1 Cosmic Microwave Background Radiation
In cosmology, the cosmic microwave background radiation (most often abbreviated CMB but
occasionally CMBR, CBR or MBR) 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.
1.1 Features
The cosmic microwave background is isotropic to roughly one part in 100,000: the root mean
square variations are only 18 µK.[1] 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
The cosmic microwave background is a prediction of the Big Bang. 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, the
cosmological redshift caused the plasma to cool until it became favorable 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 continued cooling until they reached their present 2.725 K temperature. 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 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.[3]
Two of the greatest successes of the big bang 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.[4] 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.[5]
1.2 History
The cosmic microwave background was predicted by George Gamow, Ralph Alpher, and Robert
Herman in 1948. Moreover, Alpher and Herman were able to estimate the temperature of the
cosmic microwave background to be 5 K.[9] 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.[10]
The results of Gamow were not widely discussed. However, they were rediscovered by Robert Dicke
and Yakov Zel’dovich in the early 1960s. In 1964, this prompted David Todd Wilkinson and Peter
Roll, Dicke’s colleagues at Princeton University, to begin constructing a Dicke radiometer to
measure the cosmic microwave background[11]. In 1965, Arno Penzias and Robert Woodrow
Wilson at Bell Telephone Laboratories in nearby Holmdel, 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 Holmdel, Dicke famously
quipped: ”Boys, we’ve been scooped.”[12] A meeting between the Princeton and Holmdel
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 temperatureË¡ of
interstellar space is 2 K.”[13] 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.
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.[14] Rashid Sunyaev later calculated the observable
imprint that these inhomogeneities would have on the cosmic microwave background.[15]
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.[16]
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.[17]. These measurements demonstrated
that the Universe is flat and were able to rule out cosmic strings as a theory 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.
1.3 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.
1.4 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.
1.5 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 shape 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.
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, 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 supression of anisotropies on small scales, and give
rise to the characteristic exponential damping tail seen in the very small angular scale
anisotropies.
1.6 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.
1.7 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.[18]
1.8 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 that is used to build 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.
1.9 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.
1.10 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.[19] 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.[20] 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
1.11 References
[1] This ignores the dipole anisotropy, which is due to the Doppler shift of the microwave
background radiation due to our peculiar velocity relative to the comoving cosmic rest frame. This
feature is consistent with the Earth moving at some 380 km/s towards the Constellation
Virgo.
[2] D. J. Fixen et al., ”The Cosmic Microwave Background Spectrum from the full COBE FIRAS
data set”, Astrophysical Journal 473, 576–587 (1996).
[3] The energy density of a black-body spectrum is πkB2T4∕15(ℏc)3, where T is the temperature,
kB is the Boltzmann constant, ℏ is the Planck constant and c is the speed of light. This can be
related to the critical density of the universe using the parameters of the Lambda-CDM
model.
[4] Astrophysical Journal Supplement, 148 (2003). In particular, G. Hinshaw et al. ”First-year
Wilkinson Microwave Anisotropy Probe (WMAP) observations: the angular power spectrum”,
135–159.
[5] D. N. Spergel et al., ”First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations:
determination of cosmological parameters”, Astrophysical Journal Supplement 148, 175–194
(2003).
[6] Helge Kragh, Cosmology and Controversy: The Historical Development of Two Theories of the
Universe (1999) ISBN 0-691-00546-X
[7] George Gamow, The Creation Of The Universe p.50 (Dover reprint of revised 1961 edition)
ISBN 0-486-43868-6
[8] J. Kovac et al., ”detection of polarization in the cosmic microwave background using DASI”,
Nature 420, 772-787 (2002).
[9] G. Gamow, ”The Origin of Elements and the Separation of Galaxies,” Physical Review 74
(1948), 505. G. Gamow, ”The evolution of the universe”, Nature 162 (1948), 680. R. A. Alpher and
R. Herman, ”On the Relative Abundance of the Elements,” Physical Review 74 (1948),
1577.
[10] A. K. T. Assis, M. C. D. Neves, ”History of the 2.7 K Temperature Prior to Penzias and
Wilson,” (1995, pdf — HTML) but see also N. Wright, ”Eddington did not predict the CMB”,
[1].
[11] R. H. Dicke, ”The measurement of thermal radiation at microwave frequencies”, Rev. Sci.
Instrum. 17, 268 (1946). This basic design for a radiometer has been used in most subsequent
cosmic microwave background experiments.
[12] A. A. Penzias and R. W. Wilson, ”A Measurement of Excess Antenna Temperature at 4080
MC/,” Astrophysical Journal 142 (1965), 419. R. H. Dicke, P. J. E. Peebles, P. G. Roll and D. T.
Wilkinson, ”Cosmic Black-Body Radiation,” Astrophysical Journal 142 (1965), 414. The history is
given in P. J. E. Peebles, Principles of physical cosmology (Princeton Univ. Pr., Princeton
1993).
