arXiv subjects

Ming-Chung Chu

Publications and source records attributed to Ming-Chung Chu.

At least 19 recordsLinked to original sources

arXiv2025

BEC vortices as an observational signature of ultra-light bosonic dark matter

Ultra-light bosonic dark matter (ULDM) is an interesting and promising dark matter candidate. While the wave-like nature of ULDM has been widely studied in the literature, we explore another distinctive feature of ULDM as Bose-Einstein Condensate (BEC) in this paper: the emergence of vortices in a rotating BEC-ULDM halos. Using numerical solution of the GPP equation, we demonstrate that a lattice of vortices ,underdensity columns that carry angular momentum, naturally forms in a ULDM halo under conditions similar to those of the Milky Way. Furthermore, we study the gravitational lensing by these vortices as a possible observational signature of BEC-ULDM. If the vortices are large enough and the halo's rotational axis align with the line of sight, regularly separated brightness anomalies can be produced, providing strong evidence for BEC-ULDM.

astro-ph.CO↗

arXiv2025

R-process Nucleosynthesis of Subminimal Neutron Star Explosions

We show that a minimum-mass neutron star undergoes delayed explosion after mass removal from its surface. We couple the Newtonian hydrodynamics to a nuclear reaction network of $\sim4500$ isotopes to study the nucleosynthesis and neutrino emission during the explosion. An electron antineutrino burst with a peak luminosity of $\sim3\times10^{50}$ erg s$^{-1}$ is emitted while the ejecta is heated to $\sim10^{9}$ K. A robust $r$-process nucleosynthesis is realized in the ejecta. Lanthanides and heavy elements near the second and third $r$-process peaks are synthesized as end products of nucleosynthesis, suggesting that subminimal neutron star explosions could be an important source of solar chemical elements.

astro-ph.HE↗

arXiv2025

Constraining Ultra-Light Dark Matter mass with Dwarf Galaxy Rotation Curves

While ultra-light bosonic dark matter (ULDM) in a Bose-Einstein condensate (BEC) state could naturally account for the central core in some galaxies and resolve the core-cusp problem, the dark matter density distribution in the outer regions of galaxies remains less explored. We propose a trial wavefunction to model the ULDM distribution beyond the BEC core. We derive the corresponding rotation velocity curve, which shows excellent agreement with those of 12 dwarf spheroidal galaxies. The best-fit ULDM particle mass for each dwarf galaxy falls within a strikingly narrow range of $m=(1.8-3.2)\times 10^{-23}\text{eV}$.

astro-ph.GA↗

arXiv2025

Bianchi Type I Model Cannot Explain the Observed CMB Angular Acoustic Scale Directional Variation

Anisotropic cosmological models have been gaining attention due to various observational hints of large-scale anisotropies. One of the most surprising evidences for the latter is the discovery of a dipole-like directional variation in cosmological parameters extracted from the Cosmic Microwave Background (CMB) data. In this work, we show that the directional variation of the CMB angular acoustic angle calculated with the fully asymmetric Bianchi Type I metric, a simple extension of the standard Friedmann-Lemaître-Robertson-Walker metric, cannot account for the observed dipole-like anisotropy.

astro-ph.CO↗

arXiv2025

Constraining Axially Symmetric Bianchi Type I Model with Self-Consistent Recombination History and Observables

Recent cosmological measurements suggest the possibility of an anisotropic universe. As a result, the Bianchi Type I model, being the simplest anisotropic extension to the standard Friedmann-Lemaître-Robertson-Walker metric has been extensively studied. In this work, we show how the recombination history should be modified in an anisotropic universe and derive observables by considering the null geodesic. We then constrain the axially symmetric Bianchi Type I model by performing Markov Chain Monte Carlo with the acoustic scales in Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillation data, together with local measurements of $H(z)$ and Pantheon Supernova data. Our results reveal that the anisotropic model is not worth a bare mention compared to the $Λ$ cold dark matter model, and we obtain a tight constraint on the anisotropy that generally agrees with previous studies under a maximum temperature anisotropy fraction of $2\times 10^{-5}$. To allow for a non-kinematic CMB dipole, we also present constraints based on a relaxed maximum temperature anisotropy comparable to that of the CMB dipole. We stress that there is a significant difference between the geodesic-based observables and the naive isotropic analogies when there is a noticeable anisotropy. However, the changes in recombination history are insignificant even under the relaxed anisotropy limit.

