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Do black holes from early Universe
really make up all of the dark matter?

International team comes with significant results challenging Prof. Stephen Hawking’s hypothesis

An international team of researchers from Japan, India and USA has arrived at significant results which question the predictions made by Prof. Stephen Hawking that black holes formed in early Universe could contribute significantly to dark matter in the Universe. This finding is a part of newly published research paper in peer reviewed journal Nature Astronomy which was released on April 1, 2019. The research team involved Dr. Surhud More and Dr. Anupreeta More from IUCAA.

Dark Matter is widely considered to consist of about 85% of the matter in the Universe. It is notable that until now, attempts to detect dark matter particles directly via underground experiments, or accelerator experiments such as the Large Hadron Collider (LHC) by CERN have failed. In 1971, Prof. Hawking showed the possibility that black holes could form very early in the Universe. Since the actual nature of dark matter particles is a mystery, he predicted that that these primordial black holes (made up of ordinary matter) could behave like dark matter. Given the failures of other experiments in detecting dark matter particles, there was considerable interest in the scientific community to revisit and test the predictions of Prof. Hawking on the nature of dark matter through other rigorous experiments.

The team used gravitational lensing to look for primordial black holes between us and our neighbour, the Andromeda galaxy. Albert Einstein was among the first few scientists who predicted Gravitational lensing. This results in bending of light rays, coming from a distant star, due to gravitation of a massive object in between such as a early universe black hole. In extreme cases, the light from the background star gets magnified, an effect which can be detected using powerful telescopes.

Considering how early universe black holes are expected to move in interstellar space, the research team recorded multiple images of Andromeda galaxy for a period of one night using the Hyper Suprime-Cam on the Japanese Subaru Telescope located in Hawaii. Researchers were hoping to find many instances of stars in Andromeda galaxy, gravitationally lensed by intervening early universe black holes. Gravitational lensing is very rare as it requires a distant star, black hole and the observer on earth to be well-aligned.

The use of Subaru telescope was essential to capture any rare events of gravitational lensing happening in the direction to Andromeda galaxy. Subaru telescope with a primary mirror of 8.2 meter, made it possible to image the entire disk of the Andromeda galaxy in one shot. From 190 consecutive images of Andromeda galaxy recorded from Subaru telescope, researchers expected to find 1000 events if all of dark matter was made up of primordial black holes. However the team could identify at most one case of this kind. Thus, their results have now confirmed that primordial black holes with masses similar or less massive than the moon cannot contribute more than a percent of all dark matter.

Significance of this research: Observations from this study rule out the possibility that primordial black holes with masses comparable to the moon and sizes of about 0.1 mm can make up most of the dark matter. This 'negative result' vis-a-vis the predictions of Prof. Hawking about early universe black holes, have resulted in further advancement of our knowledge about the constituent elements of the Universe. This study imposes stringent constraints on the physics of the early Universe and thus disfavours Prof. Hawking's hypothesis.

About Research Paper:

(a) Title: Microlensing constraints on primordial black holes with the Subaru/HSC Andromeda observation

Citation: Niikura, H. et al., Nature Astronomy, April 1, 2019,
DOI: https://doi.org/10.1038/s41550-019-0723-1 (link accessible to Nature subscribers)
Full Text: arXiv:https://arxiv.org/abs/1701.02151

(b) Figures - Credit: Kavli IPMU

Figuer 1
Figure 1: As the Subaru Telescope on Earth looks at the Andromeda galaxy, a star in Andromeda will become significantly brighter if a primordial black hole passes in front of the star. As the primordial black hole continues to move out of alignment, the star will also turn dimmer (go back to its original brightness).

Figuer 2
Figure 2: Constraints on the mass fraction of primordial black holes to dark matter in the Milky Way and the Andromeda galaxy as a function of primordial black hole mass. Shaded regions show excluded regions where existence of such primordial black holes are not consistent with various observation data. The red color indicates the area where the current study has contributed to the study of primordial black holes. One-night HSC/Subaru gives the most stringent constraints for primordial black holes with masses lighter than the moon, e.g. compared to the NASA Kepler 2-year data.

(c) Co-Authors and Affiliation

Authors: Hiroko Niikura(1,2), Masahiro Takada(1), Naoki Yasuda(1), Robert H. Lupton(3), Takahiro Sumi(4), Surhud More(1,5), Toshiki Kurita(1,2), Sunao Sugiyama(1,2), Anupreeta More(1,5), Masamune Oguri(1,2,6), Masashi Chiba(7)
  1. Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Chiba, Japan
  2. Physics Department, The University of Tokyo, Tokyo, Japan
  3. Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA
  4. Department of Earth and Space Science, Osaka University, Toyonaka, Japan
  5. Inter-University Centre for Astronomy and Astrophysics, Pune, India
  6. Research Center for the Early Universe, University of Tokyo, Tokyo, Japan
  7. Astronomical Institute, Tohoku University, Sendai, Japan

(d) About Nature Astronomy

Nature Astronomy is a monthly, online-only journal, launched in January 2017. It publishes the most significant research, review and comment at the cutting edge of astronomy, astrophysics, and planetary science in order to represent and foster closer interaction between all key astronomy-relevant disciplines. Find out more at https://www.nature.com/natastron/volumes/3/issues/4

Research contacts:
Surhud More Surhud More
Associate Professor,
The Inter University Center for Astronomy and Astrophysics,
Tel.: +91-20-25604-120 (Off.)
E-mail: surhud_at_iucaa.in
* please change _at_ to @
Anupreeta More Anupreeta More
Scientific & Technical Officer - E (R&D),
The Inter University Center for Astronomy and Astrophysics
Tel.: +91-20-25604-121 (Off.)
E-mail: anupreeta_at_iucaa.in
* please change _at_ to @