arXiv subjects

Hsu-Wen Chiang

Publications and source records attributed to Hsu-Wen Chiang.

14 recordsLinked to original sources

arXiv2026

Three-form dark energy: constraints and multi-probe comparison with $Λ$CDM

Three-form fields provide a theoretically well-motivated framework for dark energy, arising in higher-dimensional theories and exhibiting a rich cosmological phenomenology. We investigate a minimally coupled three-form dark energy model with a Gaussian potential and constrain it using current cosmological observations, including CMB shift parameters, DESI DR2 baryon acoustic oscillation measurements, Pantheon+ supernovae with and without SH0ES calibration, cosmic chronometers, and gamma-ray bursts. Parameter estimation is performed within a Bayesian Markov-chain Monte Carlo framework, while model comparison relies on several information criteria and the Bayesian evidence, as well as tension statistics. We find that the three-form model provides a viable and competitive description of the expansion history of the Universe. It is mildly preferred over $Λ$CDM for the combination of early and late-time datasets that are heavily tensioned (CMB+BAO and Pantheon+SH0ES). This preference decreases to neutrality for the other, less tensioned combination of early and late-time data, while for individual early-time or late-time datasets analysed separately, the information criteria are neutral or favour $Λ$CDM. This suggests that the additional degrees of freedom of the three-form field may help accommodate cosmological observations of different origins within a common framework. The reconstructed dark energy dynamics exhibit a characteristic phantom phase at intermediate redshifts while approaching a cosmological-constant-like behaviour at early and late times, providing a distinctive observational signature. Although the model does not significantly alleviate the Hubble tension despite allowing higher values of $H_0$, it remains consistent with current observations and offers a well-motivated alternative to $Λ$CDM whose predictions can be tested by future cosmological surveys.

astro-ph.CO↗

arXiv2026

Alleviating the Hubble Tension with Smooth Sign-Switching Dark Energy: Full CMB Constraints with DESI and PantheonPlus

Sign-switching dark energy has recently been proposed as a minimal modification of the late-time expansion history aimed at alleviating tensions within the standard cosmological model. In this work, we investigate ECDM, a smooth realisation of this scenario, with the dark energy density gradually transitioning from a negative to a positive value. We develop a consistent formulation of the perturbation equations that remains well behaved even when the dark energy equation-of-state parameter diverges during the transition. We confront the model with a comprehensive set of cosmological observations, including cosmic microwave background measurements from Planck 2018, ACT DR6 and SPT-3G, baryon acoustic oscillation measurements from DESI DR2, Type Ia supernova distances from Pantheon+, and local Hubble constant measurement of SH0ES. The inclusion of perturbations allows us to assess the impact of the model on structure growth and CMB anisotropies, providing a more thorough test of sign-switching dark energy. Our results show that this class of models is fully compatible with current precision cosmological observations while alleviating the Hubble tension and providing a compelling modification of the late-time dynamics of the Universe.

astro-ph.CO↗

arXiv2026

Cosmological tensions in Proca-Nuevo theory

We study the cosmological predictions of (extended) Proca-Nuevo theory. This vector-tensor theory enjoys stable homogeneous and isotropic solutions characterized by an effective dark energy fluid, with behavior that ranges from freezing quintessential to thawing phantom-like, serving as a motivated framework to scrutinize the cosmological tensions that affect the standard $Λ$CDM model. While the model we consider is sufficiently generic to encompass a large class of field theories, it distinguishes itself from scalar dark energy models (quintessential ones, kinetic ones and non-minimally coupled ones) by the presence of what would be classed as a vector degree of freedom which can be for instance inherited from more generic theories of gravity. We improve on previous work in several directions: we consider a general one-parameter class of background models; identify a so-called 'special' model and analyze observational constraints taking also into account perturbations and making use of wide up-to-date catalogs of datasets including recently released ones. We find that the one-parameter Proca-Nuevo model is preferred over $Λ$CDM at $1.5σ$ when fitting CMB and BAO data, and at $2.4σ$ when further adding low-redshift data. The Hubble tension is alleviated, dropping from $5.8σ$ to $2.3σ$ (resp. $1.5σ$) between CMB with (and resp. without) BAO data and local measurements. On the other hand, we find that the vector field generically introduces a significant enhancement of the effective Newton constant for natural values of parameters, so that matching the observed matter power spectrum requires a mild amount of tuning to suppress the impact of perturbations. Since, at the background level, Proca-Nuevo is degenerate with other classes of theories, our results are also relevant to a wider range of set-ups including and beyond vector-tensor models.

