= Solution
The <cosmic microwave background> is relic thermal <radiation> from the hot early universe. Its <temperature> anisotropy is expanded in <spherical harmonics>, with angular <power> spectrum $C_\ell^{TT}$. <Thomson scattering> of a local <radiation quadrupole> generates <linear polarization> around last scattering and again during <reionization>. Polarization maps are decomposed into <E-mode polarization> and <B-mode polarization>, giving spectra such as $EE$, $TE$ and $BB$ in addition to <temperature> information.
<Cosmic microwave background acoustic peak> positions compare the <sound horizon> with the angular-diameter distance to last scattering, constraining geometry and expansion history. Relative peak heights constrain <baryon> loading and <matter-radiation equality>; the damping tail constrains diffusion and other early-universe physics. The <scalar spectral index> and <primordial scalar amplitude> constrain inflationary perturbations. Coherent acoustic structure and temperature-polarization correlations support predominantly adiabatic initial perturbations. Large-angle <E-mode polarization> measures the <reionization> <optical depth>, helping break its degeneracy with the scalar amplitude inferred from small-scale <temperature> anisotropy.
Under a specified cosmological model, these data constrain ordinary and <cold dark matter> densities, the <Hubble constant>, spatial curvature and cosmic age. Complementary distance information helps break the geometrical degeneracy: <temperature> peaks alone do not uniquely establish a <cosmological constant>. The Planck 2013 parameter analysis used Planck <temperature> data and, in important combinations, <WMAP> low-multipole polarization; it should not be described as a full Planck polarization release.<Planck 2013 results. XVI. Cosmological parameters by Planck Collaboration>[Abstract and data-combination definitions, including WMAP polarization]{ref}
Linear <scalar cosmological perturbations> generate <E-mode polarization> but no primordial <B-mode polarization> in the simplest treatment. <tensor cosmological perturbations> can generate <B-mode polarization>, making them a possible probe of inflationary <gravitational waves>. <gravitational lensing> converts some E-mode <power> into <B-mode polarization>, however, and Galactic dust and <synchrotron radiation> also contribute polarized foregrounds. Multifrequency observations, foreground modelling, beam <calibration> and control of <temperature-to-polarization leakage> are therefore necessary. Polarization does not by itself establish a primordial tensor signal. \b[<temperature> and polarization jointly test the expansion history and primordial perturbations, while complementary data and foreground control determine how uniquely parameters can be inferred.]
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