Searcharxiv⌕ Search

arXiv subjects

Scott Dodelson

Publications and source records attributed to Scott Dodelson.

At least 127 records · Page 7Linked to original sources
arXiv1999

Improved Treatment of Cosmic Microwave Background Fluctuations Induced by a Late-decaying Massive Neutrino

A massive neutrino which decays after recombination (t > 10^{13} sec) into relativistic decay products produces an enhanced integrated Sachs-Wolfe effect, allowing constraints to be placed on such neutrinos from present cosmic microwave background anisotropy data. Previous treatments of this problem have approximated the decay products as an additional component of the neutrino background. This approach violates energy-momentum conservation, and we show that it leads to serious errors for some neutrino masses and lifetimes. We redo this calculation more accurately, by correctly incorporating the spatial distribution of the decay products. For low neutrino masses and long lifetimes, we obtain a much smaller distortion in the CMB fluctuation spectrum than have previous treatments. We combine these new results with a recent set of CMB data to exclude the mass and lifetime range m_h > 100 eV, τ> 10^{12} sec. Masses as low as 30 eV are excluded for a narrower range in lifetime.

astro-ph↗
arXiv1998

The Impact of Inhomogeneous Reionization on Cosmic Microwave Background Anisotropy

Hydrogen atoms in the Universe that initially formed at redshift z = 1100 were reionized at 5 < z < 60. It is likely that this transition proceeded through a mixed phase of partial ionization. We develop an analytic approach to calculating the effect on the cosmic microwave background (CMB) of Thomson scattering off this inhomogeneous distribution of free electrons. We study two models of the reionization process, one of which associates ionized patches with overdense regions of the smoothed density field. We find the generated anisotropy to have a power spectrum which peaks at angular scales corresponding to the extent of the ionized regions, and has a width that reflects the correlations between them. There is considerable uncertainty in the amplitude, which depends on how efficiently collapsed objects can reionize the surrounding medium. We calculate the effect of neglecting inhomogeneous reionization in the determination of cosmological parameters from high resolution CMB maps and find that it may be a significant source of systematic error.

astro-ph↗
arXiv1998

Probing Unstable Massive Neutrinos with Current Cosmic Microwave Background Observations

The pattern of anisotropies in the Cosmic Microwave Background depends upon the masses and lifetimes of the three neutrino species. A neutrino species of mass greater than 10 eV with lifetime between 10^{13} sec and 10^{17} sec leaves a very distinct signature (due to the integrated Sachs-Wolfe effect): the anisotropies at large angles are predicted to be comparable to those on degree scales. Present data exclude such a possibility and hence this region of parameter space. For $m_ν\simeq 30$ eV, $τ\simeq 10^{13}$ sec, we find an interesting possibility: the Integrated Sachs Wolfe peak produced by the decaying neutrino in low-$Ω$ models mimics the acoustic peak expected in an $Ω= 1$ model.

astro-ph↗
arXiv1998

Precision Detection of the Cosmic Neutrino Background

In the standard Big Bang cosmology the canonical value for the ratio of relic neutrinos to CMB photons is 9/11. Within the framework of the Standard Model of particle physics there are small corrections, in sum about 1%, due to slight heating of neutrinos by electron/positron annihilations and finite-temperature QED effects. We show that this leads to changes in the predicted cosmic microwave background (CMB) anisotropies that might be detected by future satellite experiments. NASA's MAP and ESA's PLANCK should be able to test the canonical prediction to a precision of 1% or better and could confirm these corrections.

astro-ph↗
arXiv1997

Likelihood Analysis of Galaxy Surveys

One of the major goals of cosmological observations is to test theories of structure formation. The most straightforward way to carry out such tests is to compute the likelihood function L, the probability of getting the data given the theory. We write down this function for a general galaxy survey. The full likelihood function is very complex, depending on all of the $n$-point functions of the theory under consideration. Even in the simplest case, where only the two point function is non-vanishing (Gaussian perturbations), L cannot be calculated exactly, primarily because of the Poisson nature of the galaxy distribution. Here we expand L about the (trivial) zero correlation limit. As a first application, we take the binned values of the two point function as free parameters and show that L peaks at $(DD - DR + RR)/DD$. Using Monte Carlo techniques, we compare this estimator with the traditional $DD/DR$ and Landy & Szalay estimators. More generally, the success of this expansion should pave the way for further applications of the likelihood function.

