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The Sunyaev–Zel'dovich effect (often abbreviated as the SZ effect) is the result of high energy electrons distorting the cosmic microwave background radiation (CMB) through inverse Compton scattering, in which the low energy CMB photons receive an average energy boost during collision with the high energy cluster electrons. Observed distortions of the cosmic microwave background spectrum are used to detect the density perturbations of the universe. Using the Sunyaev–Zel'dovich effect, dense clusters of galaxies have been observed.
The Sunyaev–Zel'dovich effect can be divided into:
- thermal effects, where the CMB photons interact with electrons that have high energies due to their temperature
- kinematic effects, a second-order effect where the CMB photons interact with electrons that have high energies due to their bulk motion (also called the Ostriker–Vishniac effect, after Jeremiah P. Ostriker and Ethan Vishniac.)
Rashid Sunyaev and Yakov Zel'dovich predicted the effect, and conducted research in 1969, 1972, and 1980. The Sunyaev–Zel'dovich effect is of major astrophysical and cosmological interest. It can help determine the value of the Hubble constant. To distinguish the SZ effect due to galaxy clusters from ordinary density perturbations, both the spectral dependence and the spatial dependence of fluctuations in the cosmic microwave background are used. Analysis of CMB data at higher angular resolution (high l values) requires taking into account the Sunyaev–Zel'dovich effect.
Current research is focused on modelling how the effect is generated by the intracluster plasma in galaxy clusters, and on using the effect to estimate the Hubble constant and to separate different components in the angular average statistics of fluctuations in the background. Hydrodynamic structure formation simulations are being studied to gain data on thermal and kinetic effects in the theory. Observations are difficult due to the small amplitude of the effect and to confusion with experimental error and other sources of CMB temperature fluctuations. However, since the Sunyaev–Zel'dovich effect is a scattering effect, its magnitude is independent of redshift. This is very important: it means that clusters at high redshift can be detected just as easily as those at low redshift. Another factor which facilitates high-redshift cluster detection is the angular scale versus redshift relation: it changes little between redshifts of 0.3 and 2, meaning that clusters between these redshifts have similar sizes on the sky. The use of surveys of clusters detected by their Sunyaev–Zel'dovich effect for the determination of cosmological parameters has been demonstrated by Barbosa et al. (1996). This might help in understanding the dynamics of dark energy in forthcoming surveys (SPT, ACT, Planck).
Timeline of observations 
- 1983 – Researchers from the Cambridge Radio Astronomy Group and the Owens Valley Radio Observatory first detect the Sunyaev–Zel'dovich effect from clusters of galaxies.
- 1993 – The Ryle Telescope is the first telescope to image a cluster of galaxies in the Sunyaev–Zel'dovich effect.
- 2003 – The WMAP spacecraft maps the Cosmic Microwave Background (CMB) over the whole sky with some (limited) sensitivity to the Sunyaev–Zel'dovich effect
- 2005 – The Atacama Pathfinder Experiment – Sunyaev-Zel'dovich camera saw first light and shortly after began pointed observations of galaxy clusters.
- 2005 – The Arcminute Microkelvin Imager and the Sunyaev–Zel'dovich Array each begin surveys for very high redshift clusters of galaxies using the Sunyaev–Zel'dovich effect
- 2007 – The South Pole Telescope (SPT) saw first light on 16 February 2007, and began science observations in March of that same year.
- 2007 – The Atacama Cosmology Telescope (ACT) saw first light on 8 June, and will soon begin an SZ survey of galaxy clusters.
- 2008 – The South Pole Telescope (SPT) discover for the first time galaxy clusters via the SZ effect.
- 2009 – The Planck spacecraft, launched on 14 May 2009, to realize a full sky SZ survey of galaxy clusters.
- 2012 – The Atacama Cosmology Telescope (ACT) performs the first detection of the kinematic SZ effect.
See also 
- Background radiation
- Compton effect
- Cosmic microwave background radiation
- Rashid Sunyaev
- Yakov Zel'dovich
- Yoel Rephaeli
- Ostriker, Jeremiah P. & Vishniac, Ethan T. (1986). "Effect of gravitational lenses on the microwave background, and 1146+111B,C". Nature 322 (6082): 804. Bibcode:1986Natur.322..804O. doi:10.1038/322804a0.
