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Scott Dodelson

Publications and source records attributed to Scott Dodelson.

At least 73 records · Page 4Linked to original sources
arXiv2010

Constraining Dark Matter in Galactic Substructure

Detecting the dark matter annihilation signal from Galactic substructure, or subhalos, is an important challenge for high-energy gamma-ray experiments. In this paper we discuss detection prospects by combining two different aspects of the gamma-ray signal: the angular distribution and the photon counts probability distribution function (PDF). The true PDF from subhalos has been shown recently (by Lee et al.) to deviate from Poisson; we extend this analysis and derive the signal PDF from a detailed Lambda-CDM-based model for the properties of subhalos. We combine our PDF with a model for Galactic and extra-Galactic diffuse gamma-ray emission to obtain an estimator and projected error on dark matter particle properties (mass and annihilation cross section) using the Fermi Gamma-Ray Space Telescope. We compare the estimator obtained from the true PDF to that obtained from the simpler Poisson analysis. We find that, although both estimators are unbaised in the presence of backgrounds, the error on dark matter properties derived from the true PDF is ~50% smaller than when utilizing the Poisson-based analysis.

astro-ph.GA↗
arXiv2010

Re-capturing cosmic information

Gravitational lensing of distant galaxies can be exploited to infer the convergence field as a function of angular position on the sky. The statistics of this field, much like that of the cosmic microwave background (CMB), can be studied to extract information about fundamental parameters in cosmology, most notably the dark energy in the Universe. Unlike the CMB, the distribution of matter in the Universe which determines the convergence field is highly non-Gaussian, reflecting the nonlinear processes which accompanied structure formation. Much of the cosmic information contained in the initial field is therefore unavailable to the standard power spectrum measurements. Here we propose a method for re-capturing cosmic information by using the power spectrum of a simple function of the observed (nonlinear) convergence field. We adapt the approach of Neyrinck et al. (2009) to lensing by using a modified logarithmic transform of the convergence field. The Fourier transform of the log-transformed field has modes that are nearly uncorrelated, which allows for additional cosmological information to be extracted from small-scale modes.

astro-ph.CO↗
arXiv2010

Will Multiple Probes of Dark Energy find Modified Gravity?

One of the most pressing issues in cosmology is whether general relativity (GR) plus a dark sector is the underlying physical theory or whether a modified gravity model is needed. Upcoming dark energy experiments designed to probe dark energy with multiple methods can address this question by comparing the results of the different methods in constraining dark energy parameters. Disagreement would signal the breakdown of the assumed model (GR plus dark energy). We study the power of this consistency test by projecting constraints in the $w_0-w_a$ plane from the four different techniques of the Dark Energy Survey in the event that the underlying true model is modified gravity. We find that the standard technique of looking for overlap has some shortcomings, and we propose an alternative, more powerful Multi-dimensional Consistency Test. We introduce the methodology for projecting whether a given experiment will be able to use this test to distinguish a modified gravity model from GR.

astro-ph.CO↗
arXiv2010

A ground-based 21cm Baryon acoustic oscillation survey

Baryon acoustic oscillations (BAO) provide a robust standard ruler with which to measure the acceleration of the Universe. The BAO feature has so far been detected in optical galaxy surveys. Intensity mapping of neutral hydrogen emission with a ground-based radio telescope provides another promising window for measuring BAO at redshifts of order unity for relatively low cost. While the cylindrical radio telescope (CRT) proposed for these measurements will have excellent redshift resolution, it will suffer from poor angular resolution (a few arcminutes at best). We investigate the effect of angular resolution on the standard ruler test with BAO, using the Dark Energy Task Force Figure of Merit as a benchmark. We then extend the analysis to include variations in the parameters characterizing the telescope and the underlying physics. Finally, we optimize the survey parameters (holding total cost fixed) and present an example of a CRT BAO survey that is competitive with Stage III dark energy experiments. The tools developed here form the backbone of a publicly available code that can be used to obtain estimates of cost and Figure of Merit for any set of parameters.