hep-th↗

arXiv2026

Observational constraints on 3-forms dark energy

3-forms are natural candidates for describing the late-time accelerated expansion of the Universe, as they can inherently reproduce a positive cosmological constant when lacking an evolving potential. When such a potential is present, a 3-form field may exhibit either quintessence-like or phantom-like behaviour. In this paper, we consider a late-time effective dark energy model described by a 3-form with a Gaussian potential, stable during the dark-energy-dominated era. We constrain this model observationally by performing a Markov Chain Monte Carlo (MCMC) analysis employing a comprehensive cosmological dataset, including Planck PR4 cosmic microwave background (CMB) data, DESI DR1 baryon acoustic oscillation (BAO) measurements, Pantheon+ Type Ia supernovae data, low-$z$ Cepheid calibrators, and DES Y1 large-scale structure observations. We demonstrate that the 3-form model successfully increases the predicted Hubble parameter of CMB and BAO data from $67.89\pm0.36{\rm km/s/Mpc}$ of $Λ$CDM model to $68.29^{+0.56}_{-0.61}{\rm km/s/Mpc}$ by approaching the potential peak at the right time, thus mildly reducing the tension with the late-time observation. Overall, the 3-form field serves as a promising candidate of phantom-like dark energy from both theoretical and observational points of view.

astro-ph.CO↗

arXiv2026

No-go theorem for spontaneous vectorization

Generalized vector-tensor theories of gravity have drawn attention for admitting hairy black hole solutions, thereby circumventing the standard no-hair theorems. It remains an open question, however, how such black holes may form starting from reasonable initial conditions. It has been suggested that vector hair may grow spontaneously as a result of the field developing a negative effective mass squared $-$ the so-called spontaneous vectorization mechanism. We demonstrate that this is not possible if the initial state is a hairless black hole, a result that applies to essentially all stationary and axisymmetric solutions of interest in general relativity. More precisely, we prove that the appearance of a negative effective mass squared for the vector field must necessarily be accompanied by ghost- or gradient-type instabilities. Demanding the absence of such instabilities translates into interesting bounds on the coupling constants of the theory as functions of the black hole parameters. In particular, we discover that a Kerr black hole may become unstable when the spin increases above a certain critical value.

gr-qc↗

arXiv2025

Black hole destabilization via trapped quasi-normal modes

In the presence of non-minimal gravitational couplings, matter field perturbations on a static black hole spacetime may develop unphysical poles in their linearized equations. Physical solutions confined in the domain between the event horizon and a pole satisfy a boundary value problem, although with boundary conditions which are different from standard quasi-normal modes. We refer to them as "trapped quasi-normal modes". Focusing on a Schwarzschild black hole in Einstein-Proca theory, we find that trapped quasi-normal modes accurately capture the behavior of perturbations under time evolution. In particular, axial-vector modes are unstable, with a growth rate that increases with multipole number. More interestingly, we uncover a new instability that affects monopole perturbations. These results confirm the existence of a novel destabilization mechanism of black holes by non-minimally coupled vector fields, with potential implications to well-studied models of modified gravity and cosmology based on vector particles.

gr-qc↗

arXiv2025

Observational constraints on generalised axion-like potentials for the late Universe

Axions have emerged as compelling candidates for describing the dark sector of the Universe. In this work, we explore quintessence models inspired by axion-like potentials as a dynamical alternative to the cosmological constant. These models naturally exhibit a tracking behaviour, reducing the need for fine-tuned initial conditions. We perform a Markov chain Monte Carlo (MCMC) analysis on a complete cosmological dataset of Planck PR4 cosmic microwave background (CMB), DESI DR1 baryon acoustic oscillation (BAO), Pantheon+ type Ia supernovae, low-$z$ Cepheid anchors, and DES Y1 large scale structure measurement. Unfortunately, neither the Hubble tension nor the $S_8$ tension is eased. The model does reach parity with $Λ$CDM model statistically according to DIC, WAIC and Bayesian ratio, suggesting that a quintessence model may still help with the cosmic tension by further extending the model we have investigated.

