By B. D. Wandelt; glossary and references added by Karen Y. Ng.
Since its Nobel-prize winning discovery in the late 1960s by Penzias and
Wilson, the CMB has become the cornerstone of cosmological astrophysics.
This radiation was emitted when the Universe was only 300,000 years old.
It therefore provides a snapshot of the early Universe 40,000 times younger
than it is now. Its main feature is its remarkably uniform brightness in
all directions on which small fluctuations are imprinted at the level of
1 part in 105 (the anisotropies). These anisotropies
were first reliably detected at low resolution by the COBE satellite
in the early 1990s. George Smoot (my
collaborator on the Planck space mission) received the Nobel
Prize in Physics for this discovery in 2006, sharing the prize with John
Mather, the principal investigator of the COBE missionand
the leader of FIRAS, the instrument that showed that the
CMB is a near-perfect blackbody.
Since then, COBE's observations have been confirmed and
vastly extended by a range of other instruments from the ground, the upper
atmosphere and space. Very recently, the presence of anisotropies
in the CMB's polarization pattern has been discovered, in
accordance with theoretical expectations. These polarization
anisotropies, while even fainter than the temperature anisotropies,
carry valuable additional information. Extracting the correlation properties
of these anisotropies on the celestial sphere reveals sensitive
measures of the global properties of the Universe, such as its overall density
and geometry, its composition, and its properties in the very first instants
of time.
This information is quantitatively encoded in cosmological parameters.
Like no other cosmological observation, detailed analyses of the CMB anisotropies
allow determining the structure, properties and ingredients of the Universe
on super-large scales and at very early times, the basis for any theoretical
description of cosmic history.
These direct observations of the 300,000-year-old Universe can be linked
with great confidence to the cosmological parameters, because the physics
that governs the CMB anisotropies is conceptually simple.
Just as we can infer much about when, where and how the surface of a lake
has been disturbed by the way a wave crests and troughs arrive at the shore,
we can use the statistical properties of the CMB temperature anisotropies
to deduce much about the history and nature of perturbations at earlier
times, all the way back to the era where our current understanding of physics
breaks down: the Planck Time.
The immediacy of the impact of CMB observations on our knowledge of the
global properties of the Universe and the earliest moments of creation has
led to an explosion of interest in this field, both theoretically and observationally.
On the theoretical side, fundamental particle physics theory increasingly
looks to cosmology for guidance on the way to a unified theory of all interactions.
On the observational side, the United States leads an international effort
to generate high quality CMB data. The culmination of this effort is in
a series of CMB observatories in space currently headed by the successfully
operating Wilkinson Microwave Anisotropy Probe (WMAP), and the development
of the Planck satellite mission.
In a globally isotropic universe the information is not contained in localized
features of the CMB anisotropies, such as the absolute
placement and shape of individual hot and cold spots, but in the overall
pattern or texture of the field. The information is contained only in properties
of the field that depend on the relative angular distance between
two locations in the sky. Mathematically, if the CMB anisotropies
are a Gaussian random field, where 2-point statistics specify
all higher order moments, this means that the angular power spectrum
coefficients Cl of the anisotropies
contain all of the information. Within the standard paradigm of cosmology
the universe is isotropic and the primordial fluctuations, and hence
the CMB, are Gaussian. Cosmological theory predicts
the Cl given a set of cosmological parameters.
Measuring the power spectrum Cl
is therefore the main goal of all CMB experiments.
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Physical properties
that are different in different directions, e.g. the strength of wood along
the grain differing from that across the grain.
There are anisotropies
in the CMB as CMB radiation fluctuates a little bit over the different directions
in the sky.
A US astronomical
satellite launched in 1989 to study cosmic background radiation. It confirmed
the Big Bang theory of the universe's creation when its Far Infrared Absolute
Spectrophotometer (FIRAS) measured the afterglow of the Big Bang. In 1992
it confirmed that radiation has a black body spectrum of temperature 2.73
K (-270.4 °C) and revealed ripples in the background radiation believed
to mark the first stage in galaxy formation. Its final instrument was turned
off on 23 December 1993.