astro-ph.CO↗
arXiv2010

Primordial Gravitational Waves and Cosmology

The observation of primordial gravitational waves could provide a new and unique window on the earliest moments in the history of the universe, and on possible new physics at energies many orders of magnitude beyond those accessible at particle accelerators. Such waves might be detectable soon in current or planned satellite experiments that will probe for characteristic imprints in the polarization of the cosmic microwave background (CMB), or later with direct space-based interferometers. A positive detection could provide definitive evidence for Inflation in the early universe, and would constrain new physics from the Grand Unification scale to the Planck scale.

astro-ph.CO↗
arXiv2010

Cross-Correlating Probes of Primordial Gravitational Waves

One of the most promising ways of detecting primordial gravitational waves generated during inflation is to observe B-modes of polarization, generated by Thomson scattering after reionization, in the cosmic microwave background (CMB). Large scale foregrounds though are expected to be a major systematic issue, so -- in the event of a tentative detection -- an independent confirmation of large scale gravitational waves would be most welcome. Previous authors have suggested searching for the analogous mode of cosmic shear in weak lensing surveys but have shown that the signal to noise of this mode is marginal at best. This argument is reconsidered here, accounting for the cross-correlations of the polarization and lensing B-modes. A lensing survey can potentially strengthen the argument for a detection of primordial gravitational waves, although it is unlikely to help constrain the amplitude of the signal.

astro-ph.CO↗
arXiv2009

Identifying Dark Matter Annihilation Products In The Diffuse Gamma Ray Background

Annihilation of cosmologically distributed dark matter is predicted to produce a potentially observable flux of high energy photons. Neglecting the contribution from local structure, this signal is predicted to be virtually uniform on the sky and, in order to be identified, must compete with various extragalactic backgrounds. We focus here on unresolved blazars and discuss several techniques for separating the dark matter signal from this background. First, the spectral shapes of the signal and background are expected to differ, a feature which can be exploited with the Fisher Matrix formalism. Second, in any given angular pixel, the number of photons from blazars is drawn from a distribution which is far from Poisson. A knowledge of this distribution enhances one's ability to extract the dark matter signal, while ignorance of it can lead to the introduction of a large systematic error.

astro-ph.CO↗
arXiv2009

Cosmic Neutrino Last Scattering Surface

Neutrinos decoupled from the rest of the cosmic plasma when the Universe was less than one second old, far earlier than the photons which decoupled at t=380,000 years. Surprisingly, though, the last scattering surface of the neutrinos is much closer to us than that of the photons. Here we calculate the properties of the last scattering surfaces of the three species of neutrinos.

astro-ph.CO↗
arXiv2009

Lensing Bias in Cosmic Shear

Only galaxies bright enough and large enough to be unambiguously identified and measured are included in galaxy surveys used to estimate cosmic shear. We demonstrate that because gravitational lensing can scatter galaxies across the brightness and size thresholds, cosmic shear experiments suffer from lensing bias. We calculate the effect on the shear power spectrum and show that - unless corrected for - it will lead analysts to cosmological parameters estimates that are biased at the 2-3σlevel in DETF Stage III experiments, such as the Dark Energy Survey.

astro-ph.CO↗
arXiv2009

Size Bias in Galaxy Surveys

Only certain galaxies are included in surveys: those bright and large enough to be detectable as extended sources. Because gravitational lensing can make galaxies appear both brighter and larger, the presence of foreground inhomogeneities can scatter galaxies across not only magnitude cuts but also size cuts, changing the statistical properties of the resulting catalog. Here we explore this size bias, and how it combines with magnification bias to affect galaxy statistics. We demonstrate that photometric galaxy samples from current and upcoming surveys can be even more affected by size bias than by magnification bias.

astro-ph.CO↗
arXiv2009

CMBPol Mission Concept Study: Probing Inflation with CMB Polarization

We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to differentiate various inflationary mechanisms. In particular, a detection of primordial B-mode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a super-Planckian distance in field space. We explain how such a detection or constraint would illuminate aspects of physics at the Planck scale. Moreover, CMB measurements can constrain the scale-dependence and non-Gaussianity of the primordial fluctuations and limit the possibility of a significant isocurvature contribution. Each such limit provides crucial information on the underlying inflationary dynamics. Finally, we quantify these considerations by presenting forecasts for the sensitivities of a future satellite experiment to the inflationary parameters.

