Black holes, not the dark matter physicists are looking for

dark matter
Composite image of the Perseus galaxy cluster using data from NASA’s Chandra X-ray Observatory, ESA’s XMM-Newton and Hitomi, a Japanese-led X-ray telescope. X-ray: NASA/CXO/Fabian

Dark matter, something that is yet to be detected and observed directly is believed to make up a majority of all matter in the universe. Physicists have now ruled out another likely candidate for dark matter- black holes.

For a brief while, physicists around the world did consider attributing dark matter to black holes, which have also not directly been imaged till now. A new study from the University of California, Berkeley, however, has conclusively ruled out black holes now.

Dark matter still remains something that astronomers seem to be in the dark about. Dark matter is believed to make up over 80 percent of all matter in the observable universe and yet, it has not been found. So far astronomers have proposed a wide range of candidates to explain dark matter from ultra-light particles like axions all the way to ultra-dense particles like MACHOs, now even black holes are off the table as far as dark matter is concerened.

This study was based on an analysis of the brightest supernovas ever detected. Physicists analysed 740 separate star explosions. The data included supernovas till 2014, reports Not even one of them were magnified or brightened "gravitational lenses," that should have been there if black holes really were made of dark matter.

Research concluded "primordial black holes"—ones that were created within the first milliseconds of the big bang—could not possibly contain more than 40 percent of the universe's dark matter.

They also found that no percentage of the universe's dark matter consists either of heavy black holes, or "massive compact halo objects" (MACHO).

"I can imagine it being two types of black holes," explains lead author Miguel Zumalacárregui of the Berkeley Center for Cosmological Physics. He also speaks of particles that are extremely dense as well as matter that is really light. "We would be going from something astrophysical to something that is truly microscopic, perhaps even the lightest thing in the universe, and that would be very difficult to explain."

Researchers reached this conclusion based on "gravitational lensing"- the effect that primordial black holes, or just about any massive object with a strong gravitational field would bend and magnify light passing through on its way to Earth. Naturally, gravitational lensing should have this effect on the light from distant supernovas, but they do not.

"You cannot see this effect on one supernova, but when you put them all together and do a full Bayesian analysis you start putting very strong constraints on the dark matter, because each supernova counts and you have so many of them," Zumalacárregui said.

Related topics : Space Universe