- Cunnama D., Faltenbacher F.; Passmoor S., Cress C.; Cress, C. & Passmoor, S. (2009). "The velocity-shape alignment of clusters and the kinetic Sunyaev-Zeldovich effect". MNRAS letters 397 (1): L41–L45. arXiv:0904.4765. Bibcode:2009MNRAS.397L..41C. doi:10.1111/j.1745-3933.2009.00680.x.
- Hand, Nick (2012). "Detection of Galaxy Cluster Motions with the Kinematic Sunyaev-Zel'dovich Effect". Physical Review Letters, submitted. arXiv:1203.4219. Bibcode:2012arXiv1203.4219H.
Further reading 
- Rephaeli, Y. (1995). "Comptonization Of The Cosmic Microwave Background: The Sunyaev-Zeldovich Effect". Annual Review of Astronomy and Astrophysics 33 (1): 541–580. Bibcode:1995ARA&A..33..541R. doi:10.1146/annurev.aa.33.090195.002545.
- Barbosa; et al. (1996). "The Sunyaev-Zel'dovich effect and the value of Ω0". Astronomy and Astrophysics 314: 14. arXiv:astro-ph/9511084. Bibcode:1996A&A...314...13B.
- Birkinshaw; et al. (1984). "The Sunyaev-Zel'dovich effect towards three clusters of galaxies". Nature 309 (5963): 34–35. Bibcode:1984Natur.309...34B. doi:10.1038/309034a0.
- Birkinshaw, Mark (1999). "The Sunyaev Zel'dovich Effect". Physics Reports 310 (2–3): 97. arXiv:astro-ph/9808050. Bibcode:1999PhR...310...97B. doi:10.1016/S0370-1573(98)00080-5.
- Cen, Renyue; Jeremiah P. Ostriker (1994). "A hydrodynamic approach to cosmology: the mixed dark matter cosmological scenario". The Astrophysical Journal 431: 1. arXiv:astro-ph/9404011. Bibcode:1994ApJ...431..451C. doi:10.1086/174499.
- Hu, Jian; Yu-Qing Lou (2004). "Magnetic Sunyaev-Zel'dovich effect in galaxy clusters". ApJL 606: L1–L4. arXiv:astro-ph/0402669. Bibcode:2004ApJ...606L...1H. doi:10.1086/420896.
- Ma, Chung-Pei; J. N. Fry (27 May 2002). "Nonlinear Kinetic Sunyaev-Zel'dovich Effect". PRL 88 (21). arXiv:astro-ph/0106342. Bibcode:2002PhRvL..88u1301M. doi:10.1103/PhysRevLett.88.211301.
- Myers, A. D.; et al. (2004). "Evidence for an Extended SZ Effect in WMAP Data". Monthly Notices of the Royal Astronomical Society 347 (4): L67–L72. arXiv:astro-ph/0306180. Bibcode:2004MNRAS.347L..67M. doi:10.1111/j.1365-2966.2004.07449.x.
- Springel, Volker; Martin White and Lars Hernquist (2001). "Hydrodynamic Simulations of the Sunyaev-Zel'dovich effect(s)". ApJ 549 (2): 681–687. arXiv:/0008133 astro-ph /0008133. Bibcode:2001ApJ...549..681S. doi:10.1086/319473.
- Sunyaev, R. A.; Ya. B. Zel'dovich (1970). "Small-Scale Fluctuations of Relic Radiation". Astrophysics and Space Science 7: 3. Bibcode:1970Ap&SS...7....3S. doi:10.1007/BF00653471 (inactive 2008-06-20).
- Sunyaev, R. A.; Ia. B Zel'dovich (1980). "Microwave background radiation as a probe of the contemporary structure and history of the universe". Annual review of astronomy and astrophysics 18 (1): 537–560. Bibcode:1980ARA&A..18..537S. doi:10.1146/annurev.aa.18.090180.002541.
- Diego, J.M; E. Martinez, J.L. Sanz, N. Benitez, J. Silk (2002). "The Sunyaev-Zel'dovich effect as a cosmological discriminator". Monthly Notices of the Royal Astronomical Society, 331 (3): 556–568. arXiv:astro-ph/0103512. Bibcode:2002MNRAS.331..556D. doi:10.1046/j.1365-8711.2002.05039.x.
- Royal Astronomical Society, Corrupted echoes from the Big Bang? RAS Press Notice PN 04/01