arXiv subjects

Zhi-Chao Li

Publications and source records attributed to Zhi-Chao Li.

13 recordsLinked to original sources

arXiv2026

Inflation on the lattice: scalar Gauss-Bonnet single field inflation

We use lattice methods to study inflation in the scalar Gauss-Bonnet (SGB) gravity theory. We focus on the ultra-slow-roll scenario with the peak frequency falls into the PTA band. In the parameter range we consider, we find that the lattice results exceed the perturbative predictions, which differs from the case in Einstein gravity. We further find that lattice corrections become significant when the peak of the primordial curvature spectrum reaches $\sim 10^{-2}$. We calculate the energy density spectra of second-order scalar induced gravitational waves (SIGWs) using the primordial power spectra obtained from both the lattice method and the traditional perturbative method, and analyze the SGB model in light of current pulsar timing array (PTA) observations. Our results indicate that lattice corrections enhance the ability of the SGB model to dominate the PTA observations.

astro-ph.CO↗

arXiv2026

Classification of Oppenheimer-Snyder Collapse: Singular, Bouncing, and Soft-Landing Scenarios

We study Oppenheimer-Snyder (OS) gravitational collapse matched to a general static, spherically symmetric exterior spacetime. Unlike the Schwarzschild case, two new features can arise in black holes with two horizons: an apparent-horizon minimum, a temporary minimum in the apparent-horizon radius during collapse, and a bounce, where the star surface stops collapsing at a nonzero radius and reverses into expansion. We identify the conditions that lead to these two features. For two-horizon exteriors, trapped-region consistency requires that the apparent-horizon turning point occurs no earlier than the surface crossing of the inner horizon. As a concrete example, the OS collapse of the Reissner-Nordström (RN) spacetime shows both effects. In contrast, regular black holes with de Sitter cores show neither: their collapse is smooth and monotonic, and the surface approaches the center only as the proper time goes to infinity. These results naturally classify the OS collapses into three categories: singular, which ends at the center in finite time; bouncing, which reverses at a finite radius; and soft-landing, which reaches the center only asymptotically. We argue that these features are consistent with Penrose's strong cosmic censorship conjecture.

gr-qc↗

arXiv2026

Matter Maps to Geometry in Gravitational Collapse

We establish an exact bidirectional map between the effective homogeneous interior density of a collapsing star and the exterior metric function in generalized Oppenheimer--Snyder collapse. The Darmois--Israel junction conditions reduce this relation to a purely algebraic form, providing a direct way to reconstruct candidate static geometries and their surface dynamics without solving the corresponding differential field equations separately. Correction powers diagnose models: integer exponents signal ultraviolet completions, while fractional powers identify phenomenological ones. Our framework not only simplifies the construction of regular black holes but also provides a universal benchmark for testing singularity resolution and cosmic censorship in quantum gravity phenomenology.

gr-qc↗

arXiv2026

Lorentz-Violating (Regular) Black Holes in Einstein Gravity

We introduce a minimally coupled dark sector based on a nonlinear electrodynamics to generate regular spacetime textures that interpolate from a regular core to an asymptotic Lorentz-violating conical geometry. This construction provides a simple mechanism, in Einstein gravity with minimally coupled matter, for obtaining Lorentz-violating Schwarzschild and Reissner-Nordström black holes. The framework further allows us to construct Lorentz-violating regular black holes, including the Bardeen and Hayward black holes, as well as a class of electrically-charged regular black holes.

gr-qc↗

arXiv2026

When Bumblebee Meets NLED: Lorentz-Violating Black Holes and Regular Spacetimes

We construct charged black hole solutions in bumblebee gravity coupled to a general class of nonlinear electrodynamics (NLED) using an auxiliary Maxwell-scalar formalism. The norm-fixed radial configuration of the bumblebee vector makes the solutions asymptotic to a conical Lorentz-violating vacuum and requires stringent nonminimal bumblebee-NLED couplings. The general black hole solutions contain independent mass and charge parameters. There are two sources of singular behavior at the center: one is due to the Schwarzschild-type pole and the other is the residual conical singularity of the Lorentz-violating vacuum. By fine-tuning the mass-charge relation, one can generally remove the pole singularity, giving rise to marginally regular black holes. For a suitable NLED theory such as Born-Infeld theory, both singularity sources can be removed at the cost of requiring both the mass and the charge to be fine-tuned to specific functions of the coupling constants. The resulting solutions describe regular horizonless spacetimes interpolating from AdS or dS cores to Lorentz-violating vacua.

gr-qc↗

arXiv2026

Probing small-scale primordial power spectra with induced gravitational waves

Large-scale primordial perturbations have been well constrained by current cosmological observations, but the properties of small-scale perturbations remain elusive. This study focuses on second-order induced gravitational waves generated by large-amplitude primordial scalar and tensor perturbations on small scales. In this case, the induced gravitational waves include contributions from three types of source terms: scalar-scalar, tensor-scalar, and tensor-tensor. To distinguish them from second-order scalar induced gravitational waves (SIGWs), we refer to those generated by these three source terms as tensor-scalar induced gravitational waves (TSIGWs). We provide the analytical expressions for the kernel functions and the corresponding energy density spectra of second-order TSIGWs. By combining observations of stochastic gravitational wave background (SGWB) across different scales, TSIGWs can be used to constrain small-scale primordial curvature perturbations and primordial gravitational waves. Furthermore, we discuss the feasibility of TSIGWs dominating the current pulsar timing array (PTA) observations under various primordial power spectra scenarios. Our results indicate that TSIGWs generated by monochromatic primordial power spectra might be more likely to dominate the current PTA observations.

