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Cosmic microwave background radiation

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

          Physical cosmology

     * Age of the universe
     * Big Bang
     * Blueshift
     * Comoving distance
     * Cosmic microwave background
     * Dark energy
     * Dark matter
     * FLRW metric
     * Friedmann equations
     * Galaxy formation
     * Hubble's law
     * Inflation
     * Large-scale structure
     * Lambda-CDM model
     * Metric expansion of space
     * Nucleosynthesis
     * Observable universe
     * Redshift
     * Shape of the universe
     * Structure formation
     * Timeline of the Big Bang
     * Timeline of cosmology
     * Ultimate fate of the universe
     * Universe

            Related topics
     * Astrophysics
     * General relativity
     * Particle physics
     * Quantum gravity


   In cosmology, the cosmic microwave background radiation (most often
   abbreviated CMB but occasionally CMBR, CBR or MBR, also referred as
   relic radiation) is a form of electromagnetic radiation discovered in
   1965 that fills the entire universe. It has a thermal 2.725 kelvin
   black body spectrum which peaks in the microwave range at a frequency
   of 160.4  GHz, corresponding to a wavelength of 1.9 mm. Most
   cosmologists consider this radiation to be the best evidence for the
   hot big bang model of the universe.

Features

   The cosmic microwave background spectrum measured by the FIRAS
   instrument on the COBE satellite is the most precisely measured black
   body spectrum in nature. The data points and error bars on this graph
   are obscured by the theoretical curve.
   Enlarge
   The cosmic microwave background spectrum measured by the FIRAS
   instrument on the COBE satellite is the most precisely measured black
   body spectrum in nature. The data points and error bars on this graph
   are obscured by the theoretical curve.

   The cosmic microwave background is isotropic to roughly one part in
   100,000: the root mean square variations are only 18 µK. The
   Far-Infrared Absolute Spectrophotometer (FIRAS) instrument on the NASA
   COsmic Background Explorer (COBE) satellite has carefully measured the
   spectrum of the cosmic microwave background. FIRAS compared the CMB
   with a reference black body and no difference could be seen in their
   spectra. Any deviations from the black body form that might still
   remain undetected in the CMB spectrum over the wavelength range from
   0.5 to 5 mm must have a weighted rms value of at most 50 parts per
   million (0.005%) of the CMB peak brightness. This made the CMB spectrum
   the most precisely measured black body spectrum in nature.

   The cosmic microwave background is a prediction of Big Bang theory. In
   the theory, the early universe was made up of a hot plasma of photons,
   electrons and baryons. The photons were constantly interacting with the
   plasma through Thomson scattering. As the universe expanded, adiabatic
   cooling (of which the cosmological redshift is an on-going symptom)
   caused the plasma to cool until it became favourable for electrons to
   combine with protons and form hydrogen atoms. This happened at around
   3,000 K or when the universe was approximately 380,000 years old
   (z=1088). At this point, the photons did not scatter off of the now
   neutral atoms and began to travel freely through space. This process is
   called recombination or decoupling (referring to electrons combining
   with nuclei and to the decoupling of matter and radiation
   respectively).

   The photons have continued cooling ever since; they have now reached
   2.725 K and their temperature will continue to drop as long as the
   universe continues expanding. Accordingly, the radiation from the sky
   we measure today comes from a spherical surface, called the surface of
   last scattering, from which the photons that decoupled from interaction
   with matter in the early universe, 13.7 billion years (13.7 G yr) ago,
   are just now reaching observers on Earth. The big bang suggests that
   the cosmic microwave background fills all of observable space, and that
   most of the radiation energy in the universe is in the cosmic microwave
   background, which makes up a fraction of roughly 5×10^-5 of the total
   density of the universe.

