arXiv subjects

Hanyu Cheng

Publications and source records attributed to Hanyu Cheng.

10 recordsLinked to original sources

arXiv2026

A Markovian Traffic Equilibrium Model for Ride-Hailing

We develop a Markovian traffic equilibrium model for ride-hailing in which vehicles, whether empty or hired, make sequential order-acceptance and link-choice decisions over a traffic network to maximize total discounted return in an infinite-horizon semi-Markov decision process. The model endogenizes both competition among empty vehicles for passenger demand and traffic congestion arising from road usage at the link level. We characterize equilibrium as the solution to a fixed-point system, establish its existence, and develop relaxed fixed-point iteration algorithms for equilibrium computation, with convergence results for specialized network structures. Computational experiments on realistic networks demonstrate the model's practical value for transportation planning. Ablation analyses reveal that ignoring either traffic congestion or drivers' forward-looking behavior can lead to potentially substantial biases in policy evaluation.

cs.GT↗

arXiv2026

Primordial Black Hole Abundance from Reionization

We derive robust constraints on the initial abundance of evaporating primordial black holes (PBHs) using the reionization history of the Universe as a cosmological probe. We focus on PBHs that inject electromagnetic (EM) energy into the intergalactic medium (IGM) after recombination, in the mass range $3.2\times 10^{13}\,\mathrm{g} \lesssim M_{\rm PBH} \lesssim 5\times 10^{14}\,\mathrm{g}$. For each PBH mass, we compute the redshift-dependent energy injection from Hawking evaporation using \texttt{BlackHawk}, fully accounting for the time evolution of the PBH mass and the complete spectrum of emitted Standard Model particles and gravitons. The resulting photons and electrons are propagated through the primordial plasma using \texttt{DarkHistory}, which self-consistently models EM cascades and determines the fraction of injected energy deposited into ionization, excitation, and heating of the IGM. These modifications to the ionization and thermal histories are incorporated into a Gaussian Process reconstruction of the free-electron fraction based on low-$\ell$ CMB polarization data from the \textit{Planck} mission. This non-parametric approach allows for a statistically well-defined separation between exotic high-redshift energy injection and late-time astrophysical reionization, allowing PBH evaporation to be constrained through its contribution to the high-redshift optical depth. Requiring consistency with current CMB measurements, we obtain upper limits on the initial PBH abundance that are robust against reionization modeling uncertainties and systematically more conservative than existing bounds, reflecting the fully numerical and time-dependent treatment of Hawking evaporation and energy deposition. Our results demonstrate the power of reionization observables as a precision probe of PBH evaporation and other scenarios involving late-time energy injection.

astro-ph.CO↗

arXiv2025

Beyond Two Parameters: Revisiting Dark Energy with the Latest Cosmic Probes

Dark energy (DE) models with many free parameters are often considered excessive, as constraining all parameters poses a significant challenge. While such models offer greater flexibility to probe the DE sector in more detail. With the rapid advancement of astronomical surveys and the availability of diverse datasets, it is timely to examine whether current combined observations can effectively constrain an extended parameter space in DE models. This article investigates a four-parameter dynamical DE model that spans a broad region of the universe's expansion history through four key parameters: present-day value of the DE equation of state ($w_0$), its initial value ($w_m$), scale factor depicting transition from $w_m$ to $w_0$ ($a_t$), and steepness of this transition ($\Delta_{\rm de}$). We constrain the model using cosmic microwave background data from Planck, BAO from DESI DR2, and three distinct compilations of Type Ia Supernovae: PantheonPlus, DESY5, and Union3. Our results show that constraining all four parameters remains challenging: $a_t$ is not constrained by any dataset, constraints on $w_m$ and $\Delta_{\rm de}$ remain weak, only $w_0$ is well constrained across all datasets. The results further show that $w_0 > -1$, while $w_m$ is negative, indicating a phantom-like behaviour at early times. Interestingly, despite its larger parameter space, the proposed model shows a preference over the $\Lambda$CDM and $w_0w_a$CDM scenarios for certain combined datasets, according to both $\Delta \chi^2$ and Bayesian evidence, although this preference is not strong.

astro-ph.CO↗

arXiv2025

Optimizing Urban Electric Vehicle Charging and Battery Swapping Infrastructure: A Location-Inventory-Grid Model

