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Wilkinson Microwave Anisotropy Probe

Phase: Past

Launch Date: June 30, 2001

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Program(s):Astrophysics Explorers, Explorers

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The WMAP mission addressed fundamental questions in cosmology: What is the geometry of the Universe? How did structures, such as galaxies and galaxy clusters, that we see in today’s sky come about? How old is the Universe, and what are its constituents?

WMAP Resolves the Universe

The detailed, all-sky picture of the infant universe created from seven 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.
Credit: NASA/WMAP Science Team

Answers to these questions lie in the Cosmic Microwave Background (CMB), the remnant background radiation left over from the Big Bang, which is remarkably uniform over the entire sky, at an effective temperature of 2.7 degrees Kelvin. The CMB, smooth as it is, nevertheless contains tiny fluctuations in its temperature, on the level of one part in 100,000. From these fluctuations grew the structures in the Universe that we see today, and with a careful measurement of the properties of these fluctuations, much can be learned about the history and content of the Universe.

WMAP used differential microwave radiometers that measured temperature differences between two points on the sky. WMAP observed the sky from an orbit about the L2 Sun-Earth Lagrange point, 1.5 million km from Earth. This vantage point offers an exceptionally stable environment for observing since the observatory can always point away from the Sun, Earth and Moon while maintaining an unobstructed view to deep space. WMAP scanned the sky in such a way as to cover ~30% of the sky each day and as the L2 point follows the Earth around the Sun WMAP observed the full sky every six months. To facilitate rejection of foreground signals from our own Galaxy, WMAP used five separate frequency bands from 22 to 90 GHz.

The Wilkinson Microwave Anisotropy Probe (WMAP) was renamed after the late Dr. David Wilkinson of Princeton University, a member of the science team and pioneer in the study of cosmic background radiation.

WMAP was decommissioned in October of 2010 after 9 years of flight. During those 9 years, WMAP helped change how we view our Universe.

  • WMAP found that today our Universe is made up of 72% Dark Energy, 23% Dark Matter and only 4.6% Atoms.
  • WMAP found the age of the Universe is 13.75± 0.13 billion years old. Known to within 1%.
  • WMAP found that the Universe was very different when it was 380,000 years old. At that time it was dominated by Dark Matter (63%), Photons (15%), Atoms (12%), and Neutrinos (10%). Dark Energy did not exist in measureable quantities at that time.
  • WMAP found that the first generation of stars to shine in the Universe ignited only 200 million years after the Big Bang.
  • WMAP found new evidence that a sea of cosmic neutrinos permeates the Universe.
  • WMAP found clear evidence the first stars took more than a half-billion years to create a cosmic fog.
  • WMAP drew tight new constraints on the burst of expansion in the Universe’s first trillionth of a second (called inflation).
  • WMAP provided the first direct detection of pre-stellar helium, providing an important test of the Big Bang prediction.
  • WMAP helped constrain the geometry of the Universe. New data show it must be flat to better than 1%. The simplest model, a flat universe with a cosmological constant, fits the data remarkably well. v
  • WMAP data places constraints on the number of neutrino-like species to between 3 and 5, with 4 as the most likely number. The standard model of particle physics has 3 neutrino species.

As WMAP greatly improved knowledge about the CMB beyond what the COBE mission learned, the recent ESA-led Planck mission has improved upon the legacy left by the WMAP mission. The Planck mission measured the CMB with increased precision and angular resolution compared to WMAP. A key goal of the Planck mission was to measure the polarization of the CMB due to gravitational radiation from the inflation period in the primordial universe.

The 2012 Gruber Cosmology Prize was awarded to Charles L. Bennet and the Wilkinson Microwave Anistropy Probe (WMAP) team for their 'exquisite measurements of anisotropies in the relic radiation from the Big Bang --- the Cosmic Microwave Background. These measurements have helped to secure rigorous constraints on the origin, content, age, and geometry of the Universe, transforming our current paradigm of structure formation from appealing scenario into precise science.'

Timeline 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. 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.
Credit: NASA/WMAP Science Team

Last Updated: April 17, 2015

Related Links
  • CMB Analyzer -
  • COBE to WMAP -
  • Planck -