Cosmological measurements of the 21-cm line of neutral hydrogen are poised to dramatically enhance our understanding of the early universe. In particular, both the epochs of reionization and cosmic dawn remain largely uncharted, and the 21-cm signal is one of the few probes to reach them. The simplest 21-cm measurement is the global signal (GS), which corresponds to the averaged absorption or emission of 21-cm photons across the entire sky. While bright radio foregrounds swamp the cosmic signal over the entire frequency range observable, presenting a formidable hurdle, they can in principle be subtracted, given enough sensitivity. Here, however, we point out an additional---and irreducible---source of uncertainty for the 21-cm GS: cosmic variance. The cosmic-variance noise arises from the finite volume of the universe accessible to 21-cm experiments. Due to the cosmological redshifting of 21-cm photons, each observed frequency probes our universe during a particular cosmic age, corresponding to a narrow redshift slice. The presence of large 21-cm fluctuations makes the GS within each slice different than the GS averaged over the entire universe. We estimate the size of this cosmic-variance noise, and find that for a standard scenario it has a size of $\sim 0.1$ mK, which is $\sim 10\%$ of the size of the expected instrumental noise of a year-long experiment. Interestingly, cosmic variance can overtake instrumental noise for scenarios with extreme 21-cm fluctuations, such as those suggested to explain the sharpness of the claimed EDGES detection. Moreover, as large-scale 21-cm fluctuations are coherent over long distances, cosmic variance correlates the measurements of the GS at nearby redshifts, leading to off-diagonal uncertainties that have so far been neglected.
Current and future generations of intensity mapping surveys promise dramatic improvements in our understanding of galaxy evolution and large-scale structure. An intensity map provides a census of the cumulative emission from all galaxies in a given region and redshift, including faint objects that are undetectable individually. Furthermore, cross-correlations between line intensity maps and galaxy redshift surveys are sensitive to the line intensity and clustering bias without the limitation of cosmic variance. Using the Fisher information matrix, we derive simple expressions describing sensit…
In the coming decade, a new generation of telescopes, including JWST and WFIRST, will probe the period of the formation of first galaxies and quasars, and open up the last frontier for structure formation. Recent simulations as well as observations have suggested that these galaxies are strongly clustered (with large scale bias $\gtrsim6$), and therefore have significant cosmic variance. In this work, we use \texttt{BlueTides}, the largest volume cosmological simulation of galaxy formation, to directly estimate the cosmic variance for current and upcoming surveys. Given its resolution and volu…
Cosmic variance introduces significant uncertainties into galaxy number density properties when surveying the high-z Universe with a small volume, such uncertainties produce the field-to-field variance of galaxy number $σ_{g}$ in observational astronomy. This uncertainty significantly affects the Luminosity Functions (LF) measurement of Lya Emitters (LAEs). For most previous Lya LF studies, $σ_{g}$ is often adopted from predictions by cosmological simulations, but barely confirmed by observations. Measuring cosmic variance requires a huge sample over a large volume, exceeding the capabilities …
The next generation of Cosmic Microwave Background experiments will produce cosmic variance limited observations over a large fraction of sky and for a large range of multipoles. In this work we discuss different consistency tests that can be performed with the upcoming data from the Simons Observatory and the Planck data. We quantify the level of expected cosmological parameter shifts probed by these tests. We discuss the effect of difference in frequency of observation and present forecasts on a direct measurement of the Planck T-to-E leakage beam. We find that instrumental systematics in ei…
Analyzing the cosmic variance limit of remote dipole measurements of the cosmic microwave background using the large-scale kinetic Sunyaev Zel'dovich effect
Pulsar timing-array correlation measurements offer an exciting opportunity to test the nature of gravity in the cosmologically novel nanohertz gravitational wave regime. The stochastic gravitational wave background is assumed Gaussian and random, while there are limited pulsar pairs in the sky. This brings theoretical uncertainties to the correlation measurements, namely the pulsar variance due to pulsar samplings and the cosmic variance due to Gaussian signals. We demonstrate a straightforward calculation of the mean and the variances on the Hellings-Downs correlation relying on a power spect…
Cosmic variance or galaxy bias? Disentangling finite-volume and galaxy formation effects in cosmological analysis
Institute for Fundamental Physics of the Universe (IFPU)（宇宙基本物理研究所）；SISSA - International School for Advanced Studies（国际高等研究院）；INAF - Osservatorio Astronomico di Trieste（的里雅斯特天文台）；INFN – National Institute for Nuclear Physics（意大利国家核物理研究所）；Instituto de Astrofísica de Canarias（加那利天体物理研究所）；Universidad de La Laguna（拉古纳大学）
Cosmic variance of the galaxy cluster weak lensing signal
