Detection of Polarization in the Cosmic Microwave Background using DASI

TL;DR

This paper reports the first robust detection of polarization in the CMB using the Degree Angular Scale Interferometer (DASI) at the South Pole. It employs a comprehensive likelihood framework, detailed instrument modeling, and extensive consistency tests to extract the polarization power spectra (E and B) and their cross-correlations with temperature, finding a clear E-mode signal and TE correlation in agreement with ΛCDM. The B-mode signal remains undetected, with a stringent upper limit, while the temperature spectrum remains consistent with prior measurements, all supporting a coherent cosmological picture of polarimetric CMB anisotropy. The results significantly validate the standard theory of CMB polarization and bolster confidence in the cosmological parameters derived from CMB data.

Abstract

We report the detection of polarized anisotropy in the Cosmic Microwave Background radiation with the Degree Angular Scale Interferometer (DASI), located at the Amundsen-Scott South Pole research station. Observations in all four Stokes parameters were obtained within two 3.4 FWHM fields separated by one hour in Right Ascension. The fields were selected from the subset of fields observed with DASI in 2000 in which no point sources were detected and are located in regions of low Galactic synchrotron and dust emission. The temperature angular power spectrum is consistent with previous measurements and its measured frequency spectral index is -0.01 (-0.16 -- 0.14 at 68% confidence), where 0 corresponds to a 2.73 K Planck spectrum. The power spectrum of the detected polarization is consistent with theoretical predictions based on the interpretation of CMB anisotropy as arising from primordial scalar adiabatic fluctuations. Specifically, E-mode polarization is detected at high confidence (4.9 sigma). Assuming a shape for the power spectrum consistent with previous temperature measurements, the level found for the E-mode polarization is 0.80 (0.56 -- 1.10), where the predicted level given previous temperature data is 0.9 -- 1.1. At 95% confidence, an upper limit of 0.59 is set to the level of B-mode polarization with the same shape and normalization as the E-mode spectrum. The TE correlation of the temperature and E-mode polarization is detected at 95% confidence, and also found to be consistent with predictions. These results provide strong validation of the underlying theoretical framework for the origin of CMB anisotropy and lend confidence to the values of the cosmological parameters that have been derived from CMB measurements.

Figures (6)

• Figure 1: Polarization maps constructed from polarized datasets that have been split by epoch, and formed into sum (left) or difference (right) data vectors, as reported in §\ref{['sec:chi2tests']}. In order to isolate the most significant signal in our data, we have used only the subset of 34 eigenmodes which, under the concordance model, are expected to have average signal/noise $>1$. Unlike conventional interferometer maps, the signal/noise selected eigenmodes reflect the gain of the primary beam. This is apparent in the difference map (right), which is consistent with noise. Comparison of this map to the sum map (left) illustrates a result also given numerically for this split/subset in Table \ref{['tab:chi2']}: that these individual modes in the polarized dataset show a significant signal.
• Figure 2: The upper panel shows the $E$ (solid blue) and B (solid red, much lower curve) parameter window functions that indicate the response to the $E$ power spectrum of the two parameters in our $E$/B analysis. The blue dashed curve shows the result of multiplying the $E$ parameter window function by the concordance $E$ spectrum, illustrating that for this CMB spectrum, most of the response of our experiment's $E$ parameter comes from the region of the second peak ($250 \lesssim l \lesssim 450$), with a substantial contribution also from the third peak and a smaller contribution from the first. The lower panel shows $E1$ -- $E5$ (blue) and $B1$ -- $B5$ (red, again much lower) parameter window functions for the $E$ power spectrum from our $E5$/$B5$ analysis. DASI's response to $E$ and B is very symmetric, so that the corresponding plots that show these parameters response to the B power spectrum are nearly identical to these, with the $E$ and B parameters reversed.
• Figure 3: Results from the two parameter shaped bandpower E/B polarization analysis assuming an $E$-mode power spectrum shape as predicted for the concordance model, in units of amplitude relative to that model. The same shape is assumed for the $B$-mode spectrum. (right panel) The point shows the maximum likelihood value with the cross indicating Fisher matrix errors. Likelihood contours are placed at levels $\exp(-n^2/2)$ relative to the maximum, i.e., for a normal distribution, the extrema of these contours along either dimension would give the marginalized $n$-sigma interval. (left panels) The corresponding single parameter likelihood distributions marginalized over the other parameter. The grey lines enclose 68% of the total likelihood. The red line indicates the 95% confidence upper limit on $B$-mode power. The green band shows the distribution of $E$ expectation values for a large grid of cosmological models weighted by the likelihood of those models given our previous temperature result (see Paper III).
• Figure 4: Results from several likelihood analyses: The T5 temperature analysis is shown in the top panel. The ten parameter $E5$/$B5$ polarization analysis is shown in the middle two panels. The 5 $TE$ bands from the $T$/$E$/$TE5$ joint analysis are shown in the bottom panel. All the results shown are flat bandpower values. The blue line shows the maximum likelihood bandpower values with the grey error bars indicating the 68% central region of the likelihood marginalizing over the other parameter values (analogous to the grey lines in Figure \ref{['fig:EBplot']}). In each case the green line is the concordance model.
• Figure 5: (left) Results from the two parameter shaped bandpower T/$\beta_T$ temperature analysis assuming the $T$ power spectrum shape as predicted for the concordance model, and in units relative to that model. The layout of the plot is analogous to Figure \ref{['fig:EBplot']}. Spectral index is relative to thermal --- in these units synchrotron emission would be expected to have an index of approximately $-3$. (right) Results of the similar E/$\beta_E$ analysis performed on the polarization data.