Introduction
The Cosmic Microwave Background (CMB) radiation is the thermal radiation left over from the Big Bang. First detected inadvertently in 1965 by Arno Penzias and Robert Wilson, it provides crucial evidence for the Big Bang model of the universe.
Discovery and Significance
The discovery of the CMB was a landmark event in the development of Big Bang cosmology. Its detection confirmed a key prediction of the Big Bang theory, providing strong evidence that the universe began in a hot, dense state.
Characteristics of the CMB
The CMB is a nearly uniform, isotropic radiation field that fills the universe. It has a blackbody temperature of approximately 
kelvin and is isotropic to roughly one part in 
.
Anisotropies and the CMB Power Spectrum
Small variations, or anisotropies, in the temperature of the CMB provide important clues about the early universe and the formation of structures within it. These anisotropies are characterized by a power spectrum, which shows the amplitude of fluctuations as a function of their scale.
The power spectrum of the CMB is given by:
where 
is the power spectrum, 
are the coefficients of the spherical harmonic decomposition of the temperature pattern, and 
is the multipole moment related to the angular scale.
Cosmic Inflation and the CMB
The pattern of anisotropies in the CMB provides strong evidence for a period of rapid expansion in the early universe known as cosmic inflation. Inflation explains why the CMB is nearly isotropic and why its anisotropies have the specific pattern observed.
Conclusion
The CMB continues to be a rich source of information about the universe’s past, present, and future. Ongoing and future CMB experiments aim to refine our understanding of cosmic inflation, the nature of dark matter and dark energy, and other fundamental questions in cosmology.
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