I want to sort out the names of various effects and what-not related to the cosmic microwave background (CMB) radiation, which probably makes for boring reading material. Oh well. In particular, I’m going to summarise four effects that affect the density fluctuations in the CMB:
- The Sunyaev-Zel’dovich effect
- The Sachs-Wolfe effect
- Diffusion damping
- Baryon Acoustic Oscillation
The Sunyaev-Zel’dovich effect is when CMB photons gain a boost in energy via their inverse Compton scattering from electrons in the hot gas surrounding galaxy clusters.
The Sachs-Wolfe effect is the gravitational redshift of photons either at the surface of last scattering, this is called the “non-integrated Sachs-Wolfe effect”, or not at the surface of last scattering, in which case it is called (you guessed it) the “integrated Sachs-Wolfe effect”.
Diffusion damping is the redshifting of photons as they escape gravitational wells caused by density anisotropies in the CMB plasma. Note that this has the effect of flattening out density anisotropies.
Lastly, the Baryon Acoustic Oscillation (BAO) is the imprint of acoustic vibrations in the primordial plasma caused by the opposing forces of pressure and gravitational attraction. Measuring the angular peaks of the BAO tells cosmologists lots of useful information about the universe and helps us constrain cosmological parameters .