Robust Signatures of the Relic Neutrinos in CMB

When the perturbations forming the acoustic peaks of the cosmic microwave background (CMB) reentered the horizon and interacted gravitationally with all the matter, neutrinos presumably comprised 41% of the universe energy. CMB experiments have reached a capacity to probe this background of relic neutrinos. I review the neutrino impact on the CMB at the onset of the acoustic oscillations. The discussion addresses the underlying physics, robustness or degeneracy of the neutrino imprints with changes of free cosmological parameters, and non-minimal models for the unseen radiation sector with detectable signatures in CMB anisotropy and polarization.Comment: 7 pages. Revised footnote f. Based on the Proceedings for the 10th International Symposium on Particles, Strings and Cosmology (PASCOS 04), Boston, August 200

The Halo Mass Function: High-Redshift Evolution and Universality

We study the formation of dark matter halos in the concordance LCDM model over a wide range of redshifts, from z=20 to the present. Our primary focus is the halo mass function, a key probe of cosmology. By performing a large suite of nested-box N-body simulations with careful convergence and error controls (60 simulations with box sizes from 4 to 256 Mpc/h, we determine the mass function and its evolution with excellent statistical and systematic errors, reaching a few percent over most of the considered redshift and mass range. Across the studied redshifts, the halo mass is probed over 6 orders of magnitude (10^7 - 10^13.5 M_sun/h). Historically, there has been considerable variation in the high redshift mass function as obtained by different groups. We have made a concerted effort to identify and correct possible systematic errors in computing the mass function at high redshift and to explain the discrepancies between some of the previous results. We discuss convergence criteria for the required force resolution, simulation box size, halo mass range, initial and final redshift, and time stepping. Because of conservative cuts on the mass range probed by individual boxes, our results are relatively insensitive to simulation volume, the remaining sensitivity being consistent with extended Press-Schechter theory. Previously obtained mass function fits near z=0, when scaled by linear theory, are in good agreement with our results at all redshifts, although a mild redshift dependence consistent with that found by Reed and collaborators exists at low redshifts.Comment: 20 pages, 15 figures. Minor changes to the text and figures; results and conclusions unchange

Gravity of Cosmological Perturbations in the CMB

First, we establish which measures of large-scale perturbations are least afflicted by gauge artifacts and directly map the apparent evolution of inhomogeneities to local interactions of cosmological species. Considering nonlinear and linear perturbations of phase-space distribution, radiation intensity and arbitrary species' density, we require that: (i) the dynamics of perturbations defined by these measures is determined by observables within the local Hubble volume; (ii) the measures are practically applicable on microscopic scales and in an unperturbed geometry retain their microscopic meaning on all scales. We prove that all measures of linear overdensity that satisfy (i) and (ii) coincide in the superhorizon limit. Their dynamical equations are simpler than the traditional ones, have a nonsingular superhorizon limit and explicit Cauchy form. Then we show that, contrary to the popular view, the perturbations of the cosmic microwave background (CMB) in the radiation era are not resonantly boosted self-gravitationally during horizon entry. (Consequently, the CMB signatures of uncoupled species which may be abundant in the radiation era, e.g. neutrinos or early quintessence, are mild; albeit non-degenerate and robust to cosmic variance.) On the other hand, dark matter perturbations in the matter era gravitationally suppress large-angle CMB anisotropy by an order of magnitude stronger than presently believed. If cold dark matter were the only dominant component then, for adiabatic perturbations, the CMB temperature power spectrum C_l would be suppressed 25-fold.Comment: 23 pages, 5 figures. Minor corrections. The text matches the published pape

