In an article published Monday, UC Berkeley researchers disproved existing notions that dark matter is mostly composed of black holes.
The research team consisted of lead author Miguel Zumalacárregui, who is a Marie Curie Global Fellow at the Berkeley Center for Cosmological Physics, and campus astronomy and physics professor Uroš Seljak. Zumalacárregui and Seljak received help from supernova experts in the campus astronomy department and the Lawrence Berkeley National Laboratory when analyzing data, Zumalacárregui said.
According to a press release from Berkeley News, the makeup of dark matter is one of astronomy’s biggest mysteries. Dark matter theoretically constitutes 84.5 percent of all matter in the universe, but “no one can find it,” according to the press release. Zumalacárregui said many scientific models suggest that much of this missing matter is black holes.
According to Zumalacárregui, he and Seljak conducted research by searching for signs of black holes using supernovas for guidance. He added that black holes are invisible, so researchers look for their effects on surrounding objects, such as stars. In this case, they looked for effects of gravitational lensing on type Ia supernovas — the same type of supernova used to prove that the universe’s expansion is accelerating.
“We know well how bright supernovas are, so we can compare whether there is a black hole between us and the supernova,” Zumalacárregui said. “For instance, the black hole would act like a lens and magnify the supernova, making it brighter.”
The amount of gravitational lensing detected determined that at most 40 percent of dark matter could be black holes, which disproves many existing models, Zumalacárregui said. He added that it was “appealing” to believe that much of dark matter consisted of black holes, but now scientists know that something else makes up the majority of dark matter.
According to the press release, Zumalacárregui and Seljak used 740 of the brightest supernovas to determine the 40 percent limit. But since completing the research included in the study published Monday, they have conducted a “reanalysis” using a more extensive list of 1,048 supernovas. Though the reanalysis is unpublished, the researchers found that the percent of dark matter that could be black holes is actually even lower, at about 23 percent rather than 40 percent.
Zumalacárregui said people are “mainly excited and interested” in his findings and that the article was met with “good reception.” He added that he had received many invitations to give talks on his findings in multiple departments.
“This is a very interesting study that sets observational limits on how much of the dark matter might consist of primordial black holes,” campus astronomy professor Alex Filippenko said in an email. “Though there were already several plausible arguments (some of them theoretical) against the hypothesis that such black holes constitute most of the dark matter, this particular research using real data supports our general skepticism.”