astro-ph.CO↗

arXiv2024

On the Nucleosynthesis in Accretion-Induced Collapse of White Dwarfs

It has long been hypothesized that accretion-induced collapse (AIC) of white dwarfs contribute to heavy chemical elements production in the universe. We present one-dimensional neutrino-radiative hydrodynamic simulations of AIC followed by post-processing nucleosynthesis calculations of the ejecta. A proto-neutron star is formed after the AIC, and a neutrino burst with peak luminosity $\sim10^{53}$ erg s$^{-1}$, comparable to that of a core-collapse supernova (CCSN), is emitted. The ejecta mass of AIC could be up to $\sim10^{-2}$ M$_\odot$, and the first neutron-capture peak elements (Sr, Y, and Zr) could be abundantly synthesized, with an overproduction of $\sim10^{6}$ relative to the solar abundances. The yield of $^{56}\text{Ni}$ could be up to at most $\sim10^{-3}$ M$_\odot$, suggesting that the electromagnetic light curve associated with AIC is at least $2$ orders dimmer than those associated with Type Ia supernovae (Type Ia SN). The inferred upper bound of AIC event rate, from nucleosynthesis calculations, is at most $\sim10\,\%$ relative to those of CCSNe and Type Ia SNe.

astro-ph.HE↗

arXiv2023

Is the Observable Universe Consistent with the Cosmological Principle?

The Cosmological Principle (CP) -- the notion that the Universe is spatially isotropic and homogeneous on large scales -- underlies a century of progress in cosmology. It is conventionally formulated through the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive $Λ$-Cold-Dark-Matter ($Λ$CDM) model. Yet, tensions have emerged within the $Λ$CDM model, most notably a statistically significant discrepancy in the value of the Hubble constant, $H_0$. Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the $H_0$ tension may extend beyond $Λ$CDM to the CP itself. This review surveys current observational hints for deviations from the expectations of the CP, highlighting synergies and disagreements that warrant further study. Setting aside the debate about individual large structures, potential deviations from the CP include variations of cosmological parameters on the sky, discrepancies in the cosmic dipoles, and mysterious alignments in quasar polarizations and galaxy spins. While it is possible that a host of observational systematics are impacting results, it is equally plausible that precision cosmology may have outgrown the FLRW paradigm, an extremely pragmatic but non-fundamental symmetry assumption.

astro-ph.CO↗

arXiv2022

Tidal Deformability of Dark Matter Admixed Neutron Stars

The tidal properties of a neutron star are measurable in the gravitational waves emitted from inspiraling binary neutron stars, and they have been used to constrain the neutron star equation of state. In the same spirit, we study the dimensionless tidal deformability of dark matter admixed neutron stars. The tidal Love number is computed in a two-fluid framework. The dimensionless tidal Love number and dimensionless tidal deformability are computed for dark matter admixed stars with the dark matter modelled as ideal Fermi gas or self-interactive bosons. The dimensionless tidal deformability shows a sharp change from being similar to that of a pure normal matter star to that of a pure dark matter star, within a narrow range of intermediate dark matter mass fraction. Based on this result, we illustrate an approach to study the dark matter parameters through the tidal properties of massive compact stars, making use of the self-similarity of the dimensionless tidal deformability-mass relations when the dark matter mass fraction is high.

astro-ph.HE↗

arXiv2022

Directional Variations of Cosmological Parameters from the Planck CMB Data

Recent observations suggest that there are violations of the isotropy of the universe at large scales, an important part of the cosmological principle. In this paper, we use the Cosmic Microwave Background (CMB) data to search for spatial variations of the cosmological parameters in the $Λ\mathrm{CDM}$ model. We fit the Planck temperature angular power spectrum $\mathcal{C}^{TT}_\ell$ for 48 different half-skies, centering on 48 different directions, to search for directional dependences of the standard cosmological parameters. There are $3(2)σ$-level directional variations in $Ω_bh^2$, $Ω_ch^2$, $n_s$, $100θ_\mathrm{MC}$, and $H_0$ $(τ$ and $\ln(10^{10}A_s))$. Furthermore, the directional distributions of the parameters follow a dipole form to good approximation. The Bayes factor between the isotropic and anisotropic hypotheses is $0.0041$, strongly disfavouring the former. The best-fit dipole axes for $Ω_bh^2$, $Ω_ch^2$, $n_s$, $100θ_\mathrm{MC}$, and $A_s e^{-2τ}$ all generally align with the mean direction of $\boldsymbol{V} \equiv (b = -5.6^{+17.0{\circ}}_{-17.4}, l = 48.8^{+14.3{\circ}}_{-14.4})$, which is roughly perpendicular to the dipole of the variation in fine structure constant, and is about $45^{\circ}$ to the directions of the CMB kinematic dipole, CMB parity asymmetry, and polarization of QSOs. Our results suggest either significant violation of the cosmological principle, or previously unknown systematic errors in the standard CMB analysis.

