arXiv subjects

Sarah Shandera

Publications and source records attributed to Sarah Shandera.

At least 37 records · Page 2Linked to original sources

arXiv2019

Probing the origin of our Universe through cosmic microwave background constraints on gravitational waves

The next generation of instruments designed to measure the polarization of the cosmic microwave background (CMB) will provide a historic opportunity to open the gravitational wave window to the primordial Universe. Through high sensitivity searches for primordial gravitational waves, and tighter limits on the energy released in processes like phase transitions, the CMB polarization data of the next decade has the potential to transform our understanding of the laws of physics underlying the formation of the Universe.

astro-ph.CO↗

arXiv2019

Cosmological Probes of Dark Matter Interactions: The Next Decade

Cosmological observations offer unique and robust avenues for probing the fundamental nature of dark matter particles-they broadly test a range of compelling theoretical scenarios, often surpassing or complementing the reach of terrestrial and other experiments. We discuss observational and theoretical advancements that will play a pivotal role in realizing a strong program of cosmological searches for the identity of dark matter in the coming decade. Specifically, we focus on measurements of the cosmic-microwave-background anisotropy and spectral distortions, and tracers of structure (such as the Lyman-$α$ forest, galaxies, and the cosmological 21-cm signal).

astro-ph.CO↗

arXiv2018

A cosmological open quantum system

We derive the evolution equation for the density matrix of a UV- and IR- limited band of comoving momentum modes of the canonically normalized scalar degree of freedom in two examples of nearly de Sitter universes. Including the effects of a cubic interaction term from the gravitational action and tracing out a set of longer wavelength modes, we find that the evolution of the system is non-Hamiltonian and non-Markovian. We find linear dissipation terms for a few modes with wavelength near the boundary between system and bath and nonlinear dissipation terms for all modes. The non-Hamiltonian terms persist to late times when the scalar field dynamics is such that the curvature perturbation continues to evolve on super-Hubble scales.

hep-th↗

arXiv2018

Gravitational Waves from Binary Mergers of Sub-solar Mass Dark Black Holes

We explore the possible spectrum of binary mergers of sub-solar mass black holes formed out of dark matter particles interacting via a dark electromagnetism. We estimate the properties of these dark black holes by assuming that their formation process is parallel to Population-III star formation; except that dark molecular cooling can yield smaller opacity limit. We estimate the binary coalescence rates for the Advanced LIGO and Einstein telescope, and find that scenarios compatible with all current constraints could produce dark black holes at rates high enough for detection by Advanced LIGO.

astro-ph.CO↗

arXiv2018

The ANITA Anomalous Events as Signatures of a Beyond Standard Model Particle, and Supporting Observations from IceCube

The ANITA collaboration have reported observation of two anomalous events that appear to be $\varepsilon_{\rm cr} \approx 0.6$ EeV cosmic ray showers emerging from the Earth with exit angles of $27^\circ$ and $35^\circ$, respectively. While EeV-scale upgoing showers have been anticipated as a result of astrophysical tau neutrinos converting to tau leptons during Earth passage, the observed exit angles are much steeper than expected in Standard Model (SM) scenarios. Indeed, under conservative extrapolations of the SM interactions, there is no particle that can propagate through the Earth with probability $p > 10^{-6}$ at these energies and exit angles. We explore here whether "beyond the Standard Model" (BSM) particles are required to explain the ANITA events, if correctly interpreted, and conclude that they are. Seeking confirmation or refutation of the physical phenomenon of sub-EeV Earth-emergent cosmic rays in data from other facilities, we find support for the reality of the ANITA events, and three candidate analog events, among the Extremely High Energy Northern Track neutrinos of the IceCube Neutrino Observatory. Properties of the implied BSM particle are anticipated, at least in part, by those predicted for the "stau" slepton ($\tildeτ_R$) in some supersymmetric models of the fundamental interactions, wherein the stau manifests as the next-to-lowest mass supersymmetric partner particle.

astro-ph.HE↗

arXiv2018

Methods for the detection of gravitational waves from sub-solar mass ultracompact binaries