   Two of the greatest successes of the big bang theory are its prediction
   of its almost perfect black body spectrum and its detailed prediction
   of the anisotropies in the cosmic microwave background. The recent
   Wilkinson Microwave Anisotropy Probe has precisely measured these
   anisotropies over the whole sky down to angular scales of 0.2 degrees.
   These can be used to estimate the parameters of the standard Lambda-CDM
   model of the big bang. Some information, such as the shape of the
   Universe, can be obtained straightforwardly from the cosmic microwave
   background, while others, such as the Hubble constant, are not
   constrained and must be inferred from other measurements.

History

                              Timeline of the CMB
                           Important people and dates
    1940          Andrew McKellar The observational detection of an average
              bolometric temperature of 2.3 K based on the study of interstellar
             absorption lines is reported from the Dominion Observatory, British
                                                                       Columbia
    1946       Robert Dicke predicts ".. radiation from cosmic matter" at <20 K,
                                      but did not refer to background radiation
    1948          George Gamow calculates a temperature of 50 K (assuming a
                3-billion year old Universe), commenting it ".. is in reasonable
          agreement with the actual temperature of interstellar space", but does
                                               not mention background radiation.
    1948    Ralph Alpher and Robert Herman estimate "the temperature in the
           Universe" at 5 K. Although they do not specifically mention microwave
                                       background radiation, it may be inferred.
    1950        Ralph Alpher and Robert Herman re-re-estimate the temperature at
                                                                           28 K.
    1953                                             George Gamow estimates 7 K.
    1956                                             George Gamow estimates 6 K.
   1960s        Robert Dicke re-estimates a MBR (microwave background radiation)
                                                            temperature of 40 K
    1964        A. G. Doroshkevich and Igor Novikov publish a brief paper, where
                           they name the CMB radiation phenomenon as detectable.
 1964-65  Arno Penzias and Robert Woodrow Wilson measure the temperature
         to be approximately 3 K. Robert Dicke, P. J. E. Peebles, P. G. Roll and
              D. T. Wilkinson interpret this radiation as a signature of the big
                                                                           bang.
    1990             FIRAS measures the black body form of the CMB spectrum with
                                                            exquisite precision.
    1992       COBE DMR reveals the primary temperature anisotropy for the first
                                                                           time.
    2002                                        Polarization discovered by DASI.

   The cosmic microwave background was predicted in 1948 by George Gamow
   and Ralph Alpher, and by Alpher and Robert Herman. Moreover, Alpher and
   Herman were able to estimate the temperature of the cosmic microwave
   background to be 5 K, though two years later, they re-estimated it at
   28 K.. Although there were several previous estimates of the
   temperature of space (see timeline), these suffered from two flaws.
   First, they were measurements of the effective temperature of space,
   and did not suggest that space was filled with a thermal Planck
   spectrum: Second, they are dependent on our special place at the edge
   of the Milky Way galaxy and did not suggest the radiation is isotropic.
   Moreover, they would yield very different predictions if Earth happened
   to be located elsewhere in the universe.

   The 1948 results of Gamow and Alpher were not widely discussed.
   However, they were rediscovered by Robert Dicke and Yakov Zel'dovich in
   the early 1960s. The first published recognition of the CMB radiation
   as a detectable phenomenon appeared in a brief paper by Soviet
   astrophysicists A. G. Doroshkevich and Igor Novikov, in the spring of
   1964. In 1964, David Todd Wilkinson and Peter Roll, Dicke's colleagues
   at Princeton University, began constructing a Dicke radiometer to
   measure the cosmic microwave background. In 1965, Arno Penzias and
   Robert Woodrow Wilson at the Crawford Hill location of Bell Telephone
   Laboratories in nearby Holmdel Township, New Jersey had built a Dicke
   radiometer that they intended to use for radio astronomy and satellite
   communication experiments. Their instrument had an excess 3.5 K antenna
   temperature which they could not account for. After receiving a
   telephone call from Crawford Hill, Dicke famously quipped: "Boys, we've
   been scooped." A meeting between the Princeton and Crawford Hill groups
   determined that the antenna temperature was indeed due to the microwave
   background. Penzias and Wilson received the 1978 Nobel Prize in Physics
   for their discovery.

