The squeezed matter bispectrum covariance with responses

We present a calculation of the angle-averaged squeezed matter bispectrum covariance ${\rm Cov}\left(B_{m}(k_1, k_1', s_1), B_{m}(k_2, k_2', s_2)\right)$, $s_i \ll k_i,k_i'$ ($i=1,2$), that uses matter power spectrum responses to describe the coupling of large- to short-scale modes in the nonlinear regime. The covariance is given by a certain configuration of the 6-point function, which we show is dominated by response-type mode-coupling terms in the squeezed bispectrum limit. The terms that are not captured by responses are small, effectively rendering our calculation complete and predictive for linear $s_1,s_2$ values and any nonlinear values of $k_1,k_1',k_2,k_2'$. Our numerical results show that the squeezed bispectrum super-sample covariance is only a negligible contribution. We also compute the power spectrum-bispectrum cross-covariance using responses. Our derivation for the squeezed matter bispectrum is the starting point to calculate analytical covariances for more realistic galaxy clustering and weak-lensing applications. It can also be used in cross-checks of numerical ensemble estimates of the general bispectrum covariance, given that it is effectively noise-free and complete in the squeezed limit.Comment: 29 pages + appendices; 3 figures and 1 table. The busy reader can focus on the summary in the Introduction, Table 1 and Section 6 with results. Comments welcomed. v2: minor corrections to the text; version published on JCA

The local PNG bias of neutral Hydrogen, H-I

We use separate universe simulations with the IllustrisTNG galaxy formation model to predict the local PNG bias parameters b(phi) and b(phi delta) of atomic neutral hydrogen, H-I. These parameters and their relation to the linear density bias parameter b(1) play a key role in observational constraints of the local PNG parameter f(NL) using the H-I power spectrum and bispectrum. Our results show that the popular calculation based on the universality of the halo mass function overpredicts the b(phi)(b(1)) and b(phi delta)(b(1)) relations measured in the simulations. In particular, our results show that at z less than or similar to 1 the H-I power spectrum is more sensitive to f(NL) compared to previously thought (b(phi) is more negative), but is less sensitive at other epochs (b(phi) is less positive). We discuss how this can be explained by the competition of physical effects such as that large-scale gravitational potentials with local PNG (i) accelerate the conversion of hydrogen to heavy elements by star formation, (ii) enhance the effects of baryonic feedback that eject the gas to regions more exposed to ionizing radiation, and (iii) promote the formation of denser structures that shield the H-I more efficiently. Our numerical results can be used to revise existing forecast studies on f(NL) using 21 cm line-intensity mapping data. Despite this first step towards predictions for the local PNG bias parameters of H-I, we emphasize that more work is needed to assess their sensitivity on the assumed galaxy formation physics and H-I modeling strategy

Predictions for local PNG bias in the galaxy power spectrum and bispectrum and the consequences for f NL constraints

We use hydrodynamical separate universe simulations with the IllustrisTNG model to predict the local primordial non-Gaussianity (PNG) bias parameters bϕ and bϕδ, which enter at leading order in the galaxy power spectrum and bispectrum. This is the first time that bϕδ is measured from either gravity-only or galaxy formation simulations. For dark matter halos, the popular assumption of universality overpredicts the bϕδ(b1) relation in the range 1 ≲ b1 ≲ 3 by up to Δ bϕδ ∼ 3 (b1 is the linear density bias). The adequacy of the universality relation is worse for the simulated galaxies, with the relations bϕ(b1) and bϕδ(b1) being generically redshift-dependent and very sensitive to how galaxies are selected (we test total, stellar and black hole mass, black hole mass accretion rate and color). The uncertainties on bϕ and bϕδ have a direct, often overlooked impact on the constraints of the local PNG parameter fNL, which we study and discuss. For a survey with V = 100 Gpc3/h3 at z=1, uncertainties Δ bϕ ≲ 1 and Δ bϕδ ≲ 5 around values close to the fiducial can yield relatively unbiased constraints on fNL using power spectrum and bispectrum data. We also show why priors on galaxy bias are useful even in analyses that fit for products fNLbϕ and fNLbϕδ. The strategies we discuss to deal with galaxy bias uncertainties can be straightforwardly implemented in existing fNL constraint analyses (we provide fits for some of the bias relations). Our results motivate more works with galaxy formation simulations to refine our understanding of bϕ and bϕδ towards improved constraints on fNL

