arXiv subjects

Kendrick M. Smith

Publications and source records attributed to Kendrick M. Smith.

At least 73 records · Page 4Linked to original sources

arXiv2014

Using Large Scale Structure to test Multifield Inflation

Primordial non-Gaussianity of local type is known to produce a scale-dependent contribution to the galaxy bias. Several classes of multi-field inflationary models predict non-Gaussian bias which is stochastic, in the sense that dark matter and halos don't trace each other perfectly on large scales. In this work, we forecast the ability of next-generation Large Scale Structure surveys to constrain common types of primordial non-Gaussianity like $f_{NL}$, $g_{NL}$ and $τ_{NL}$ using halo bias, including stochastic contributions. We provide fitting functions for statistical errors on these parameters which can be used for rapid forecasting or survey optimization. A next-generation survey with volume $V = 25 h^{-3}$Gpc$^3$, median redshift $z = 0.7$ and mean bias $b_g = 2.5$, can achieve $σ(f_{NL}) = 6$, $σ(g_{NL}) = 10^5$ and $σ(τ_{NL}) = 10^3$ if no mass information is available. If halo masses are available, we show that optimally weighting the halo field in order to reduce sample variance can achieve $σ(f_{NL}) = 1.5$, $σ(g_{NL}) = 10^4$ and $σ(τ_{NL}) = 100$ if halos with mass down to $M_{min} = 10^{11}$ $h^{-1} M_\odot $ are resolved, outperforming Planck by a factor of 4 on $f_{NL}$ and nearly an order of magnitude on $g_{NL}$ and $τ_{NL}$. Finally, we study the effect of photometric redshift errors and discuss degeneracies between different non-Gaussian parameters, as well as the impact of marginalizing Gaussian bias and shot noise.

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arXiv2014

New natural shapes of non-Gaussianity from high-derivative interactions and their optimal limits from WMAP 9-year data

Given the fantastic experimental effort, it is important to thoroughly explore the signature space of inflationary models. The fact that higher derivative operators do not renormalize lower derivative ones allows us to find a large class of technically natural single-clock inflationary models where, in the context of the Effective Field Theory of Inflation, the leading interactions have many derivatives. We systematically explore the 3-point function induced by these models and their overlap with the standard equilateral and orthogonal templates. We find that in order to satisfactorily cover the signature space of these models, two new additional templates need to be included. We then perform the optimal analysis of the WMAP 9-year data for the resulting four templates, finding that the overall significance of a non-zero signal is between 2--2.5$σ$, depending on the choice of parameter space, partially driven by the preference for nonzero $f_{NL}^{\rm orth}$ in WMAP9.

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arXiv2014

On quantifying and resolving the BICEP2/Planck tension over gravitational waves

The recent BICEP2 measurement of primordial gravity waves (r = 0.2^{+0.07}_{-0.05}) appears to be in tension with the upper limit from WMAP (r<0.13 at 95% CL) and Planck (r<0.11 at 95% CL). We carefully quantify the level of tension and show that it is very significant (around 0.1% unlikely) when the observed deficit of large-scale temperature power is taken into account. We show that measurements of TE and EE power spectra in the near future will discriminate between the hypotheses that this tension is either a statistical fluke, or a sign of new physics. We also discuss extensions of the standard cosmological model that relieve the tension, and some novel ways to constrain them.

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arXiv2013

Non-Gaussian structure of the lensed CMB power spectra covariance matrix

Gravitational lensing of the Cosmic Microwave Background (CMB) encodes cosmological information in the observed anisotropies of temperature and polarization. Accurate extraction of this additional information requires precise modeling of the covariance matrix of the power spectra of observed CMB fields. We introduce a new analytical model to describe the non-Gaussian structure of this covariance matrix and display the importance of second-order terms that were previously neglected. When compared with direct numerical simulations our model captures parameter errors to better than a few percent for cases where the non-Gaussianity causes an order unity degradation in errors. We also provide a detailed comparison between the information content of lensed CMB power spectra and ideal reconstruction of the lensing potential. We illustrate the impact of the non-Gaussian terms in the power spectrum covariance by providing Fisher errors on the sum of the masses of the neutrinos, the dark energy equation of state, and the curvature of the Universe.

