Cosmic microwave background fluctuations
The remarkable isotropy of the cosmic microwave background (CMB) was the first real evidence that Einstein’s cosmological principle is a good approximation for the universe. The first deviation from perfect isotropy was found in the 1970s, with the discovery of the CMB “dipole” anisotropy: the background is slightly hotter in one direction of the sky and cooler in the opposite direction. This is due to our galaxy’s motion at a velocity of about 600 km s–1, as a result of the combined gravitational pull of galaxies and clusters within about 300 million light years of us.
In 1992 the team working with COBE, the Cosmic Background Explorer satellite, found fluctuations in the temperature of the cosmic microwave background, at a level of about one part in 100,000. This was the crucial clue to how galaxies and clusters of galaxies formed in the universe. A later mission, WMAP, made precision measurements of the CMB fluctuations from which researchers were able to determine several cosmological parameters (density of ordinary and dark matter, the cosmological constant, age of the universe, Hubble constant) rather precisely. So today a tight consensus exists on the kind of universe we inhabit, with just 4% being in the form of ordinary matter, 21% in the form of dark matter, and 75% in the form of dark energy.
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