CMB Polarization Experiments

We discuss the analysis of polarization experiments with particular emphasis on those that measure the Stokes parameters on a ring on the sky. We discuss the ability of these experiments to separate the $E$ and $B$ contributions to the polarization signal. The experiment being developed at Wisconsin university is studied in detail, it will be sensitive to both Stokes parameters and will concentrate on large scale polarization, scanning a $47^o$ degree ring. We will also consider another example, an experiment that measures one of the Stokes parameters in a $1^o$ ring. We find that the small ring experiment will be able to detect cosmological polarization for some models consistent with the current temperature anisotropy data, for reasonable integration times. In most cosmological models large scale polarization is too small to be detected by the Wisconsin experiment, but because both $Q$ and $U$ are measured, separate constraints can be set on $E$ and $B$ polarization.Comment: 27 pages with 12 included figure

The Dipole Observed in the COBE DMR Four-Year Data

The largest anisotropy in the cosmic microwave background (CMB) is the $\approx 3$ mK dipole assumed to be due to our velocity with respect to the CMB. Using the four year data set from all six channels of the COBE Differential Microwave Radiometers (DMR), we obtain a best-fit dipole amplitude $3.358 \pm 0.001 \pm 0.023$ mK in the direction $(l,b)=(264\deg.31 \pm 0\deg.04 \pm 0\deg.16, +48\deg.05 \pm 0\deg.02 \pm 0\deg.09)$, where the first uncertainties are statistical and the second include calibration and combined systematic uncertainties. This measurement is consistent with previous DMR and FIRAS resultsComment: New and improved version; to be published in ApJ next mont

Coasting cosmologies with time dependent cosmological constant

The effect of a time dependent cosmological constant is considered in a family of scalar tensor theories. Friedmann-Robertson-Walker cosmological models for vacumm and perfect fluid matter are found. They have a linear expansion factor, the so called coasting cosmology, the gravitational "constant" decreace inversely with time; this model satisfy the Dirac hipotesis. The cosmological "constant" decreace inversely with the square of time, therefore we can have a very small value for it at present time.Comment: 7 pages, latex file (ijmpal macro), accepted for publication in Int. Mod. Phys.

Observations of the Cosmic Microwave Background and Implications for Cosmology and Large Scale Structure

Observations of the Cosmic Microwave Background (CMB) are discussed, with particular emphasis on current ground-based experiments and on future satellite, balloon and interferometer experiments. Observational techniques and the effects of contaminating foregrounds are highlighted. Recent CMB data is used with large scale structure (LSS) data to constrain cosmological parameters and the complementary nature of CMB, LSS and supernova distance data is emphasized.Comment: 23 pages, 10 figures. Phil. Trans. R. Soc. Lond. A., 1998, in pres

Dark-Energy Dynamics Required to Solve the Cosmic Coincidence

Dynamic dark energy (DDE) models are often designed to solve the cosmic coincidence (why, just now, is the dark energy density $\rho_{de}$, the same order of magnitude as the matter density $\rho_m$?) by guaranteeing $\rho_{de} \sim \rho_m$ for significant fractions of the age of the universe. This typically entails ad-hoc tracking or oscillatory behaviour in the model. However, such behaviour is neither sufficient nor necessary to solve the coincidence problem. What must be shown is that a significant fraction of observers see $\rho_{de} \sim \rho_m$. Precisely when, and for how long, must a DDE model have $\rho_{de} \sim \rho_{m}$ in order to solve the coincidence? We explore the coincidence problem in dynamic dark energy models using the temporal distribution of terrestrial-planet-bound observers. We find that any dark energy model fitting current observational constraints on $\rho_{de}$ and the equation of state parameters $w_0$ and $w_a$, does have $\rho_{de} \sim \rho_m$ for a large fraction of observers in the universe. This demotivates DDE models specifically designed to solve the coincidence using long or repeated periods of $\rho_{de} \sim \rho_m$.Comment: 16 pages, 8 figures, Submitted to Phys. Rev.

