Kashlinsky, an astrophysicist at NASA Goddard. "If
this is correct, then all galaxies, including our own, are embedded
within a vast sphere of black holes each about 30 times the sun's
mass."
In 2005, Kashlinsky led a team of astronomers using NASA's Spitzer Space Telescope to explore the background glow of infrared light in one part of the sky. The researchers reported
excessive patchiness in the glow and concluded it was likely caused by
the aggregate light of the first sources to illuminate the universe more
than 13 billion years ago. Follow-up studies confirmed that this cosmic infrared background (CIB) showed similar unexpected structure in other parts of the sky.
In 2013, another study compared how the cosmic X-ray background (CXB) detected by NASA's Chandra X-ray Observatory
compared to the CIB in the same area of the sky. The first stars
emitted mainly optical and ultraviolet light, which today is stretched
into the infrared by the expansion of space, so they should not
contribute significantly to the CXB.
Yet the irregular glow of low-energy X-rays in the CXB matched the
patchiness of the CIB quite well. The only object we know of that can be
sufficiently luminous across this wide an energy range is a black hole.
The research team concluded that primordial black holes must have been
abundant among the earliest stars, making up at least about one out of
every five of the sources contributing to the CIB.
The nature of dark matter remains one of the most important
unresolved issues in astrophysics. Scientists currently favor
theoretical models that explain dark matter as an exotic massive
particle, but so far searches have failed to turn up evidence these
hypothetical particles actually exist. NASA is currently investigating
this issue as part of its Alpha Magnetic Spectrometer and Fermi Gamma-ray Space Telescope missions.
"These studies are providing increasingly sensitive results, slowly
shrinking the box of parameters where dark matter particles can hide,"
Kashlinsky said. "The failure to find them has led to renewed interest
in studying how well primordial black holes -- black holes formed in the
universe's first fraction of a second -- could work as dark matter."
Physicists have outlined several ways
in which the hot, rapidly expanding universe could produce primordial
black holes in the first thousandths of a second after the Big Bang. The
older the universe is when these mechanisms take hold, the larger the
black holes can be. And because the window for creating them lasts only a
tiny fraction of the first second, scientists expect primordial black
holes would exhibit a narrow range of masses.
On Sept. 14, gravitational waves produced by a pair of merging black holes 1.3 billion light-years away were captured by
the Laser Interferometer Gravitational-Wave Observatory
(LIGO) facilities in Hanford, Washington, and Livingston, Louisiana.
This event marked the first-ever detection of gravitational waves as
well as the first direct detection of black holes. The signal provided
LIGO scientists with information about the masses of the individual
black holes, which were 29 and 36 times the sun's mass, plus or minus
about four solar masses. These values were both unexpectedly large and
surprisingly similar.
"Depending on the mechanism at work, primordial black holes could
have properties very similar to what LIGO detected," Kashlinsky
explained. "If we assume this is the case, that LIGO caught a merger of
black holes formed in the early universe, we can look at the
consequences this has on our understanding of how the cosmos ultimately
evolved."
Primordial black holes, if they exist, could be
similar to the merging black holes detected by the LIGO team in 2014.
This computer simulation shows in slow motion what this merger would
have looked like up close. The ring around the black holes, called an
Einstein ring, arises from all the stars in a small region directly
behind the holes whose light is distorted by gravitational lensing. The
gravitational waves detected by LIGO are not shown in this video,
although their effects can be seen in the Einstein ring. Gravitational
waves traveling out behind the black holes disturb stellar images
comprising the Einstein ring, causing them to slosh around in the ring
even long after the merger is complete. Gravitational waves traveling in
other directions cause weaker, shorter-lived sloshing everywhere
outside the Einstein ring. If played back in real time, the movie would
last about a third of a second.Credits: SXS Lensing. Youtube
In his new paper, published May 24 in The Astrophysical Journal Letters, Kashlinsky analyzes what might have happened if dark matter consisted of a population of black holes similar to those detected by LIGO.
The black
holes distort the distribution of mass in the early universe, adding a
small fluctuation that has consequences hundreds of millions of years
later, when the first stars begin to form.
For much of the universe's first 500 million years, normal matter remained too hot to coalesce
into the first stars. Dark matter was unaffected by the high
temperature because, whatever its nature, it primarily interacts through
gravity. Aggregating by mutual attraction, dark matter first collapsed
into clumps called minihaloes, which provided a gravitational seed
enabling normal matter to accumulate. Hot gas collapsed toward the
minihaloes, resulting in pockets of gas dense enough to further collapse
on their own into the first stars.
Kashlinsky shows that if black holes
play the part of dark matter, this process occurs more rapidly and
easily produces the lumpiness of the CIB detected in Spitzer data even
if only a small fraction of minihaloes manage to produce stars.
As cosmic gas fell into the minihaloes, their constituent black holes
would naturally capture some of it too. Matter falling toward a black
hole heats up and ultimately produces X-rays. Together, infrared light
from the first stars and X-rays from gas falling into dark matter black
holes can account for the observed agreement between the patchiness of
the CIB and the CXB.
Occasionally, some primordial black holes will pass close enough to
be gravitationally captured into binary systems. The black holes in each
of these binaries will, over eons, emit gravitational radiation, lose
orbital energy and spiral inward, ultimately merging into a larger black
hole like the event LIGO observed.
"Future LIGO observing runs will tell us much more about the
universe's population of black holes, and it won't be long before we'll
know if the scenario I outline is either supported or ruled out,"
Kashlinsky said.
Kashlinsky leads science team centered at Goddard that is participating in the European Space Agency's Euclid mission, which is currently scheduled to launch in 2020. The project, named LIBRAE,
will enable the observatory to probe source populations in the CIB with
high precision and determine what portion was produced by black holes.
By Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland
Source: NASA/Dark Energy/Matter