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arXiv2012

Stochastic Bias from Non-Gaussian Initial Conditions

In this article, we show that a stochastic form of scale-dependent halo bias arises in multi-source inflationary models, where multiple fields determine the initial curvature perturbation. We derive this effect for general non-Gaussian initial conditions and study various examples, such as curvaton models and quasi-single field inflation. We present a general formula for both the stochastic and the non-stochastic parts of the halo bias, in terms of the N-point cumulants of the curvature perturbation at the end of inflation. At lowest order, the stochasticity arises if the collapsed limit of the four-point function is boosted relative to the square of the three-point function in the squeezed limit. We derive all our results in two ways, using the barrier crossing formalism and the peak-background split method. In a companion paper, we prove that these two approaches are mathematically equivalent.

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arXiv2012

On the Equivalence of Barrier Crossing, Peak-Background Split, and Local Biasing

Several, apparently distinct, formalisms exist in the literature for predicting the clustering of dark matter halos. It has been noticed on a case-by-case basis that the predictions of these different methods agree in specific examples, but there is no general proof that they are equivalent. In this paper, we give a simple proof of the mathematical equivalence of barrier crossing, peak-background split, and local biasing.

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arXiv2011

Beyond the power spectrum: primordial and secondary non-Gaussianity in the microwave background

Cosmic microwave background observations are most commonly analyzed by estimating the power spectrum. In the limit where the CMB statistics are perfectly Gaussian, this extracts all the information, but the CMB also contains detectable non-Gaussian contributions from secondary, and possibly primordial, sources. We review possible sources of CMB non-Gaussianity and describe statistical techniques which are optimized for measuring them, complementing the power spectrum analysis. The machinery of $N$-point correlation functions provides a unifying framework for optimal estimation of primordial non-Gaussian signals or gravitational lensing. We review recent results from applying these estimators to data from the WMAP satellite mission.

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arXiv2011

Local stochastic non-Gaussianity and N-body simulations

Large-scale clustering of highly biased tracers of large-scale structure has emerged as one of the best observational probes of primordial non-Gaussianity of the local type (i.e. f_{NL}^{local}). This type of non-Gaussianity can be generated in multifield models of inflation such as the curvaton model. Recently, Tseliakhovich, Hirata, and Slosar showed that the clustering statistics depend qualitatively on the ratio of inflaton to curvaton power ξafter reheating, a free parameter of the model. If ξis significantly different from zero, so that the inflaton makes a non-negligible contribution to the primordial adiabatic curvature, then the peak-background split ansatz predicts that the halo bias will be stochastic on large scales. In this paper, we test this prediction in N-body simulations. We find that large-scale stochasticity is generated, in qualitative agreement with the prediction, but that the level of stochasticity is overpredicted by ~30%. Other predictions, such as ξindependence of the halo bias, are confirmed by the simulations. Surprisingly, even in the Gaussian case we do not find that halo model predictions for stochasticity agree consistently with simulations, suggesting that semi-analytic modeling of stochasticity is generally more difficult than modeling halo bias.

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arXiv2011

No evidence for the cold spot in the NVSS radio survey

We revisit recent claims that there is a "cold spot" in both number counts and brightness of radio sources in the NVSS survey, with location coincident with the previously detected cold spot in WMAP. Such matching cold spots would be difficult if not impossible to explain in the standard LCDM cosmological model. Contrary to the claim, we find no significant evidence for the radio cold spot, after including systematic effects in NVSS, and carefully accounting for the effect of a posteriori choices when assessing statistical significance.

