arXiv subjects

Tomohiro Nakama

Publications and source records attributed to Tomohiro Nakama.

25 records · Page 2Linked to original sources

arXiv2015

Cosmological long-wavelength solutions and primordial black hole formation

We construct cosmological long-wavelength solutions without symmetry in general gauge conditions compatible with the long-wavelength scheme. We then specify the relationship among the solutions in different time slicings. Applying this general framework to spherical symmetry, we derive the correspondence relation between long-wavelength solutions in the constant mean curvature slicing with conformally flat spatial coordinates and asymptotic quasihomogeneous solutions in the comoving gauge and compare the numerical results of PBH formation in these two different approaches. To discuss the PBH formation, it is convenient and conventional to use $\tildeδ_{c}$, the value which the averaged density perturbation at threshold in the comoving slicing would take at horizon entry in the lowest-order long-wavelength expansion. We numerically find that within compensated models, the sharper the transition from the overdense region to the FRW universe is, the larger the $\tildeδ_{c}$ becomes. We suggest that, for the equation of state $p=(Γ-1)ρ$, we can apply the analytic formulas for the minimum $\tildeδ_{c, {\rm min}}\simeq [3Γ/(3Γ+2)]\sin^{2}\left[π\sqrt{Γ-1}/(3Γ-2)\right]$ and the maximum $\tildeδ_{c, {\rm max}}\simeq 3Γ/(3Γ+2)$. As for the threshold peak value of the curvature variable $ψ_{0,c}$, we find that the sharper the transition is, the smaller the $ψ_{0,c}$ becomes. We analytically explain this feature. Using simplified models, we also analytically deduce an environmental effect that $ψ_{0,c}$ can be significantly larger (smaller) if the underlying density perturbation of much longer wavelength is positive (negative).

gr-qc↗

arXiv2014

Testing scenarios of primordial black holes being the seeds of supermassive black holes by ultracompact minihalos and CMB $μ$-distortions

Supermassive black holes and intermediate mass black holes are believed to exist in the Universe. There is no established astrophysical explanation for their origin and considerations have been made in the literature that those massive black holes (MBHs) may be primordial black holes (PBHs), black holes which are formed in the early universe (well before the matter-radiation equality) due to the direct collapse of primordial overdensities. This paper aims at discussing the possibility of excluding the PBH scenario as the origin of the MBHs. We first revisit the constraints on PBHs obtained from the CMB distortion that the seed density perturbation causes. By adopting a recent computation of the CMB distortion sourced by the seed density perturbation and the stronger constraint on the CMB distortion set by the COBE/FIRAS experiment used in the literature, we find that PBHs in the mass range $6\times 10^4~M_\odot \sim 5 \times 10^{13}~M_\odot$ are excluded. Since PBHs lighter than $6 \times 10^4~M_\odot$ are not excluded from the non-observation of the CMB distortion, we propose a new method which can potentially exclude smaller PBHs as well. Based on the observation that large density perturbations required to create PBHs also result in the copious production of ultracompact minihalos (UCMHs), compact dark matter halos formed at around the recombination, we show that weakly interacting massive particles (WIMPs) as dark matter annihilate efficiently inside UCMHs to yield cosmic rays far exceeding the observed flux. Our bound gives severe restriction on the compatibility between the particle physics models for WIMPs and the PBH scenario as the explanation of MBHs.

astro-ph.CO↗

arXiv2014

The double formation of primordial black holes

Primordial black holes (PBHs) are a useful tool in cosmology to probe primordial inhomogeneities on small scales that reenter the Hubble radius during the radiation dominated epoch. In this paper, a phenomenon we call the double formation of PBHs, described below, is explored. Suppose there exists a highly perturbed region which will collapse to form a PBH after the horizon crossing of this region, and farther that this region is superposed on much larger region, which also collapses upon reentry. One then expects the collapse of the central smaller region at the time of the crossing of this region, followed by another collapse of the larger perturbation at the time of its respective crossing. The smaller PBH, formed earlier, should be swallowed in the second collapse leading to a single larger PBH as the final state. This paper reports the first direct numerical confirmation of such double PBH formation. Related to this, we also discuss the effects of high-frequency modes on the formation of PBHs.

gr-qc↗

arXiv2014

Reheating the Universe Once More: The Dissipation of Acoustic Waves as a Novel Probe of Primordial Inhomogeneities on Even Smaller Scales

We provide a simple but robust bound on the primordial curvature perturbation in the range $10^4 {\rm Mpc}^{-1} < k < 10^5 {\rm Mpc}^{-1}$, which has not been constrained so far unlike low wavenumber modes. Perturbations on these scales dissipate the energy of their acoustic oscillations by the Silk damping after primordial nucleosynthesis but before the redshift $z \sim 2 \times 10^6$ and reheat the photon bath without invoking CMB distortions. This {\it acoustic reheating} results in the decrease of the baryon-photon ratio. By combining independent measurements probing the nucleosynthesis era and around the recombination epoch, we find an upper bound on the amplitude of the curvature perturbation over the above wavenumber range as ${\cal P}_ζ< 0.06$. Implications for super massive black holes are also discussed.

astro-ph.CO↗

arXiv2014

Investigating formation condition of primordial black holes for generalized initial perturbation profiles

gr-qc↗

arXiv2014

Identifying the most crucial parameters of the initial curvature profile for primordial black hole formation

Primordial black holes (PBHs) are an important tool in cosmology to probe the primordial spectrum of small-scale curvature perturbations that reenter the cosmological horizon during radiation domination epoch. We numerically solve the evolution of spherically symmetric highly perturbed configurations to clarify the criteria of PBHs formation using an extremely wide class of curvature profiles characterized by five parameters, (in contrast to only two parameters used in all previous papers) which specify the curvature profiles not only at the central region but also at the outer boundary of configurations. It is shown that formation or non-formation of PBHs is determined entirely by only two master parameters one of which can be presented as an integral of curvature over initial configurations and the other is presented in terms of the position of the boundary and the edge of the core.

gr-qc↗

arXiv2012

Self-consistent initial conditions for primordial black hole formation

For an arbitrary strong, spherically symmetric super-horizon curvature perturbation, we present analytical solutions of the Einstein equations in terms of asymptotic expansion over the ratio of the Hubble radius to the length-scale of the curvature perturbation under consideration. To obtain this solution we develop a recursive method of quasi-linearization which reduces the problem to a system of coupled ordinary differential equations for the $N$-th order terms in the asymptotic expansion with sources consisting of a non-linear combination of the lower order terms. We use this solution for setting initial conditions for subsequent numerical computations. For an arbitrary precision requirement predetermined by the intended accuracy and stability of the computer code, our analytical solution yields optimal truncated asymptotic expansion which can be used to find the upper limit on the moment of time when the initial conditions expressed in terms of such truncated expansion should be set. Examples of how these truncated (up to eighth order) solutions provide initial conditions with given accuracy for different radial profiles of curvature perturbations are presented.

gr-qc↗