astro-ph↗
arXiv1997

Cosmic microwave background measurements can discriminate among inflation models

Quantum fluctuations during inflation may be responsible for temperature anisotropies in the cosmic microwave background (CMB). Observations of CMB anisotropies can be used to falsify many currently popular models. In this paper we discuss the prospectus for observations of CMB anisotropies at the accuracy of planned satellite missions to reject currently popular inflation models and to provide some direction for model building.

astro-ph↗
arXiv1997

Anisotropies in the Cosmic Microwave Background: Theory

Anisotropies in the Cosmic Microwave Background (CMB) contain a wealth of information about the past history of the universe and the present values of cosmological parameters. I ouline some of the theoretical advances of the last few years. In particular, I emphasize that for a wide class of cosmological models, theorists can accurately calculate the spectrum to better than a percent. The specturm of anisotropies today is directly related to the pattern of inhomogeneities present at the time of recombination. This recognition leads to a powerful argument that will enable us to distinguish inflationary models from other models of structure formation. If the inflationary models turn out to be correct, the free parameters in these models will be determined to unprecedented accuracy by the upcoming satellite missions.

astro-ph↗
arXiv1996

Comment on a Paper by Fang, Huang, and Wu

Before decoupling in the early universe, the tightly coupled photon/electron gas underwent acoustic oscillations. These oscillations should be visible today in the spectrum of anisotropies. Recently Fang, Huang, and Wu (1996) claimed that when random processes are accounted for, the phases of these oscillations are no longer coherent. In fact, they claim that the well-defined peaks will be completely smoothed out. We show here that their claim is incorrect. The standard Boltzmann treatment determining the anisotropies is sufficient; random processes do not change the standard result.

astro-ph↗
arXiv1996

Cold Dark Matter Models

Motivated by inflation, the theory of big-bang nucleosynthesis and the quest for a deeper understanding of the fundamental forces and particles, a very successful paradigm for the development of structure in the Universe has evolved. It holds that most of the matter exists in the form of slowly moving elementary particles left over from the earliest moments (cold dark matter or CDM) and that the small density inhomogeneities that seed structure formation arose from quantum fluctuations around $10^{-34}\sec$ after the bang. A flood of observations is now testing the cold dark matter paradigm -- from determinations of the Hubble constant to measurements of the anisotropy of the Cosmic Background Radiation (CBR) -- and could reveal the details of the theory as well.

astro-ph↗
arXiv1995

Determining cosmic microwave background anisotropies in the presence of foregrounds

Separating foregrounds from the signal is one of the big challenges in cosmic microwave background (CMB) experiments. A simple way to estimate the CMB temperature in a given pixel is to fit for the amplitudes of the CMB and the various foreground components. The variance squared of this estimator is shown to be equal to $[(FDF)^2\ \sigzt + \sigsh^2]$, where $\sigz$ is the variance in the absence of foregrounds; $\sigsh$ is the variance due to the uncertainty in the shapes of the foreground components; and FDF is the {\it foreground degradation factor}. This one number, the FDF, gives a good indication of the ability of a given experiment to disentangle the CMB from foreground sources. A variety of applications relating to the planning and analyzing of experiments is presented.

astro-ph↗
arXiv1995

Cold + Hot Dark Matter and the Cosmic Microwave Background

We examine the cosmic microwave background power spectrum for adiabatic models with a massive neutrino component. We present the results of a detailed numerical evolution of cold + hot dark matter (CHDM) models and compare with the standard cold dark matter (CDM) spectrum. The difference is of order $5-10%$ for $400 < l < 1000 $ for currently popular CHDM models. With semi-analytic approximations, we also discuss the relevant physics involved. Finally we remark on the ability of future experiments to differentiate between these models. An all-sky experiment with a beam size smaller than 30 arcminutes can distinguish between CHDM and CDM if other cosmological parameters are known. Even allowing other parameters to vary, it may be possible to distinguish CDM from CHDM.