astro-ph.CO↗

arXiv2023

Resonant orbits of rotating black holes beyond circularity: Discontinuity along parameter shift

According to general relativity, an isolated black hole in vacuum shall be described by the Kerr metric, whose geodesic equations are integrable. The violation of integrability leads to chaos for particles moving around the black hole. This chaotic dynamics could leave imprints on the associated gravitational waveform and could be tested with upcoming observations. In this paper, we investigate the chaotic orbital dynamics induced by the violation of a certain spacetime symmetry, the circularity. Specifically, we focus on the resonant orbits of a particular noncircular spacetime as an example and find that they form chains of Birkhoff islands on Poincaré surfaces of section. We compare the island structures with those generated in typical nonintegrable but circular spacetimes. The islands of stability induced by noncircularity appear asymmetric on the most common Poincaré surface of section at the equatorial plane. The asymmetric patterns of islands vary discontinuously when the spacetime parameters transit through integrable regions. The origin of such features is explained in the context of perturbation analysis by considering the orbits associated with stable fixed points on the section. Possible observational implications about testing circularity through gravitational wave detection are discussed.

gr-qc↗

arXiv2022

Eikonal quasinormal modes and photon orbits of deformed Schwarzschild black holes

The geometric optics approximation provides an interpretation for eikonal correspondence that, in black-hole-containing spacetimes, connects high-frequency black hole quasinormal modes with closed photon orbits around said black hole. This correspondence has been identified explicitly for Schwarzschild, Reissner-Nordström, Kerr, and Kerr-Newman black holes, the violation of which can be a potential hint toward physics beyond General Relativity. Notably, the aforementioned black hole spacetimes have sufficient symmetries such that both the geodesic equations and the master wave equations are separable. The identification of the correspondence seems to largely rely on these symmetries. One naturally asks how the eikonal correspondence would appear if the spacetime were less symmetric. For a pioneering work in this direction, we consider in this paper a deformed Schwarzschild spacetime retaining only axisymmetry and stationarity. We show that up to the first order of spacetime deformations the eikonal correspondence manifests through the definition of the \textit{averaged} radius of trapped photon orbits along their one period. This averaged radius overlaps the potential peak in the master wave equation, which can be defined up to the first order of spacetime deformations, allowing the explicit identification of the eikonal correspondence.

gr-qc↗

arXiv2021

A generic unitary black-hole evaporation model based on first principles

Based on the discretized horizon picture, we introduce a macroscopic effective model of the horizon area quanta that encapsulates the features necessary for black holes to evaporate consistently. The price to pay is the introduction of a "hidden sector" that represents our lack of knowledge about the final destination of the black hole entropy. We focus on the peculiar form of the interaction between this hidden sector and the black hole enforced by the self-consistency. Despite the expressive power of the model, we arrive at several qualitative statements. Furthermore, we identify these statements as features inside the microscopic density of states of the horizon quanta, with the dimension of the configuration space being associated with the area per quanta in Planck unit, a UV cutoff proportional to the amount of excess entropy relative to Bekenstein's law at the end of evaporation, and a zero-frequency-pole-like structure corresponding to, similarly, the amount of excess entropy at IR limit. We then relate this nearly-zero-frequency structure to the soft hairs proposed by Strominger et al., and argue that we should consider deviating away from the zero frequency limit for soft hairs to participate in the black hole evaporation.

hep-th↗

arXiv2020

Modification to the Hawking temperature of a dynamical black hole by a flow-induced supertranslation

