The Cosmic Microwave Background
The Cosmic Microwave Background
Most objects in the Universe emit radiation that is proportional to the temperature of that object. This type of light is called blackbody radiation and is emitted by stars, galaxies, and the Universe itself. Even humans emit a blackbody spectrum. If you put on some night-vision goggles (ones that are tuned to the wavelengths in which humans glow), you see a glow around a relatively warm person with respect to the cooler surroundings. At our body temperature, we humans radiate most of our energy in the infrared. Stars, galaxies, and the Universe radiate much of their energy in different regions of the spectrum—see “Electromagnetic Spectrum” for more information.
In modern cosmology, the Big Bang Theory best describes the origins of the Universe. Briefly, the theory suggests that origins of space and time, matter, and energy come from a time when the Universe was extremely hot and dense. A little less than 14 billion years ago, the Universe exploded, forming space itself, which would later be populated with the galaxies we see today.
Very soon after the Big Bang, matter in the Universe was so hot that it formed a plasma, a state in which atoms are separated into nuclei and free electrons, similar to the Sun's interior. These free electrons prohibit the free flow of light in the Universe. The Universe was opaque to light in these early times. Later, as the Universe expanded and cooled, the electrons combined with the nuclei and formed hydrogen atoms. With few free electrons, light was now free to travel the Universe. This transition took place about 379,000 years after the Big Bang, ushering in a new era in the Universe.
The Universe was then transparent to light as photons, which traveled freely across the Universe, encountering few collisions that would absorb or scatter the light. If we could have seen it, the Universe would have been about 3,000 Kelvin and would have glowed orange in visible light. In fact, the peak intensity of the Universe at that time was around 1 micron (in the infrared). Since that time, the Universe has expanded 1,000-fold and the light waves have expanded along with space. The light, then, appears redshifted as space itself expands. Today, the peak intensity of the Universe is 1,000 times lower than it was then, so the wavelength is 1,000 times greater and is close to 1 millimeter (1 micron x 1,000=1mm). This peak intensity corresponds to a gas temperature 1,000 times less, or 3 Kelvin (3,000 K÷1,000=3 K). Light at 1 mm is in the microwave portion of the EM spectrum, and this is where we see the remnant light from the Big Bang, called the Cosmic Microwave Background (CMB) radiation.
The CMB radiation pervades the Universe. It is an imprint of the time of recombination in the Universe, when protons and electrons combined to form hydrogen, when light began to travel in space unimpeded. When we look at that light, we are looking back in time to that event, 379,000 years after the Big Bang. Before that time, the Universe was opaque, so we can never see the Big Bang event itself.
The CMB, then, defines that part of the Universe that we can see, what we call our observable Universe. While the CMB is everywhere in the Universe, we can also think of it as our outer limit, that which we cannot see beyond.
The best observations of the CMB come from the Wilkinson Microwave Anisotropy Probe [learn more about this space telescope in the section on the WMAP all-sky survey]. The image is a snapshot, a baby picture if you will, of the early Universe, tracing out the farthest reaches of the observable Universe. It shows the minute variations in temperature that were present in the Universe at that early era. These temperature variations are on the order of millionths of degrees around the average temperature of 2.725 Kelvin. In the next section, we will see just how important these minute fluctuations are in determining the structure and evolution of the Universe.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott