What is the cosmic microwave background radiation?
Erik M. Leitch of the University of Chicago explains. The Cosmic Microwave Background radiation, or CMB for short, is a faint glow of light that fills the universe, falling on Earth from every direction with nearly uniform intensity. It is the residual heat of creation–the afterglow of the big bang–streaming through space these last 14 billion years like the heat from a sun-warmed rock, reradiated at night. Since the early twentieth century, two concepts have transformed the way astronomers think about observing the universe. The first is that it is fantastically large; the portion of the universe visible today is a sphere nearly 15 billion light-years in radius, and that, we believe, is just the tip of the iceberg. The second is that light travels at a fixed speed. A simple consequence of these ideas is that as you look at more and more distant objects, you’re seeing farther and farther back in time–sometimes very far back indeed. When you see Jupiter shining in the night sky, for
In 1964, two Bell Laboratories scientists, Arno Penzias and Robert Wilson, accidentally discovered that the earth is bathed in low-intensity microwave electromagnetic radiation coming equally from all directions. The intensities and wavelengths of these microwaves are characteristic of thermal radiation. Everything emits thermal radiation. Any object at a particular temperature emits thermal radiation with a particular set of intensities and wavelengths. For example, a white-hot light bulb filament at 3000°C emits radiation at high intensities and short wavelengths all across the visible light part of the electromagnetic spectrum. A red-hot coal at 700° C emits lower intensity radiation at longer wavelengths, some of it at the red end of the visible light part of the spectrum and some of it at invisible “infrared” wavelengths. A soldier’s body at 37°C emits yet lower intensities and longer wavelengths; his thermal radiation is not visible to the eye, being fully in the infrared part of