We describe detection methods for extensions of gravitational wave searches to sub-solar mass compact binaries. Sub-solar mass searches were previously carried out using Initial LIGO, and Advanced LIGO boasts a detection volume approximately 1000 times bigger than Initial LIGO at design sensitivity. Low masses present computational difficulties, and we suggest a way to rein in the increase while retaining a sensitivity much greater than previous searches. Sub-solar mass compact objects are of particular interest because they are not expected to form astrophysically. If detected they could be evidence of primordial black holes (PBH). We consider a particular model of PBH binary formation that would allow LIGO/Virgo to place constraints on this population within the context of dark matter, and we demonstrate how to obtain conservative bounds for the upper limit on the dark matter fraction.

astro-ph.IM↗

arXiv2018

Statistical anisotropies in temperature and polarization fluctuations from a scale-dependent trispectrum

We study statistical anisotropies generated in the observed two-point function of the cosmic microwave background (CMB) fluctuations if the primordial statistics are non-Gaussian. Focusing on the dipole modulations of the anisotropies, we find that the hemispherical power asymmetry observed in the CMB temperature fluctuations can be modeled by a local-type trispectrum with amplitude $τ_{\rm NL}(k_p=0.05~{\rm Mpc}^{-1}) \approx 2 \times 10^4$ and a large red tilt $n\approx -0.68$. We numerically evaluate the non-Gaussian covariance of the modulation estimators for both temperature and E-mode polarization fluctuations and discuss the prospects of constraining the model using Planck satellite data. We then discuss other effects of the scale-dependent trispectrum that could be used to distinguish this scenario from other explanations of the power asymmetry: higher-order modulations of the two-point function and the non-Gaussian angular power spectrum covariance. As an important consequence of the non-Gaussian power spectrum covariance, we discuss how the CMB-inferred spectral index of primordial scalar fluctuations can be significantly biased in the presence of a scale-dependent local-type trispectrum.

astro-ph.CO↗

arXiv2017

Power-counting during single-field slow-roll inflation

We elucidate the counting of the relevant small parameters in inflationary perturbation theory. Doing this allows for an explicit delineation of the domain of validity of the semi-classical approximation to gravity used in the calculation of inflationary correlation functions. We derive an expression for the dependence of correlation functions of inflationary perturbations on the slow-roll parameter $ε= -\dot{H}/H^2$, as well as on $H/M_p$, where $H$ is the Hubble parameter during inflation. Our analysis is valid for single-field models in which the inflaton can traverse a Planck-sized range in field values and where all slow-roll parameters have approximately the same magnitude. As an application, we use our expression to seek the boundaries of the domain of validity of inflationary perturbation theory for regimes where this is potentially problematic: models with small speed of sound and models allowing eternal inflation.

hep-th↗

arXiv2016

Constraining primordial and gravitational mode coupling with the position-dependent bispectrum of the large-scale structure

We develop and study the position-dependent bispectrum. It is a generalization of the recently proposed position-dependent power spectrum method of measuring the squeezed-limit bispectrum. The position-dependent bispectrum can similarly be used to measure the squeezed-limit trispectrum in which one of the wavelengths is much longer than the other three. In this work, we will mainly consider the case in which the three smaller wavelengths are nearly the same (the equilateral configuration). We use the Fisher information matrix to forecast constraints on bias parameters and the amplitude of primordial trispectra from the position-dependent bispectrum method. We find that the method can constrain the local-type $g_{\rm NL}$ at a level of $σ(g_{\rm NL}^{\rm local}) \approx 3 \times 10^5$ for a large volume SPHEREx-like survey; improvements can be expected by including all the triangular configurations of the bispectra rather than just the equilateral configuration. However, the same measurement would also constrain a much larger family of trispectra than local $g_{\rm NL}$ model. We discuss the implications of the forecasted reach of future surveys in terms of super cosmic variance uncertainties from primordial non-Gaussianities.

astro-ph.CO↗

arXiv2016

CMB-S4 Science Book, First Edition

This book lays out the scientific goals to be addressed by the next-generation ground-based cosmic microwave background experiment, CMB-S4, envisioned to consist of dedicated telescopes at the South Pole, the high Chilean Atacama plateau and possibly a northern hemisphere site, all equipped with new superconducting cameras. CMB-S4 will dramatically advance cosmological studies by crossing critical thresholds in the search for the B-mode polarization signature of primordial gravitational waves, in the determination of the number and masses of the neutrinos, in the search for evidence of new light relics, in constraining the nature of dark energy, and in testing general relativity on large scales.

