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Cosmic microwave background radiation


Cosmic microwave
background radiation
June 2001
The Cosmic Microwave Background Radiation is a form of electromagnetic radiation that fills the whole of the universe. It has the characteristics of black body radiation at a temperature of 2.726 Kelvin. \nIt has a frequency in the microwave range.

Table of contents
1 CMR and the Big Bang
2 Features
3 Detection, Prediction and Discovery
4 Experiments
5 CBR and Non-Standard Cosmologies
6 See also
7 Bibliography
8 References and external links

CMR and the Big Bang

This radiation is regarded as the best available evidence of the Big Bang (BB) theory and its discovery in the mid-1960s marked the end of general interest for alternatives such as the steady state theory. The CMR gives a snapshot of the Universe when, according to standard cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 300,000 years after the Big Bang -- this point in time is generally known as the "last scattering surface" -- the temperature of the Universe was about 6000 K. Since then it has dropped because of the expansion of the Universe, which cools radiation inversely proportional to the fourth power of the Universe's scale length.

Features

One of the microwave background's most salient features is a high degree of isotropy. There are some anisotropies, the most pronounced of which is the dipole anisotropy at a level of about 10-4 at a scale of 180 degrees of arc. It is due to the motion of the observer against the CBR, which is some 700 km/s for the Earth. Variations due to external physics also exist; the\nSunyaev-Zel'dovich Effect is one of the major factors here, in which an cloud of high energy electrons scatters the radiation transferring some energy to the CMB photons. Even more interesting are anisotropies at a level of roughly 1/100000 and on a scale of a few arc minutes. Those very small variations correspond to the density fluctuations at the last scattering surface and give valuable information about the seeds for the large scale structures we observe now. These density fluctations arise because different parts of the universe are not in contact with each other. In addition, the Sachs-Wolfe effect causes photons from the Cosmic microwave background to be gravitationally redshifted. These small-scale variations give observational constraints on the properties of universe, and are therefore one important test for cosmological models.

Detection, Prediction and Discovery

The CBR was predicted by George Gamow, Ralph Alpher, and Robert Hermann in the 1940s and was accidentally discovered in 1964 by Penzias and Wilson, who received a Nobel Prize in Physics in 1978 for this discovery. The CBR had, however, been detected and its temperature deduced in 1941, seven years before Gamow's prediction. Based on the study of narrow absorption line features in the spectra of stars, the astronomer Andrew McKellar wrote: "It can be calculated that the 'rotational' temperature of interstellar space is 2 K." Because water absorbs microwave radiation, a fact that is used to build microwave ovens, it is rather difficult to observe the CMB with ground-based instruments. CMB research therefore makes increasing use of air and space-borne experiments. Main article: Discovery of cosmic microwave background radiation

Experiments

Of these experiments, the Cosmic Background Explorer (COBE) satellite that was flown in 1989-1996 is probably the most famous and which made the first detection of the large scale anisotropies (other than the dipole). In June 2001, NASA launched a second CBR space mission, WMAP, to make detailed measurements of the anisotropies over the full sky. Results from this mission provide a detailed measurement of the angular power spectrum down to degree scales, giving detailed constraints on various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation as well as various other competing theories, and are available in detail at NASA's data center for Cosmic Microwave Background (CMB) [ed. see links below], A third space mission, Planck, is to be launched in 2007. \nUnlike the previous two space missions, Planck is a collaboration between NASA and ESA (the European Space Agency).

CBR and Non-Standard Cosmologies

During the mid-1990s, the lack of detection of anisotropies in the CBR led to some interest in nonstandard cosmologies (such as plasma cosmology) mostly as a backup in case detectors failed to find anisotropy in the CBR. The discovery of these anisotropies combined with a large amount of new data coming in has greatly reduced interest in these alternative theories. Some supporters of non-standard cosmology argue that the \nprimodorial background radiation is uniform (which is\ninconsistent with the big bang) and that the variations\nin the CBR are due to the Sunyaev-Zel'dovich effect mentioned above (among other effects).

WMAP data reviewed (early 2004), casts doubt on anisotropy

See here for a recent reappriasal of the WMAP data by Professor Tom Shanks of the University of Durham that casts some doubt on some of the anisotropic evidence.

See also

Main:\nBackground radiation,\nCOBE,\nCosmic inflation,\nCosmic background radiation,\nGravity wavess,\nMicrowave,\nUnsolved problems in physics,\nWMAP Physics and Astronomy: \nAnisotropy (or Anisotropic),\nBaryonic dark matter,\nBig bang nucleosynthesis, \nBlack body,\nBlack dwarf,\nCold dark matter,\nDark energy,\nGreisen-Zatsepin-Kuzmin limit,\nHistory of astronomy,\nHubble's law,\nIntegrated Sachs Wolfe effect,\nNobel Prize in Physics,\nObservation,\nOlbers' paradox,\nRadio astronomy,\nRedshift Theories: \nBig Bang,\nBig Crunch,\nPlasma cosmology,\nReciprocal System of Theory People: \nArno Allan Penzias,\nFred Hoyle,\nGeorges Lemaître,\nRobert Wilson,\nRobert Woodrow Wilson Timelines and lists: \nList of astronomical topics,\nList of famous experiments,\nTimeline of cosmic microwave background astronomy,\nTimeline of knowledge about galaxies, clusters of galaxies, and large-scale structure,\nTimeline of the Big Bang,\nTimeline of white dwarfs, neutron stars, and supernovae Other:\n1 E12 s,\nBackground,\nHolmdel Township, New Jersey

Bibliography

  • Seife, Charles BREAKTHROUGH OF THE YEAR: Illuminating the Dark Universe, Science 2003 302: 2038-2039\n* Partridge, R. B. "3K: The Cosmic Microwave Background Radiation". New York, Cambridge University Press, 1995.

References and external links

"#3 pencils and quadrille pads." - Seymoure Cray (1925-1996) when asked what CAD tools he used to design the Cray I supercomputer; he also recommended using the back side of the pages so that the lines were not so dominant.