   The interpretation of the cosmic microwave background was a
   controversial issue in the 1960s with some proponents of the steady
   state theory arguing that the microwave background was the result of
   scattered starlight from distant galaxies. Using this model, and based
   on the study of narrow absorption line features in the spectra of
   stars, the astronomer Andrew McKellar wrote in 1941: "It can be
   calculated that the ' rotational temperatureˡ of interstellar space is
   2 K." However, during the 1970s the consensus was established that the
   cosmic microwave background is a remnant of the big bang. This was
   largely because new measurements at a range of frequencies showed that
   the spectrum was a thermal, black body spectrum, a result that the
   steady state model was unable to reproduce.
   The Horn antenna on which Penzias and Wilson discovered the cosmic
   microwave background.
   Enlarge
   The Horn antenna on which Penzias and Wilson discovered the cosmic
   microwave background.

   Harrison, Peebles and Yu, and Zel'dovich realized that the early
   universe would have to have inhomogeneities at the level of 10^-4 or
   10^−5. Rashid Sunyaev later calculated the observable imprint that
   these inhomogeneities would have on the cosmic microwave background.
   Increasingly stringent limits on the anisotropy of the cosmic microwave
   background were set by ground based experiments, but the anisotropy was
   first detected by the Differential Microwave Radiometer instrument on
   the COBE satellite.

   Inspired by the COBE results, a series of ground and balloon-based
   experiments measured cosmic microwave background anisotropies on
   smaller angular scales over the next decade. The primary goal of these
   experiments was to measure the scale of the first acoustic peak, which
   COBE did not have sufficient resolution to resolve. The first peak in
   the anisotropy was tentatively detected by the Toco experiment and the
   result was confirmed by the BOOMERanG and MAXIMA experiments.. These
   measurements demonstrated that the Universe is approximately flat and
   were able to rule out cosmic strings as a major component of cosmic
   structure formation, and suggested cosmic inflation was the right
   theory of structure formation.

   The second peak was tentatively detected by several experiments before
   being definitively detected by WMAP, which has also tentatively
   detected the third peak. Several experiments to improve measurements of
   the polarization and the microwave background on small angular scales
   are ongoing. These include DASI, WMAP, BOOMERanG and the Cosmic
   Background Imager. Forthcoming experiments include the Planck
   satellite, Atacama Cosmology Telescope and the South Pole Telescope.
   WMAP image of the CMB anisotropy.
   Enlarge
   WMAP image of the CMB anisotropy.

Relationship to the Big Bang

   The standard hot big bang model of the universe requires that the
   initial conditions for the universe are a Gaussian random field with a
   nearly scale invariant or Harrison-Zel'dovich spectrum. This is, for
   example, a prediction of the cosmic inflation model. This means that
   the initial state of the universe is random, but in a clearly specified
   way in which the amplitude of the primeval inhomogeneities is 10^-5.
   Therefore, meaningful statements about the inhomogeneities in the
   universe need to be statistical in nature. This leads to cosmic
   variance in which the uncertainties in the variance of the largest
   scale fluctuations observed in the universe are difficult to accurately
   compare to theory.