The rapid rise of electric vehicles (EVs) places unprecedented stress on both urban mobility systems and low-voltage power grids. Designing battery swapping and charging networks that are cost-efficient, grid-compatible, and sustainable is therefore a pressing yet complex challenge: service providers must jointly optimize station locations, battery inventory, and grid interaction under high-dimensional uncertainty. We develop an integrated location-inventory-grid model and employ a continuous approximation approach to overcome the intractability of discrete formulations. Our analysis compares centralized versus decentralized charging, with and without participation in frequency regulation. The results reveal that centralized charging, when combined with frequency regulation, not only reduces cost but also strengthens grid stability. However, it may constrain operational flexibility near the optimum, potentially challenging efforts to mitigate environmental impacts by lowering battery inventories. These results offer actionable guidance for cost-efficient, environmentally sustainable, and grid-compatible scaling of urban EV infrastructure to meet the demands of large-scale EV adoption.

math.OC↗

arXiv2025

Dynamic Carbon Intensity Indicator (CII) Management in Stochastic Tramp Shipping Market

In the maritime sector, tramp shipping companies manage fleets to maximize profit while navigating market uncertainties. The International Maritime Organization (IMO) recently introduced the Carbon Intensity Indicator (CII) to reduce greenhouse gas emissions, further complicating deployment decisions. This paper introduces a novel two-stage stochastic programming model for long-term fleet deployment under market uncertainty and CII regulation. It is the first to integrate key operational uncertainties such as fuel prices, freight rates, and cargo demand into a unified tactical planning framework under CII regulation, simultaneously optimizing routing, cargo allocation, and speed. Furthermore, we develop an novel efficient heuristic algorithm that reliably converges to solutions within a 5\% optimality gap, enabling practical decision-support under uncertainty. Numerical analysis highlights two key findings based on our model: (1) It uncovers the ``CII paradox,'' a critical counterintuitive phenomenon where the present Supply-based CII regulation may increase total emissions significantly and drastically reduce profits. This challenges the conventional wisdom that stricter carbon-intensity rules invariably reduce emissions. (2) It demonstrates the advantage of stochastic modeling, showing that accounting for future uncertainties significantly narrows the revenue gap with perfect-foresight solutions, thereby offering superior economic performance over deterministic approaches. Collectively, these results deepen the understanding of environmental regulation's operational impacts and pave the way for more effective and sustainable fleet management strategies.

math.OC↗

arXiv2025

Constraining the axiverse with reionization

Axions that couple to electromagnetism are produced in the early Universe by, among other channels, freeze-in via the Primakoff process. For sufficiently large axion masses, the same coupling causes the axions to decay into two photons, which subsequently ionize the intergalactic medium. If this decay occurs in the redshift range $20 \lesssim z \lesssim 1100$, then the contribution to the cosmic microwave background optical depth $\tau_{\rm reio}$ can lead to a conflict with observations, excluding models with sufficiently strongly coupled, heavy axions and high reheating temperatures, $T_{\rm reh}$. Using large ensembles of explicit type IIB string theory models with up to $h^{1,1} = 100$ axions, we compute the full cosmic reionization history caused by the decays of multiple axions. We compare this to the posterior on the high-$z$ component of $\tau_{\rm reio}$ derived from parametric-independent constraints on the ionization state of the Universe, obtained in a full \textit{Planck} analysis presented in a companion paper. For $h^{1,1} = 20, 50, 100$, we find that approximately 15\%, 15\%, and 10\% of the models in the ensemble prefer $T_{\rm reh} \lesssim 10^{10}\,\text{GeV}$ at 95\% CL. We provide a publicly available code at:~\href{https://github.com/ZiwenYin/Reionization-with-multi-axions-decay}{github.com/ZiwenYin/Reionization-with-multi-axions-decay}, which computes the reionization history for arbitrary ensembles of decaying axions. Our analysis opens the door for future large-scale work studying the preference for low-temperature reheating in models with multiple axions.

hep-ph↗

arXiv2025

Constraining exotic high-$z$ reionization histories with Gaussian processes and the Cosmic Microwave Background

The large-angle polarization anisotropies in the Cosmic Microwave Background (CMB) arise from Thomson scattering of CMB photons off free electrons in the post-recombination Universe. In the standard $\Lambda$ cold dark matter cosmological model, the free electron density increases at redshifts $z \lesssim 10$ as the first stars form, reionizing the intergalactic medium. We use \emph{Gaussian processes} to perform a model-independent reconstruction of the cosmic reionization history constrained by \textit{Planck} CMB data. Our approach recovers the standard reionization at $z \lesssim 10$ and places stringent limits on any additional high-$z$ reionization. From this reconstruction, we define a new derived parameter, the high-redshift contribution to the CMB optical depth, $\tau_{\mathrm{highz}}$, whose posterior distribution provides robust constraints on exotic energy injection scenarios. We demonstrate this for decaying dark matter with particle masses in the range $\mathcal{O}(1\,\text{MeV})$. A companion paper applies this framework to multi-axion models. All data and code are publicly available at: \href{https://github.com/Cheng-Hanyu/CLASS_reio_gpr}{github.com/Cheng-Hanyu/CLASS\_reio\_gpr}.

