New research published in Physical Review Letters this week may have put the final nail in the coffin for a theory that suggests dark matter is made of massive, primordial black holes.
In February 2016, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) ushered in a “new era of astronomy” when they announced they had detected gravitational waves from two colliding black holes. Aside from the remarkable nature of the discovery itself, it also revived an old theory of dark matter based on massive astrophysical compact halo objects (MACHOs), ultradense objects that don’t emit light.
Dark matter is thought to account for 85 percent of matter in the universe, but so far physicists have been unable to detect it. Its existence was theorized by Vera Rubin in the late 70s because she couldn’t account for observed galactic rotations based on the amount of visible matter alone. Although the stars on the outskirts of a galaxy should’ve been rotating far slower than those closer to the galactic center, observations revealed that the galactic rotations of inner and outer stars was effectively the same. This suggested that there was some hidden matter exerting gravitational force on the outer stars. The big question was what this “dark” matter was made of.
Over the last few decades, a number of dark matter candidates have been proposed. Today, some of the leading candidates are particles like axions or weakly interacting massive particles (WIMPs). But a few decades ago, MACHOs were considered to be one of the most likely explanations for dark matter. The MACHOs theory suggests that dark matter actually consists of baryonic matter (the ordinary matter that you can see) that is so dense it hardly emits any radiation. These massive objects include neutron stars, rogue planets, and theoretical primordial black holes that were formed shortly after the Big Bang and have been roaming the cosmos ever since.
MACHOs fell out of vogue in the late 90s as scientists started looking at particle candidates instead, but LIGO’s discovery renewed interest in black holes as a possible explanation for the missing dark matter.
Since MACHOs don’t emit any radiation and would thus appear totally “dark” to an observer, researchers expected to detect them through gravitational lensing. This phenomenon bends light toward an observer due to the gravitational field of an ultradense object passing between the observer and a distant star. This has the effect of increasing the brightness of the star as it passes behind the black hole.
Miguel Zumalacaregui and Uros Seljak, two physicists from the University of California, Berkeley, recently did a sophisticated analysis of data on 740 supernovae—an extremely bright star explosion—to look for evidence of gravitational lensing caused by primordial black holes. Supernovae are often used by astronomers to measure large distances in the universe because they are so bright and their slow fade in brightness can be precisely measured.
According to their analysis, the researchers concluded that eight supernovae from the 740 should be brighter by a few tenths of a percent due to the effects of gravitational lensing from primordial black holes if these did actually account for the missing dark matter in the universe. Yet when the researchers examined the data, they didn’t find a single supernova that exhibited an increase in brightness indicative of a primordial black hole.
While this doesn’t entirely eradicate the possibility that black holes are the source of dark matter, it does place an upper limit on how much dark matter black holes can account for. According to the researchers, their analysis means that no more than 40 percent of the dark matter in the universe can be attributed to black holes. According to the researchers, an unpublished follow up study of 1,048 black holes brings that maximum all the way down to 23 percent.
Although it is possible that dark matter can be attributed to several different sources, say a mix of ultralight particles and ultradense black holes, trying to explain why the mass of dark matter ranges over 90 orders of magnitude makes for incredibly complex theories.
“We are back to the standard discussions—what is dark matter?” Seljak said in a statement. “Indeed, we are running out of good options. This is a challenge for future generations."