One interesting proposal to solve the black hole information loss paradox without modifying either general relativity or quantum field theory, is the soft hair, a diffeomorphism charge that records the anisotropic radiation in the asymptotic region. This proposal, however, has been challenged, given that away from the source the soft hair behaves as a coordinate transformation that forms an Abelian group, thus unable to store any information. To maintain the spirit of the soft hair but circumvent these obstacles, we consider Hawking radiation as a probe sensitive to the entire history of the black hole evaporation, where the soft hairs on the horizon are induced by the absorption of a null anisotropic flow, generalizing the shockwave considered by Hawking et al. To do so we introduce two different time-dependent extensions of the diffeomorphism associated with the soft hair, where one is the backreaction of the anisotropic null flow, and the other is a coordinate transformation that produces the Unruh effect and a Doppler shift to the Hawking spectrum. Together, they form an exact BMS charge generator on the entire manifold that allows the nonperturbative analysis of the black hole horizon, whose surface gravity, i.e. the Hawking temperature, is found to be modified. The modification depends on an exponential average of the anisotropy of the null flow with a decay rate of 4M, suggesting the emergence of a new 2-D degree of freedom on the horizon, which could be a way out of the information loss paradox.

gr-qc↗

arXiv2016

A Quantized Spacetime Based on $Spin(3,1)$ Symmetry

We introduce a new type of the spacetime quantization based on the spinorial description suggested by loop quantum gravity. Specifically, we build our theory on a string theory inspired $Spin(3,1)$ worldsheet action. Because of its connection with quantum gravity theories, our proposal may in principle link back to string theory, connect to loop quantum gravity where $SU(2)$ is suggested as the fundamental symmetry, or serve as a Lorentzian spin network. We derive the generalized uncertainty principle and demonstrate the holographic nature of our theory. Due to the quantization of spacetime, geodesics in our theory are fuzzy, but the fuzziness is shown to be much below conceivable astrophysical bounds.

hep-th↗

arXiv2016

Quantization of Spacetime Based on Spacetime Interval Operator

Motivated by both concepts of R.J. Adler's recent work on utilizing Clifford algebra as the linear line element $ds = \left\langle γ_μ\right\rangle dX^μ$, and the fermionization of the cylindrical worldsheet Polyakov action, we introduce a new type of spacetime quantization that is fully covariant. The theory is based on the reinterpretation of Adler's linear line element as $ds = γ_μ\left\langle λγ^μ\right\rangle$, where $λ$ is the characteristic length of the theory. We name this new operator as "spacetime interval operator", and argue that it can be regarded as a natural extension to the one-forms in the $U(\mathfrak{s}u(2))$ non-commutative geometry. By treating Fourier momentum as the particle momentum, the generalized uncertainty principle of the $U(\mathfrak{s}u(2))$ non-commutative geometry, as an approximation to the generalized uncertainty principle of our theory, is derived, and is shown to have a lowest order correction term of the order $p^2$ similar to that of Snyder's. The holography nature of the theory is demonstrated, and the predicted fuzziness of the geodesic is shown to be much smaller than conceivable astrophysical bounds.

hep-th↗

arXiv2013

A new method to determine large scale structure from the luminosity distance

The luminosity distance can be used to determine the properties of large scale structure around the observer. To this purpose we develop a new inversion method to map luminosity distance to a LTB metric based on the use of the exact analytical solution for Einstein equations. The main advantages of this approach are an improved numerical accuracy and stability, an exact analytical setting of the initial conditions for the differential equations which need to be solved and the validity for any sign of the functions determining the LTB geometry. Given the fully analytical form of the differential equations, this method also simplifies the calculation of the red-shift expansion around the apparent horizon point where the numerical solution becomes unstable. We test the method by inverting the supernovae Ia luminosity distance function corresponding to the the best fit $ΛCDM$ model. We find that only a limited range of initial conditions is compatible with observations, or a transition from red to blue shift can occur at relatively low redshift. Despite LTB solutions without a cosmological constant have been shown not to be compatible with all different set of available observational data, those studies normally fit data assuming a special functional ansatz for the inhomogeneity profile, which often depend only on few parameters. Inversion methods on the contrary are able to fully explore the freedom in fixing the functions which determine a LTB solution. Another important possible application is not about LTB solutions as cosmological models, but rather as tools to study the effects on the observations made by a generic observer located in an inhomogeneous region of the Universe where a fully non perturbative treatment involving exact solutions of Einstein equations is required.

astro-ph.CO↗