Scientists challenge Hawking’s black hole theory

Pune: Challenging Stephen Hawking’s theory about primordial black holes as dark matter, two Indian scientists from city working at the city-based Inter-University Centre for Astronomy and Astrophysics (IUCAA), along with their counterparts in Japan and the US, showed that dark matter cannot be entirely made up lunar mass primordial black holes.

Primordial black holes could form immediately after the Big Bang. 

“We do not know what dark matter is made up of. But now, we have mostly eliminated the possibility that it was made up of only primordial black holes as small as the mass of the moon,” said Surhud More and Anupreeta More while talking to Sakal Times.
 
“The possibility of dark matter being made up of such primordial black holes is at most a per cent,” they said. 

“Our observations were sensitive to black holes of mass in the range between 10^{-6} to 10^{-11} of the mass of the Sun and with sizes less than 0.1 millimetre,” said Surhud. 

 “Primordial black holes (PBHs) have long been suggested as a viable candidate for the elusive dark matter (DM). The abundance of such primordial black holes has been constrained using a number of astrophysical observations, except for a hitherto unexplored mass window of MPBH = [10-14, 10-9 ]Msun, Anupreeta added.

Explaining micro-lensing, Anupreeta said, “If a primordial black hole comes in between us and a background star in Andromeda, it can cause the star to change its brightness for a short period of time due to microlensing. After carrying out a seven-hour long observation of the Andromeda galaxy by using the Hyper Suprime-Cam camera on the Subaru telescope, we expected to find many such microlensing events due to primordial black holes, but we could identify only a single candidate event, which translated into the most stringent upper bounds on the abundance of primordial black holes ranged between MPBH 10-11, 10-6) M.”

The Hyper Suprime Cam, an 870-megapixel camera attached to the 8.2 metre Subaru telescope at the summit of Maunakea, allowed researchers to study it and make observations. The researchers took the observations on the night of November 23, 2014, and the paper was accepted for publications in Nature Astronomy in 2019. The entire team was working on it since 2014. The paper went through a rigorous peer review before it was published,” said Surhud. 

The idea of this paper was first put forth by Masahiro Takada, (Kavli IPMU, Japan), while Hiroko Niikura (Kavli IPMU, Japan) worked on the entire analysis. Robert Lupton (Princeton USA), Naoki Yasuda (Kavli IPMU, Japan) developed the software to differentiate the images. 

Surhud worked in the collaboration on the analysis of the events and the resultant constraints, while Anupreeta worked on designing the observation and following up the detected candidate event and understanding its nature.