[13] A. McKellar, Publ. Dominion Astrophys. Obs. 7, 251.
[14] E. R. Harrison, ”Fluctuations at the threshold of classical cosmology,” Phys. Rev. D1 (1970),
2726. P. J. E. Peebles and J. T. Yu, ”Primeval adiabatic perturbation in an expanding universe,”
Astrophysical Journal 162 (1970), 815. Ya. B. Zel’dovich, ”A hypothesis, unifying the structure
and entropy of the universe,” Monthly Notices of the Royal Astronomical Society 160
(1972).
[15] R. A. Sunyaev, ”Fluctuations of the microwave background radiation,” in Large Scale
Structure of the Universe ed. M. S. Longair and J. Einasto, 393. Dordrecht: Reidel 1978. While this
is the first paper to discuss the detailed observational imprint of density inhomogeneities as
anisotropies in the cosmic microwave background, some of the groundwork was laid in Peebles and
Yu, above.
[16] G. F. Smoot et al. ”Structure in the COBE DMR first year maps”, Astrophysical Journal
396 L1–L5 (1992). C. L. Bennett et al. ”Four year COBE DMR cosmic microwave
background observations: maps and basic results.”, Astrophysical Journal 464 L1–L4
(1996).
[17] A. D. Miller et al., ”A measurement of the angular power spectrum of the cosmic microwave
background from l = 100 to 400”, Astrophysical Journal 524, L1–L4 (1999). A. E. Lange et al.,
”Cosmological parameters from the first results of Boomerang”. P. de Bernardis et al., ”A flat
universe from high-resolution maps of the cosmic microwave background”, Nature 404,
955 (2000). S. Hanany et al. ”MAXIMA-1: A measurement of the cosmic microwave
background anisotropy on angular scales of 10’-5°”, Astrophysical Journal 545 L5–L9
(2000).
[18] A. Lewis and A. Challinor (2006). ”Weak gravitational lensing of the CMB”. Phys. Rep.. (to
appear)
[19.] A. de Oliveira-Costa, M. Tegmark, M. Zaldarriga and A. Hamilton (2004). ”The
significance of the largest scale CMB fluctuations in WMAP”. Phys. Rev. D69: 063516.
arXiv:astro-ph/0307282. D. J. Schwarz, G. D. Starkman, D. Huterer and C. J. Copi
(2004). ”Is the low-l microwave background cosmic?”. Phys. Rev. Lett. 93: 221301.
arXiv:astro-ph/0403353. P. Bielewicz, K. M. Gorski and A. J. Banday (2004). ”Low-order
multipole maps of CMB anisotropy derived from WMAP”. Mon. Not. Roy. Astron. Soc. 355: 1283.
arXiv:astro-ph/0405007.
[20] C. L. Bennett et al. (WMAP collaboration) (2003). ”First-year Wilkinson Microwave
Anisotropy Probe (WMAP) observations: preliminary maps and basic results”. Astrophysical
Journal Supplement 148: 1. arXiv:astro-ph/0302207. G. Hinshaw et al. (WMAP collaboration)
(March 2006). ”Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations:
temperature analysis”. preprint. M. Tegmark, A. de Oliveira-Costa and A. Hamilton
(2003). ”A high resolution foreground cleaned CMB map from WMAP”. Phys. Rev. D68:
123523. arXiv:astro-ph/0302496. The first year WMAP paper warns: ”the statistics of
this internal linear combination map are complex and inappropriate for most CMB
analyses.” The third year paper states: ”Not surprisingly, the two most contaminated
multipoles are [the quadrupole and octopole], which most closely trace the galactic plane
morphology.”
[21] A. Slosar and U. Seljak (2004). ”Assessing the effects of foregrounds and sky removal in
WMAP”. Phys. Rev. D70: 083002. arXiv:astro-ph/0404567. P. Bielewicz, H. K. Eriksen, A. J.
Banday, K. M. Gorski and P. B. Lilije (2005). ”Multipole vector anomalies in the first-year WMAP
data: a cut-sky analysis”. Astrophys. J. 635: 750–60. arXiv:astro-ph/0507186. C. J. Copi, D.
Hueterer, D. J. Schwarz and G. D. Starkman (2006). ”On the large-angle anomalies of the
microwave sky”. Mon. Not. Roy. Astron. Soc. 367: 79–102. arXiv:astro-ph/0508047. A. de
Oliveira-Costa and M. Tegmark (2006). ”CMB multipole measurements in the presence of
foregrounds”. preprint. arXiv:astro-ph/0603369.
This entry is a derivative of the cosmic microwave background radiation article from Wikipedia,
the Free Encyclopedia. Authors of the orginial article include: TexMurphy, Peripitus, Wdanwatts,
Kungfuadam and Profero. History page of the original is here
