Published on May 18, 2008

Wilkinson Microwave Anisotropy Probe

The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA Explorer mission. It has produced a wealth of precise and accurate cosmological information. WMAP produced the first full-sky map of the microwave sky with a resolution of under a degree, about the angular size of the moon.

The patterns in the map result from well-understood physical processes that happened when the universe was young. By matching the patterns in the map to the physics we know, WMAP has produced a convincing consensus on the contents of the universe, erasing lingering doubts about the existence of dark energy, and severely limiting the density of hot dark matter.
WMAP has determined the age of the universe, the epochs of the key transitions of the universe, and the geometry of the universe, while providing the most stringent data yet on events in the first fraction of a second of the universe.

The Cosmic Microwave Background (CMB) radiation is the light left over from the Big Bang, shifted to microwave wavelengths due to the expansion of the universe. The whole universe is bathed in this afterglow light. This is the oldest light in the universe and has been traveling across the Universe for about 13.7 billion years. The light patterns across the sky encode a wealth of details about the history, shape, content, and ultimate fate of the Universe.

The Microwave Sky

The cosmic microwave temperature fluctuations from the 5-year WMAP data seen over the full sky. The average temperature is 2.725 Kelvin (degrees above absolute zero; equivalent to -270 C or -455 F), and the colors represent the tiny temperature fluctuations, as in a weather map. Red regions are warmer and blue regions are colder by about 0.0002 degrees.

Five Year Microwave Sky. The detailed, all-sky picture of the infant universe from three years of WMAP data. The image reveals 13.7 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. The signal from the our Galaxy was subtracted using the multi-frequency data. This image shows a temperature range of ± 200 microKelvin.
Credit: NASA / WMAP Science Team

Content of the Universe

WMAP measures the composition of the universe. The top chart shows a pie chart of the relative constituents today.
A similar chart (bottom) shows the composition at 380,000 years old (13.7 billion years ago) when the light WMAP observes emanated.

The composition varies as the universe expands: the dark matter and atoms become less dense as the universe expands, like an ordinary gas, but the photon and neutrino particles also lose energy as the universe expands, so their energy density decreases faster than the matter. They formed a larger fraction of the universe 13.7 billion years ago. It appears that the dark energy density does not decrease at all, so it now dominates the universe even though it was a tiny contributor 13.7 billion years ago.

Content of the Universe. WMAP data reveals that its contents include 4.6% atoms, the building blocks of stars and planets. Dark matter comprises 23% of the universe. This matter, different from atoms, does not emit or absorb light. It has only been detected indirectly by its gravity. 72% of the universe, is composed of "dark energy", that acts as a sort of an anti-gravity. This energy, distinct from dark matter, is responsible for the present-day acceleration of the universal expansion. WMAP data is accurate to two digits, so the total of these numbers is not 100%. This reflects the current limits of WMAP's ability to define Dark Matter and Dark Energy.
Credit: NASA / WMAP Science Team

Time Line of the Universe

Timeline of the Universe: A representation of the evolution of the universe over 13.7 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 380,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe.
Credit: NASA / WMAP Science Team

Temperature Fluctuation by Angular Size

This graph illustrates how much the temperature fluctuates on different anglular sizes in the map. Very large angles are on the left, and smaller angles are on the right. Note that there is a large first peak, illustrating a preferred spot size in the map. This means that there is a preferred length for the sound waves in the early universe, just as a guitar string length produces a specific note. The second and third peaks are the harmonic overtones of the first peak. The third overtone is now clearly captured in the new 5-year WMAP data. It helps provide evidence for neutrinos.

CMB Angular Spectrum: The "angular spectrum" of the fluctuations in the WMAP full-sky map. This shows the relative brightness of the "spots" in the map vs. the size of the spots. The shape of this curve contain a wealth of information about the history the universe.
Credit: NASA / WMAP Science Team