astro-ph.CO↗

arXiv2025

The Upper Bound of Event Horizon Formation Time in Generalized Oppenheimer-Snyder Collapse

We prove that, in the framework of the Oppenheimer-Snyder collapse, the Schwarzschild exterior maximizes the event horizon formation time $ΔT_{\text{eh}}=\frac{19}{6}m$ among all asymptotically flat, static, spherically-symmetric black holes with the same ADM mass $m$ that satisfy the weak energy condition. This bound extends the typical black hole inequalities--such as the Penrose inequality, which constrains spatial geometry--to temporal setting.

gr-qc↗

arXiv2025

Cosmological constraints on small-scale primordial non-Gaussianity

In contrast to the large-scale primordial power spectrum $\mathcal{P}_ζ(k)$ and primordial non-Gaussianity $f_{\mathrm{NL}}$, which are strictly constrained, the small-scale $\mathcal{P}_ζ(k)$ and $f_{\mathrm{NL}}$ remain less restricted. Considering local-type primordial non-Gaussianity, we study the PBH and SIGW caused by large-amplitude small-scale primordial power spectrum. By analyzing current observational data from PTA, CMB, BAO, and abundance of PBH, and combining them with the SNR analysis of LISA, we rigorously constrain the parameter space of $\mathcal{P}_ζ(k)$ and $f_{\mathrm{NL}}$. Furthermore, we examine the effects of different shapes of the primordial power spectrum on these constraints and comprehensively calculate the Bayes factors for various models. Our results indicate that SIGW generated by a monochromatic primordial power spectrum are more likely to dominate current PTA observations, with the corresponding constraint on the primordial non-Gaussian parameter being $-10.0<f_{\mathrm{NL}}<1.2$.

astro-ph.CO↗

arXiv2025

Tensor induced gravitational waves

Primordial gravitational waves on small scales are not tightly constrained by current cosmological observations, which allows for the possibility of large amplitudes at small scales. We investigate second-order tensor induced gravitational waves (TIGWs) sourced by primordial gravitational waves and present the corresponding corrections to the total energy density spectrum of gravitational wave. We analyze primordial gravitational waves with large amplitudes generated by various models at small scales. Our results indicate that when primordial gravitational waves on small scales sufficiently dominate the current PTA observations, corrections to the total energy density spectrum from second-order TIGWs may become pronounced in certain frequency bands.

astro-ph.CO↗

arXiv2025

Can tensor-scalar induced GWs dominate PTA observations ?

Observational constraints on small-scale primordial gravitational waves are considerably weaker than those on large scales. We focus on scenarios with significant primordial gravitational waves and curvature perturbations on small scales, studying the energy density spectrum of the second-order TSIGW. By leveraging current data from CMB, BAO, and PTA, combined with the SNR analysis of LISA, we can investigate how tensor-scalar induced gravitational waves affect observations on various scales, thus constraining the parameter space for primordial gravitational waves and curvature perturbations. The Bayes factor analysis suggests that TSIGW+PGW might be more likely to dominate current PTA observations compared to SMBHB.

astro-ph.CO↗

arXiv2024

Regular Electric Black Holes from EMS Gravity

We construct Einstein-Maxwell-Scalar (EMS) theories that admit regular electric black holes. Such a Maxwell-scalar theory is equivalent to some nonlinear electrodynamics (NLED) at the level of equations of motion, but it has the advantage of circumventing the no-go theorem of regular electric black holes under a given Lagrangian of NLED. We study the thermodynamics and show that the mass of the regular black hole can be determined solely by the Maxwell field, without having to know the metric profile function. Our formalism allows to study the applications of the electrically-charged regular black holes in areas that were previously available only to the non-regular ones.

gr-qc↗

arXiv2023

Regular Black Holes and Stars from Analytic $f(F^2)$

We construct regular black holes and stars that are geodesically complete and satisfy the dominant energy condition from Einstein-$f(F^2)$ gravities with several classes of analytic $f(F^2)$ functions that can be viewed as perturbations to Maxwell's theory in weak field limit. We establish that regular black holes with special static metric ($g_{tt} g_{rr}=-1$) violate the strong energy condition and such a regular black hole with Minkowski core violates the null energy condition. We develop a formalism to perform electromagnetic duality transformations in $f(F^2)$. We obtain two new explicit examples where the duality is a symmetry. We study the properties of the corresponding dyonic black holes. We study the geodesic motions of a particular class of solutions that we call repulson stars or black holes.

gr-qc↗

arXiv2013

Detection of a Majorana-fermion zero mode by a T-shaped quantum-dot structure

Electron transport through the T-shaped quantum-dot (QD) structure is theoretically investigated, by considering a Majorana zero mode coupled to the terminal QD. It is found that in the double-QD case, the presence of the Majorana zero mode can efficiently dissolve the antiresonance point in the conductance spectrum and induce a conductance peak to appear at the same energy position whose value is equal to $e^2/2h$. This antiresonance-resonance change will be suitable to detect the Majorana bound states. Next in the multi-QD case, we observe that in the zero-bias limit, the conductances are always the same as the double-QD result, independent of the parity of the QD number. We believe that all these results can be helpful for understanding the properties of Majorana bound states.

cond-mat.mes-hall↗