When scientists working at the Laser Interferometer Gravitational-Wave Observatory (LIGO) discovered gravitational waves back in February, it marked the end of a century-long search for something physicists knew existed, but couldn’t quite pin down and identify.
Perhaps, we stumbled upon a rare gem with that discovery, and inadvertently killed two 100-year-old birds with one stone. A new paper written by physicists at Johns Hopkins University and published in Physical Review Letters investigates whether the black hole merger that produced gravitational waves and were observed by LIGO also contained a signal that confirms the existence of dark matter.
Dark matter, first hypothesized in 1922, makes up 85 percent of all matter in the universe. But unlike ordinary matter, scientists have never been able to observe it and measure it. We know it exists, because we’ve seen some strange things happening in the universe that could only be the result of some massive accumulation of matter creating a gravitational effect. Unfortunately, it remains hidden — and scientists have spent practically a century looking for it to no avail.
Back in February, scientists used a pair of interferometers to observe the extremely faint signals — chirps — that result from gravitational waves (essentially ripples in spacetime produced by high-energy events). In this instance, it was two black holes — each about 30 times more massive than our sun — colliding into one another 1.3 billion light-years from Earth. That collision released 5.3 × 10^47 joules in energy.
The JHU research team began wondering if the LIGO signal also contained something more — something related to dark matter. The whole investigation focuses around something called primordial black holes (PBHs) — a hypothetical first pitched by Stephen Hawking in 1971 that suggests the early universe was composed of several different dense regions stocked with the raw cosmic materials that make up stellar bodies. While normal black holes are the result of collapsed stars, PBHs were these regions collapsing on themselves. Thus, early stars would have formed close to these PBHs, which are small enough to prop up frequently around the galactic halo — the part of a galaxy where dark matter is thought to primarily exist.
Although the idea that PBHs are really out there has taken a dive in the last decade or so, some physicists still think they might be possible. The JHU team does not necessarily offer up any affirmative evidence that the LIGO signal illustrates dark matter; instead they conclude that the measurements don’t exclude the notion of an anticipated rate for merging PBHs within a galactic halo.
“Distinguishing whether any individual [gravitational wave] event, or even some population of events, are from PBH [dark matter] or more traditional astrophysical sources will be daunting. Still, there are some prospects. Most apparently, PBH mergers will be distributed more like small-scale [dark matter] halos and are thus less likely to be found in or near luminous galaxies than [black hole] mergers from more traditional astrophysical sources.”
In other words, we can’t yet conclude that the LIGO signal wasn’t caused by a PBH associated with dark matter. The JHU team suggests focusing studies on astrophysical masses within galactic halos that can’t conclusively be linked to known sources.
One thing’s for sure: the search for dark matter just got weirder.
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