As the Universe grows older, the observable Universe gets
bigger, due to the finite speed of light. The particle horizon of
an observer is the distance to the farthest object that could
have affected that observer. Any objects further than this point
are not, and never have been, in causal contact with the
observer. At the last scattering surface the particle horizon
corresponds to
as seen from Earth today. No physical processes will act on
scales larger than this. At the epoch of recombination,
fluctuations on scales larger than
must have been produced by matter perturbations already present
at this time. It is thought these fluctuations were probably laid
down only
seconds after the big bang, and have their origin in quantum
fluctuations of a scalar field. On smaller scales, corresponding
to regions in causal contact at recombination, we should be able
to see in the CMB the effects of physical processes occurring at
recombination, such as acoustic oscillation of the coupled
photon-baryon fluid, which thus also gives us a direct link with
the physics of galaxy formation.
In 1992, the NASA Cosmic Microwave Background Explorer (COBE)
satellite was the first experiment to detect the bumps [93]. These initial measurements were in the form of a statistical
detection rather than individual physical features (Some features
in the first year maps were real CMB anisotropies. but it was not
possible to distinguish these from the noise except in a
statistical way.). Today, experiments all around the world are
finding these bumps, both causally connected and non-causally
connected, that eventually grew into galaxies and clusters of
galaxies. The required sensitivities called for new techniques in
astronomy. The main principle behind all of these experiments is
that, instead of measuring the actual brightness, they measure
the difference in brightness between different regions of the
sky. The experiments at Tenerife produced what was probably the
first detection of the real, individual CMB fluctuations[52] on scales comparable to the beam size of the experiment. These
particular features were later confirmed by the COBE two-year
data[70].
There are many different theories of how the universe began its life and how it evolved into the structures seen today. Each of these theories make slightly different predictions of how the universe looked at the very early stages which up until now have been impossible to prove or disprove. Knowing the structure of the CMB, within a few years it should be possible for astronomers to tell us where the universe came from, how it developed and what will happen to it in the future.
Useful overall reviews of the physics of the production of CMB fluctuations, which will complement the informal presentation given in the next section, are contained in White et al., 1994 [98], Scott et al., 1995 [88] and Hu et al., 1995 [59]. This review is intended to be an extension and update of Lasenby and Jones, 1997 [68], and parts of that review are reproduced here so that the arguments are easily followed.
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The Cosmic Microwave Background
Aled W. Jones and Anthony N. Lasenby http://www.livingreviews.org/lrr-1998-11 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |