Why gravitational wave detection may have also revealed dark matter
by Graham Templeton
Gravitational waves communicate information in a wholly different medium than any type of radiation. What that means is that astronomers could use them to study things that were fundamentally invisible before. Black holes and dark matter come to mind as the most prominent examples of parts of the universe that are, at least, difficult to see with light. So, it can hardly be surprising, the controversy now swirling around the idea that the gravitational waves detected earlier this year, marking a watershed confirmation of Einstein’s general relativity, could turn out to contain information on both of those topics.
There’s one theory of dark matter that’s a little different than most of the others, in that it doesn’t posit that the mysterious substance is made of some equally mysterious new WIMP-y particle with airy-fairy new physical attributes, but rather out of regular old black holes. The idea goes back to the moments immediately following the Big Bang, when there could have existed areas of the new universe with great enough gas density that they collapsed straight from gas to a black hole, without becoming stars first.
These black holes, if they were formed, would exist in a different overall mass range than the regular, star-born black holes — some could be as light as an asteroid, and thus incredibly tiny. Some models say that these could have been created in the distribution and abundance necessary for them to cluster in the ways we know dark matter clusters: ringed around spiral galaxies and strung three-dimensionally throughout the universe like a massive, invisible crystal lattice. Could it be, that dark matter is really just regular matter condensed into black holes with attributes we simply haven’t thought to look for?
That’s part of the idea tentatively put forward by researchers from Johns Hopkins University, in a study published in Physical Review Letters. It’s based purely around feasibility — they haven’t shown that these were primordial black holes, but rather that based on the readings collected there’s no way we can rule out that possibility. One major factor pointing in that direction is the weight of the two black holes involved in the historic LIGO detection: they were both too heavy, and too light.
At 36 and 29 times the mass of the sun, respectively, the two black holes were too massive to come from any sensible stellar collapse event, but too light to be the “supermassive” black holes that are thought to lie at the center of galaxies. But primordial black holes, if they do exist, could go from the very small, right up to the masses observed here. These would be very large for primordial black holes, but technically allowable based on preexisting model for their creation.
What this means is not only that gravitational wave detectors could be used to investigate the nature of dark matter, via investigating these black holes, but that gravitational wave detectors already have investigated dark matter. It’s just up to astronomers to do the hard work of looking at the data through the right (conceptual) lens.
Of course, there are problems with this idea as well. For one, we do know that not all of dark matter could be made of primordial black holes. Some of these primordial black holes would be in the stellar collapse mass range, and thus detectable by the gravitational lensing that normally reveals black holes. The lensing that has been done has proven that at least some of the observed discrepancies in the behavior of mass must be due to the gravitational influence of some form of mass that does not interact normally with light.
There could be black holes we’ve missed up until now, but they’d have to be below a certain mass threshold to have remained invisible — and the effects of dark matter are too enormous to be explained by those on their own.
While primordial black holes might exist, and might actually account for some of the mass currently thought of as dark matter, they also can’t account for all of the mass currently thought of as dark matter. It also means that these LIGO black holes really could be “dark matter” black holes, but that such a discovery wouldn’t necessarily solve the mystery of exotic dark matter as most people think of it.
Rather, it would prove that the portion of dark matter that is truly mysterious is smaller than we had thought — or rather, that the universe as it seems to make sense to us might actually comprise a larger proportion of it than we had thought.
Discoveries like this, and similar theories about alternate explanations for dark energy, drive home their nature as catch-all theories, more defined by what they aren’t than what they are. Even today, dark matter is basically defined as anything that makes the laws of physics make sense, in the context of the observed movement of the universe. As physicists learn more about the way the world is, the borders of that negative space contract, slowly honing its definition over time.
The problem with this approach, necessary as it is, is that it can lead you to treat complex systems as monolithic and simple. For instance, it could lead you to over-estimate the amount of dark matter that can’t be regular matter, by lumping invisible-for-now primordial black holes in which invisible-forever dark matter made of the theoretical dark matter particles (WIMPs).
On the other hand, this primordial black hole hypothesis is far from widely accepted. The researchers are careful to note that they haven’t collected any evidence that these were primordial black holes, but they have notably failed to find any indication that they couldn’t be. With increasingly sensitive techniques like “galactoseismology” increasing in prominence, it might not be long before we can start to make definitive statements about what dark matter is — rather than just what it isn’t.
Note: This article was originally published on ExtremeTech.com.