astro-ph.CO↗

arXiv2016

On the space of non-Gaussian fields with single-clock bispectra

We develop a mathematical construction of non-Gaussian fields whose bispectra satisfy the single-clock inflation consistency relation. At the same order that our basis for bispectra recovers the two simplest single clock templates, we also find a third orthogonal template which has the single clock squeezed limit, peaks in folded configurations, and has very small coupling in the equilateral limit. We explore the map between templates and operators in a very general Lagrangian for single-clock fluctuations and find no significant overlap between the new template and models in the literature. We comment on the physical implications of this conclusion. Our findings add support for the idea that both theory and data driven considerations will be best served if next generation non-Gaussianity constraints are made in a basis that uses the degree of coupling between long and short wavelength modes as an organizing principle.

astro-ph.CO↗

arXiv2016

Cosmic variance in inflation with two light scalars

We examine the squeezed limit of the bispectrum when a light scalar with arbitrary non-derivative self-interactions is coupled to the inflaton. We find that when the hidden sector scalar is sufficiently light ($m\lesssim0.1\,H$), the coupling between long and short wavelength modes from the series of higher order correlation functions (from arbitrary order contact diagrams) causes the statistics of the fluctuations to vary in sub-volumes. This means that observations of primordial non-Gaussianity cannot be used to uniquely reconstruct the potential of the hidden field. However, the local bispectrum induced by mode-coupling from these diagrams always has the same squeezed limit, so the field's locally determined mass is not affected by this cosmic variance.

astro-ph.CO↗

arXiv2016

The extended Baryon Oscillation Spectroscopic Survey (eBOSS): a cosmological forecast

We present a science forecast for the eBOSS survey, part of the SDSS-IV project, which is a spectroscopic survey using multiple tracers of large-scale structure, including luminous red galaxies (LRGs), emission line galaxies (ELGs) and quasars (both as a direct probe of structure and through the Ly-$α$ forest). Focusing on discrete tracers, we forecast the expected accuracy of the baryonic acoustic oscillation (BAO), the redshift-space distortion (RSD) measurements, the $f_{\rm NL}$ parameter quantifying the primordial non-Gaussianity, the dark energy and modified gravity parameters. We also use the line-of-sight clustering in the Ly-$α$ forest to constrain the total neutrino mass. We find that eBOSS LRGs ($0.6 0.6$), ELGs ($0.6<z<1.2$) and Clustering Quasars (CQs) ($0.6<z<2.2$) can achieve a precision of 1%, 2.2% and 1.6% precisions, respectively, for spherically averaged BAO distance measurements. Using the same samples, the constraint on $fσ_8$ is expected to be 2.5%, 3.3% and 2.8% respectively. For primordial non-Gaussianity, eBOSS alone can reach an accuracy of $σ(f_{\rm NL})\sim10-15$, depending on the external measurement of the galaxy bias and our ability to model large-scale systematic errors. eBOSS can at most improve the dark energy Figure of Merit (FoM) by a factor of $3$ for the Chevallier-Polarski-Linder (CPL) parametrisation, and can well constrain three eigenmodes for the general equation-of-state parameter (Abridged).

astro-ph.CO↗

arXiv2016

Large-scale anomalies in the cosmic microwave background as signatures of non-Gaussianity

We derive a general expression for the probability of observing deviations from statistical isotropy in the cosmic microwave background (CMB) if the primordial fluctuations are non-Gaussian and extend to superhorizon scales. The primary motivation is to properly characterize the monopole and dipole modulations of the primordial power spectrum that are generated by the coupling between superhorizon and subhorizon perturbations. Unlike previous proposals for generating the hemispherical power asymmetry, we do not assume that the power asymmetry results from a single large superhorizon mode. Instead, we extrapolate the observed power spectrum to superhorizon scales and compute the power asymmetry that would result from a specific realization of non-Gaussian perturbations on scales larger than the observable universe. Our study encompasses many of the scenarios that have been put forward as possible explanations for the CMB hemispherical power asymmetry. We confirm our analytic predictions for the probability of a given power asymmetry by comparing them to numerical realizations of CMB maps. We find that non-local models of non-Gaussianity and scale-dependent local non-Gaussianity produce scale-dependent modulations of the power spectrum, thereby potentially producing both a monopolar and a dipolar power modulation on large scales. We then provide simple examples of finding the posterior distributions for the parameters of the bispectrum from the observed monopole and dipole modulations.