A European Space Agency
mission scheduled for launch in 2007 to examine details of the cosmic background
radiation that was the first electromagnetic radiation to fill the Universe
after the Big Bang. Planck carries a telescope of 1.5 m diameter with detectors
that will map the cosmic microwave background with far greater resolution
and sensitivity than before. It will be positioned at the L2 Lagrangian point
of the Earth's orbit, 1.5 million km from Earth in a direction opposite that
of the Sun. Planck's results should help evaluate important cosmological values
such as the Hubble constant as well as elucidating the nature of the dark
matter that dominates the present Universe.
A hypothetical body
that absorbs all the radiation falling on it. It thus has an absorptance and
an emissivity of 1. While a true black body is an imaginary concept, a small
hole in the wall of an enclosure at uniform temperature is the nearest approach
that can be made to it in practice.Black-body radiation is the electromagnetic
radiation emitted by a black body. It extends over the whole range of wavelengths
and the distribution of energy over this range has a characteristic form with
a maximum at a certain wavelength. The position of the maximum depends on
temperature, moving to shorter wavelengths with increasing temperature. See
Stefan's law; Wien's displacement law.
In fact, the CMB radiation
is the best blackbody that has ever been observed. For more information, see
http://lambda.gsfc.nasa.gov/product/cobe/firas_image.cfm
The phenomenon in
which electromagnetic waves, such as light waves, vibrate in a preferred plane
or planes; or the process of confining the vibrations to certain planes. In
unpolarized light the vibrations are equally distributed in all directions
perpendicular to the direction of propagation of the wave. If all the vibrations
are confined to one plane, the light is said to be plane-polarized (or linearly
polarized). If the light in one plane is out of phase with the light in the
plane at right angles to it (i.e. if the peaks and troughs of the waves are
not in step), then the light is said to be circularly polarized. If all these
phenomena occur together, the light is said to be elliptically polarized.
Plane polarization is usually caused by scattering, and circular polarization
by strong magnetic fields. Circularly and elliptically polarized light can
also be produced by a wave plate .
Anisotropies in the
CMB are partially polarized . This polarization contains a large amount of
extra information about the properties of the universe and its initial conditions.
A NASA probe designed
to make a full-sky map of temperature fluctuations in the cosmic background
radiation with much higher resolution, sensitivity, and accuracy than NASA's
1992 Cosmic Background Explorer satellite. WMAP detects anisotropy (tiny fluctuations)
in the temperature of the cosmic background radiation to provide a detailed
picture of the early universe. Its scientific observation point lies about
1.5 million km from the Earth, in order to prevent interference from terrestrial
or solar microwave emissions. WMAP was launched on 30 June 2001 for an 18-month
mission. In 2002 it completed full sky scans in April and August, and the
WMAP team extended its mission for several years.
Gaussian random field
is a random spatial pattern. It can be defined by the joint probability
density of its values at all space points. As an example, a random field can
be the dark and bright patterns of floor tiles or marble or wood patterns.
Even though both a piece of marble and a piece of wood both have patterns,
the form/shape of these patterns distinguish a piece of wood from a piece of marble.
The joint probability density is a function that describes the (statistical) nature of the pattern.
The NASA website contains a video clip about where the image is obtained.
The distribution of
the energy of a waveform among its different frequency components.
The power spectrum is
one of the properties of a random field. For a Gaussian random field without preferred direction
the power spectrum tells you everything there is to now about the average properties of the field. The picture below shows the angular power spectrum of the
CMB data. It is a plot of the temperature fluctuation (like the color spots
on the CMB image above) against the angular size of the fluctuation. The
shape of the power spectrum tells us conditions of the early universe and the cosmological parameters.
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The Cosmic Microwave Background
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