Seminars

I introduce a higher-derivative action for dark matter whose energy-momentum tensor describes an imperfect fluid with nonzero pressure, energy flux, and anisotropic stress. On a homogeneous background (e.g. in cosmology), the dynamics coincides with that of pressureless dust, while in the presence of inhomogeneities the higher-derivative terms generate nonzero acceleration and vorticity. This makes it possible to avoid the formation of caustic singularities, even if the strong energy condition is satisfied.
[Based on arXiv:2510.23838 [gr-qc]]

In this talk, I will present our study of the Schwinger effect for charged scalar fields in charged black holes embedded in de Sitter spacetime. We analyze charge emission in two near-extremal limits of charged de Sitter black holes: the cold black hole and the Nariai black hole. In the cold black hole limit, the near-horizon geometry is AdS, and the Breitenlohner–Freedman bound can prevent the evolution toward a singular spacetime. In contrast, the near-horizon geometry of the Nariai black hole is dS, which allows for catastrophic charge emission. Furthermore, we uncover a reciprocal relation between the mean number of produced fermions in de Sitter and anti-de Sitter spaces, provided that the spacetime curvature is analytically continued.

We consider cubic interactions of massless fields of arbitrary spin in 4d spacetime, within the lightcone formalism. We extend two key results from flat to (Anti-)de Sitter spacetime. First, we present a simple explicit expression for all the cubic vertices. Second, we prove that the various self-dual/chiral theories, such as Self-Dual Yang-Mills, Self-Dual General Relativity and Chiral Higher-Spin Gravity, are fully consistent with no need for vertices beyond cubic. 

We study the quantum aspects of the conformal gravity in four dimensions, specifically addressing a known discrepancy in beta functions between general quadratic curvature theories and conformal gravity, which corresponds to two scalar degrees of freedom. We demonstrate that this mismatch is resolved by carefully introducing gauge-fixing and ghost terms via the BRST symmetry, which effectively adds the two scalar modes. Drawing lessons from two-dimensional quantum gravity and Liouville theory, we proceed to integrate the four-dimensional trace anomaly to derive a consistent Liouville action, which is given by a free-field action for the conformal mode with a consistent conformal anomaly. Finally we give the condition that the BRST transformation is anomaly free.

Primordial black holes (PBHs) are believed to form through the gravitational collapse of overdense regions in the early Universe. They may serve as seeds for galaxy formation and are also considered one of the important candidates for cold dark matter (DM). 

In particular, I will focus on several representative toy models of single-field inflation. The enhanced primordial perturbations in these models can not only produce PBHs, but also generate gravitational waves through higher-order effects. I will further extend the discussion to the possibility of a PBH-dominated era, which could leave observable signatures if PBH evaporation produces stable relics. These studies demonstrate the significant potential of PBHs as probes of the early Universe, naturally leading to the important question of how to accurately estimate the PBH abundance. In the latter part of the talk, I will introduce a method based on peaks theory for estimating the abundance of primordial black holes. Our approach works well for arbitrary forms of the power spectrum, and by incorporating more systematic statistical methods, we expect it to provide useful cross-checks in combination with future gravitational-wave observations and related cosmological probes.

Understanding the quantum nature of gravity is an important challenge in modern physics. In recent years, following the direct detection of gravitational waves, the search for new physics using gravitational waves has progressed. Against this background, we analyze gravitational waves from binary black holes within the framework of quantum theory. We show that these gravitational waves can be well described by coherent states. We also estimate the magnitude of the squeezing parameter for binary black hole systems. In this talk, I will explain these results in detail and discuss their possible applications as a method for probing the physics of the early Universe.

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