From Physics: “Controversy Continues over Black Holes as Dark Matter” and “Viewpoint: Supernova Study Dampens Dark Matter Theory”

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“Controversy Continues over Black Holes as Dark Matter”

October 1, 2018
Michael Schirber

Following recent gravitational-wave detections, black holes have emerged as a possible, though contentious, dark matter candidate.

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This artist’s conception shows two merging black holes, like those detected by the LIGO and Virgo facilities on several occasions in recent years. The detections have led some cosmologists to suggest that primordial black holes may be so numerous that they are responsible for at least some of the dark matter. LIGO/Caltech/MIT/Sonoma State (A. Simonnet)

Most of the matter in the Universe is dark, and its composition remains a mystery. Among the proposed ingredients, there’s perhaps no darker dark matter candidate than black holes, which could have formed in large numbers in the early Universe. The idea is not new, but it has seen a huge resurgence with the recent gravitational-wave observations of black holes. Support among cosmologists for the black-holes-as-dark-matter hypothesis has been mixed, with some researchers seeing black holes as the cure for a number of outstanding issues in cosmology and others pointing to astronomical observations that constrain black hole numbers. The ongoing debate between these two sides, regardless of its outcome, is pushing researchers to more carefully examine the available data.

The picture that most cosmologists have for dark matter is that it’s some sort of subatomic particle, such as a weakly interacting massive particle (WIMP) or a so-called axion. However, searches for these particles continue to come up empty, opening the door to alternative possibilities. Among the options are black holes, but not the “ordinary” black holes that are known to form when a massive star explodes. To account for the dark matter, cosmologists need a separate population of black holes that formed long before stars. Such “primordial” black holes were first proposed decades ago, but they never garnered a lot of interest, as no hints of their existence were found.

Things changed in 2016, when the LIGO-Virgo collaboration reported the first detection of gravitational waves, and the source was a binary black hole merger (see 11 February 2016 Viewpoint). The sizes of these black holes (around 30 solar masses) were slightly larger—and their rotation rates were slower—than expected from stellar models, so many theorists began considering a primordial origin. “The field has grown dramatically,” says black hole expert Christian Byrnes from the University of Sussex in the UK. “A lot of people now want to get their fingers into gravitational waves and black holes.”

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This chart shows the masses of the five confirmed black hole mergers (and one candidate merger) observed by LIGO and Virgo. LSC/LIGO/Caltech/Sonoma State (A. Simonnet)

The basic recipe for making primordial black holes is actually pretty simple: all you need is a region in the very early Universe with a density that is roughly twice the average density, and gravity will do the rest. The problem is making the overdense region in the first place: the early Universe appears to have been very uniform—at least on the large scales that astronomers can see by observing the cosmic microwave background (CMB). “If all density fluctuations are as small as those in the CMB, then you wouldn’t produce any black holes in the early Universe,” explains astrophysicist Anne Green from the University of Nottingham in the UK.

However, theorists have devised models of the early Universe that can generate large density fluctuations at small scales, while still matching CMB observations. Juan García-Bellido, a particle physicist from the Autonomous University of Madrid, has one such model, called hybrid inflation [1]. It predicts that the early Universe spat out large numbers of primordial black holes with a wide range of masses. García-Bellido says that these black holes could resolve several open problems in cosmology. For example, the merging of some of these primordial black holes might explain the high numbers of supermassive black holes observed at large distances in x-ray studies. Primordial black holes might also play a role in generating the fluctuations measured in the cosmic infrared background [2].

But of course, the most exciting possibility is that primordial black holes are numerous enough to account for the dark matter. According to García-Bellido’s model, the Milky Way Galaxy should be swimming in a sea of roughly a trillion black holes. “When you tell people that space may be full of black holes, they get nervous,” García-Bellido says. But he adds that the separations are so large that the probability of a black hole coming near our Solar System is very small.

Still, these black holes should occasionally run into things, in particular, each other. The LIGO-Virgo detections of black hole mergers—five so far—are evidence of a large population of black holes in the 10 solar-mass range. But whether or not this population could be large enough to constitute the dark matter is still under debate.

Recent calculations have shown that if dark matter were made up of black holes weighing between 10 and 300 solar masses, then LIGO should have detected hundreds more mergers in its first run [3]. Turning the argument around: multi-solar-mass black holes can only account for about 1% of the dark matter. “But there are caveats,” admits Yacine Ali-Haïmoud from New York University, who helped set those limits. One of these caveats relates to clustering. García-Bellido explains that primordial black holes could form gravitationally-bound groups similar to star clusters. He believes that gravitational perturbations within these groups would lower the overall merger rate, but the full effect of clustering is still a domain of active research [4].

But there are other reasons to question whether black holes are viable contenders for the mantle of dark matter. Researchers have set bounds on how much of the dark matter could be in black holes based on a variety of data, such as CMB observations [5] and stellar density profiles of dwarf galaxies [6]. One of the earliest constraints came from surveys that searched for the temporary brightening of stars that occurs when a black hole passes between a star and Earth—a so-called gravitational lensing effect. A new study of lensing of supernovae claims to rule out black holes weighing more than 0.01 solar masses being a major component of dark matter, subject to some assumptions (see today’s Viewpoint). “There are lots of constraints that together probably exclude black holes with mass around 10 solar masses making up all the dark matter,” Green says. Even so, she calls herself “on the fence,” as there could still be scenarios—such as clustering—that might skirt around the observational bounds.