astro-ph.CO↗

arXiv2019

Dark Matter Admixed Type Ia Supernovae

We perform two-dimensional hydrodynamic simulations for the thermonuclear explosion of Chandrasekhar-mass white dwarfs with dark matter (DM) cores in Newtonian gravity. We include a 19-isotope nuclear reaction network and make use of the pure turbulent deflagration model as the explosion mechanism in our simulations. Our numerical results show that the general properties of the explosion depend quite sensitively on the mass of the DM core M$_{\rm DM}$: a larger M$_{\rm DM}$ generally leads to a weaker explosion and a lower mass of synthesized iron-peaked elements. In particular, the total mass of $^{56}$Ni produced can drop from about 0.3 to 0.03 $M_{\odot}$ as M$_{\rm DM}$ increases from 0.01 to 0.03 $M_{\odot}$. We have also constructed the bolometric light curves obtained from our simulations and found that our results match well with the observational data of sub-luminous Type-Ia supernovae.

astro-ph.CO↗

arXiv2019

Accretion-Induced Collapse of Dark Matter Admixed White Dwarfs -- I: Formation of Low-mass Neutron Stars

Recently observed pulsars with masses $\sim 1.1 ~M_{\odot}$ challenge the conventional neutron star (NS) formation path by core-collapse supernova (CCSN). Using spherically symmetric hydrodynamics simulations, we follow the collapse of a massive white dwarf (WD) core triggered by electron capture, until the formation of a proto-NS (PNS). For initial WD models with the same central density, we study the effects of a static, compact dark matter (DM) admixed core on the collapse and bounce dynamics and mass of the PNS, with DM mass $\sim 0.01 ~M_{\odot}$. We show that increasing the admixed DM mass generally leads to slower collapse and smaller PNS mass, down to about 1.0 $M_{\odot}$. Our results suggest that the accretion-induced collapse of dark matter admixed white dwarfs can produce low-mass neutron stars, such as the observed low-mass pulsar J0453+1559, which cannot be obtained by conventional NS formation path by CCSN.

astro-ph.HE↗

arXiv2019

Accretion-Induced Collapse of Dark Matter Admixed White Dwarfs -- II: Rotation and Gravitational-wave Signals

We present axisymmetric hydrodynamical simulations of accretion-induced collapse (AIC) of dark matter (DM) admixed rotating white dwarfs (WD) and their burst gravitational-wave (GW) signals. For initial WD models with the same central baryon density, the admixed DM is found to delay the plunge and bounce phases of AIC, and decrease the central density and mass of the proto-neutron star (PNS) produced. The bounce time, central density and PNS mass generally depend on two parameters, the admixed DM mass $M_\mathrm{DM}$ and the ratio between the rotational kinetic and gravitational energies of the inner core at bounce $β_\mathrm{ic,b}$. The emitted GWs have generic waveform shapes and the variation of their amplitudes $h_+$ show a degeneracy on $β_\mathrm{ic,b}$ and $M_\mathrm{DM}$. We found that the ratios between the GW amplitude peaks around bounce allow breaking the degeneracy and extraction of both $β_\mathrm{ic,b}$ and $M_\mathrm{DM}$. Even within the uncertainties of nuclear matter equation of state, a DM core can be inferred if its mass is greater than 0.03 $M_{\odot}$. We also discuss possible DM effects on the GW signals emitted by PNS g-mode oscillations. GW may boost the possibility for the detection of AIC, as well as open a new window in the indirect detection of DM.

astro-ph.HE↗

arXiv2019

Effects of neutrino mass and asymmetry on cosmological structure formation

Light but massive cosmological neutrinos do not cluster significantly on small scales, due to their high thermal velocities. With finite masses, cosmological neutrinos become part of the total matter field and contribute to its smoothing. Structure formation in the presence of massive neutrinos is therefore impeded compared to that in the standard $Λ$CDM cosmology with massless neutrinos. Neutrinos' masses also distort the anisotropy power spectrum of cosmic microwave background (CMB). Furthermore, a finite chemical potential $μ$ for cosmological neutrinos, still allowed by current data, would have a non-negligible impact on CMB and structure formation. We consistently evaluate effects of neutrino masses and chemical potentials on the matter power spectrum by use of a neutrino-involved N-body simulation, with cosmological parameters obtained from a Markov-Chian Moonte-Carlo (MCMC) refitting of CMB data. Our results show that while a finite averaged neutrino mass $m_ν$ tends to suppress the matter power spectrum in a range of wave numbers, the neutrino degeneracy parameters ${ξ_i \equiv μ_i /T}$ ($i=$1, 2, 3) enhance the latter, leading to a large parameter degeneracy between $m_ν$ and $ξ_i$. We provide an empirical formula for the effects on the matter power spectrum in a selected range of wave numbers induced by $m_ν$ and $η\equiv \sqrt{\sum_i ξ^2_i}$. Observing a strong correlation between $m_ν$ and $η$, we propose a single redshift-independent parameter $m_ν- \frac{4}{3}η^2$ to characterize the neutrino effects on the matter power spectrum.