astro-ph.CO↗

arXiv2015

Non-local bispectra from super cosmic variance

We present examples of non-Gaussian statistics that can induce bispectra matching local and non-local (including equilateral) templates in biased sub-volumes. We find cases where the biasing from coupling to long wavelength modes affects only the power spectrum, only the bispectrum or both. Our results suggest that ruling out multi-field scenarios is quite difficult: some measurements of the scalar correlations, including the shape of the bispectrum, can be consistent with single-clock predictions even when cosmic variance from super-horizon modes is at work. Furthermore, if observations of the density perturbations rule out single-clock inflation, we will face a serious cosmic variance obstacle in drawing any further conclusions about the particle physics origin of the scalar fluctuations.

astro-ph.CO↗

arXiv2014

Testing Inflation with Large Scale Structure: Connecting Hopes with Reality

The statistics of primordial curvature fluctuations are our window into the period of inflation, where these fluctuations were generated. To date, the cosmic microwave background has been the dominant source of information about these perturbations. Large scale structure is however from where drastic improvements should originate. In this paper, we explain the theoretical motivations for pursuing such measurements and the challenges that lie ahead. In particular, we discuss and identify theoretical targets regarding the measurement of primordial non-Gaussianity. We argue that when quantified in terms of the local (equilateral) template amplitude $f_{\rm NL}^{\rm loc}$ ($f_{\rm NL}^{\rm eq}$), natural target levels of sensitivity are $Δf_{\rm NL}^{\rm loc, eq.} \simeq 1$. We highlight that such levels are within reach of future surveys by measuring 2-, 3- and 4-point statistics of the galaxy spatial distribution. This paper summarizes a workshop held at CITA (University of Toronto) on October 23-24, 2014.

astro-ph.CO↗

arXiv2014

Weighing the Giants IV: Cosmology and Neutrino Mass

We employ robust weak gravitational lensing measurements to improve cosmological constraints from measurements of the galaxy cluster mass function and its evolution, using X-ray selected clusters detected in the ROSAT All-Sky Survey. Our lensing analysis constrains the absolute mass scale of such clusters at the 8 per cent level, including both statistical and systematic uncertainties. Combining it with the survey data and X-ray follow-up observations, we find a tight constraint on a combination of the mean matter density and late-time normalization of the matter power spectrum, $σ_8(Ω_m/0.3)^{0.17}=0.81\pm0.03$, with marginalized, one-dimensional constraints of $Ω_m=0.26\pm0.03$ and $σ_8=0.83\pm0.04$. For these two parameters, this represents a factor of two improvement in precision with respect to previous work, primarily due to the reduced systematic uncertainty in the absolute mass calibration provided by the lensing analysis. Our new results are in good agreement with constraints from cosmic microwave background (CMB) data, both WMAP and Planck (plus WMAP polarization), under the assumption of a flat $Λ$CDM cosmology with minimal neutrino mass. Consequently, we find no evidence for non-minimal neutrino mass from the combination of cluster data with CMB, supernova and baryon acoustic oscillation measurements, regardless of which all-sky CMB data set is used (and independent of the recent claimed detection of B-modes on degree scales). We also present improved constraints on models of dark energy (both constant and evolving), modifications of gravity, and primordial non-Gaussianity. Assuming flatness, the constraints for a constant dark energy equation of state from the cluster data alone are at the 15 per cent level, improving to $\sim 6$ per cent when the cluster data are combined with other leading probes.

astro-ph.CO↗

arXiv2014

Higher moments of primordial non-Gaussianity and N-body simulations

We perform cosmological N-body simulations with non-Gaussian initial conditions generated from two independent fields. The dominant contribution to the perturbations comes from a purely Gaussian field, but we allow the second field to have local non-Gaussianity that need not be weak. This scenario allows us to adjust the relative importance of non-Gaussian contributions beyond the skewness, producing a scaling of the higher moments different from (and stronger than) the scaling in the usual single field local ansatz. We compare semi-analytic prescriptions for the non-Gaussian mass function, large scale halo bias, and stochastic bias against the simulation results. We discuss applications of this work to large scale structure measurements that can test a wider range of models for the primordial fluctuations than is usually explored.

astro-ph.CO↗