Dark matter is one of the most accurately and least accurately named creatures in physics. The “dark” designation is not a mistake. Its noninteraction with any passing photons renders it inaccessible to optical telescopes and current scientific theories cannot yet illuminate its composition. And yet, precisely this lack of understanding is why Neil deGrasse Tyson argues “dark matter” is just as compelling a title as “Fred.” Indeed, physicists have inferred dark matter through its gravitational interaction with baryonic matter, which is the ordinary matter whose building block are atoms. The what remains cloaked in darkness, and this disappointing gap in knowledge translates to an unfamiliarity with around 80% of the observable universe’s content. For all pursuing a scientifically-accurate model of the universe, this is disappointing.
Fortunately, cosmic mysteries beckon our species, compelling us to allocate our intellectual resources to uncovering their nature. And so it has been the task of many a physicist to uncover dark matter, to figuratively illuminate the unilluminable. From the use of mathematical supersymmetry to posit fundamental particles responsible for a galaxy’s strange rotation curve, to the engineering of xenon tanks for operation in such extraordinary areas as abandoned underground mines – dark matter searchers are the epitome of resilience in the midst of perpetual failure.
A more radical explanation includes the idea of primordial black holes, with recent formulations emerging from Harvard’s Center for Astrophysics (CfA). A team of five researchers led by Qirong Zhu analysed a series of computer simulations to determine whether dwarf galaxies are in possession of these black holes namely in their halos. Because the halo’s density distribution should be noticeably different if the halo was composed of exotic particles or primordial black holes, the simulations suggested that the black holes could influence the distribution of a dwarf galaxy’s constituent stars. This becomes an optical indication that these cosmic curiosities lurk within the halo.
This work has largely been an exercise in scientific speculation, and the two prevailing questions that remain are about the origins of these black holes, and whether experimental evidence is attainable. Both invite additional guesswork, with the former entertaining the cosmic microwave background density fluctuations as giving rise to black holes in very dense regions, or even an inherent instability afflicting the Higgs field; an answer to the latter question, however, currently holds more promise.
As the laser interferometer gravitational-wave observatory (LIGO) becomes more sensitive, and once the laser interferometer space antenna (LISA) enters orbit, there will be a likelihood of detecting signals from merging primordial black holes with galaxy halos. The masses of the theoretical black holes are considerably lower than those responsible for the measured GW signals we have recorded until now, so more sensitive instruments may translate to a plethora of signal data to sift through and, by extension, turn this idea into a tenable hypothesis. Even if the findings are fruitless, the narrowing of dark matter candidates will only benefit our chore of unraveling the tangled mess that is the universe.