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Cosmologists propose new way to form primordial black holes

What is dark matter? How do supermassive black holes form? ‘Primordial’ black holes might hold the answer to these long-standing questions. Leiden and Chinese cosmologists have identified a new way in which these hypothetical objects could be produced just after the Big Bang. Publication in Physical Review Letters.

In their quest to understand the Universe, scientists are faced with some major unsolved puzzles. For example, stars move around galaxies as if there is five times more mass present than we can see. What makes up this spooky, so-called dark matter? And another riddle: galaxies harbor enormous black holes in their cores, weighing millions of solar masses. In young galaxies, collapsed stars did not have enough time to grow that big. So then how did these so-called supermassive black holes form?

Primordial black holes

Cosmologists pose a hypothetical solution which could solve one of both riddles. Primordial black holes, spawned shortly after the Big Bang, have the ability to either remain tiny or quickly gain mass. In the former case, they are candidates for dark matter. In the latter case, they could serve as seeds for supermassive black holes. Leiden cosmologist Dong-Gang Wang and his Chinese colleagues Yi-Fu Cai, Xi Tong and Sheng-Feng Yan of USTC University have just found a new way in which primordial black holes could have formed around the time of the Big Bang.


After the Big Bang, the Universe contained small density perturbations caused by random quantum fluctuations. These are large enough to eventually form stars and galaxies, but too small to grow into primordial black holes on their own. Wang and his collaborators have identified a new resonance effect which makes primordial black holes possible by enhancing certain perturbations selectively. This leads to the prediction that all primordial black holes should have approximately the same mass. The narrow peaks in figure 1 show a range of possible masses as a consequence of the resonance.

Viable model

‘Other calculations have different ways to enhance perturbations, but run into problems,’ says Wang. ‘We use resonance during inflation, when the Universe grew exponentially shortly after the Big Bang. Our calculations are simple enough so that we can work with it. In reality the mechanism might be more complicated, but this is a start. The narrow peaks that we get are inherent to the mechanism, because it uses resonance.’

Dong-Gang Wang is Leiden de Sitter Cosmology fellow currently working in Ana Achúcarro's group.


Yi-Fu Cai, Xi Tong, Dong-Gang Wang, and Sheng-Feng Yan, ‘Primordial Black Holes from Sound Speed Resonance during Inflation’, Phys. Rev. Lett. 121, 081306

Figure 1: This figure shows the fraction of dark matter due to primordial black holes (vertical axis), as a function of their individual mass in solar masses (horizontal axis). The shaded areas are excluded by astronomical observations. The resonance effect manifests itself as narrow peaks (red and blue dotted lines) that show the mass distribution of primordial black holes. Because the peaks are narrow, all primordial black holes are predicted to have the same mass. For our Universe, there is only one real peak, depending on (still unknown) details of the Big Bang. For instance, the blue peak corresponds to black holes of about 10 – 100 solar masses—the range recently detected by the LIGO/VIRGO gravitational wave experiment.
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