Remember how the world’s top physicists celebrated the discovery of gravitational waves and esteemed Albert Einstein once again?
The Laser Interferometer Gravitational-Wave Observatory — two detectors based in Louisiana and Washington and more popularly known as LIGO — picked up a faint tremble of a cataclysmic event in deep universe. The signal matched what would be anticipated from the merging of two black holes, those estimated to be 29 and 36 times the mass of the sun, during an event some 1.3 billion years ago.
That report observed gravitational waves for the first time ever, with the world waiting with bated breath for other detections.
In the meantime, a new study proposes that perhaps black holes are the dark matter themselves.
Primordial Black Holes
Let’s have a quick refresher: dark matter is a mysterious substance making up most of the material universe. It is “dark” because of its non-interaction with electromagnetic radiation, thus scientists cannot build a telescope to detect it.
Black holes, on the other hand, are collapsed stars. Here, the matter becomes so compressed that its gravitational field warps space-time and no light can escape within the so-called “event horizon” of the black hole.
One alternative view is that dark matter is made up of black holes formed during the first second of the existence of the universe, known as primordial black holes (PBHs).
Writing in the Astrophysical Journal Letters, astrophysicist Alexander Kashlinsky suggested that this is aligned with existing knowledge of cosmic infrared and X-ray background glows, and potentially explains the surprisingly high masses of merging black holes unearthed last year.
"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,” explained Kashlinsky of the NASA Goddard Space Flight Center.
The LIGO black holes, while difficult to imagine as forming from stars or mergers, fall within the upper range of PBH models. Kashlinsky concluded that PBHs must have been plenty among the earliest stars, forming at least one-fifth of the sources making up the cosmic infrared background.
The cosmologist delved on fluctuations in the cosmic infrared background (CIB). The CIB – unlike the cosmic microwave background or the thermal remains of the big bang – resulted from a wide range of functions producing infrared light, including heated gas in galaxies. Fluctuations occurring in this background lend insight about its sources’ structure.
He compared infrared fluctuations with the distribution of galaxies and other known sources, finding that some of those fluctuations could not be accounted for by familiar sources.
The scale of the fluctuations are consistent with a dark matter distribution of LIGO-mass black holes, meaning black holes could be key in explaining dark matter after all.
Black Holes As Dark Matter?
In a phone interview with the Washington Post, Kashlinsky pointed out that the black holes with 30 solar masses are a key to this hypothesis, as it is an “unusual mass” for typical black holes that form in today’s universe from stars.
If black holes are actually dark matter, they would primarily exist in a halo that surrounds galaxies, including the Milky Way. And they would be numerous, some 10 billion to 30 billion black holes, Kashlinsky continued.
In a nutshell: primordial black holes could maintain properties highly similar to what LIGO detected – in this case, LIGO found a merger of black holes formed in the beginnings of the universe.
Kashlinsky’s argument is similar to that of Simeon Bird from Johns Hopkins University, raising the possibility that LIGO has found primordial black hole dark matter. If the black holes are usually less massive, that would pinpoint the standard origin through collapsed stars. But since the black holes were in the 30-plus solar mass range, they can be speculated as PBHs.
Some PBHs occasionally get close enough to be gravitationally trapped into binary systems. Ultimately, black holes in each of the binaries will merge into a larger one like the event described by LIGO.
For Kashlinsky it’s only a matter of time before future LIGO observations give out more information to support or rule out his proposed scenario.
The Debate Continues
Dark matter and its nature remain a hotly debated topic in astrophysics, with scientists currently favoring theoretical models explaining this mystery as an exotic massive particle. So far, however, there isn’t any proof yet that these particles exist.
Some experts have disagreed with Kashlinsky’s paper, with Michael Turner of University of Chicago, for instance, saying he will eat the paper if the theory is proven correct. Rainer Weiss, one of the LIGO founders, also said in an email that he is “quite skeptical.”
“If [those black holes] are primeval and are the dark matter they should have distorted the spectrum of the cosmic background radiation to be non-thermal,” he said, adding they should have also appeared in the data of weak lensing measurements as a function of time.
Kashlinsky leads a team joining the Euclid mission of the European Space Agency, which is poised to launch in 2020. The project will investigate galactic and stellar populations in the CIB using high precision instruments and see what part was produced by black holes.
