text: The “Big Bang” really refers to one simple hypothesis:  that the Universe is expanding from a state when it was once much hotter and denser than it is today.  One can think of a “zero point” at which the Universe was unimaginably tiny, etc., and that point is what we often refer to as the “Big Bang,” but that presupposes a bit more than we need to.\n\nSo…  If the Universe was indeed once much hotter and denser than it is today, it may have been too hot and too dense for light to travel very far—a bit like the interior of a cloud, which is too dense for us to see inside.  We would then only “see” (i.e., be able to detect) radiation from a time after that epoch, after the Universe had cooled to the point that photons could zip around relatively unimpeded, as they do today.  That point in time corresponds to the equivalent of the surface of a cloud in our sky—also called the “surface of last scattering,” which isn’t a distinct, smooth surface at all, but kind of a fuzzy region that represents the limit of how far we can see into the cloud.  (Or, in the case of the Universe, the surface of last scattering, is the fuzzy boundary between when the Universe was too hot for light to travel far and when it was cool enough for photons to travel basically inifintely far.)\n\nSo far, so good (I hope).  It turns out that we can calculate the temperature at which this transition would took place—something like 3,000 Kelvins (or 5,000° Fahrenheit).  Something that hot should radiate light in a highly characteristic way, which we call “blackbody” radiation—the same effect that causes the Sun or other stars or a red-hot poker to glow a certain color.  As it turns out, a 3,000-Kelvin blackbody would glow reddish-orange—not just a single wavelength that we’d call “reddish-orange,” but a collection of wavelengths that peak around the color reddish-orange.\n\nBut wait!  The Universe is expanding, and light expands with it.  So that reddish-orange gets redshifted down into microwave light.  COBE and WMAP detect exactly the kind of microwave light we’d expect from a blackbody some 13 billion years in the past.  This is a remarkable confirmation of the Big Bang hypothesis.  Furthermore, we can look at the characteristics of the blackbody “curve” (another name for the collection of wavelengths of light emitted by stuff of a certain temperature) and determine lots of other things about the early Universe.  Including the time that the light was emitted, about 370,000 years after the “Big Bang.”\n\nOf course, other sources of microwave radiation get in the way, but because we’re looking for a collection of wavelengths (not just a single color, but a collection of colors that correspond to a specific temperature), we can weed out interference.  So we can determine which photons have travelled to us all the way from the early Universe, some 13 billion years ago.\n\nThe WMAP website explains all this pretty well, if you want another description:\nhttp://map.gsfc.nasa.gov/m_mm.html\n\nI hope that helps…\n\n\nRyan Wyatt\nRose Center for Earth & Space\nNew York, New York