Fluctuations In the CMB
The
adjacent intensity
map shows the temperature variations of the microwave background
in different directions
taken by the Differential Microwave Radiometer (DMR) on NASA's COBE satellite,
plotted in galactic coordinates. It
is not completely uniform, though it is very nearly so. In this plot, regions
in red are slightly hotter and regions in blue are slightly cooler than the
average temperature of the CMB.
To obtain this image, the average dipole
anisotropy has been subtracted out, since it represents a Doppler shift due to
the Earth's motion. In addition, emission from the plane of the galaxy has been suppressed
since it represents local and not cosmological effects.
Thus, what remains should represent true variations in the temperature of the
cosmic microwave background.
Tiny Anisotropies
In this image,
red denotes hotter fluctuations and blue denotes cooler
fluctuations around the average blackbody temperature. These fluctuations are extremely small.
The typical deviation from average in the positive and negative directions
represents a fractional difference of only about 0.00001 (one part in 100,000)
from the average temperature for the observed background radiation. As noted in the right panel,
the structures in the top right figure are the oldest objects
known in the Universe, dating from less than
400,000
years after its birth.
That is, the microwaves that created this image started their
journey to us as higher-energy radiation
about 400,000 years after the big bang. They have just now arrived, having been
redshifted
into the microwave portion of the spectrum by the expansion of
the Universe
during
their long journey.