Many other researchers have come down on one side or the other of that fence. “The community is a bit polarized,” Ali-Haïmoud says. In his mind, it’s too soon to rule in or rule out black holes as dark matter. In fact, the answer may be something in between, with black holes providing some portion of the dark matter and the rest being accounted for by some sort of new particle, he says. “We have no reason to believe that nature was so kind as to only give us one type of dark matter.”

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“Viewpoint: Supernova Study Dampens Dark Matter Theory”

October 1, 2018
Simeon Bird

A search for lensing of supernovae by black holes comes up empty, leading researchers to conclude that black holes cannot account for all dark matter.

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Figure 1: A supernova explosion of a massive star appears brighter to an observer on Earth if a black hole sits between the explosion and the observer. The black hole’s gravity distorts the path of light emitted by the supernova, acting as a lens that magnifies the light. APS/Carin Cain

Black holes are one of the oldest candidates for dark matter—the unidentified “stuff” that underpins galaxies and makes up 85% of the matter in the Universe. But whether the total mass of black holes is sufficient to account for all the dark matter is unclear. A new analysis of supernovae makes this possibility seem very unlikely [1]. A black hole of sufficient mass passing in front of a supernova should act as a magnifying lens and make the star appear brighter, allowing the black hole to be spotted. But when Miguel Zumalacárregui and Uroš Seljak from the University of California, Berkeley, searched for this effect in over a thousand supernovae, they came up empty handed. Their analysis applies to all black holes with masses greater than 0.01 times that of our Sun and indicates that such objects can account for at most 40% the Universe’s dark matter.

Stephen Hawking first proposed the idea that black holes might contain the Universe’s missing matter in 1974. The idea quickly attracted the attention of other physicists, as it didn’t require some, as yet, undiscovered particle. Black holes can form when massive stars implode, but there haven’t been enough such stars to account for dark matter. However, current theories suggest that huge numbers of black holes could have formed in the early Universe from the gravitational collapse of dense regions of matter and that the total mass of these so-called primordial black holes could be enough to account for dark matter [2] (see accompanying Feature: Controversy Continues over Black Holes as Dark Matter). But so uncertain is this era of the Universe that our best theories make no solid prediction for the mass of primordial black holes: if they exist, they could be as light as 10^−5 g, roughly the same weight as an eyelash, or as heavy as a billion Suns.

Black holes that originate from a collapsed star usually have a visible “halo” of gas and debris swirling around them. However, primordial black holes, if they exist, formed before the first atoms, leaving them disc-free and completely dark. To see such a black hole, researchers therefore look for its gravitational effect on light. Previous searches for black holes via gravitational effects ruled out the existence of large numbers of primordial black holes with masses ranging from 10 down to 10−8

times that of our Sun [3, 4], restricting possible models that allow black holes to explain dark matter. Then, in 2015, the LIGO-Virgo collaboration detected the first gravitational signal from two merging black holes [5]. Their analysis indicated that the black holes they detected each had a mass more than 25 times greater than that of our Sun—heavier than the mass expected for a typical black hole formed from a dying star. The finding fueled speculation that the LIGO-Virgo collaboration had detected primordial black holes, and interest in black holes as dark matter candidates was renewed [6].

In their study, Zumalacárregui and Seljak looked for black holes using the effect of gravitational lensing, the same gravitational effect probed in previous studies. This effect arises when the path of light emitted by an object, say a star, is bent by the gravity of some other object, like a black hole, that sits between the star and Earth. The black hole acts as a lens, making the star appear brighter, with the degree of brightening depending on the black hole’s mass (Fig. 1). The previous lensing experiments searched for black-hole-brightened stars in the Magellanic Clouds—two dwarf galaxies that closely orbit the Milky Way [3, 4, 7]. The duration of the experiments made them sensitive to changes in a star’s brightness over timescales from a few minutes to five years—long enough to detect the transit of black holes with masses of up to 10 times that of our Sun.

The same approach isn’t practical for observing larger black holes, which could take a decade or more to pass by a star. Instead, Zumalacárregui and Seljak turned to lensing-induced brightening of supernovae. The variety of supernovae they observed, type 1a, are standard candles—they have an intrinsic brightness that can be determined by measuring how quickly they fade. But because of lensing, a supernova erupting fortuitously behind a passing black hole will appear brighter than expected from its fade time. Looking for artificially bright supernovae eliminates the need to wait for the black hole to transit the object, providing sensitivity to black hole masses ranging from 0.01 to several thousand times that of the Sun.

The duo analyzed the signals from over 1300 supernovae spread across most of the sky in the Northern hemisphere, finding no brighter-than-expected supernovae. Assuming different black hole abundances, they also calculated the likelihood that a black hole—in the mass range detectable in the experiment—would randomly pass between Earth and one of the supernovae in the time frame of the measurements. Based on this calculation, they conclude that the cumulative mass of these black holes can account for only 40% of dark matter.

The work of Zumalacárregui and Seljak shows that black holes with masses greater than 0.01 times that of our Sun occur in insufficient numbers to account for all the dark matter. The result, taken together with those from other experiments [8], rules out the idea that dark matter could be entirely accounted for with black holes. But the possibility remains that black holes could make up a small fraction of dark matter, with the rest coming from some other potential source like weakly interacting massive particles (WIMPs), sterile neutrinos, or axions. For this reason, experiments will continue to search for black holes. Future supernova surveys will likely further tighten the constraints placed by Zumalacárregui and Seljak on the mass and abundance of black holes in the Universe. For example, the upcoming start-up of the Large Synoptic Survey Telescope, which will be able to image the entire sky in three days, should significantly boost our ability to detect transient events, be they related to black hole lensed supernovae and stars, or something else.

This research is published in Physical Review Letters.

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Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).