astro-ph↗
arXiv1995

Large Scale Structure Tests of Warm Dark Matter

Warm dark matter (WDM) is an intriguing model of structure formation from the point of view of both cosmology and particle physics. We consider a one-parameter family of WDM models. The linear power spectra for these models is calculated and compared with the corresponding spectra for cold dark matter (CDM), hot dark matter (HDM) and mixed dark matter (MDM) as well as the power spectrum derived from observations. Our linear analyses suggest that a model universe dominated by a particle whose mass to temperature ratio $m_x/T_x$ is increased by a factor of two as compared with the standard HDM neutrino gives a reasonable fit to the data on large $(>8h^{-1} {\rm ~Mpc})$ scales. $N$-body simulations for this particular WDM model show features of both HDM and CDM. As in HDM, the first objects to collapse are large pancake-like structures. The final matter distribution is rather smooth and structures as small as galaxy halos are excluded. However, there appear to be virialized rich clusters evident in the CDM but not the HDM simulations. Unfortunately, a simple comparison of the matter distribution and its statistical properties with observations indicates that WDM, like CDM, has too much power at small scales. This is particularly evident in the small-scale pairwize velocity dispersion. The cluster multiplicity function has the wrong shape with too many rich clusters being produced, though this conclusion is based on the simple assumption that light traces mass in groups of galaxies.

astro-ph↗
arXiv1994

Noise Correlation in Cosmic Microwave Background Experiments

Many analyses of microwave background experiments neglect the correlation of noise in different frequency or polarization channels. We show that these correlations, should they be present, can lead to severe misinterpretation of an experiment. In particular, correlated noise arising from either electronics or atmosphere may mimic a cosmic signal. We quantify how the likelihood function for a given experiment varies with noise correlation, using both simple analytic models and actual data. For a typical microwave background anisotropy experiment, noise correlations at the level of 1\% of the overall noise can seriously {\it reduce} the significance of a given detection.

astro-ph↗
arXiv1994

Analysis of Small and Medium-Scale Cosmic Microwave Background Experiments

Anisotropies in the temperature of the cosmic microwave background have been detected on a range of scales by several different experiments. These anisotropies reflect the primordial spectrum of metric perturbations in the early universe. In principle, the largest barrier to a clean interpretation of the experimental results is contamination by foreground sources. We address this issue by projecting out likely sources of foreground contamination {}from seven separate small-angle and medium-angle experiments. We then calculate likelihood functions for models with adiabatic perturbations, first for the amplitude of the spectrum while constraining the spectral index to be $n=1$, and then jointly for the amplitude and spectrum of the fluctuations. All of the experiments are so far consistent with the simplest inflationary models; for $n=1$ the experiments' combined best-fit quadrupole amplitude is $Q_{\rm rms-ps}= 18^{+3}_{-1} \,\,μK$, in excellent agreement with the COBE two-year data. In ($Q_{\rm rms-ps}$, n) space, the allowed region incorporating intermediate and small-scale experiments is substantially more constrained than {}from COBE alone. We briefly discuss the expected improvement in the data in the near future and corresponding constraints on cosmological models.

astro-ph↗
arXiv1994

Testing Inflation with the Cosmic Microwave Background

Cosmic microwave background (CMB) anisotropy may result from both scalar and tensor perturbations. For a sufficiently narrow range of angular scales, CMB perturbations can be characterized by four parameters. Results from the Cosmic Background Explorer fix one combination of the parameters, reducing the parameters to three. If CMB perturbations are from inflation, there is an additional relation, reducing the parameters to two. An appropriate combination of a medium-angle and a small-angle CMB observation can test the inflation hypothesis because inflation cannot explain a high signal in one experiment and a low signal in the other.

astro-ph↗
arXiv1994

Is a Massive Tau Neutrino Just What Cold Dark Matter Needs?

The cold dark matter (CDM) scenario for structure formation in the Universe is very attractive and has many successes; however, when its spectrum of density perturbations is normalized to the COBE anisotropy measurement the level of inhomogeneity predicted on small scales is too large. This can be remedied by a tau neutrino of mass $1\MeV - 10\MeV$ and lifetime $0.1\sec - 100\sec$ whose decay products include electron neutrinos because it allows the total energy density in relativistic particles to be doubled without interfering with nucleosynthesis. The anisotropies predicted on the degree scale for ``$τ$CDM'' are larger than standard CDM. Experiments at $e^\pm$ colliders may be able to probe such a mass range.

astro-ph↗