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Microwave (WMAP) All-Sky Survey


Microwave (WMAP) All-Sky Survey

Group Name wmap
Reference Wilkinson Microwave Anisotropy Probe (2003)
Prepared by Brian Abbott (AMNH/Hayden)
Labels No
Files wmap.speck
Dependencies wmap1024blue.sgi
Wavelength 13.0 mm, 9.09 mm, 7.32 mm, 4.92 mm, 3.23 mm
Frequency 23 GHz, 33 GHz, 41 GHz, 61 GHz, 93 GHz

In 1948, the astronomers Ralph Alpher (b. 1921), Hans Bethe (1906-2005), and George Gamow (1904-1968) published their assertion that the gas in the early Universe must have been very hot and dense and that this gas should be present throughout today's Universe, albeit much cooler and far less dense.

Alpher searched for this cool gas, but it would be another 16 years before it was discovered, not by astronomers but by two physicists working at Bell Telephone Laboratories in New Jersey. In 1964, Arno Penzias (b. 1933) and Robert Wilson (b. 1936) were trying to communicate with a recently launched communications satellite and could not remove “noise” from their transmissions. This weak hiss was a constant nuisance that was present during the day, the night, and throughout the year. This fact ruled out possibilities such as equipment interference, atmospheric effects, or even bird droppings on the radio telescope built to communicate with their satellite.

Penzias and Wilson tried their best to remove this noise but were unsuccessful. In the end, they acknowledged that the faint microwave signal must be real and is not from some defect or artificial interference [see “Electromagnetic Spectrum” for information on microwave light].

In the meantime, researchers down the road at Princeton University were on Alpher's trail, investigating this gas from the early Universe. They maintained that the hot radiation would have been redshifted from gamma rays into X-rays, ultraviolet, visible light, and into the radio range of the EM spectrum. Furthermore, astronomers expected this radiation to be thermal, or what astronomers call blackbody, radiation. An object is said to be a blackbody when it emits all the radiation it absorbs. In the early Universe, with only free electrons and nuclei (protons and neutrons), light scattered off electrons just as light travels through a dense fog and would have produced a blackbody spectrum.

If the signal detected at Bell Labs corresponded to a blackbody, it would have a temperature of about 3 Kelvin, which is equivalent to -270oC or -454oF. (In practical terms, most astronomical objects can be approximated by a blackbody spectrum, which has an inverse relation between the object's peak intensity and its temperature called Wien's Law. Given the wavelength or frequency of the object's peak intensity, Wein's Law tells us the object's temperature.) But the Bell Labs observations could not confirm that the radiation was in fact from a blackbody, and they therefore could not conclude with certainty that this was the radiation left from the Big Bang.

In 1989, the Cosmic Background Explorer (COBE) was launched into orbit to see, once and for all, whether the cosmic microwave background (CMB) was a blackbody (thermal radiation). COBE observed light in the range from a few microns to about 1 cm, covering a broad swath in the radio spectrum. The results were indisputable. COBE had confirmed decades of theories with observational proof that the CMB was indeed the light left over from the Big Bang.

COBE also confirmed that the light was remarkably uniform. No matter where the telescope looked, it observed radiation equivalent to a 2.73-Kelvin blackbody with deviations on the order of one part in a hundred thousand. While COBE's angular resolution on the sky was about 7o, it was able to see small differences in temperature.

In 1995, a new mission to explore the CMB to greater resolution was proposed to NASA. Called MAP (Microwave Anisotropy Probe), it was approved by NASA and launched on June 30, 2001, aboard a Delta II rocket. With the death of David Wilkinson, one of the founding members of MAP and COBE, the mission was named in his honor in 2002.

The WMAP observations have a much higher temperature resolution than COBE, allowing astronomers to see these temperature fluctuations in more detail. At present, the WMAP is the best image of the CMB, and the results from the mission have narrowed down many of the open questions in cosmology, including the age of the Universe and its ultimate fate.


Origin of the CMB

In the beginning, the Universe was very hot and free electrons (those not attached to any atom) prohibited radiation from traveling freely. As the Universe began to expand, the temperature dropped several thousand Kelvin, allowing protons and electrons to combine to form hydrogen atoms. This occurred about 379,000 years after the Big Bang. Once most of the free electrons were bound to hydrogen atoms, the Universe became transparent to light, allowing the cooled radiation left over from the Big Bang to travel freely throughout the Universe.

When this recombination event took place, the light from the Big Bang peaked at about 1 micrometer in the infrared. At that time the gas would have been about 3,000 Kelvin and would have glowed orange-red in the visible spectrum. However, the Universe has expanded 1,000 times since, and the light within space has been redshifted to longer and longer wavelengths. Today the peak wavelength is close to 1 mm (1 micrometer x 1,000 = 1 mm) and corresponds to a gas temperature around 3 Kelvin (3, 000K ÷ 1, 000 = 3K).

The WMAP Image

The WMAP results show the CMB to be 2.725 Kelvin and resolve temperature fluctuations that vary by millionths of a degree. In the image, red patches are warmer by the slightest deviations from the 2.725 K average, while blue patches are cooler by the same slight difference. Remember, though, that the overall temperature would appear uniform to our eyes. If we could see in microwave wavelengths, the sky would be a uniform brightness everywhere, for our eyes cannot see such small fluctuations.

The red regions are the seeds of the large-scale structure we see in today's Universe. The WMAP data give astronomers another piece in the puzzle of the evolution of the Universe. Astronomers continue to ponder how these small temperature fluctuations from 379,000 years after the Big Bang ultimately became the galaxy clusters and filaments we see in the present Universe.

Currently, we miss much of the universe's childhood. The WMAP is the baby picture taken when the Universe was 379,000 years old. Our next picture is not until the Universe is about 900 million years old, when quasars and galaxies appear. What occurred between those times is under the microscope—or telescope, in this case—of astronomers the world over.

Placement of the WMAP

The WMAP all-sky image is a two-dimensional image taken from a space telescope in orbit around Earth. We place the image on a sphere whose radius represents the furthest extent of light from the recombination era. This is a bit deceiving, since the CMB is everywhere in the Universe; however, the sphere marks the farthest reaches of the Universe as seen from Earth, where hydrogen formed 379,000 years after the Big Bang. Currently, this places our horizon at about 42 billion light-years in all directions. Beyond this distance, the Universe will forever be opaque to us.

© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott