arXiv subjects

Yue Nan

Publications and source records attributed to Yue Nan.

9 recordsLinked to original sources

arXiv2026

Unveiling $f(R)$ Gravity with Void-Galaxy Cross-Correlation Multipoles

Cosmic voids provide low-density environments where the scalar fifth force predicted by $f(R)$ modified gravity can be weakly screened. We present a semi-analytical calculation of the monopole, dipole, and quadrupole of the void-galaxy cross-correlation function $\xi^{s}(s,\mu)$ in redshift space for the Hu-Sawicki $f(R)$ model ($n=1$), combining scale-dependent growth induced by the scalaron with nonlinear spherical shell dynamics. The same framework can be generalized to metric $f(R)$ theories for which $G_{\rm eff}(k,a)/G$ is specified in the quasi-static limit. Our key results are: (1)~the monopole deviation from $\Lambda{\rm CDM}$ grows from $+2.8\%$ for large voids ($r_v=30 h^{-1}{\rm Mpc}$) to $+29.7\%$ for small voids ($r_v=11.7 h^{-1}{\rm Mpc}$) at $|f_{R0}|=10^{-5}$, a distinctive size-dependent signature of the Compton-scale scalaron response, with $\lambda_C\approx 8 h^{-1}{\rm Mpc}$; (2)~nonlinear evolution amplifies the modified-gravity signal by $\mathcal{A}_0\approx 4$, bringing it within reach of ongoing and upcoming spectroscopic surveys such as DESI, Subaru PFS, Euclid, and Roman; (3) the gravitational potential contains a finite-range Yukawa component, producing a radially dependent dipole signature complementary to the density and velocity multipoles; (4) for the fiducial Hu-Sawicki evolution, the signal generally decreases toward higher redshift as the scalaron Compton wavelength becomes shorter, but remains potentially detectable at Stage-IV spectroscopic void samples. We show that the void-scale transition in the modified-gravity response, the joint sensitivity to density, velocity, and fifth-force contributions, and the nonlinear amplification around void shells make redshift-space void-galaxy multipoles a powerful semi-analytical probe of $f(R)$ gravity and effective dark-energy inhomogeneities in modified gravity.

astro-ph.CO↗

arXiv2025

On the equivalence between galaxy angular correlation function and power spectrum in constraining primordial non-Gaussianity

We investigate the angular power spectrum ($C_\ell)$ and angular correlation function ($w(\theta)$) of galaxy number density field in the presence of the local-type primordial non-Gaussianity (PNG), explicitly accounting for the integral constraint in an all-sky survey. We show that the PNG signature in $C_{\ell}$ is confined to low multipoles in the linear regime, whereas its signature in $w(\theta)$ extends across a wide range of angular scales, including those below the nonlinear scale. Therefore, the equivalence between $C_\ell$ and $w(\theta)$ can be violated when scale cuts of multipoles or angular scales -- for example, to mitigate systematic effects -- are applied in the analysis. Assuming samples of photometric galaxies divided into multiple redshift bins in the range $0<z<7$, we forecast the precision of constraining the PNG parameter ($f_{\rm NL}$) from the hypothetical measurements of $C_\ell$ or $w(\theta)$ assuming different scale cuts in the multipoles or angular scales, respectively. Our results imply that the PNG information can be extracted from $w(\theta)$ on relatively small angular scales such as $\lesssim 10$ degree for a high-redshift galaxy sample or from $w(\theta)$ measured in a survey with partial area coverage.

astro-ph.CO↗

arXiv2022

Large-scale structure with superhorizon isocurvature dark energy

The standard cosmological model assumes a homogeneous and isotropic universe as the background spacetime on large scales called the cosmological principle. However, some observations suggest the possibility of an inhomogeneous and anisotropic universe at large scales. In this paper, we investigate a model of the universe with random inhomogeneities and anisotropies on very large scales, motivated by the supercurvature dark energy model in Nan et al. [Phys. Rev. D 99, 103512 (2019)]. In this model, the authors introduced a scalar field with $\mathcal{O}(1)$ inhomogeneities on a scale sufficiently larger than the current horizon scale (superhorizon scale), and the potential energy of the scalar field explains the accelerating expansion, with slight deviations from the cosmological principle. We aim at clarifying the theoretical prediction on the large-scale structure (LSS) of the matter component in this model. Based on the work on the superhorizon scale fluctuations (superhorizon mode) presented in Nan and Yamamoto [Phys. Rev. D 105, 063518 (2022)], we derive the equations that the perturbative components to the LSS obey as a generalization of the cosmological perturbations theory, which is solved to find the influence of the dark energy inhomogeneities on the formation of the LSS. Finally, we show that the model can be consistent with observations by comparing the $σ_8$ predicted by the numerical solution of the model with the $σ_8$ indicated by observations such as Planck and SDSS.

astro-ph.CO↗

arXiv2022

Dark energy model with very large-scale inhomogeneity

We consider a dynamical model for dark energy based on an ultralight mass scalar field with very large-scale inhomogeneities. This model may cause observable impacts on the anisotropic properties of the cosmic microwave background (CMB) intensity and luminosity distance. We formulate the model as the cosmological perturbations of the superhorizon scales, focusing on the local region of our universe. Moreover, we investigated the characteristic properties of the late-time evolution of inhomogeneous dark energy. Our numerical solutions show that the model can mimic the standard $Λ$CDM cosmology while including spatially dependent dark energy with flexible ranges of the model parameters. We put a constraint on the amplitude of these inhomogeneities of the dark energy on very large scales with the observations of the CMB anisotropies. We also discuss their influence on the estimation of the luminosity distance.