Temperature

   The cosmic microwave background radiation and the cosmological red
   shift are together regarded as the best available evidence for the Big
   Bang (BB) theory. The discovery of the CMB in the mid-1960s curtailed
   interest in alternatives such as the steady state theory. The CMB 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 400,000 years after the Big Bang — this time period
   is generally known as the "time of last scattering" or the period of
   recombination or decoupling — the temperature of the Universe was about
   3,000 K. This corresponds to an energy of about 0.25 eV, which is much
   less than the 13.6 eV ionization energy of hydrogen. Since then, the
   temperature of the radiation has dropped by a factor of roughly 1100
   due to the expansion of the Universe. As the universe expands, the CMB
   photons are redshifted, making the radiation's temperature inversely
   proportional to the Universe's scale length. For details about the
   reasoning that the radiation is evidence for the Big Bang, see Cosmic
   background radiation of the Big Bang.
   The power spectrum of the cosmic microwave background radiation
   anisotropy in terms of the angular scale (or multipole moment). The
   data shown come from the WMAP (2006), Acbar (2004) Boomerang (2005),
   CBI (2004) and VSA (2004) instruments.
   Enlarge
   The power spectrum of the cosmic microwave background radiation
   anisotropy in terms of the angular scale (or multipole moment). The
   data shown come from the WMAP (2006), Acbar (2004) Boomerang (2005),
   CBI (2004) and VSA (2004) instruments.

Primary anisotropy

   The anisotropy of the cosmic microwave background is divided into two
   sorts: primary anisotropy – which is due to effects which occur at the
   last scattering surface and before – and secondary anisotropy – which
   is due to effects, such as interactions with hot gas or gravitational
   potentials, between the last scattering surface and the observer.

   The structure of the cosmic microwave background anisotropies is
   principally determined by two effects: acoustic oscillations and
   diffusion damping (also called collisionless damping or Silk damping).
   The acoustic oscillations arise because of a competition in the photon-
   baryon plasma in the early universe. The pressure of the photons tends
   to erase anisotropies, whereas the gravitational attraction of the
   baryons – which are moving at speeds much less than the speed of light
   – makes them tend to collapse to form dense haloes. These two effects
   compete to create acoustic oscillations which give the microwave
   background its characteristic peak structure. The peaks correspond,
   roughly, to resonances in which the photons decouple when a particular
   mode is at its peak amplitude.

   The peaks contain interesting physical signatures. The first peak
   determines the curvature of the Universe (but not the topology of the
   Universe). The second peak – truly the ratio of the odd peaks to the
   even peaks – determines the reduced baryon density. The third peak can
   be used to extract information about the dark matter density.

   The locations of the peaks also gives important information about the
   nature of the primordial density perturbations. There are two
   fundamental types of density perturbations -- called "adiabatic" and
   "isocurvature." A general density perturbation is a mixture of these
   two types, and different theories that purport to explain the
   primordial density perturbation spectrum predict different mixtures.
     * For adiabatic density perturbations, the fractional overdensity in
       each matter component ( baryons, photons ...) is the same. That is,
       if there is 1% more energy in baryons than average in one spot,
       then with a pure adiabatic density perturbations there is also 1%
       more energy in photons, and 1% more energy in neutrinos, than
       average. Cosmic inflation predicts that the primordial
       perturbations are adiabatic.

     * With isocurvature density perturbations, the sum of the fractional
       overdensities is zero. That is, a perturbation where at some spot
       there is 1% more energy in baryons than average, 1% more energy in
       photons than average, and 2% lower energy in neutrinos than
       average, would be a pure isocurvature perturbation. Cosmic strings
       would produce mostly isocurvature primordial perturbations.

   The CMB spectrum is able to distinguish these two because these two
   types of perturbations produce different peak locations. Isocurvature
   density perturbations produce a series of peaks whose angular scales
   (l-values of the peaks) are roughly in the ratio 1 : 3 : 5 ..., while
   adiabatic density perturbations produce peaks whose locations are in
   the ratio 1 : 2 : 3 ... Observations are consistent with the primordial
   density perturbations being entirely adiabatic, providing key support
   for inflation, and ruling out many models of structure formation
   involving, for example, cosmic strings.

   Collisionless damping is caused by two effects, when the treatment of
   the primordial plasma as a fluid begins to break down:
     * the increasing mean free path of the photons as the primordial
       plasma becomes increasingly rarefied in an expanding universe
     * the finite thickness of the last scattering surface (LSS), which
       causes the mean free path to increase rapidly during decoupling,
       even while some Compton scattering is still occurring.