astro-ph↗
arXiv2009

Observing the Evolution of the Universe

How did the universe evolve? The fine angular scale (l>1000) temperature and polarization anisotropies in the CMB are a Rosetta stone for understanding the evolution of the universe. Through detailed measurements one may address everything from the physics of the birth of the universe to the history of star formation and the process by which galaxies formed. One may in addition track the evolution of the dark energy and discover the net neutrino mass. We are at the dawn of a new era in which hundreds of square degrees of sky can be mapped with arcminute resolution and sensitivities measured in microKelvin. Acquiring these data requires the use of special purpose telescopes such as the Atacama Cosmology Telescope (ACT), located in Chile, and the South Pole Telescope (SPT). These new telescopes are outfitted with a new generation of custom mm-wave kilo-pixel arrays. Additional instruments are in the planning stages.

astro-ph.CO↗
arXiv2009

21 cm Intensity Mapping

Using the 21 cm line, observed all-sky and across the redshift range from 0 to 5, the large scale structure of the Universe can be mapped in three dimensions. This can be accomplished by studying specific intensity with resolution ~ 10 Mpc, rather than via the usual galaxy redshift survey. The data set can be analyzed to determine Baryon Acoustic Oscillation wavelengths, in order to address the question: 'What is the nature of Dark Energy?' In addition, the study of Large Scale Structure across this range addresses the questions: 'How does Gravity effect very large objects?' and 'What is the composition our Universe?' The same data set can be used to search for and catalog time variable and transient radio sources.

astro-ph.IM↗
arXiv2008

CMBPol Mission Concept Study: A Mission to Map our Origins

Quantum mechanical metric fluctuations during an early inflationary phase of the universe leave a characteristic imprint in the polarization of the cosmic microwave background (CMB). The amplitude of this signal depends on the energy scale at which inflation occurred. Detailed observations by a dedicated satellite mission (CMBPol) therefore provide information about energy scales as high as $10^{15}$ GeV, twelve orders of magnitude greater than the highest energies accessible to particle accelerators, and probe the earliest moments in the history of the universe. This summary provides an overview of a set of studies exploring the scientific payoff of CMBPol in diverse areas of modern cosmology, such as the physics of inflation, gravitational lensing and cosmic reionization, as well as foreground science and removal .

astro-ph↗
arXiv2008

Weighing the spatial and temporal fluctuations of the dark universe

A generic prediction of the standard cosmology, based on general relativity (GR), dark matter and the cosmological constant (and more generally, smooth dark energy), is that, the two gravitational potentials describing the spatial and temporal scalar perturbations of the universe are equivalent. Modifications in GR or dark energy clustering in general violate this relation. Thus this ratio serves as a smoking gun of the dark universe. We propose a method to extract this ratio at various cosmological scales and redshifts from a set of measurements, in a model independent way. The ratio measured by future surveys has strong discriminating power for a variety of dark universe scenarios.

astro-ph↗
arXiv2008

Weak Lensing Effects on the Galaxy Three-Point Correlation Function

We study the corrections to the galaxy three-point correlation function (3PCF) induced by weak lensing magnification due to the matter distribution along the line of sight. We consistently derive all the correction terms arising up to second order in perturbation theory and provide analytic expressions as well as order of magnitude estimates for their relative importance. The magnification contributions depend on the geometry of the projected triangle on the sky plane, and scale with different powers of the number count slope and redshift of the galaxy sample considered. We evaluate all terms numerically and show that, depending on the triangle configuration as well as the galaxy sample considered, weak lensing can in general significantly contribute to and alter the three-point correlation function observed through galaxy and quasar catalogs.

astro-ph↗
arXiv2008

Universal Weak Lensing Distortion of Cosmological Correlation Functions

Gravitational lensing affects observed cosmological correlation functions because observed images do not coincide with true source locations. We treat this universal effect in a general way here, deriving a single formula that can be used to determine to what extent this effect distorts any correlation function. We then apply the general formula to the correlation functions of galaxies, of the 21-cm radiation field, and of the CMB.

astro-ph↗