On the impact of galaxy bias uncertainties on primordial non-Gaussianity constraints

We study the impact that uncertainties on assumed relations between galaxy bias parameters have on constraints of the local PNG $f_{\rm NL}$ parameter. We focus on the relation between the linear density galaxy bias $b_1$ and local PNG bias $b_\phi$ in an idealized forecast setup with multitracer galaxy power spectrum and bispectrum data. We consider two parametrizations of galaxy bias: 1) one inspired by the universality relation where $b_\phi = 2\delta_c\left(b_1 - p\right)$ and $p$ is a free parameter; and 2) another in which the product of bias parameters and $f_{\rm NL}$, like $f_{\rm NL} b_\phi$, is directly fitted for. The constraints on the $f_{\rm NL}-p$ plane are markedly bimodal, and both the central value and width of marginalized constraints on $f_{\rm NL}$ depend sensitively on the priors on $p$. Assuming fixed $p=1$ in the constraints with a fiducial value of $p=0.55$ can bias the inferred $f_{\rm NL}$ by $0.5\sigma$ to $1\sigma$; priors $\Delta p \approx 0.5$ around this fiducial value are however sufficient in our setup to return unbiased constraints. In power spectrum analyses, parametrization 2, that makes no assumptions on $b_\phi$, can distinguish $f_{\rm NL} \neq 0$ with the same significance as parametrization 1 assuming perfect knowledge of $b_\phi$ (the value of $f_{\rm NL}$ is however left unknown). A drawback of parametrization 2 is that the addition of the bispectrum information is not as beneficial as in parametrization 1. Our results motivate strongly the incorporation of mitigation strategies for bias uncertainties in PNG constraint analyses, as well as further theoretical studies on the relations between bias parameters to better inform those strategies.Comment: 16 pages + 2 appendices; 7 figures; comments welcomed

Structure formation in modified gravity cosmologies

We study linear and nonlinear structure formation in cosmologies where the accelerated expansion is driven by modifications to general relativity (GR). We focus on Galileon and Nonlocal gravity, which are two classes of models that have been attracting much attention. We derive the linearly perturbed model equations and solve them with suitably modified versions of Einstein-Boltzmann codes. We also derive the perturbed equations keeping the relevant nonlinear terms for small scale structure formation, which we solve using N- body codes and semi-analytical techniques that were developed for these models. Using CMB, SNIa and BAO data we find strong evidence for nonzero active neutrino masses (Σmν ≈ 0.6 eV) in all three main branches of covariant Galileon cosmologies, known as the Cubic, Quartic and Quintic models. However, in all branches, the lensing potential does not decay at late times on sub-horizon scales, which contradicts the measured positive sign of the ISW effect, thereby ruling out the Galileon model. The Nonlocal model we study should be able to fit the CMB with similar parameter values as ΛCDM. The N-body simulation results show that the covariant Galileon model admits realistic halo occupation distributions of luminous red galaxies, even for model parameters whose linear growth is noticieably enhanced (σ8 ≈ 1) relative to ΛCDM. In the Cubic Galileon model the screening mechanism is very efficient on scales smaller than 1Mpc, but in the Quartic and Quintic sectors, as well as in the Nonlocal model, we identify potential tensions with Solar System bounds. We illustrate that, despite the direct modifications to the lensing potential in the Cubic Galileon and Nonlocal models, cluster masses estimated from lensing remain the same as in GR. The lensing effects produced by cosmic voids found in the simulations of the Cubic Galileon are significanly boosted (≈ 100%) compared to GR, which strongly motivates using voids in tests of gravity. The combination of linear and nonlinear theory results presented here for Galileon and Nonlocal gravity is an example of what it could be done for any serious alternative models to ΛCDM, which will be tested by future experiments