   These effects contribute about equally to the suppression of
   anisotropies on small scales, and give rise to the characteristic
   exponential damping tail seen in the very small angular scale
   anisotropies.

   The thickness of the LSS refers to the fact that the decoupling of the
   photons and baryons does not happen instantaneously, but instead
   requires an appreciable fraction of the age of the Universe up to that
   era. One method to quantify exactly how long this process took uses the
   photon visibility function (PVF). This function is defined so that,
   denoting the PVF by P(t), the probability that a CMB photon last
   scattered between time t and t+dt is given by P(t)dt.

   The maximum of the PVF (the time where it is most likely that a given
   CMB photon last scattered) is known quite precisely. The first-year
   WMAP results put the time at which P(t) is maximum as 372 +/- 14 kyr .
   This is often taken as the "time" at which the CMB formed. However, to
   figure out how long it took the photons and baryons to decouple, we
   need a measure of the width of the PVF. The WMAP team finds that the
   PVF is greater than half of its maximum value (the "full width at half
   maximum", of FWHM) over an interval of 115 +/- 5 kyr. By this measure,
   decoupling took place over roughly 115,000 years, and when it was
   complete, the universe was roughly 487,000 years old.

Late time anisotropy

   After the creation of the CMB, it is modified by several physical
   processes collectively referred to as late-time anisotropy or secondary
   anisotropy. After the emission of the CMB, ordinary matter in the
   universe was mostly in the form of neutral hydrogen and helium atoms,
   but from observations of galaxies it seems that most of the volume of
   the intergalactic medium (IGM) today consists of ionized material
   (since there are few absorption lines due to hydrogen atoms). This
   implies a period of reionization in which the material of the universe
   breaks down into hydrogen ions.

   The CMB photons scatter off free charges such as electrons that are not
   bound in atoms. In an ionized universe, such electrons have been
   liberated from neutral atoms by ionizing (ultraviolet) radiation. Today
   these free charges are at sufficiently low density in most of the
   volume of the Universe that they do not measurably affect the CMB.
   However, if the IGM was ionized at very early times when the universe
   was still denser, then there are two main effects on the CMB:
    1. Small scale anisotropies are erased (just as when looking at an
       object through fog, details of the object appear fuzzy).
    2. The physics of how photons scatter off free electrons ( Thomson
       scattering) induces polarization anisotropies on large angular
       scales. This large angle polarization is correlated with the large
       angle temperature perturbation.

   Both of these effects have been observed by the WMAP satellite,
   providing evidence that the universe was ionized at very early times,
   at a redshift of larger than 17. The detailed provenance of this early
   ionizing radiation is still a matter of scientific debate. It may have
   included starlight from the very first population of stars ( population
   III stars), supernovae when these first stars reached the end of their
   lives, or the ionizing radiation produced by the accretion disks of
   massive black holes.

   The period after the emission of the cosmic microwave background and
   before the observation of the first stars is semi-humorously referred
   to by cosmologists as the dark age, and is a period which is under
   intense study by astronomers (See 21 centimeter radiation).

   Other effects that occur between reionization and our observation of
   the cosmic microwave background which cause anisotropies include the
   Sunyaev-Zel'dovich effect, in which a cloud of high energy electrons
   scatters the radiation, transferring some energy to the CMB photons,
   and the Sachs-Wolfe effect, which causes photons from the cosmic
   microwave background to be gravitationally redshifted or blue shifted
   due to changing gravitational fields.
   E polarization measurements as of March 2006 in terms of angular scale
   (or multipole moment). The polarization is much more poorly measured
   than the temperature anisotropy.
   Enlarge
   E polarization measurements as of March 2006 in terms of angular scale
   (or multipole moment). The polarization is much more poorly measured
   than the temperature anisotropy.