astro-ph.CO↗

arXiv2025

Cosmic Strings as Dynamical Dark Energy: Novel Constraints

Cosmic strings, topological defects predicted by high-energy theories, may contribute to the late-time expansion of the Universe, effectively mimicking dynamical dark energy. We investigate four phenomenological extensions of the $\Lambda$CDM model involving a residual string network: (i) a non-relativistic component with positive energy density (Model~1), (ii) a velocity-dependent extension (Model~2), (iii) a non-relativistic string network with energy density allowed to take both positive and negative values (Model~3), and (iv) a general scenario with free energy and velocity parameters (Model~4). These models are constrained using \textit{Planck} CMB data, SDSS or DESI baryon acoustic oscillations, and Type Ia supernovae observations. Models~1 and~2 yield strong upper bounds on the string density, for example, $\Omega_{\mathrm{s}} < 0.00901$ at 95\% CL from the CMB+DESI+DESY5 combination for Model~2, and mildly shift the inferred value of $H_0$ upward, though they are not favored by Bayesian evidence. For the same combination, the bulk velocity is bound as $v_{\mathrm{s}} < 0.569$. Models~3 and~4 exhibit a consistent preference for slightly negative values of $\Omega_{\mathrm{s}}$, with CMB-only data yielding $\Omega_{\mathrm{s}} = -0.038^{+0.029}_{-0.022}$ and $v_{\mathrm{s}}< 0.574$ in Model~4, and a best-fit improvement of $\Delta \chi^2 = -6.07$. However, these improvements are not sufficient to overcome the Occam penalty, and the Bayesian evidence continues to favor $\Lambda$CDM. These findings demonstrate the power of current data to constrain exotic energy components and encourage further exploration of string-inspired extensions to $\Lambda$CDM, particularly those involving negative-tension networks.

astro-ph.CO↗

arXiv2025

Pressure Parametrization of Dark Energy: First and Second-Order Constraints with Latest Cosmological Data

We explore an extension of the $\Lambda$CDM model in which the pressure $p$ of the dark energy (DE) fluid evolves with the expansion of the Universe, expressed as a function of the scale factor $a$. The corresponding energy density $\rho$ is derived from the continuity equation, resulting in a dynamical equation-of-state parameter $w \equiv p/\rho$ during the late-time expansion of the Universe. The pressure is modeled using a Taylor expansion around the present epoch ($a = 1$), introducing deviations from a cosmological constant within the dynamical dark energy (DDE) framework. At first order, a single new parameter $\Omega_1$ captures linear deviations, while a second-order parameter, $\Omega_2$, accounts for quadratic evolution in the pressure. We constrain the first- and second-order DDE models using multiple observational datasets and compare their performance against $\Lambda$CDM and the CPL parameterization. A joint analysis of Planck CMB, DESI, and DESY5 data yields the strongest evidence for DDE, with a $2.7\sigma$ deviation in the first-order model and over $4\sigma$ in the second-order model, providing strong statistical support for a departure from a cosmological constant. The reconstructed DE evolution in the second-order case reveals a distinctive non-monotonic behavior in both energy density and $w_{\rm DE}(a)$, including clear phantom-crossing phenomena. Notably, the late-time evolution of $w_{\rm DE}(a)$ remains consistent across datasets and shows strong agreement with the CPL parameterization, underscoring the robustness of the pressure-based approach.

astro-ph.CO↗

arXiv2024

Future targets for light gauge bosons from cosmic strings

Cosmic strings, theoretical one-dimensional topological defects from the early universe, provide a valuable opportunity for exploring dark matter (DM) production and gravitational wave (GW) emission. Our study investigates the production of gauge bosons and GW emission from cosmic string decay, considering the constraints imposed by cosmological observations. We specifically examine how gauge bosons radiated from strings contribute to DM and dark radiation, with limits set by the observed DM abundance and cosmic microwave background data, respectively. Additionally, we analyze the gravitational wave spectrum of this model across both low and high frequencies. Notably, the spectrum typically follows a $f^{-1/3}$ pattern at high frequencies, beyond the pivot frequency $f_*$. We derive an analytical expression for the frequency $f_*$ and confirm its accuracy through numerical verification. Furthermore, we compare the GW spectrum of this model with the forecasted capabilities of future GW observatories, such as the Einstein Telescope, the Laser Interferometer Space Antenna, the Big Bang Observer, and $\mu$Ares. Finally, we discuss how these considerations impact the model parameters, specifically the gauge boson mass and the energy scale of U(1) symmetry breaking, providing insights into how cosmic strings could enhance our understanding of DM and GW astronomy.

hep-ph↗