2-Point Correlations in the COBE DMR 4-Year Anisotropy Maps

The 2-point temperature correlation function is evaluated from the 4-year COBE DMR microwave anisotropy maps. We examine the 2-point function, which is the Legendre transform of the angular power spectrum, and show that the data are statistically consistent from channel to channel and frequency to frequency. The most likely quadrupole normalization is computed for a scale-invariant power-law spectrum of CMB anisotropy, using a variety of data combinations. For a given data set, the normalization inferred from the 2-point data is consistent with that inferred by other methods. The smallest and largest normalization deduced from any data combination are 16.4 and 19.6 uK respectively, with a value ~18 uK generally preferred.Comment: Sumbitted to ApJ Letter

Concerning Parameter Estimation Using the Cosmic Microwave Background

Most parameter constraints obtained from cosmic microwave background (CMB) anisotropy data are based on power estimates and rely on approximate likelihood functions; computational difficulties generally preclude an exact analysis based on pixel values. With the specific goal of testing this kind of approach, we have performed a complete (un-approximated) likelihood analysis combining the COBE, Saskatoon and MAX data sets. We examine in detail the ability of certain approximate techniques based on band-power estimates to recover the full likelihood constraints. The traditional $\chi^2$-method does not always find the same best-fit model as the likelihood analysis (a bias), due mainly to the false assumption of Gaussian likelihoods that makes the method overly sensitive to data outliers. Although an improvement, other approaches employing non-Gaussian flat-band likelihoods do not always faithfully reproduce the complete likelihood constraints either; not even when using the exact flat-band likelihood curves. We trace this to the neglect of spectral information by simple flat band-power estimates. A straightforward extension incorporating a local effective slope (of the power spectrum, $C_l$) provides a faithful representation of the likelihood surfaces without significantly increasing computing cost. Finally, we also demonstrate that the best-fit model to this particular data set is a {\em good fit}, or that the observations are consistent with Gaussian sky fluctuations, according to our statistic

Current cosmological constraints from a 10 parameter CMB analysis

We compute the constraints on a ``standard'' 10 parameter cold dark matter (CDM) model from the most recent CMB and data and other observations, exploring 30 million discrete models and two continuous parameters. Our parameters are the densities of CDM, baryons, neutrinos, vacuum energy and curvature, the reionization optical depth, and the normalization and tilt for both scalar and tensor fluctuations. Our strongest constraints are on spatial curvature, -0.24 < Omega_k < 0.38, and CDM density, h^2 Omega_cdm <0.3, both at 95%. Including SN 1a constraints gives a positive cosmological constant at high significance. We explore the robustness of our results to various assumptions. We find that three different data subsets give qualitatively consistent constraints. Some of the technical issues that have the largest impact are the inclusion of calibration errors, closed models, gravity waves, reionization, nucleosynthesis constraints and 10-dimensional likelihood interpolation.Comment: Replaced to match published ApJ version. More details added. 13 ApJ pages. CMB movies and color figs at http://www.hep.upenn.edu/~max/10par_frames.html or from [email protected]

MAX 4 and MAX 5 CMB anisotropy measurement constraints on open and flat-Lambda CDM cosmogonies

We account for experimental and observational uncertainties in likelihood analyses of cosmic microwave background (CMB) anisotropy data from the MAX 4 and MAX 5 experiments. These analyses use CMB anisotropy spectra predicted in open and spatially-flat Lambda cold dark matter cosmogonies. Amongst the models considered, the combined MAX data set is most consistent with the CMB anisotropy shape in Omega_0 ~ 0.1-0.2 open models and less so with that in old (t_0 >~ 15 - 16 Gyr, i.e., low h), high baryon density (Omega_B >~ 0.0175/h^2), low density (Omega_0 ~ 0.2 - 0.4), flat-Lambda models. The MAX data alone do not rule out any of the models we consider at the 2-sigma level. Model normalizations deduced from the combined MAX data are consistent with those drawn from the UCSB South Pole 1994 data, except for the flat bandpower model where MAX favours a higher normalization. The combined MAX data normalization for open models with Omega_0 ~ 0.1-0.2 is higher than the upper 2-sigma value of the DMR normalization. The combined MAX data normalization for old (low h), high baryon density, low-density flat-Lambda models is below the lower 2-sigma value of the DMR normalization. Open models with Omega_0 ~ 0.4-0.5 are not far from the shape most favoured by the MAX data, and for these models the MAX and DMR normalizations overlap. The MAX and DMR normalizations also overlap for Omega_0 = 1 and some higher h, lower Omega_B, low-density flat-Lambda models.Comment: Latex, 37 pages, uses aasms4 styl