The University of Tokyo · Physics and Astronomy
Professor Jun’ichi Yokoyama’s research lab specializes in theoretical cosmology, focusing on the formation and astrophysical implications of primordial black holes (PBHs) in the early universe. The lab investigates inflationary dynamics, particularly chaotic and new inflation scenarios with non-trivial scalar field potentials, and examines how primordial density fluctuations lead to PBH formation. Key research directions include the spectrum of curvature perturbations during inflation, the statistical and gravitational collapse dynamics of density peaks, and cosmological constraints on PBH abundance across a wide mass range—from Planck-scale relics to supermassive black hole candidates.
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It is shown that in a number of scalar potentials with an unstable local maximum at the origin chaotic inflation is followed by new inflation if model parameters are appropriately chosen. In this model density fluctuation can have a large-amplitude peak on the comoving Hubble scale at the onset of the slow-roll new inflation and can result in the formation of an appreciable amount of primordial black holes on astrophysically interesting mass scales.
We show that it is extremely difficult and perhaps even impossible to have inflation supported by thermal effects.
The mass function of primordial black holes created through the near-critical gravitational collapse is calculated in a manner fairly independent of the statistical distribution of underlying density fluctuation, assuming that it has a sharp peak on a specific scale. Comparing it with various cosmological constraints on their mass spectrum, some newly excluded range is found in the volume fraction of the region collapsing into black holes as a function of the horizon mass.
The spectrum of curvature perturbation generated during inflation is studied in the case the inflation-driving scalar field (inflaton) φ crosses over its potential extremum. It is shown that the nondecaying mode of perturbation has a finite value and a proper formula is given. The result is also extended to more general cases where φ̈ is nonnegligible.
We update the constraints on the fraction of the Universe that may have gone into primordial black holes (PBHs) over the mass range 10<sup>-5</sup>to 10<sup>50</sup> g. Those smaller than ∼10<sup>15</sup> g would have evaporated by now due to Hawking radiation, so their abundance at formation is constrained by the effects of evaporated particles on big bang nucleosynthesis, the cosmic microwave background (CMB), the Galactic and extragalactic<i>γ</i>-ray and cosmic ray backgrounds and the possib
As a nonbaryonic explanation of massive compact halo objects, a phenomenological model is presented which predicts formation of primordial black holes at a desired mass scale. The required feature of initial density fluctuation is realized making use of the primordially isocurvature fluctuation generated in an inflationary universe model with multiple scalar fields.
A new scenario of cosmic strings is presented which is free from the gravitational-radiation constraints imposed on their line density by the primordial nucleosynthesis and the timing data of a millisecond pulsar. In this scenario the phase transition is induced nonthermally during the inflation so that it is not necessary to assume Friedmann-Robertson-Walker universe in thermal equilibrium at the outset.
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