Could Black Holes Be the Dark Matter Everyone Has Been Looking For?

You remember LIGO, right? Cast your mind back to February and the big hoo-hah at the National Press Club at which many of the world's top physicists celebrated the discovery of gravitational waves. Einstein affirmed again. Decades of hard work rewarded. The Laser Interferometer Gravitational-Wave Observatory - two sprawling detectors, one in Louisiana and one in Washington state - had picked up the faint tremble of a cataclysmic event in deep space. The signal precisely matched what would be expected from the merger of two black holes. The event was so violent that it rippled the fabric of the universe. Cool!

But that presser reported only one detection, amid rumors that the LIGO scientists were chewing on other possible detections and would soon tell the world about them. But now it's almost June. Rumors still abound, but where's the next paper? The public demands: Show us the data! (Pretending, here, that we live in a science-obsessed society.)

We asked the LIGO folks when we might see more results.

"We are still finishing up the analyses of the second set of LIGO data from our first observing run, and [will] be reporting results from those analyses sometime in the near future, hopefully in June," LIGO's David Reitze, of Caltech, told us by email.

In the meantime, one maverick theory has entered the conversation. Two different papers have been published recently suggesting that LIGO may have stumbled upon the solution to the enduring mystery of dark matter. Maybe black holes are the dark matter, these papers say.

A quick refresher: Dark matter is "dark" because it doesn't interact at all with electromagnetic radiation. Thus you can't build a telescope to detect it. We infer its existence through its gravitational effects on galaxies and clusters of galaxies. We know it's there. Indeed it accounts for a lot of the overall composition of the universe. But for many decades, scientists have wondered what might comprise this ghostly matter. The best guess: exotic particles.

A black hole, meanwhile, is a collapsed star. The matter gets so compressed that its gravitational field warps space-time itself, and within the "event horizon" of the black hole, not even light can escape. A black hole does not have a "size," per se - it is infinitely dense. It has no real spatial dimensions, but it does have a mass - described typically in units of "solar masses," which is to say, in comparison to the mass of our friend the Sun.

The black-hole merger detected by LIGO involved two black holes of roughly 30 solar masses each. The default assumption is that these black holes were formed through the standard process of stellar evolution. But the maverick theorists have suggested that they might be "primordial" black holes formed at the dawn of time when the universe was just started to expand and inflate and puff out its chest.

The first paper, in Physical Review Letters, from Simeon Bird of Johns Hopkins University, and a number of co-authors, said that the first round of LIGO results raises "the possibility that LIGO has detected [primordial black hole] dark matter."

This conversation centers on the masses of the black holes that LIGO detected. If the black holes are typically less massive, say 10 or 20 solar masses or so, that would point to the standard origin via collapsed stars, but if the black holes usually are much bigger, in the 30-plus solar mass range, that would leave open the possibility that they're primordial black holes (PBHs), Bird said by email. Bird wrote:

"If LIGO sees a smooth distribution of events with masses from 10 to, say, 40, with most of them at 10, some at 20, and a few at 30, one of which just happened to be first, that would probably favour a pure stellar binary origin (ie; no PBH dark matter).

"However, if LIGO saw a 30 solar mass merger once per month and nothing else, that would be extremely suggestive! A stellar binary model producing that would be a little strange. Those merger rates would I think lie within our theoretical uncertainties; we could explain them.

"It still wouldn't be proof of PBH dark matter; but it would be interesting."

A similar argument has just been published in the Astrophysical Journal Letters by Alexander Kashlinsky, a cosmologist at Nasa's Goddard Space Flight Center. In a phone interview, Kashlinsky said the 30-solar-mass black holes open the door to the maverick hypothesis: "It's an unusual mass for normal black holes that form in today's universe from stars."

If, in fact, black holes are the dark matter, they would exist primarily in a halo surrounding galaxies, including our own Milky Way galaxy. And there would be a lot of them. How many?

"Roughly 10 billion, 30 billion of these black holes," Kashlinsky said. And that's just around this galaxy. So the universe would be lousy with primordial black holes.

However, we asked some luminaries in the cosmology business, and none cottoned to the black holes/dark matter concept.

For example, here's Michael Turner of the University of Chicago, when we asked about the Kashlinsky paper: "Obviously it's not impossible, but I'll eat the paper if it's correct."

We'll put that down as a "no."

Rainer Weiss, one of the founders of LIGO, was also not a fan of the hypothesis. An excerpt from his email (complete with some abstruse physics):

"I am quite skeptical of the claim. If they are primeval and are the dark matter they should have distorted the spectrum of the cosmic background radiation to be non-thermal and they should have showed up in the statistics of weak lensing measurements as a function of time....We will find out if the hypothesis is tenable with further measurements by LIGO...."

David Spergel of Princeton raises a number of objections to the Kashlinsky paper, including: "The mechanism proposed in the paper would produce individual black holes, not black hole binaries. The paper does not provide a viable mechanism for producing binaries."

So it sounds as though there are many reasons for being skeptical. For one thing, there's nothing weird in what LIGO detected. The usual stellar evolution/collapse can make a 30-mass black hole. Observational data also constrain the likelihood of primordial black holes.

This will all get settled, in any event, when LIGO produces more of its data and shows us some more black hole mergers.

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