Distribuição de vídeo sobre uma rede IEEE 802.11n

A recente norma IEEE 802.11n oferece um elevado débito em redes locais sem fios sendo por isso esperado uma adopção massiva desta tecnologia substituindo progressivamente as redes 802.11b/g. Devido à sua elevada capacidade esta recente geração de redes sem fios 802.11n permite um crescimento acentuado de serviços audiovisuais. Neste contexto esta dissertação procura estudar a rede 802.11n, caracterizando o desempenho e a qualidade associada a um serviço de transmissão de vídeo, recorrendo para o efeito a uma arquitectura de simulação da rede 802.11n. Desta forma é caracterizado o impacto das novas funcionalidades da camada MAC introduzidas na norma 801.11n, como é o caso da agregação A-MSDU e A-MPDU, bem como o impacto das novas funcionalidades da camada física como é o caso do MIMO; em ambos os casos uma optimização da parametrização é realizada. Também se verifica que as principais técnicas de codificação de vídeo H.264/AVC para optimizar o processo de distribuição de vídeo, permitem optimizar o desempenho global do sistema de transmissão. Aliando a optimização e parametrização da camada MAC, da camada física, e do processo de codificação, é possível propor um conjunto de configurações que permitem obter o melhor desempenho na qualidade de serviço da transmissão de conteúdos de vídeo numa rede 802.11n. A arquitectura de simulação construída nesta dissertação é especificamente adaptada para suportar as técnicas de agregação da camada MAC, bem como para suportar o encapsulamento em protocolos de rede que permitem a transmissão dos pacotes de vídeo RTP, codificados em H.264/AVC.The recent standard IEEE 802.11n offersa high speed wireless local areas networks, consequently is expected a progressively and massive acceptance of this technology to replacing the 802.11b/g networks. The high capacity of this modern generation of 802.11n wireless network also enables a fast increase of audiovisual services. In this context this dissertation provides a study of the 802.11n networks, showing the video transmission quality of service performance, using a simulation architecture for this matter. The study of new features introducedin the MAC layer of 802.11n such as AMSDU and A-MPDU aggregation as wellas the study of the new features in the physical layer such as MIMO, allow an optimal parameterization in the both layers of the standard. The H.264/AVC video encoder main techniques studies and the use of these same techniques in the simulation process also allow achieving an optimal overall performance of the transmission system. Combining the optimization with the MAC layer, physical layer and also the codification process, parameterization, it’s possible to propose a configuration set, that can provide a better video transmission quality of service performance, using a 802.11n network. The simulation architecture that was build on this dissertation it is adapted to support the aggregation techniques of the MAC layer, as well, to support the encapsulation of network protocols that allow the transmission of RTP packets encoded with the H.264/AVC

Responses of Halo Occupation Distributions: a new ingredient in the halo model & the impact on galaxy bias

Halo occupation distribution (HOD) models describe the number of galaxies that reside in different haloes, and are widely used in galaxy-halo connection studies using the halo model (HM). Here, we introduce and study HOD response functions $R_\mathcal{O}^g$ that describe the response of the HODs to long-wavelength perturbations $\mathcal{O}$. The linear galaxy bias parameters $b_\mathcal{O}^g$ are a weighted version of $b_\mathcal{O}^h + R_\mathcal{O}^g$, where $b_\mathcal{O}^h$ is the halo bias, but the contribution from $R_\mathcal{O}^g$ is routinely ignored in the literature. We investigate the impact of this by measuring the $R_\mathcal{O}^g$ in separate universe simulations of the IllustrisTNG model for three types of perturbations: total matter perturbations, $\mathcal{O}=\delta_m$; baryon-CDM compensated isocurvature perturbations, $\mathcal{O}=\sigma$; and potential perturbations with local primordial non-Gaussianity, $\mathcal{O}\propto f_{\rm NL}\phi$. Our main takeaway message is that the $R_\mathcal{O}^g$ are not negligible in general and their size should be estimated on a case-by-case basis. For stellar-mass selected galaxies, the responses $R_\phi^g$ and $R_\sigma^g$ are sizeable and cannot be neglected in HM calculations of the bias parameters $b_\phi^g$ and $b_\sigma^g$; this is relevant to constrain inflation using galaxies. On the other hand, we do not detect a strong impact of the HOD response $R_1^g$ on the linear galaxy bias $b_1^g$. These results can be explained by the impact that the perturbations have on stellar-to-total-mass relations. We also look into the impact on the bias of the gas distribution and find similar conclusions. We show that a single extra parameter describing the overall amplitude of $R_\mathcal{O}^g$ recovers the measured $b_\mathcal{O}^g$ well, which indicates that $R_\mathcal{O}^g$ can be easily added to HM/HOD studies as a new ingredient.Comment: 22 pages, 12 figures, 1 table. Comments are welcome! Accepted by JCA

