Koushiki

The 21-centimetre (21-cm) signal is one of the most promising probes of the high-redshift Universe. It has typically been modelled without accounting for the effects of active galactic nuclei (AGNs) in the pre-JWST era, primarily due to the lack of observational evidence for AGNs at z > 6. However, following the discovery of several AGNs at redshifts as high as z ∼ 10 by JWST, it has become imperative to incorporate the impact of these early AGNs when predicting the 21-cm signal. Based on the assumption that these AGNs are seeded by primordial black holes (PBHs), we studied their impact with a semi-numerical model setup. Specifically, we extended the explicitly photon-conserving reionisation framework, SCRIPT , including essential cosmic dawn physics and PBH contributions. This enabled us to compute both the global signal and the power spectrum of the 21-cm line over the redshift range z ∼ 30 − 5 within a self-consistent framework. Building on this set-up, we investigated the impact of different PBH mass functions (obeying current observational constraints) on the resulting signal. We find the X-ray heating from PBHs can substantially make the global 21-cm signal shallower and suppress the expected power spectrum amplitude during cosmic dawn. We also find that the choice of mass function plays a crucial role in shaping the 21-cm signal, and can, in fact, lead to significantly different predictions.

We examine the existence of an apparent horizon in the collapse of an isolated dust cloud in spherically symmetric spacetime. The interior geometry is expressed in the Lemaître–Tolman–Bondi coordinates, while the outside geometry is the standard Schwarzschild. The novelty of the approach lies in using the Lemaître coordinates for the Schwarzschild part of the geometry when discussing junction conditions. We provide general formulas for expansions in the marginally bound cloud case and use them to determine the position of the apparent horizon. Our results indicate that in the region of spacetime far from the gravitational singularity, the system under consideration has a horizon, and thus the singularity is not naked. The results are expected to be correct away from the singularity, where quantum effects can be neglected. Near the singularity, general relativity breaks down, and quantum gravity would be of primary importance. The classical formulas for expansion scalars can serve as a starting point for more detailed quantum analysis.

In this paper, working in a geometrical, frame-independent and ansatz-independent formalism, we analytically investigate the gravitational collapse a mass-less scalar field in a spherically symmetric space-time. This is one of the fundamental matter fields determined by the Lagrangian formulation. We identify a single dimensionless parameter, that determines the end-state to be a black-hole or complete dispersal and a locally naked null singularity, which exists as a critical case between the previous two end-states. The governing parameter, being dimensionless, makes the result scale independent, thus confirming the universality and criticality that was observed in extensive numerical studies by various authors.

In this paper, we observe the collapse of a mass-less scalar field covariantly. We show that the strengths of the collapsing and dispersing modes of this scalar field will decide whether the collapse will end up in a black-hole or disperse. We find a locally naked null singularity as a critical case between these two and confirm that there is a single dimensionless parameter that determine the end state. This work is ansatz-independent, hence, true for all mass-less scalar field families. We also show that the geometrical mass of these singularities go to zero.

This paper explores the cosmological implications of a scalar field with a specific potential, crucial for achieving the final equilibrium state of gravitational collapse. We consider a system with two fluids: minimally coupled matter representing dust-like dark matter and a scalar field acting as dark energy. Our model, akin to the top-hat collapse model, focuses on isolated over-dense regions within a closed FLRW metric, while the background follows a flat FLRW metric. We analyze spacetime configurations where these regions undergo initial expansion followed by contraction, deriving the scalar field potential responsible for their equilibrium state. Our fully relativistic approach offers a comprehensive understanding of stable cosmic over-dense regions, without the need for ad-hoc Newtonian virialization.

This paper explores the cosmological implications of a scalar field with a specific potential, crucial for achieving the final equilibrium state of gravitational collapse. We consider a system with two fluids: minimally coupled matter representing dust-like dark matter and a scalar field acting as dark energy. Our model, akin to the top-hat collapse model, focuses on isolated over-dense regions within a closed FLRW metric, while the background follows a flat FLRW metric. We analyze spacetime configurations where these regions undergo initial expansion followed by contraction, deriving the scalar field potential responsible for their equilibrium state. Our fully relativistic approach offers a comprehensive understanding of stable cosmic over-dense regions, without the need for ad-hoc Newtonian virialization.

We study the model of spherically symmetric and spatially homogeneous gravitational collapse of a minimally coupled scalar field. Our study focuses on obtaining the scalar field potential that leads to a final equilibrium state in the gravitational collapse. We demonstrate the existence of a class of scalar field solutions that can indeed result in such an end equilibrium state.