Signatures of Relativistic Neutrinos in CMB Anisotropy and Matter Clustering

We present a detailed analytical study of ultra-relativistic neutrinos in cosmological perturbation theory and of the observable signatures of inhomogeneities in the cosmic neutrino background. We note that a modification of perturbation variables that removes all the time derivatives of scalar gravitational potentials from the dynamical equations simplifies their solution notably. The used perturbations of particle number per coordinate, not proper, volume are generally constant on superhorizon scales. In real space an analytical analysis can be extended beyond fluids to neutrinos. The faster cosmological expansion due to the neutrino background changes the acoustic and damping angular scales of the cosmic microwave background (CMB). But we find that equivalent changes can be produced by varying other standard parameters, including the primordial helium abundance. The low-l integrated Sachs-Wolfe effect is also not sensitive to neutrinos. However, the gravity of neutrino perturbations suppresses the CMB acoustic peaks for the multipoles with l>~200 while it enhances the amplitude of matter fluctuations on these scales. In addition, the perturbations of relativistic neutrinos generate a *unique phase shift* of the CMB acoustic oscillations that for adiabatic initial conditions cannot be caused by any other standard physics. The origin of the shift is traced to neutrino free-streaming velocity exceeding the sound speed of the photon-baryon plasma. We find that from a high resolution, low noise instrument such as CMBPOL the effective number of light neutrino species can be determined with an accuracy of sigma(N_nu) = 0.05 to 0.09, depending on the constraints on the helium abundance.Comment: 38 pages, 7 figures. Version accepted for publication in PR

Position-Space Description of the Cosmic Microwave Background and Its Temperature Correlation Function

We suggest that the cosmic microwave background (CMB) temperature correlation function C(theta) as a function of angle provides a direct connection between experimental data and the fundamental cosmological quantities. The evolution of inhomogeneities in the prerecombination universe is studied using their Green's functions in position space. We find that a primordial adiabatic point perturbation propagates as a sharp-edged spherical acoustic wave. Density singularities at its wavefronts create a feature in the CMB correlation function distinguished by a dip at theta ~ 1.2 deg. Characteristics of the feature are sensitive to the values of cosmological parameters, in particular to the total and the baryon densities.Comment: The version accepted for publication in Phys. Rev. Letters. 4 pages, 3 figure

Inhomogeneities in the early universe from the configuration space perspective

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2001.Includes bibliographical references (p. 103-106).We develop a new technique of analyzing the dynamics of cosmological perturbations in the linear regime. The gist of our method, described in Chapters 2 and 3, is to solve the corresponding evolution equations in 1 + 1 space-time dimensions with the initial conditions proportional to a spatial delta function. An arbitrary cosmological perturbation can be described as a superposition of the resulting Green's functions. In Chapter 2 we calculate the Green's functions assuming that prior to recombination photons are tightly coupled to baryons by scattering and neglecting neutrino free-streaming. We find that in the conformal Newtonian gauge a primordial perturbation does not affect the space beyond its acoustic horizon and propagates predominantly as a spherical "shock" that is trailed by a region of finite density and metric perturbation. In Chapter 4 we apply these solutions to analyze the anisotropy of the cosmic microwave background (CMB) radiation. In particular, we observe that wavefront singularities in Green's functions cause temperature anti-correlation for two regions that just established an acoustic contact. The experimental signature of this effect is a distinctive dip in the CMB temperature angular correlation function C([theta]) at [theta][is approzimately equal to] 1.2⁰. Variation of the cosmological parameters [Omega], [Omega]bh2, [Omega]c, n predictably affects the location and the shape of the dip and other C([theta]) characteristics. The acoustic oscillations in CMB angular power spectrum C, appear due to oscillations in the Fourier transform of the Green's functions, having finite spatial extent.The finite extent and monotonicity of Green's functions greatly reduce CPU time required for numerical calculation of perturbation dynamics. The geometrical part of the calculations connecting the early universe inhomogeneities with the CMB anisotropy spectrum C, in momentum space, can also be sped up by averaging the contributions from different momentum modes over rapid oscillations of a geometrical term. In Chapter 5 we describe implementation of these two ideas as a computer program calculating CMB spectrum within the fluid approximation of Chapter 2. In Chapter 3 we extend Green's function techniques to Boltzmann phase space to incorporate diffusion of massless particles. We obtain a solution for adiabatic perturbations in the radiation dominated universe that includes the dynamics of perturbations in the cosmic background of massless neutrinos.by Sergei V. Bashinsky.Ph.D