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arXiv2011

Algorithms for bispectra: forecasting, optimal analysis, and simulation

We propose a factorizability ansatz for angular bispectra which permits fast algorithms for forecasting, analysis, and simulation, yet is general enough to encompass many interesting CMB bispectra. We describe a suite of general algorithms which apply to any bispectrum which can be represented in factorizable form. First, we present algorithms for Fisher matrix forecasts and the related problem of optimizing the factorizable representation, giving a Fisher forecast for Planck as an example. We show that the CMB can give independent constraints on the amplitude of primordial bispectra of both local and equilateral shape as well as those created by secondary anisotropies. We also show that the ISW-lensing bispectrum should be detected by Planck and could bias estimates of the local type of non-Gaussianity if not properly accounted for. Second, we implement a bispectrum estimator which is fully optimal in the presence of sky cuts and inhomogeneous noise, extends the generality of fast estimators which have been limited to a few specific forms of the bispectrum, and improves the running time of existing implementations by several orders of magnitude. Third, we give an algorithm for simulating random, weakly non-Gaussian maps with prescribed power spectrum and factorizable bispectrum.

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arXiv2011

A universal bound on N-point correlations from inflation

Models of inflation in which non-Gaussianity is generated outside the horizon, such as curvaton models, generate distinctive higher-order correlation functions in the CMB and other cosmological observables. Testing for violation of the Suyama-Yamaguchi inequality tauNL >= (6/5 fNL)^2, where fNL and tauNL denote the amplitude of the three-point and four-point functions in certain limits, has been proposed as a way to distinguish qualitative classes of models. This inequality has been proved for a wide range of models, but only weaker versions have been proved in general. In this paper, we give a proof that the Suyama-Yamaguchi inequality is always satisfied. We discuss scenarios in which the inequality may appear to be violated in an experiment such as Planck, and how this apparent violation should be interpreted. We analyze a specific example, the "ungaussiton" model, in which leading-order scaling relations suggest that the Suyama-Yamaguchi inequality is eventually violated, and show that the inequality always holds.

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arXiv2011

Supersonic baryon-CDM velocities and CMB B-mode polarization

It has recently been shown that supersonic relative velocities between dark matter and baryonic matter can have a significant effect on formation of the first structures in the universe. If this effect is still non-negligible during the epoch of hydrogen reionization, it generates large-scale anisotropy in the free electron density, which gives rise to a CMB B-mode. We compute the B-mode power spectrum and find a characteristic shape with acoustic peaks at l ~ 200, 400, ... The amplitude of this signal is a free parameter which is related to the dependence of the ionization fraction on the relative baryon-CDM velocity during the epoch of reionization. However, we find that the B-mode signal is undetectably small for currently favored reionization models in which hydrogen is reionized promptly at z ~ 10, although constraints on this signal by future experiments may help constrain models in which partial reionization occurs at higher redshift, e.g. by accretion onto primordial black holes.

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arXiv2011

Testing Inflation with Dark Matter Halos

Cosmic inflation provides a mechanism for generating the early density perturbations that seeded the large-scale structures we see today. Primordial non-Gaussianity is among the most promising of few observational tests of physics at this epoch. At present non-Gaussianity is best constrained by the cosmic microwave background, but in the near term large-scale structure data may be competitive so long as the effects of primordial non-Gaussianity can be modeled through the non-linear process of structure formation. We discuss recent work modeling effects of a few types of primordial non-Gaussianity on the large-scale halo clustering and the halo mass function. More specifically, we compare analytic and N-body results for two variants of the curvaton model of inflation: (i) a "tauNL" scenario in which the curvaton and inflaton contribute equally to the primordial curvature perturbation and (ii) a "gNL" model where the usual quadratic fNL term in the potential cancels, but a large cubic term remains.

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arXiv2011

The Non-Gaussian Halo Mass Function with fNL, gNL and tauNL

Primordial non-Gaussianity has emerged as one of the most promising probes of the inflationary epoch. While the cosmic microwave background and large-scale halo bias currently provide the most stringent constraints on the non-Gaussian parameter fNL, the abundance of dark matter halos is a complementary probe which may allow tests of Gaussianity which are independent of the precise form of non-Gaussian initial conditions. We study the halo mass function in N-body simulations with a range of non-Gaussian initial conditions. In addition to the usual fNL model, we consider gNL Phi^3-type non-Gaussianity and models where the 4-point amplitude tauNL is an independent parameter. We introduce a new analytic form for the halo mass function in the presence of primordial non-Gaussianity, the "log-Edgeworth" mass function, and find good agreement with the N-body simulations. The log-Edgeworth mass function introduces no free parameters and can be constructed from first principles for any model of primordial non-Gaussianity.

