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Overview of the Cosmic Microwave Background Radiation
Shortly after the Big Bang, the universe was filled with hot plasma and radiation in thermal equilibrium. As the universe expanded and cooled, protons and electrons combined to form hydrogen, causing the universe to become transparent to electromagnetic radiation. The Cosmic Microwave Background (CMB) radiation decoupled at this time and now travels freely through the universe, redshifting with the Hubble expansion. At the present day, about 14 billion years after the Big Bang, we see the CMB as a nearly isotropic 2.7K blackbody spectrum.
The CMB has three measurable properties
- Spectrum – The COBE experiment confirmed that the CMB is a perfect blackbody with an average temperature of 2.7K.
- Temperature anisotropies – The spatial distribution of the CMB temperature on the sky is uniform to a few parts in 105. The tiny temperature anisotropies have been measured by many experiments, most recently the WMAP satellite. These measurements have demonstrated that the universe is spatially flat, and provide the best evidence to date that there was a period of inflation in the very early universe.
- Polarization anisotropies – The polarization of CMB anisotropies has been detected by a number of experiments.
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| CMB Polarization
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Anisotropic Thomson scattering polarizes the CMB. Wayne Hu’s website gives an excellent and comprehensive high-level treatment of the CMB and its polarization. The figure opposite, taken from his website, shows how polarization is produced. Hot radiation is incident on the scattering electron from the left (and right) and cooler radiation is incident from the top (and bottom). A linearly polarized wave is scattered into our line of sight.
The CMB is only a few percent polarized, so this signal is intrinsically fainter than the temperature anisotropies. CMB polarization has been detected by a number of experiments, including DASI, WMAP, CBI, CAPMAP and Boomerang. However detailed measurements remain to be made. Polarization in the CMB is generated during the last few scatterings that the photons make off of free electrons before decoupling from the hot plasma.
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The temperature and polarization pattern of CMB anisotropies form a 2x2 tensor which can be decomposed into a scalar field, usually referred to as the E mode of polarization, and a pseudo-scalar field, usually referred to as the B mode of polarization. The E and B mode definitions are chosen by analogy to electric and magnetic fields since B modes have curl and E modes are curl-free. The E mode signal is dominated by contributions from the density perturbations in the primordial plasma that are in turn the dominant source of the temperature anisotropy signal. The much weaker B mode signal has two sources: (i) relic gravitational wave radiation generated in the early universe by inflation and (ii) gravitation lensing of E mode polarization by intervening structure in the Universe. The lensing B mode signal is useful in its own right as a probe of structure in the early Universe. It must also be thoroughly measured and understood if it is to be removed from the inflationary B mode signal. |
QUaD Science
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QUaD is in the process of mapping a region of low foreground sky at 100 GHz and 150 GHz. These frequencies span the range over which other astronomical signals are at a minimum and the CMB signal is close to its maximum. The figure below left shows foregrounds at 150 GHz from the South Pole, where red is the brightest and blue the faintest foreground signal. We are mapping a portion of clean sky located in the region outlined in white. The maps will be used to construct power spectra for the E and B modes. The figure below right shows the expected coverage of QUaD in terms of multipole number, l. The theoretical power spectra are color-coded as follows: black -- temperature; green -- E mode; pink -- B mode. The grey dot-dash line in the contribution to the B mode spectrum from gravitational lensing. The B mode spectrum shown assumes the maximum amount of gravitational waves allowed by current observations; the true signal may be orders of magnitude fainter.
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