In their way towards us the CMB photons from primary anisotropies interact with the cosmic structures, and their frequency, energy or direction of propagation are affected. These are the so-called secondary effects. The induced secondary anisotropies are generated by the same physical processes as the primary (namely gravity and scattering off free electrons), however they usually act at smaller angular scales (typically the ones associated with the cosmic structures themselves).
With its broad frequency coverage (from 30 to about 900 GHz), its high angular resolution (down to 5 arcmin in the CMB channels) and its polarisation sensitive detectors (sensitivity down to 2e-6 in relative temperature variation), Planck is definitely a fantastic instrument for detecting and measuring the secondary anisotropies. Their study is already becoming one of the most lively field of research in CMB science. This is mainly due to the fact that the small scale secondary anisotropies are directly related to the structure formation, evolution and clustering in the universe, whereas the polarisation signal constrains the early universe, but at the same time it probes the reionisation of the universe, i.e. the formation of the first emitting objects.
The secondary CMB anisotropies can arise from the scattering of the CMB photons by a plasma of free electrons . A general case is the Thomson scattering and a very specific example is the so-called Sunyaev-Zel’dovich effect where the hot gas in clusters of galaxies up-scatters the cold microwave photons. At matter-radiation decoupling around 400 million years after big-bang, the energy of the electrons is low, of the order of one electron Volt. The free electrons thus scatter with the CMB photons by simple Thomson scattering. The Thomson scattering additionally induces a linear polarisation when the incident radiation has at least a quadrupolar geometry. This is the case in the CMB context for which the temperature anisotropies induced by density perturbations posses a quadrupole moment. The CMB photons can interact with more energetic electrons in hot ionised gas. In that situation and if the electron rest mass is much larger that the gas temperature and than the photon energy, the scattering brings the photon gas to equilibrium: This is the Comptonisation. The photon spectrum is consequently changed due to a net energy transfer from electrons to photons. However, the total photon number is conserved. This process which is simply the inverse Compton interaction between the CMB cold photons and the free electron of a hot ionised gas in galaxy clusters is known as the Sunyaev-Zel’dovich (SZ) effect (Zel’dovich & Sunyaev 1969, Sunyaev & Zel’dovich 1972, 1980). It is observed in an increasing number of clusters of galaxies and is predicted for other astrophysical sources of hot plasma.
The secondary anisotropies arising from so-called gravitational effects can be of two kinds. Some are due to time-variable metric perturbations and the others are due to gravitational lensing. In the first case, the CMB photons are affected by the change in the gravitational potential through which they pass. In the second case, the intervening large scale structure gravitationally lense the primary anisotropy. However, lensing does not generate any new temperature anisotropies. Lensing mainly modifies the distribution of the initial anisotropies by magnifying some patches in the sky and demagnifying others.
