"These primordial black holes could account for a
significant portion, if not all, of dark matter."
Ripples in the very fabric of space and time called
"gravitational waves" may have provided the first tantalizing
evidence of tiny black holes born during the Big Bang. These primordial black
holes could, in turn, account for most if not all of the universe's most
mysterious stuff, known as dark matter.
Unlike stellar mass black holes, primordial black holes
weren't born when massive stars died, but instead from fluctuations in density
that occurred immediately after the birth of the cosmos. That means they can be
much smaller than stellar mass black holes, which have at least the same mass
as several suns. These Big-Bang-born "non-astrophysical" black holes
can have masses as small as that of an average asteroid or as large as a
massive planet.
Yet, primordial black holes remain frustratingly
hypothetical despite being first proposed by Stephen Hawking in the 1970s. But
now, the first potential hint of their existence comes in the form of a
gravitational wave signal "heard" last by LIGO (Laser Interferometer
Gravitational-Wave Observatory), which indicated a collision between two black
holes, at least one of which has a mass smaller than the mass of the sun.
"The most common black holes form as the result of a
supernova, the death of a massive star. So, their masses can range from a few
times the sun’s mass to billions of solar masses," University of Miami
researcher Nico Cappelluti said in a statement. "We believe our study will
aid in confirming that they [primordial black holes] actually do exist."
There remains the possibility that the gravitational wave
signal mentioned above was a false alarm, the result of interference or
"noise" in LIGO's massive interferometer laser arms. However,
Cappelluti and his University of Miami colleague, Alberto Magaraggia, believe
that the unusual signal couldn't be caused by anything but a primordial black
hole.
And they intend to prove it.
"We attempted to estimate how many primordial black
holes may exist in the universe and how many of them LIGO should be able to
detect, and our results are encouraging," Magaraggia said. "We
predict that subsolar black holes like the one LIGO may have observed should
indeed be rare, consistent with how infrequently such events have been seen so
far.
"The most plausible explanation for the LIGO signal,
which lacks any conventional astrophysical explanation, is the detection of a
primordial black hole. And our research indicates that these primordial black
holes could account for a significant portion, if not all, of dark
matter."
Connecting dark matter and primordial black holes
Dark matter is a pressing puzzle for physicists because,
despite accounting for 85% of the universe's matter and thus outweighing the
"everyday matter" comprising stars, planets, moons, asteroids, our
bodies, and everything we see around us by a ratio of five to one, they have no
idea what this stuff actually is. That is partially because, unlike the
particles that account for that everyday matter, dark matter doesn't interact
with electromagnetic radiation, light to you and me. That makes it effectively
invisible, with scientists only able to infer the presence of dark matter due
to its interaction with gravity and the knock-on effect this has on light and
everyday matter.
In fact, the gravitational influence of dark matter is
crucial as the gravity of the visible matter in galaxies alone isn't sufficient
to hold them together.
The unusual characteristics of dark matter have prompted
scientists to search beyond the standard model of particle physics for
particles that could comprise it. Thus far, this search has turned up
empty-handed. That has led some scientists to postulate that dark matter could
be partially or wholly accounted for by primordial black holes. Like all black
holes, primordial black holes have mass and thus interact with gravity and are
effectively invisible due to the fact that they are bounded by a light-trapping
surface called an event horizon. That makes them a good fit for dark matter.
However, as Cappelluti and Magaraggia concede, as convinced
as they are that this mystery signal indicates the existence of primordial
black holes, a lot more evidence of these non-astrophysical black holes will be
needed before they can be firmly connected to dark matter.
With U.S.-based LIGO and its gravitational wave detector
partners, Virgo in Italy and KAGRA in Japan, set for sensitivity boosts and a
future wealth of highly sensitive gravitational wave detectors such as the
space-based LISA (Laser Interferometer Space Antenna), on the horizon, this
could be merely a case of waiting for technology to catch up to theory. But
that is nothing new. Considering that gravitational waves were first predicted
by Einstein in 1915 and the first successful detection was only made 100 years
later in 2015, hunting these ripples in spacetime has always been a waiting
game requiring a lot of patience.
"LIGO picked up what is very strong evidence that these
types of black holes exist. But we’ll need to detect another such signal or
even several others to get the smoking-gun confirmation that they are
real," Cappelluti said. "But what is clear is that they cannot be
excluded as being real."