K-mouflage gravity models that pass Solar System and cosmological constraints

We show that Solar System tests can place very strong constraints on K-mouflage models of gravity, which are coupled scalar field models with nontrivial kinetic terms that screen the fifth force in regions of large gravitational acceleration. In particular, the bounds on the anomalous perihelion of the Moon imposes stringent restrictions on the K-mouflage Lagrangian density, which can be met when the contributions of higher-order operators in the static regime are sufficiently small. The bound on the rate of change of the gravitational strength in the Solar System constrains the coupling strength $\beta$ to be smaller than $0.1$. These two bounds impose tighter constraints than the results from the Cassini satellite and Big Bang Nucleosynthesis. Despite the Solar System restrictions, we show that it is possible to construct viable models with interesting cosmological predictions. In particular, relative to $\Lambda$-CDM, such models predict percent-level deviations for the clustering of matter and the number density of dark matter haloes. This makes these models predictive and testable by forthcoming observational missions.Comment: 15 page

Accurate cosmic shear errors: do we need ensembles of simulations?

Accurate inference of cosmology from weak lensing shear requires an accurate shear power spectrum covariance matrix. Here, we investigate this accuracy requirement and quantify the relative importance of the Gaussian (G), super-sample covariance (SSC) and connected non-Gaussian (cNG) contributions to the covariance. Specifically, we forecast cosmological parameter constraints for future wide-field surveys and study how different covariance matrix components affect parameter bounds. Our main result is that the cNG term represents only a small and potentially negligible contribution to statistical parameter errors: the errors obtained using the G+SSC subset are within lesssim 5% of those obtained with the full G+SSC+cNG matrix for a Euclid-like survey. This result also holds for the shear two-point correlation function, variations in survey specifications and for different analytical prescriptions of the cNG term. The cNG term is that which is often tackled using numerically expensive ensembles of survey realizations. Our results suggest however that the accuracy of analytical or approximate numerical methods to compute the cNG term is likely to be sufficient for cosmic shear inference from the next generation of surveys

Halo assembly bias from a deep learning model of halo formation

We build a deep learning framework that connects the local formation process of dark matter halos to the halo bias. We train a convolutional neural network (CNN) to predict the final mass and concentration of dark matter halos from the initial conditions. The CNN is then used as a surrogate model to derive the response of the halos' mass and concentration to long-wavelength perturbations in the initial conditions, and consequently the halo bias parameters following the "response bias" definition. The CNN correctly predicts how the local properties of dark matter halos respond to changes in the large-scale environment, despite no explicit knowledge of halo bias being provided during training. We show that the CNN recovers the known trends for the linear and second-order density bias parameters $b_1$ and $b_2$, as well as for the local primordial non-Gaussianity linear bias parameter $b_\phi$. The expected secondary assembly bias dependence on halo concentration is also recovered by the CNN: at fixed mass, halo concentration has only a mild impact on $b_1$, but a strong impact on $b_\phi$. Our framework opens a new window for discovering which physical aspects of the halo's Lagrangian patch determine assembly bias, which in turn can inform physical models of halo formation and bias.Comment: 11 pages, 5 figures, to be submitted to MNRAS, comments welcom