astro-ph.CO↗

arXiv2021

Entanglement of the Vacuum between Left, Right, Future, and Past: Dirac spinor in Rindler spaces and Kasner spaces

We study the relations of the positive frequency mode functions of Dirac field in 4-dimensional Minkowski spacetime covered with Rindler and Kasner coordinates, and describe the explicit form of the Minkowski vacuum state with the quantum states in Kasner and Rindler regions, and analytically continue the solutions. As a result, we obtain the correspondence of the positive frequency mode functions in Kasner region and Rindler region in a unified manner which derives vacuum entanglement.

gr-qc↗

arXiv2019

Large-scale inhomogeneity of dark energy produced in the ancestor vacuum

We investigate large-scale inhomogeneity of dark energy in the bubble nucleation scenario of the universe. In this scenario, the present universe was created by a bubble nucleation due to quantum tunneling from a metastable ancestor vacuum, followed by a primordial inflationary era. During the bubble nucleation, supercurvature modes of some kind of a scalar field are produced, and remain until present without decaying; thus they can play a role of the dark energy, if the mass of the scalar field is sufficiently light in the present universe. The supercurvature modes fluctuate at a very large spatial scale, much longer than the Hubble length in the present universe. Thus they create large-scale inhomogeneities of the dark energy, and generate large-scale anisotropies in the cosmic microwave background (CMB) fluctuations. This is a notable feature of this scenario, where quantum fluctuations of a scalar field are responsible for the dark energy. In this paper, we calculate imprints of the scenario on the CMB anisotropies through the integrated Sachs-Wolfe (ISW) effect, and give observational constraints on the curvature parameter $Ω_K$ and on an additional parameter $ε$ describing some properties of the ancestor vacuum.

astro-ph.CO↗

arXiv2018

Gravitational redshift in the void-galaxy cross-correlation function in redshift space

We construct an analytic model for the void-galaxy cross-correlation function that enables theoretical predictions of the dipole signal produced dominantly by the gravitational redshift within voids for the first time. By extending a theoretical formulation for the redshift-space distortion of galaxies to include the second order terms of the galaxy peculiar velocity $\bm v$ and the gravitational potential, we formulate the void-galaxy cross-correlation function multipoles in the redshift space, the monopole $ξ_0^{(s)}$, dipole $ξ_1^{(s)}$ and quadrupole $ξ_2^{(s)}$. We find that the dipole $ξ_1^{(s)}$ is dominated by the gravitational redshift, which provide a unique opportunity to detect the gravitational potential of voids. Thus, for the dipole $ξ_1^{(s)}(s)$, the gravitational redshift is crucial. Although the higher order effect is almost negligible on the monopole $ξ_0^{(s)}$, it has an influence on the quadrupole $ξ_2^{(s)}$. The effects from the random velocity of galaxies and the definition of the void center on the dipole signal are also discussed. Our model offers a new theoretical probe for the detection of gravitational redshift with voids and further tests on cosmology and gravity.

astro-ph.CO↗

arXiv2018

Higher multipoles of the galaxy bispectrum in redshift space

As a generalization of our previous work [Phys. Rev. D 95 043528 (2017)], in which an analytic model for the galaxy bispectrum in redshift space was developed on the basis of the halo approach, we here investigate its higher multipoles that have not been known so far. The redshift-space bispectrum includes the two variables $ω$ and $ϕ$ for the line-of-sight direction, and the higher multipole bispectra are defined by the coefficients in the expansion of the redshift-space bispectrum using the spherical harmonics. We find 6 new nonvanishing components out of $25$ total components up to $\ell=4$, in addition to 3 components discussed in the previous work (monopole, quadruple, and hexadecapole of $m=0$). The characteristic behaviors of the new nonvanishing multipoles are compared with the results of galaxy mock catalogs that match the halo occupation distribution of the Sloan Digital Sky Survey Baryonic Oscillation Spectroscopic Survey low-redshift sample. We find that the multipoles with $m\ne 0$ are also sensitive to redshift-space distortion (RSD) as well as those with $m=0$ and thus are key ingredients in the RSD analysis using the galaxy bispectrum. Analytic approximation formulas for these nonzero components are also presented; these are useful for understanding the characteristic behaviors.

astro-ph.CO↗

arXiv2017

An analytic halo approach to the bispectrum of galaxies in redshift space

We present an analytic formula for the galaxy bispectrum in redshift space on the basis of the halo approach description with the halo occupation distribution of central galaxies and satellite galaxies. This work is an extension of a previous work on the galaxy power spectrum, which illuminated the significant contribution of satellite galaxies to the higher multipole spectrum through the non-linear redshift space distortions of their random motions. Behaviors of the multipoles of the bispectrum are compared with results of numerical simulations assuming a halo occupation distribution of the LOWZ sample of the SDSS-III BOSS survey. Also presented are analytic approximate formulas for the multipoles of the bispectrum, which is useful to understanding their characteristic properties. We demonstrate that the Fingers of God effect is quite important for the higher multipoles of the bispectrum in redshift space, depending on the halo occupation distribution parameters.

astro-ph.CO↗