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arXiv2011

Halo clustering and g_{NL}-type primordial non-Gaussianity

A wide range of multifield inflationary models generate non-Gaussian initial conditions in which the initial adiabatic fluctuation is of the form (zeta_G + g_{NL} zeta_G^3). We study halo clustering in these models using two different analytic methods: the peak-background split framework, and brute force calculation in a barrier crossing model, obtaining agreement between the two. We find a simple, theoretically motivated expression for halo bias which agrees with N-body simulations and can be used to constrain g_{NL} from observations. We discuss practical caveats to constraining g_{NL} using only observable properties of a tracer population, and argue that constraints obtained from populations whose observed bias is <~ 2.5 are generally not robust to uncertainties in modeling the halo occupation distribution of the population.

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arXiv2011

Non-Gaussianities in Single Field Inflation and their Optimal Limits from the WMAP 5-year Data

Using the recently developed effective field theory of inflation, we argue that the size and the shape of the non-Gaussianities generated by single-field inflation are generically well described by two parameters: f_NL^equil, which characterizes the size of the signal that is peaked on equilateral configurations, and f_NL^orthog, which instead characterizes the size of the signal which is peaked both on equilateral configurations and flat-triangle configurations (with opposite signs). The shape of non-Gaussianities associated with f_NL^orthog is orthogonal to the one associated to f_NL^equil, and former analysis have been mostly blind to it. We perform the optimal analysis of the WMAP 5-year data for both of these parameters. We find no evidence of non-Gaussianity, and we have the following constraints: -125 < f_NL^equil < 435, -369 < f_NL^orthog < 71 at 95% CL. We show that both of these constraints can be translated into limits on parameters of the Lagrangian of single-field inflation. For one of them, the speed of sound of the inflaton fluctuations, we find that it is either bounded to be c_s > 0.011 at 95% CL. or alternatively to be so small that the higher-derivative kinetic term dominate at horizon crossing. We are able to put similar constraints on the other operators of the inflaton Lagrangian.

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arXiv2010

Delensing CMB Polarization with External Datasets

One of the primary scientific targets of current and future CMB polarization experiments is the search for a stochastic background of gravity waves in the early universe. As instrumental sensitivity improves, the limiting factor will eventually be B-mode power generated by gravitational lensing, which can be removed through use of so-called delensing algorithms. We forecast prospects for delensing using lensing maps which are obtained externally to CMB polarization: either from large-scale structure observations, or from high-resolution maps of CMB temperature. We conclude that the forecasts in either case are not encouraging, and that significantly delensing large-scale CMB polarization requires high-resolution polarization maps with sufficient sensitivity to measure the lensing B-mode. We also present a simple formalism for including delensing in CMB forecasts which is computationally fast and agrees well with Monte Carlos.

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arXiv2010

CMB lensing and primordial non-Gaussianity

We study the effects of gravitational lensing on the estimation of non-Gaussianity from the bispectrum of the cosmic microwave background (CMB) temperature anisotropies. We find that the effect of lensing on the bispectrum may qualitatively be described as a smoothing of the acoustic features analogous to the temperature power spectrum. In contrast to previous results, for a Planck-like experiment which is cosmic-variance limited to L=2000, we find that lensing causes no significant degradation of our ability to constrain the non-Gaussianity amplitude fNL for both local and equilateral configurations, provided that the biases due to the cross correlation between the lensing potential and the integrated-Sachs-Wolfe (ISW) contribution to the CMB temperature are adequately understood. With numerical simulations, we also verify that low-order Taylor approximations to the lensed bispectrum and ISW-lensing biases are accurate.

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