In every direction, there is a very low energy and very uniform radiation that we see filling the Universe. This is called the 3 Degree Kelvin Background Radiation, or the Cosmic Background Radiation, or the Microwave Background. These names come about because this radiation is essentially a black body with temperature slightly less than 3 degrees Kelvin (about 2.76 K), which peaks in the microwave portion of the spectrum. This radiation is the strongest evidence for the validity of the hot big bang model. The adjacent figure shows the essentially perfect blackbody spectrum obtained by NASA's Cosmic Background Explorer (COBE) satellite.
The following image was taken by COBE. It shows the temperature of the cosmic background radiation plotted in galactic coordinates, with red cooler and blue and violet hotter (Ref). This dipole anisotropy is because of the Doppler effect. If the Earth moves with respect to the microwave background, it will be blue shifted to a higher effective temperature in the direction of the Earth's motion and red shifted to a lower effective temperature in the direction opposite the Earth's motion.
The indication of the above image is that the local group of galaxies, to which the Earth belongs, is moving at about 600 km/s with respect to the background radiation. It is not know why the Earth is moving with such a high velocity relative to the background radiation.
Evidence for the Big Bang
The cosmic background radiation (sometimes called the CBR), is the afterglow of the big bang, cooled to a faint whisper in the microwave spectrum by the expansion of the Universe for 15 billion years (which causes the radiation originally produced in the big bang to redshift to longer wavelengths). As shown in the adjacent intensity map of the background radiation in different directions taken by the Differential Microwave Radiometer on NASA's COBE satellite, it is not completely uniform, though it is very nearly so (Ref). To obtain this image, the average dipole anisotropy exhibited in the image above has been subtracted out, since it represents a Doppler shift due to the Earth's motion. Thus, what remains should represent true variations in the temperature of the background radiation.
In this image, red denotes hotter fluctuations and blue and black denote cooler fluctuations around the average. These fluctuations are extremely small, representing deviations from the average of only about 1/100,000 of the average temperature of the observed background radiation.
Problems with the Uniformity
The highly isotropic nature of the cosmic background radiation indicates that the early stages of the Universe were almost completely uniform. This raises two problems for the big bang theory.
First, when we look at the microwave background coming from widely separated parts of the sky it can be shown that these regions are too separated to have been able to communicate with each other even with signals travelling at light velocity. Thus, how did they know to have almost exactly the same temperature? This general problem is called the horizon problem.
Second, the present Universe is homogenous and isotropic, but only on very large scales.