Polarization

   The cosmic microwave background is polarized at the level of a few
   microkelvins. There are two types of polarization, called E-modes and
   B-modes. This is in analogy to electrostatics, in which the electric
   field (E-field) has a vanishing curl and the magnetic field (B-field)
   has a vanishing divergence. The E-modes arise naturally from Thomson
   scattering in an inhomogeneous plasma. The B-modes, which have not been
   measured and are thought to have an amplitude of at most a 0.1 µK, are
   not produced from the plasma physics alone. They are a signal from
   cosmic inflation and are determined by the density of primordial
   gravitational waves. Detecting the B-modes will be extremely difficult,
   particularly given that the degree of foreground contamination is
   unknown, and the weak gravitational lensing signal mixes the relatively
   strong E-mode signal with the B-mode signal.

Microwave background observations

   The design of cosmic microwave background experiments is a very
   challenging task. The greatest problems are:
     * Detectors The challenge of observing differences of a few
       microkelvins on top of a 2.7 K signal is difficult. Many improved
       microwave detector technologies have been designed for microwave
       background applications. Some technologies used are HEMT, MMIC, SIS
       (Superconductor-Insulator-Superconductor) and bolometers.
       Experiments generally use elaborate cryogenic systems to keep the
       detectors cool. Often, experiments are interferometers which only
       measure the spatial fluctuations in signals on the sky, and are
       insensitive to the average 2.7 K background. Another problem is the
       1/f noise intrinsic to all detectors. Usually the experimental scan
       strategy is designed to minimize the effect of such noise.
     * Optics To minimize side lobes, microwave optics usually utilize
       elaborate lenses and feed horns.
     * Water vapor Because water absorbs microwave radiation (a fact
       utilized in the operation of microwave ovens), it is rather
       difficult to observe the microwave background with ground-based
       instruments. CMB research therefore makes increasing use of air and
       space-borne experiments. Ground-based observations are usually made
       from dry, high altitude locations such as the Chilean Andes and the
       South Pole.

Analyses

   The analysis of cosmic microwave background data to produce maps, an
   angular power spectrum and ultimately cosmological parameters is a
   complicated, computationally difficult problem. Although computing a
   power spectrum from a map is in principle a simple Fourier transform,
   decomposing the map of the sky into spherical harmonics, in practice it
   is hard to take the effects of noise and foregrounds into account.
   Constraints on many cosmological parameters can be obtained from their
   effects on the power spectrum, and results are often calculated using
   Markov Chain Monte Carlo sampling techniques.

Low multipoles

   With the increasingly precise data provided by WMAP, there have been a
   number of claims that the CMB suffers from anomalies, such as
   non-gaussianity. The most longstanding of these is the low-l multipole
   controversy. Even in the COBE map, it was observed that the quadrupole
   (l = 2 spherical harmonic) has a low amplitude compared to the
   predictions of the big bang. Some observers have pointed out that the
   anisotropies in the WMAP data did not appear to be consistent with the
   big bang picture. In particular, the quadrupole and octupole (l = 3)
   modes appear to have an unexplained alignment with each other and with
   the ecliptic plane. A number of groups have suggested that this could
   be the signature of new physics at the largest observable scales.
   Ultimately, due to the foregrounds and the cosmic variance problem, the
   largest modes will never be as well measured as the small angular scale
   modes. The analyses were performed on two maps that have had the
   foregrounds removed as best as is possible: the "internal linear
   combination" map of the WMAP collaboration and a similar map prepared
   by Max Tegmark and others. Later analyses have pointed out that these
   are the modes most susceptible to foreground contamination from
   synchrotron, dust and free-free emission, and from experimental
   uncertainty in the monopole and dipole. While the low quadrupole does
   appear to be robust (The measured value has a likelihood of roughly
   2–4% in the Lambda-CDM model.), carefully accounting for the procedure
   used to remove the foregrounds from the full sky map reduces the
   significance of the alignment, and may suggest that it is due to
   foreground contamination.

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