astro-ph.CO↗

arXiv2019

Cosmological Simulation for Fuzzy Dark Matter Model

Fuzzy Dark Matter (FDM), motivated by string theory, has recently become a hot candidate for dark matter. The rest mass of FDM is believed to be $\sim 10^{-22}$eV and the corresponding de-Broglie wave length is $\sim 1$kpc. Therefore, the quantum effect of FDM plays an important role in structure formation. In order to study the cosmological structure formation in FDM model, several simulation techniques have been introduced. We review the current status and challenges in the cosmological simulation for the FDM model in this paper.

astro-ph.CO↗

arXiv2018

The Importance of Quantum Pressure of Fuzzy Dark Matter on Lyman-Alpha Forest

With recent Lyman-alpha forest data from BOSS and XQ-100, some studies suggested that the lower mass limit on the fuzzy dark matter (FDM) particles is lifted up to $10^{-21}\,\mathrm{eV}$. However, such a limit was obtained by $Λ$CDM simulations with the FDM initial condition and the quantum pressure (QP) was not taken into account which could have generated non-trivial effects in large scales structures. We investigate the QP effects in cosmological simulations systematically, and find that the QP leads to further suppression of the matter power spectrum at small scales, as well as the halo mass function in the low mass end. We estimate the suppressing effect of QP in the 1D flux power spectrum of Lyman-alpha forest and compare it with data from BOSS and XQ-100. The rough uncertainties of thermal gas properties in the flux power spectrum model calculation were discussed. We conclude that more systematic studies, especially with QP taken into account, are necessary to constrain FDM particle mass using Lyman-alpha forest.

astro-ph.CO↗

arXiv2018

New leaves of the tree: percolation analysis for cosmic web with discrete points

Percolation analysis has long been used to quantify the connectivity of the cosmic web. Most of the previous work is based on density fields on grids. By smoothing into fields, we lose information about galaxy properties like shape or luminosity. Lack of mathematical model also limits our understanding of percolation analysis. In order to overcome these difficulties, we have studied percolation analysis based on discrete points. Using a Friends-of-Friends (FoF) algorithm, we generate the S-bb relation, between the fractional mass of the largest connected group (S) and the FoF linking length (bb). We propose a new model, the Probability Cloud Cluster Expansion Theory (PCCET) to relate the S-bb relation with correlation functions. We show that the S-bb relation reflects a combination of all orders of correlation functions. Using N-body simulation, we find that the S-bb relation is robust against redshift distortion and incompleteness in observation. From the Bolshoi simulation, with Halo Abundance Matching (HAM), we have generated a mock galaxy catalogue. Good matching of the projected two-point correlation function with observation is confirmed. However, comparing the mock catalogue with the latest galaxy catalogue from SDSS DR12, we have found significant differences in their S-bb relations. This indicates that the mock galaxy catalogue cannot accurately retain higher order correlation functions than the two-point correlation function, which reveals the limit of HAM method. As a new measurement, S-bb relation is applicable to a wide range of data types, fast to compute, robust against redshift distortion and incompleteness, and it contains information of all orders of correlation function.

astro-ph.CO↗

arXiv2018

Ultra-Light Axion Dark Matter and its impacts on dark halo structure in $N$-body simulation

The Ultra-Light Axion (ULA) is a dark matter candidate with mass $\mathcal{O}(10^{-22})$eV and de Broglie wavelength of order kpc. Such an axion, also called the Fuzzy Dark Matter (FDM), thermalizes via the gravitational force and forms a Bose-Einstein condensate. Recent studies suggested that the quantum pressure from the FDM can significantly affect the structure formation in small scales, thus alleviating the so-called "small-scale crisis". In this paper, we develop a new technique to discretize the quantum pressure and illustrate the interactions among FDM particles in the $N$-body simulation, which accurately simulates the formation of the dark-matter halo and its inner structure in the region outside the softening length. In a self-gravitationally-bound virialized halo, we find a constant density, solitonic core, which is consistent with the theoretical prediction. The existence of the solitonic core reveals the non-linear effect of quantum pressure and impacts the structure formation in the FDM model.

astro-ph.CO↗

arXiv2015

A parametrization of the cosmic-ray muon flux at sea-level

Based on the standard Gaisser's formula, a modified parametrization for the sea-level cosmic-ray muon flux is introduced. The modification is verified against experimental results. The average vertical cosmic-ray muon intensity as a function of depth of standard rock is simulated using the modified formula as input to the MUSIC code. The calculated muon intensities is consistent with the experimental measurements.

hep-ex↗