固定质量星系团投影密度剖面的内禀变化是星系团弱透镜不确定性的一个来源。我们提出了一个半分析模型来解释这种效应，基于晕浓度，椭圆度和方向的变化，以及相关晕的存在。我们在10%的水平上校准了我们模型的参数，以匹配M_200 m = 10^14时星系团轮廓的经验宇宙方差。10^15 h ^-1 M_sol，z = 0.25. 0.5在宇宙学模拟中。我们表明，弱透镜测量集群显着低估质量的不确定性，如果内在的轮廓变化被忽略，我们的模型可以用来提供正确的质量可能性。对于质量最大的星系团和深层观测（在M_200 m = 10^15 h ^-1 M_sol和z = 0.25时，仅宇宙方差的不确定性就有约20%），弱透镜星系团质量测量的可实现精度受到的影响特别大，但在典型的地面条件下也很明显。我们发现，忽略内在轮廓的变化导致的偏见，在质量可观测的关系约束与弱透镜，无论是内在的散射和整体规模（后者在15%的水平）。这些偏差超过了即将进行的调查的统计误差，如果考虑到星系团分布的宇宙方差，就可以避免这些偏差。
The impact of the cosmic variance on $H_0$ on cosmological analyses
$H_0$的局部（arXiv：1804.10655）和全局（arXiv：1605.02985）测量之间的当前$3.8 σ$张力无法通过一致性$Λ$ CDM模型完全解释。它可能是由未知的系统学或超出标准模型的物理学产生的。另一方面，众所周知，线性微扰理论预测哈勃参数$H_0$的宇宙方差，这导致其局部测定的系统误差。在这里，我们研究如何包括在可能性的宇宙方差$H_0$影响统计推断。特别是，我们认为$γ$ CDM，$w$ CDM和$γw$ CDM参数扩展的标准模型，我们约束与最新的CMB，BAO，SNe Ia，RSD和$H_0$数据。我们学到了两个重要的教训。首先，来自宇宙方差的系统误差--与模型无关--在考虑红移范围$0.0233 \le z \le 0.15$时约为$σ_{\text{cv}}\approx 0.88$ km s $^{-1}$ Mpc $^{-1}$（1.2\% $H_0^{\text{loc}}$），这与（arXiv：1804.10655）的主要分析相关，而在考虑更宽的红移范围$0.01 \le z \le 0.15$时约为$σ_{\text{cv}}\approx 1.5$ km s $^{-1}$ Mpc $^{-1}$（2.1\% $H_0^{\text{loc}}$）。尽管$σ_{\text{cv}}$影响局部$H_0$上的总误差预算，但它并不能显著缓解$…
A number of independent observations suggest that the intergalactic medium was significantly neutral at $z=7$ and that reionization was, perhaps, still in progress at $z=5.7$. The narrowband survey, SILVERRUSH, has mapped over $2,000$ Lyman-$α$ emitters (LAEs) at these redshifts. Previous analyses have assumed that reionization was over by $z=5.7$, but this data may actually sample the final stages of reionization when the last neutral islands were relegated to the cosmic voids. Motivated by these developments, we reexamine LAE void and peak statistics and their ability to constrain reionizati…
The Impact of Cosmic Variance and Satellites on JWST Clustering Measurements at Redshift around 6
University of California, Santa Barbara（加州大学圣塔芭芭拉分校）；Center for Astrophysics | Harvard & Smithsonian（哈佛-史密森天体物理中心）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）
In the context of cosmic microwave background polarization studies and the characterization of the Galactic foregrounds, the power spectrum analysis of the thermal dust polarization sky has led to intriguing evidence of an E/B asymmetry and a positive TE correlation. In this work, we produce synthesized dust polarization maps from a set of global magneto-hydrodynamic (MHD) simulations of Milky-Way-sized galaxies, and analyze their power spectra at intermediate angular scales (angular multipoles $\ell \in \left[60 ,\, 140\right]$). We study the role of the initial configuration of the large-sca…
We examine all-sky cosmic microwave background (CMB) temperature maps on large angular scales to compare their consistency with two scenarios: the standard inflationary quantum picture, and a distribution constrained to have a universal variance of primordial curvature perturbations on great circles. The latter symmetry is not a property of standard quantum inflation, but may be a symmetry of holographic models with causal quantum coherence on null surfaces. Since the variation of great-circle variance is dominated by the largest angular scale modes, in the latter case the amplitude and direct…
Cosmic variance limits the accuracy of cosmological N-body simulations, introducing bias in statistics such as the power spectrum, halo mass function, or the cosmic shear. We provide new methods to measure and reduce the effect of cosmic variance in existing and new simulations. We ran pairs of simulations using phase-shifted initial conditions with matching amplitudes. We set the initial amplitudes of the Fourier modes to ensure that the average power spectrum of the pair is equal to the cosmic mean power spectrum from linear theory. The average power spectrum of a pair of such simulations re…
It is extremely important to understand the processes through which the thermal state of the inter-galactic medium (IGM) evolved in the early universe in order to study the evolution of HI 21-cm signal during cosmic dawn. Here, we consider the heating of the IGM due to cosmic ray protons generated by the supernovae from both early Pop III and Pop II stars. The low energy cosmic ray protons from Pop III supernovae can escape from minihalos and heat the IGM via collision and ionization of hydrogen. Furthermore, high energy protons generated in Pop II supernovae can escape the hosting halos and h…
Forthcoming surveys will extend the understanding of cosmological large scale structures up to unprecedented redshift. According to this perspective, we present a fully relativistic framework to evaluate the impact of stochastic inhomogeneities on the determination of the Hubble constant. To this aim, we work within linear perturbation theory and relate the fluctuations of the luminosity distance-redshift relation, in the Cosmic Concordance model, to the intrinsic uncertainty associated to the measurement of $H_0$ from high-redshift surveys ($0.15\le z\le3.85$). We first present the detailed d…
We study the spherically averaged bispectrum of the 21-cm signal from the Epoch of Reionization (EoR). This metric provides a quantitative measurement of the level of non-Gaussianity of the signal which is expected to be high. We focus on the impact of the light-cone effect on the bispectrum and its detectability with the future SKA-Low telescope. Our investigation is based on a single reionization light-cone model and an ensemble of 50 realisations of the 21-cm signal to estimate the cosmic variance errors. We calculate the bispectrum with a new, optimised direct estimation method, DviSukta w…
