arXiv subjects

Shek Yeung

Publications and source records attributed to Shek Yeung.

9 recordsLinked to original sources

arXiv2025

Impact of light sterile neutrinos on cosmological large scale structure

Sterile neutrinos with masses on the $\mathrm{eV}$ scale are promising candidates to account for the origin of neutrino mass and the reactor neutrino anomalies. The mixing between sterile and active neutrinos in the early universe could result in a large abundance of relic sterile neutrinos, which depends on not only their physical mass $m_{\rm phy}$ but also their degree of thermalization, characterized by the extra effective number of relativistic degrees of freedom $\Delta N_{\rm eff}$. Using neutrino-involved N-body simulations, we investigate the effects of sterile neutrinos on the matter power spectrum, halo pairwise velocity, and halo mass and velocity functions. We find that the presence of sterile neutrinos suppress the matter power spectrum and halo mass and velocity functions, but enhance the halo pairwise velocity. We also provide fitting formulae to quantify these effects.

astro-ph.CO↗

arXiv2024

Measuring the Hubble constant through the galaxy pairwise peculiar velocity

The Hubble constant $H_0$, the current expansion rate of the universe, is one of the most important parameters in cosmology. The cosmic expansion regulates the mutually approaching motion of a pair of celestial objects due to their gravity. Therefore, the mean pairwise peculiar velocity of celestial objects, which quantifies their relative motion, is sensitive to both $H_0$ and the dimensionless total matter density $\Omega_m$. Based on this, using the Cosmicflows-4 data, we measured $H_0$ for the first time via the galaxy pairwise velocity in the nonlinear and quasi-linear range. Our results yield $H_0=75.5\pm1.4$ km s$^{-1}$ Mpc$^{-1}$ and $\Omega_m=0.311^{+0.029}_{-0.028}$ . The uncertainties of $H_0$ and $\Omega_m$ can be improved to around 0.6% and 2%, respectively, if the statistical errors become negligible in the future.

astro-ph.CO↗

arXiv2024

Refitting cosmological data with neutrino mass and degeneracy

A simple and natural extension of the standard Lambda cold dark matter ($\Lambda$CDM) model is to allow relic neutrinos to have finite chemical potentials. We confront this $\Lambda$CDM$\xi$ model, a $\Lambda$CDM with neutrino mass $M_\nu$ and degeneracy $\xi_3$ as additional parameters, with various cosmological data sets. We find that the $H_0$ and $S_8$ tensions become significant only in the presence of the cosmic microwave background (CMB) polarization data. Specifically, the global and local measurements agree to within 0.8$\sigma$ and 1.6$\sigma$ for the $H_0$ and $S_8$ tensions, respectively, when the CMB polarization data are not included. Therefore, the $H_0$ and $S_8$ tensions exist between CMB temperature and polarization data, both being global measurements. Fitting the $\Lambda$CDM$\xi$ model to the CMB temperature data, we find 3$\sigma$ evidence for nonzero neutrino mass ($M_\nu=0.57^{+0.17}_{-0.13}\,\mathrm{eV}$) and degeneracy ($\xi_3=1.13^{+0.41}_{-0.19}$), and the O(1) neutrino degeneracy parameter is compatible with Big Bang nucleosynthesis data. The scalar index $n_s$ exceeds 1 slightly, which is compatible with some hybrid inflation models. Furthermore, the recent DESI baryon acoustic oscillation data prefer the $\Lambda$CDM$\xi$ model to the Planck $\Lambda$CDM model. Similar results are obtained when including additional supernova data, while the inclusion of the Atacama Cosmology Telescope $\tau$ prior shifts the preferred $M_\nu$ and $\xi_3$ values closer to zero and brings $n_s$ back to the values favored when the polarization data are included.

hep-ph↗

arXiv2023

Measuring neutrino mass and asymmetry with matter pairwise velocities

Neutrinos are believed to be the most abundant fermions in the Universe, but their masses are unknown, except for being non-zero but much smaller than other fermions. Cosmological relic neutrinos could also have non-zero chemical potentials (or asymmetries). Using neutrino-involved N-body simulations, we investigate the neutrino effects on the matter pairwise velocity, which itself is an interesting probe of cosmology. We find that for light-halo ($[10^{11},10^{13}]\ M_\odot$) mean pairwise velocity, in the transition range ($[4,15]\ \mathrm{Mpc}$), the effects of neutrino masses overwhelm the effects of neutrino asymmetries, while in the two-halo-group range ($[25,50]\ \mathrm{Mpc}$), for both light and heavy haloes ($[10^{13},10^{15}]\ M_\odot$), the effects of neutrino asymmetries dominate, making it possible to disentangle the two effects. We provide fitting formulae to quantify the effects of neutrino mass and asymmetry on halo-halo pairwise velocities.

astro-ph.CO↗

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

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↗

arXiv2021

Relic Neutrino Degeneracies and Their Impact on Cosmological Parameters

In the standard $Λ$CDM model, neutrinos are treated as radiation, with their masses and possible degeneracy ignored. In this paper, we compute the impact of a finite relic neutrino degeneracy $ξ$ on the CMB angular power spectra, and obtain constraints on $ξ$ using current cosmological data sets. We find that $ξ\approx O(1)$ is still allowed. We also study the correlations between $ξ$, the Hubble parameter $H_0$, and the spectral index $n_s$. Due to these correlations, the CMB constraints on inflation models are loosened when $ξ$ is fitted together with other cosmological parameters, such that some models excluded at 95% confidence level by standard fittings without $ξ$ could be revived. Furthermore, the tension in CMB and local measurements of $H_0$ is slightly alleviated. Our results suggest that $ξ$ is a non-negligible physical parameter for cosmological analyses.

astro-ph.CO↗

arXiv2019

Evidence of Neutrino Enhanced Clustering in a Complete Sample of Sloan Survey Clusters, Implying $\sum m_ν= 0.119 \pm 0.034$ eV

The clustering amplitude of 7143 clusters from the Sloan Digital Sky Survey (SDSS) is found to increase with cluster mass, closely agreeing with the Gaussian random field hypothesis for structure formation. The amplitude of the observed cluster correlation exceeds the predictions from pure cold dark matter (CDM) simulation by $\simeq 6\%$ for the standard Planck-based values of the cosmological parameters. We show that this excess can be naturally accounted for by free streaming of light neutrinos, which opposes gravitational growth, so clusters formed at fixed mass are fewer and hence more biased than for a pure CDM density field. An enhancement of the cluster bias by 7\% matches the observations, corresponding to a total neutrino mass, $m_ν = 0.119 \pm 0.034$ eV at 67\% confidence level, for the standard relic neutrino density. If ongoing laboratory experiments favor a normal neutrino mass hierarchy then we may infer a somewhat larger total mass than the minimum oscillation based value, $\sum m_ν \simeq 0.056eV$, with 90\% confidence. Much higher precision can be achieved by applying our method to a larger sample of more distant clusters with weak lensing derived masses.

astro-ph.CO↗

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↗