Sachs–Wolfe effect
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The Sachs–Wolfe effect, named after
Rainer K. Sachs Rainer Kurt "Ray" Sachs (born June 13, 1932) is a German-American mathematical physicist, with interests in general relativistic cosmology and astrophysics, as well as a computational radiation biologist. He is professor emeritus of Mathematics an ...
and Arthur M. Wolfe, is a property of the cosmic microwave background radiation (CMB), in which photons from the CMB are gravitationally redshifted, causing the CMB spectrum to appear uneven. This effect is the predominant source of fluctuations in the CMB for angular scales larger than about ten degrees.


Non-integrated Sachs–Wolfe effect

The non-integrated Sachs–Wolfe effect is caused by gravitational redshift occurring at the surface of last scattering. The effect is not constant across the sky due to differences in the matter/energy density at the time of last scattering.


Integrated Sachs–Wolfe effect

The integrated Sachs–Wolfe (ISW) effect is also caused by gravitational redshift, but it occurs between the surface of last scattering and the
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, so it is not part of the primordial
CMB In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
. It occurs when the
Universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...
is dominated in its energy density by something other than matter. If the Universe is dominated by matter, then large-scale gravitational potential energy wells and hills do not evolve significantly. If the Universe is dominated by
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
, or by
dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univer ...
, though, those potentials do evolve, subtly changing the energy of
photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...
passing through them. There are two contributions to the ISW effect. The "early-time" ISW occurs immediately after the (non-integrated) Sachs–Wolfe effect produces the primordial CMB, as photons course through density fluctuations while there is still enough
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
around to affect the Universe's expansion. Although it is physically the same as the late-time ISW, for observational purposes it is usually lumped in with the primordial CMB, since the matter fluctuations that cause it are in practice undetectable.


Late-time integrated Sachs–Wolfe effect

The "late-time" ISW effect arises quite recently in cosmic history, as
dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univer ...
, or the cosmological constant, starts to govern the Universe's expansion. Unfortunately, the nomenclature is a bit confusing. Often, "late-time ISW" implicitly refers to the late-time ISW effect to linear/first order in density perturbations. This linear part of the effect entirely vanishes in a flat universe with only matter, but dominates over the higher-order part of the effect in a universe with dark energy. The full
nonlinear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
(linear + higher-order) late-time ISW effect, especially in the case of individual voids and clusters, is sometimes known as the Rees–Sciama effect, since Martin Rees and
Dennis Sciama Dennis William Siahou Sciama, (; 18 November 1926 – 18/19 December 1999) was a British physicist who, through his own work and that of his students, played a major role in developing British physics after the Second World War. He was the PhD ...
elucidated the following physical picture. Accelerated expansion due to dark energy causes even strong large-scale potential wells (
supercluster A supercluster is a large group of smaller galaxy clusters or galaxy groups; they are among the largest known structures in the universe. The Milky Way is part of the Local Group galaxy group (which contains more than 54 galaxies), which in turn ...
s) and hills (
voids Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a s ...
) to decay over the time it takes a
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
to travel through them. A photon gets a kick of energy going into a potential well (a supercluster), and it keeps some of that energy after it exits, after the well has been stretched out and shallowed. Similarly, a photon has to expend energy entering a supervoid, but will not get all of it back upon exiting the slightly reduced potential hill. A signature of the late-time ISW is a non-zero cross-
correlation function A correlation function is a function that gives the statistical correlation between random variables, contingent on the spatial or temporal distance between those variables. If one considers the correlation function between random variables rep ...
between the galaxy density (the number of galaxies per square degree) and the temperature of the CMB, because superclusters gently heat photons, while supervoids gently cool them. This correlation has been detected at moderate to high significance. A detailed analysis of how parameters such as shot noise, maximum multipole or redshift ranges can influence the significance of radio continuum surveys was presented by Rahman in 2014. In May 2008, Granett, Neyrinck & Szapudi showed that the late-time ISW can be pinned to discrete supervoids and superclusters identified in the SDSS Luminous Red Galaxy catalog. Their ISW detection traces the localised ISW effect produced by supervoids and superclusters have on the CMB. However, the amplitude of this localised detection is controversial, as it is significantly larger than the expectations and depends on several assumptions of the analysis.


See also

*
Sunyaev–Zeldovich effect The Sunyaev–Zeldovich effect (named after Rashid Sunyaev and Yakov Zeldovich, Yakov B. Zeldovich and often abbreviated as the SZ effect) is the Cosmic microwave background spectral distortions , spectral distortion of the cosmic microwave back ...
* Cosmic microwave background spectral distortions


References


External links

* Sam LaRoque,
The Integrated Sachs–Wolfe Effect
'. University of Chicago, IL. * Aguiar, Paulo, and Paulo Crawford,
Sachs–Wolfe effect in some anisotropic models
'. (
PDF Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. ...
format) *
Sachs–Wolfe effect
Level 5.
"Dark Energy and the Imprint of Super-Structures on the Microwave Background"
a webpage by Granett, Neyrinck & Szapudi. {{DEFAULTSORT:Sachs-Wolfe effect Physical cosmology