Black holes as dark matter candidates

As the search for WIMPs (Weakly Interacting Massive Particles) continues to be fruitless, advocates for alternatives to dark matter (such as Modified Newtonian Dynamics or MOND theories) to explain cosmic puzzles have been encouraged. But if the history of science is any guide, we should not expect the larger community of scientists to quickly embrace these alternative theories. This is because once a paradigm becomes dominant, as dark matter has, then it will not be dislodged until most of the variations within that framework have been explored and rejected. In the case of dark matter, a fresh search for WIMPs has been launched using extremely sensitive detectors that, if they still fail to detect them, will likely result in WIMPs being ruled out.

Physicists are hatching a plan to give a popular but elusive dark-matter candidate a last chance to reveal itself. For decades, physicists have hypothesized that weakly interacting massive particles (WIMPs) are the strongest candidate for dark matter — the mysterious substance that makes up 85% of the Universe’s mass. But several experiments have failed to find evidence for WIMPs, meaning that, if they exist, their properties are unlike those originally predicted. Now, researchers are pushing to build a final generation of supersensitive detectors — or one ‘ultimate’ detector — that will leave the particles no place to hide.

Over the coming months, operations will begin at three existing underground detectors — in the United States, Italy and China — that search for dark-matter particles by looking for interactions in supercooled vats of xenon. Using a method honed over more than a decade, these detectors will watch for telltale flashes of light when the nuclei recoil from their interaction with dark-matter particles.

Physicists hope that these experiments — or rival WIMP detectors that use materials such as germanium and argon — will make the first direct detection of dark matter. But if this doesn’t happen, xenon researchers are already designing their ultimate WIMP detectors. These experiments would probably be the last generation of their kind because they would be so sensitive that they would reach the ‘neutrino floor’ — a natural limit beyond which dark matter would interact so little with xenon nuclei that its detection would be clouded by neutrinos, which barely interact with matter but rain down on Earth in their trillions every second. “It would be sort of crazy not to cover this gap,” says Laura Baudis, a physicist at the University of Zurich in Switzerland. “Future generations may ask us, why didn’t you do this?”

“The WIMP hypothesis will face its real reckoning after these next-generation detectors run,” says Mariangela Lisanti, a physicist at Princeton University in New Jersey.

But ruling out WIMPs does not mean ruling out dark matter because there are alternative candidates for dark matter such as gravitationally-interacting massive particles (GIMPs), supersymmetric particles, MACHOs (Massive Compact Halo Objects such as primordial black holes), and others.

Black holes as a candidate is enjoying a resurgence of sorts since the detection of gravitational waves by the LIGO detectors suggested that there may be many more black holes around than previously thought, and larger than expected too.

The discovery of these strange specimens breathed new life into an old idea — one that had, in recent years, been relegated to the fringe. We know that dying stars can make black holes. But perhaps black holes were also born during the Big Bang itself. A hidden population of such “primordial” black holes could conceivably constitute dark matter, a hidden thumb on the cosmic scale. After all, no dark matter particle has shown itself, despite decades of searching. What if the ingredients we really needed — black holes — were under our noses the whole time?

“Yes, it was a crazy idea,” said Marc Kamionkowski, a cosmologist at Johns Hopkins University whose group came out with one of the many eye-catching papers that explored the possibility in 2016. “But it wasn’t necessarily crazier than anything else.”

Note that the black hole candidates for dark matter are not the commonly thought of black holes formed by collapsing stars but instead are what are known as ‘primordial black holes’ (PBH) that were created soon after the Big Bang, long before stars, and indeed ordinary matter itself, were formed.

This paper explains the difference between PBH and regular black holes, and why the latter cannot be the source of dark matter.

Ordinary black holes come from baryonic progenitors (i.e. stars) and are hence classified with the baryonic dark matter of the Universe. (They are of course not “baryonic”in other respects, since among other things their baryon number is not defined.) Ordinary black holes are therefore subject to the nucleosynthesis bound, which limits them to less than 5% of the critical density. PBHs are not subject to this bound because they form during the radiation-dominated era, before nucleosynthesis begins. Nothing prevents them from making up most of the density in the Universe. Moreover they constitute cold dark matter because their velocities are low. (That is, they collectively obey a dust-like equation of state, even though they might individually be better described as “radiation-like” than baryonic.) PBHs were first proposed as dark-matter candidates by Zeldovich and Novikov in 1966 and Hawking in 1971.

The idea of these other kinds of black holes permeating ours and other galaxies may sound strange but any time that a dominant paradigm is under stress, stranger and stranger alternatives get explored. It will take a long time to explore all of them.

One way to shorten the process is if a dark matter alternative theory make a prediction that would not have been conceived of under the dark matter paradigm or, better still, contradicts a dark matter prediction. Some predictions have been made and now observational and experimental astrophysicists will have to look for those signatures. If any of those predictions are borne out, that will give a huge boost to the alternative theory.

Ultimately, no single result will be determinative. It is the preponderance of evidence, as adjudicated by the community of scientists in that field, that will determine which theory ends up being dominant. This is why paradigm shifts take such a long time. There are always modifications that can be suggested to patch up the dominant paradigm and ruling those out is not easy.


  1. Rob Grigjanis says

    In the case of dark matter, a fresh search for WIMPs has been launched using extremely sensitive detectors that, if they still fail to detect them, will likely result in WIMPs being ruled out.

    Based on the principle that if you haven’t found it after a set time, it doesn’t exist? The simplest WIMP hypothesis does seem to have been ruled out by recent experiments: a supersymmetric partner of a known particle, interacting via the weak interaction*. But that doesn’t constitute being ruled out.

    To be clear, I’m not married to the WIMP idea. But phrases like “ruled out” should be used with caution.

    *The ‘Weakly Interacting’ in WIMP does not refer to the weak interaction of the Standard Model. It means some interaction as weak as, or weaker, than that.

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